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POWER SYSTEM COMMISSIONING AND MAINTENANCE PRACTICE
DET 310
CHAPTER 6
UNDERGROUND CABLES
60 INTRODUCTION
A considerable amount of transmission and distribution of electrical energy especially in densely populated urban areas is carried out by means of underground cable
The underground cable are rugged in construction and provide greater service reliability increased safety better appearance and trouble free service under a variety of environmental conditions
61 Applications Of Underground Cables
Underground cables are necessary for supply connection in the electrical plants in generating stations transmission system and distribution systems utilization plants and so on List of example of underground cable application for connecting one apparatus with the others for the following
- Supply power to the individual machine apparatus in electrical plants- Connection between switchgear and individual load group load- Connection between auxiliary transformer and switchgear- Subtransmission line between receiving substation and distribution substation
62 Underground Distribution System Vs Overhead Line
Safety Reliability of supply Interference Disturbance Maintenance Environment impact Economics
63 Cable Constructions
A cable consists of three main components-bull Conductorbull Insulationbull Sheath
External protection is provided by the sheath against
mechanical damage chemical reaction moisture an so on
63 Cable Construction (continue)-
bull Conductorndash An element design to
transmit electricityndash A single core has one
conductor while a three-core has 3 conductors
ndash A cable may be has single core 3 core or multiple conductor
04172023
6
ETE503 Underground Cable
63 Cable Construction (continue)-
bull Insulationndash Is a material that reduces
or prevents the transmission of electricity
ndash Each conductor is covered by insulation
ndash Insulation is phase to ground and phase to phase
XLPE
PAPER
04172023
7
ETE503 Underground Cable
63 Cable Construction (continue)-
bull Sheathndash Cable protective covering ndash Metallic or nonmetallic
protective covering over the conductor insulation shield
ndash External protection is provided by the sheath against mechanical damage chemical reaction moisture an so on
04172023
8
ETE503 Underground Cable
64 Types of Underground Cables
bullThe identification of the cable are based on the several items
bull Insulationbull Voltage Systembull Cable Sizing And Corebull Technical Specification Characteristics Of The
Cable
9
64 Types of Underground Cables (continue)-
bullUsually the operating voltage decides the types of insulation and cable placed in various categories depending upon the voltage for which they are designed
ndashLow Voltage Cable (LV) 11kVndashHigh Voltage Cable (HV) 11 kV
10
64 Types of Underground Cables (continue)-
bullPaper Insulationndash3 core belted 11kV PILC cablendashSingle core screened 11 kV PILC cable
bullPolymer Insulationndash3 core XLPE 11 kV cablendashSingle core XLPE 11 kV cable
11
64 Types of Underground Cables (continue)-
bull A = Conductor (Aluminum)
bull B = Strand Screen (carbon black paper )bull C = Insulation (Paper)bull D = Insulation Screen
(carbon black paper)bull E = Sheath (copper
lead)bull F = Jacket
Example of Single core screened 11 kV PILC cable
12
65 Types of Underground Cables (continue)-
bull A = Conductor (Aluminum)
bull B = Strand Screen (extruded
semiconducting)bull C = Insulation (XLPE)bull D = Insulation Screen (extruded
semiconducting)bull E = Shield (copper
tape)bull F = Jacket Example of Single core
XLPE 11 kV cable
13
65 Types of Underground Cables (continue)-
ndash Excellent Electrical amp Physical Properties
ndash Capable Of Carrying Large Current At High Temperaturebull Normal ~ 90ocbull Emergency ~ 130ocbull Short Circuit Conditions
~250ocndash Easy To Install ndash XLPE Easier
To Jointndash No Need For Metallic Sheath
14
66 (CABLE FAULT) INTRODUCTION
1048707 Cable faults are undesirable causes because-
1 Power supply is interrupted
2 Locating fault in a long underground cable is difficult and time consuming
3 Repairing faulty cable is difficult and time consuming
66 Cable Fault Introduction (continue)-
662 CAUSES OF UNDERGROUND CABLE FAILURE
Major factors that cause failure of a cable are-
bull Damaged accidentally by external mechanical means
bull Damage caused as a results of mishandling the cable
during layout
bull Poor workmanship in cable jointing
bull Natural causes due to aging
of cable
bull Damaged caused by movement
of soil and erosion
661 MECHANICAL
663 MISHANDLING
Mishandling of cable may be occurred during installation
Some of the examples are
1 Excessive pull
2 Sharp bend
3 Accident crush
664 Poor workmanship During Cable Jointing
The cable are jointed together with poor workmanship can lead to cable fault after a period of time
665 NATURAL CAUSES DUE TO AGING OF CABLE
CONTINUE-
CONTINUE-
66 TYPES OF CABLE FAULT
GENERAL
bull Series (open circuit) Fault
- Failure of continuity (conductor (s) or cable)
bull Shunt (short circuit) fault
- failure of insulation
66 TYPES OF FAULT (CONTINUE)-
66 TYPES OF FAULT (CONTINUE)-
661 SERIES AND SHUNT FAULT
Are subsided into the following categories
Low Resistance Faultof ZR 10
Where Zo= cable surge impedance
=10 ndash 100 ohm
Usually happens in series fault
High Resistance Fault
Where ZoR f 10
662 INTERMITTENT OR FLASH FAULT
- Usually not apparent to insulation resistance measuring
instrument
- Does not manifest itself at lower voltages or a surge
- Breakdown will appear under application of high voltage
dc or DC pressure test
67 FAULT LOCATION PROCEDURE
The proper sequence of cable fault location are as follows
a) Analysis of fault
b) Pre-location
c) Pin Pointing
d) Confirmation and re-test
671 Analysis of Cable Fault
To analyze a cable fault is to determine and confirm the nature or characteristics of the fault
Objective to select which of the cable fault locator equipment of method is best suited to locate the particular fault
Analysis of cable fault normally consists of insulation resistance test and continuity test carried out using 1000V or 5000 V insulation tester
671 Insulation Resistance test
With all earth connections removed from the cable conductor terminal using IR tester of 100025005000V range measure and record the IR in M-ohms K-ohms or ohms between conductors and between conductors and earth Six measurement are to be taken R-E Y-E B-E R-Y Y-B B-R An IR of 100M-ohm indicates a healthy cable
672 Continuity Test
With the cable conductor looped or shorted at the tail endremote end using IR tester selected at continuity range perform continuity test
This test will determine whether any of the cable is open circuited
672 Continuity test (cont-)
Continuity Test - With the cable conductor shorted or looped at the remote end perform continuity test on the cable
- Measure and record the results in ohm
- Three measurements are to be carried out between R-Y Y-B B-R
- The test will determine whether any of cable is open circuited - The resistance per-conductor per km is provided in Table VI VII VIII and IX (refer appendix A)
672 (cont-)
if the continuity of the cable is sound insulation resistance from one end are sufficient
If continuity is broken IR test should be carried out at both ends of the cable
68 BURNING A FAULT
The continuity and IR test may indicate that burning of fault by means of HT pressure test set is required
-
68 BURNING A FAULT (continue-)
Burning a fault is achieve by passing current from a DC HT test set through the fault
Other conductors not under test should be earthed
HT is applied for about 5 to 10 minutes to burn the fault
HT test is used to determine which fault location equipment is suitable to be used
HT is the last resort often used because it sometimes produce ambiguous and unpredictable results
Therefore fault location equipment should be attempted first
69 Pre- Location of fault
Pre-location is the application of a test at the terminals of a
given cable to give an indication of the distance to the fault
from the test point
Whilst the measurement should be accurate as condition will
allow the primary purpose of pre-location is to give an
indication as quick as possible of the vicinity in which to
commence the final pin-pointing tests
69 Pre- Location of fault (cont)-
Generally there are four pre-location methods which are practised and the are as follows
1Bridge or loop Method2 Pulse Echo Time Domain Reflectometry (TDR) method3 Impulse Current Method4 Arc Reflection and secondary Impulse method
691 Bridge method
6911 Direct reading fault localizer
Conditions required
i) No break in continuity if possible one sound core available
ii) The fault resistance of cable should not be more then 20 k-ohm
iii) Applied voltage not to exceed 600 V (DC)
Case 1 Single conductor to earth with sound core available
Fault distance from front end X= n100 x L
Where n = localiser reading in
L= Cable route length
Case2 Conductor to conductor with sound core available
Fault distance from front end X= n100 x L
Where n = localiser reading in
L= Cable route length
Case 3 Three phase fault
By Open loop test and close loop test
(i) Rf2Rf1 gt 5 and (ii) Rf2 lt 20 k-ohm
Open loop test
Case 3 Three phase fault
Close loop test
Open loop and close loop test (cont)
Fault distance from front end
Lxm
mn
X 2
1001
)200200
(
Where m = open loop bridge reading
n = close loop bridge reading
L = cable route length
High Tension Bridge
Conditions required
i) No break in continuity if possible one sound core available
ii) The fault resistance of cable should be more then 20 k-ohm
iii) Applied voltage not to exceed 20kV (DC)
High Tension Bridge
Case 1Single Conductor to earth with sound core available
Fault Distance from test end X = n100 x 2L
High Tension Bridge
Case 2Conductor to conductor with sound core available
Fault Distance from test end X = n100 x 2L
High Tension Bridge
Case 3Three phase Fault with no sound core available
Rf2Rf1gt5 Rf2gt20 kohm
Lxm
)mn
(X 2
1002
1
100100
Where m = open loop bridge reading
n = close loop bridge reading
L = cable route length
6912 Universal Bridge to measure capacitance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be more then 100 ohm
iii) Earth all the conductors not under test
6912 Universal Bridge to measure capacitance of Cable
Open circuit with sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with sound core available
Fault Distance from test end x = CA x L CAB
CA = Capacitance of open circuited coreCAB = Capacitance of sound coreL = conductor length
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
Fault Distance from test endfront end = ((CA(CA+CB))x L
6913 Universal Bridge to measure Inductance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be less then 30 ohm
iii) All conductors be left free from earth
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
Fault distance X = LA x L LAB
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
Fault distance X = LA x L LA + LB
692 Pulse Echo (PE) Time Domain Reflectometry (TDR)
A simple method which works on travelling wave principles It is applicable to all series and shunt faults with Rf gt=Zo10 (5 ohm) and Rflt=10 Zo (500 ohm) respectively For power cables typical Zo=50 ohm
6921 Travelling wave principles
A pulse injected into a cable is reflected back to the source by any change in characteristics impedance (Zo) of the cable
The waveform generated can be monitored using an oscilloscope as shown in Figure below
6921 Continue-
Waveform interpretion
Shunt Fault
Series Fault
6921 Continue-
Waveform interpretion
Low Resistance Shunt Fault (Rflt=500 Ohm)
-Reflection Negative
-Amplitude depends on fault resistance
Series Fault (Rfgt=5 ohm)
- Reflection Positive
- Amplitude depends on fault resistance
Zo = Characteristics impedance = radic(LC)
L = Inductance of cable C = Capacitance of Cable
69211 Reflection Factor (r)
- Is to determine the degree of reflection
Shunt fault r (V) = -Zo1(2Rf + Zo1)
Series Fault r (V) = Rf(2Zo1 + Rf)
Example shunt fault fault resistance = 5 ohm Zo = 50 ohm
r(V) = -50 (2 x5 +50) = - 833If fault resistance = 500 ohm
r(V) = -50 (2x 500 + 50) = -475
69212 Velocity of Propagation (Vp)
- Vp depends on the cable dielectric and is defined as
Vp = Vs radicEr
- where Vs = Velocity of light in free space
Er = Relative permittivity of dielectric
Vp = 300radicEr
Dielectrics Velocity of PropagationVp (mmicroS)
Impregnated Paper 150 -171
PVC 152-175
PE Approx 200
XLPE 156-176
Typical Vp for different Dielectric
69212 Continue (Vp)
Example
Determine the velocity of propagation Vp2 for a cable length of 2500 meters t1= 3164 S and t2 = 1745 S Calculate the fault distances
6921 Continue-
The distance to the fault Lf (m) is given by
Lf = T x Vp2
6921 Continue
Solutions
Velocity of propagation = Vp2 = 25003164 = 79 mS
Distance of Fault from test end = Vp2 x t2
= 79 x 1745
= 137855
693 Impulse Current Method
Working on Travelling wave principles it is applicable to all types of fault
694 Arc Reflection or Secondary Impulse method
Basically PE and TDR associated with fault treatment It is also applicable to faults of all nature but with easily interpreted breakdown waveforms
610 PIN POINTING
Pin Pointing is the application of a test that positively confirms the exact position of the fault
Before the commencement of pin pointing the prelocated fault distance should mark on the cable route which is measured by means of a trumeter
Pin pointing is normally carried out by the shock wave discharge method The fault can be detected by the use of semisphone
611 Confirmation and Re-Test
After the pin-pointed position of the fault has been marked and exposed check for physical sign of fault If there is the fault is confirm
After confirmation the fault should be cut away IR and continuity test should be carried out on the remaining cable sections to determine the soundeness of these cable sections
The insulation resistance test are again carried out after jointing followed by pressure test before supply can be restored
Flowchart for Fault location Procedure
Example
Find a fault distance using direct reading fault localiser from end test which has a test data as below
Bridge Balance Reading =60 Length of Cable 140 meters
Solutions
Fault Distance= = 60100 x 140 = 84 meterxL
n
100
For cables route with combination of 2 or more different type of cable
Determine the resistance of cable for each section separately from Table 1 and Table 2
Example Cable 11kV Al Length 150 m size = 16mm2
From Table 1= Resistance for 1km =226 ohm
For 150 m = 1501000 x 226 = 0339 ohm
From Table 1 Calculate the equivalent resistance for all the combined section
Example for Al = 0339 ohm for copper = 0150 ohm
Equivalent Resistance = 0339+0150 =0489 ohm
Continue
Calculate equivalent resistance from obtained bridge reading
Example Bridge Reading = 60
Equivalent resistance = 06 x 0489 ohm = 02934 ohm
Determine which section of cable the fault occurs
Total resistance at Al section = 0339 ohm
As equivalent resistance at Al section is higher then 02934(Equivalent resistance from bridge test) the fault is
located at Aluminium section of cable
Table 1 Impedance data for 635011000 V Cable Aluminium
Table II Impedance data 11000 V Cable Copper
TABLE VI ndash 6001000V (ALUMINIUM)
TABLE VII ndash 38006600V (ALUMINIUM)
TABLE VIII- 635011000 (ALUMINIUM)
TABLE V
610 COMMISSIONING OF UNDERGROUND CABLE
High Voltage DC testing
High voltage testing is carried out in order to determine the electrical strength of cable insulation Site tests are performed by applying a predetermined high voltage to the insulation DC site test voltages regardless of insulation type are used to ensure that the cable cable joints and termination are correctly made and installed
High Voltage Testing (cont-)
IEC Standards specify maintenance tests after installation should be 70 of permissible factory test DC voltages (15 minutes) = 175kV
Insulation Resistance Test (cont-)
Test voltages should be slowly raised to the required value over a period of about 1 minute and the test period starts once the full voltage is reached In this way the capacitive and absorption currents will have decreased and circuit conditions stabilized such that true leakage current may be measured
60 INTRODUCTION
A considerable amount of transmission and distribution of electrical energy especially in densely populated urban areas is carried out by means of underground cable
The underground cable are rugged in construction and provide greater service reliability increased safety better appearance and trouble free service under a variety of environmental conditions
61 Applications Of Underground Cables
Underground cables are necessary for supply connection in the electrical plants in generating stations transmission system and distribution systems utilization plants and so on List of example of underground cable application for connecting one apparatus with the others for the following
- Supply power to the individual machine apparatus in electrical plants- Connection between switchgear and individual load group load- Connection between auxiliary transformer and switchgear- Subtransmission line between receiving substation and distribution substation
62 Underground Distribution System Vs Overhead Line
Safety Reliability of supply Interference Disturbance Maintenance Environment impact Economics
63 Cable Constructions
A cable consists of three main components-bull Conductorbull Insulationbull Sheath
External protection is provided by the sheath against
mechanical damage chemical reaction moisture an so on
63 Cable Construction (continue)-
bull Conductorndash An element design to
transmit electricityndash A single core has one
conductor while a three-core has 3 conductors
ndash A cable may be has single core 3 core or multiple conductor
04172023
6
ETE503 Underground Cable
63 Cable Construction (continue)-
bull Insulationndash Is a material that reduces
or prevents the transmission of electricity
ndash Each conductor is covered by insulation
ndash Insulation is phase to ground and phase to phase
XLPE
PAPER
04172023
7
ETE503 Underground Cable
63 Cable Construction (continue)-
bull Sheathndash Cable protective covering ndash Metallic or nonmetallic
protective covering over the conductor insulation shield
ndash External protection is provided by the sheath against mechanical damage chemical reaction moisture an so on
04172023
8
ETE503 Underground Cable
64 Types of Underground Cables
bullThe identification of the cable are based on the several items
bull Insulationbull Voltage Systembull Cable Sizing And Corebull Technical Specification Characteristics Of The
Cable
9
64 Types of Underground Cables (continue)-
bullUsually the operating voltage decides the types of insulation and cable placed in various categories depending upon the voltage for which they are designed
ndashLow Voltage Cable (LV) 11kVndashHigh Voltage Cable (HV) 11 kV
10
64 Types of Underground Cables (continue)-
bullPaper Insulationndash3 core belted 11kV PILC cablendashSingle core screened 11 kV PILC cable
bullPolymer Insulationndash3 core XLPE 11 kV cablendashSingle core XLPE 11 kV cable
11
64 Types of Underground Cables (continue)-
bull A = Conductor (Aluminum)
bull B = Strand Screen (carbon black paper )bull C = Insulation (Paper)bull D = Insulation Screen
(carbon black paper)bull E = Sheath (copper
lead)bull F = Jacket
Example of Single core screened 11 kV PILC cable
12
65 Types of Underground Cables (continue)-
bull A = Conductor (Aluminum)
bull B = Strand Screen (extruded
semiconducting)bull C = Insulation (XLPE)bull D = Insulation Screen (extruded
semiconducting)bull E = Shield (copper
tape)bull F = Jacket Example of Single core
XLPE 11 kV cable
13
65 Types of Underground Cables (continue)-
ndash Excellent Electrical amp Physical Properties
ndash Capable Of Carrying Large Current At High Temperaturebull Normal ~ 90ocbull Emergency ~ 130ocbull Short Circuit Conditions
~250ocndash Easy To Install ndash XLPE Easier
To Jointndash No Need For Metallic Sheath
14
66 (CABLE FAULT) INTRODUCTION
1048707 Cable faults are undesirable causes because-
1 Power supply is interrupted
2 Locating fault in a long underground cable is difficult and time consuming
3 Repairing faulty cable is difficult and time consuming
66 Cable Fault Introduction (continue)-
662 CAUSES OF UNDERGROUND CABLE FAILURE
Major factors that cause failure of a cable are-
bull Damaged accidentally by external mechanical means
bull Damage caused as a results of mishandling the cable
during layout
bull Poor workmanship in cable jointing
bull Natural causes due to aging
of cable
bull Damaged caused by movement
of soil and erosion
661 MECHANICAL
663 MISHANDLING
Mishandling of cable may be occurred during installation
Some of the examples are
1 Excessive pull
2 Sharp bend
3 Accident crush
664 Poor workmanship During Cable Jointing
The cable are jointed together with poor workmanship can lead to cable fault after a period of time
665 NATURAL CAUSES DUE TO AGING OF CABLE
CONTINUE-
CONTINUE-
66 TYPES OF CABLE FAULT
GENERAL
bull Series (open circuit) Fault
- Failure of continuity (conductor (s) or cable)
bull Shunt (short circuit) fault
- failure of insulation
66 TYPES OF FAULT (CONTINUE)-
66 TYPES OF FAULT (CONTINUE)-
661 SERIES AND SHUNT FAULT
Are subsided into the following categories
Low Resistance Faultof ZR 10
Where Zo= cable surge impedance
=10 ndash 100 ohm
Usually happens in series fault
High Resistance Fault
Where ZoR f 10
662 INTERMITTENT OR FLASH FAULT
- Usually not apparent to insulation resistance measuring
instrument
- Does not manifest itself at lower voltages or a surge
- Breakdown will appear under application of high voltage
dc or DC pressure test
67 FAULT LOCATION PROCEDURE
The proper sequence of cable fault location are as follows
a) Analysis of fault
b) Pre-location
c) Pin Pointing
d) Confirmation and re-test
671 Analysis of Cable Fault
To analyze a cable fault is to determine and confirm the nature or characteristics of the fault
Objective to select which of the cable fault locator equipment of method is best suited to locate the particular fault
Analysis of cable fault normally consists of insulation resistance test and continuity test carried out using 1000V or 5000 V insulation tester
671 Insulation Resistance test
With all earth connections removed from the cable conductor terminal using IR tester of 100025005000V range measure and record the IR in M-ohms K-ohms or ohms between conductors and between conductors and earth Six measurement are to be taken R-E Y-E B-E R-Y Y-B B-R An IR of 100M-ohm indicates a healthy cable
672 Continuity Test
With the cable conductor looped or shorted at the tail endremote end using IR tester selected at continuity range perform continuity test
This test will determine whether any of the cable is open circuited
672 Continuity test (cont-)
Continuity Test - With the cable conductor shorted or looped at the remote end perform continuity test on the cable
- Measure and record the results in ohm
- Three measurements are to be carried out between R-Y Y-B B-R
- The test will determine whether any of cable is open circuited - The resistance per-conductor per km is provided in Table VI VII VIII and IX (refer appendix A)
672 (cont-)
if the continuity of the cable is sound insulation resistance from one end are sufficient
If continuity is broken IR test should be carried out at both ends of the cable
68 BURNING A FAULT
The continuity and IR test may indicate that burning of fault by means of HT pressure test set is required
-
68 BURNING A FAULT (continue-)
Burning a fault is achieve by passing current from a DC HT test set through the fault
Other conductors not under test should be earthed
HT is applied for about 5 to 10 minutes to burn the fault
HT test is used to determine which fault location equipment is suitable to be used
HT is the last resort often used because it sometimes produce ambiguous and unpredictable results
Therefore fault location equipment should be attempted first
69 Pre- Location of fault
Pre-location is the application of a test at the terminals of a
given cable to give an indication of the distance to the fault
from the test point
Whilst the measurement should be accurate as condition will
allow the primary purpose of pre-location is to give an
indication as quick as possible of the vicinity in which to
commence the final pin-pointing tests
69 Pre- Location of fault (cont)-
Generally there are four pre-location methods which are practised and the are as follows
1Bridge or loop Method2 Pulse Echo Time Domain Reflectometry (TDR) method3 Impulse Current Method4 Arc Reflection and secondary Impulse method
691 Bridge method
6911 Direct reading fault localizer
Conditions required
i) No break in continuity if possible one sound core available
ii) The fault resistance of cable should not be more then 20 k-ohm
iii) Applied voltage not to exceed 600 V (DC)
Case 1 Single conductor to earth with sound core available
Fault distance from front end X= n100 x L
Where n = localiser reading in
L= Cable route length
Case2 Conductor to conductor with sound core available
Fault distance from front end X= n100 x L
Where n = localiser reading in
L= Cable route length
Case 3 Three phase fault
By Open loop test and close loop test
(i) Rf2Rf1 gt 5 and (ii) Rf2 lt 20 k-ohm
Open loop test
Case 3 Three phase fault
Close loop test
Open loop and close loop test (cont)
Fault distance from front end
Lxm
mn
X 2
1001
)200200
(
Where m = open loop bridge reading
n = close loop bridge reading
L = cable route length
High Tension Bridge
Conditions required
i) No break in continuity if possible one sound core available
ii) The fault resistance of cable should be more then 20 k-ohm
iii) Applied voltage not to exceed 20kV (DC)
High Tension Bridge
Case 1Single Conductor to earth with sound core available
Fault Distance from test end X = n100 x 2L
High Tension Bridge
Case 2Conductor to conductor with sound core available
Fault Distance from test end X = n100 x 2L
High Tension Bridge
Case 3Three phase Fault with no sound core available
Rf2Rf1gt5 Rf2gt20 kohm
Lxm
)mn
(X 2
1002
1
100100
Where m = open loop bridge reading
n = close loop bridge reading
L = cable route length
6912 Universal Bridge to measure capacitance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be more then 100 ohm
iii) Earth all the conductors not under test
6912 Universal Bridge to measure capacitance of Cable
Open circuit with sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with sound core available
Fault Distance from test end x = CA x L CAB
CA = Capacitance of open circuited coreCAB = Capacitance of sound coreL = conductor length
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
Fault Distance from test endfront end = ((CA(CA+CB))x L
6913 Universal Bridge to measure Inductance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be less then 30 ohm
iii) All conductors be left free from earth
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
Fault distance X = LA x L LAB
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
Fault distance X = LA x L LA + LB
692 Pulse Echo (PE) Time Domain Reflectometry (TDR)
A simple method which works on travelling wave principles It is applicable to all series and shunt faults with Rf gt=Zo10 (5 ohm) and Rflt=10 Zo (500 ohm) respectively For power cables typical Zo=50 ohm
6921 Travelling wave principles
A pulse injected into a cable is reflected back to the source by any change in characteristics impedance (Zo) of the cable
The waveform generated can be monitored using an oscilloscope as shown in Figure below
6921 Continue-
Waveform interpretion
Shunt Fault
Series Fault
6921 Continue-
Waveform interpretion
Low Resistance Shunt Fault (Rflt=500 Ohm)
-Reflection Negative
-Amplitude depends on fault resistance
Series Fault (Rfgt=5 ohm)
- Reflection Positive
- Amplitude depends on fault resistance
Zo = Characteristics impedance = radic(LC)
L = Inductance of cable C = Capacitance of Cable
69211 Reflection Factor (r)
- Is to determine the degree of reflection
Shunt fault r (V) = -Zo1(2Rf + Zo1)
Series Fault r (V) = Rf(2Zo1 + Rf)
Example shunt fault fault resistance = 5 ohm Zo = 50 ohm
r(V) = -50 (2 x5 +50) = - 833If fault resistance = 500 ohm
r(V) = -50 (2x 500 + 50) = -475
69212 Velocity of Propagation (Vp)
- Vp depends on the cable dielectric and is defined as
Vp = Vs radicEr
- where Vs = Velocity of light in free space
Er = Relative permittivity of dielectric
Vp = 300radicEr
Dielectrics Velocity of PropagationVp (mmicroS)
Impregnated Paper 150 -171
PVC 152-175
PE Approx 200
XLPE 156-176
Typical Vp for different Dielectric
69212 Continue (Vp)
Example
Determine the velocity of propagation Vp2 for a cable length of 2500 meters t1= 3164 S and t2 = 1745 S Calculate the fault distances
6921 Continue-
The distance to the fault Lf (m) is given by
Lf = T x Vp2
6921 Continue
Solutions
Velocity of propagation = Vp2 = 25003164 = 79 mS
Distance of Fault from test end = Vp2 x t2
= 79 x 1745
= 137855
693 Impulse Current Method
Working on Travelling wave principles it is applicable to all types of fault
694 Arc Reflection or Secondary Impulse method
Basically PE and TDR associated with fault treatment It is also applicable to faults of all nature but with easily interpreted breakdown waveforms
610 PIN POINTING
Pin Pointing is the application of a test that positively confirms the exact position of the fault
Before the commencement of pin pointing the prelocated fault distance should mark on the cable route which is measured by means of a trumeter
Pin pointing is normally carried out by the shock wave discharge method The fault can be detected by the use of semisphone
611 Confirmation and Re-Test
After the pin-pointed position of the fault has been marked and exposed check for physical sign of fault If there is the fault is confirm
After confirmation the fault should be cut away IR and continuity test should be carried out on the remaining cable sections to determine the soundeness of these cable sections
The insulation resistance test are again carried out after jointing followed by pressure test before supply can be restored
Flowchart for Fault location Procedure
Example
Find a fault distance using direct reading fault localiser from end test which has a test data as below
Bridge Balance Reading =60 Length of Cable 140 meters
Solutions
Fault Distance= = 60100 x 140 = 84 meterxL
n
100
For cables route with combination of 2 or more different type of cable
Determine the resistance of cable for each section separately from Table 1 and Table 2
Example Cable 11kV Al Length 150 m size = 16mm2
From Table 1= Resistance for 1km =226 ohm
For 150 m = 1501000 x 226 = 0339 ohm
From Table 1 Calculate the equivalent resistance for all the combined section
Example for Al = 0339 ohm for copper = 0150 ohm
Equivalent Resistance = 0339+0150 =0489 ohm
Continue
Calculate equivalent resistance from obtained bridge reading
Example Bridge Reading = 60
Equivalent resistance = 06 x 0489 ohm = 02934 ohm
Determine which section of cable the fault occurs
Total resistance at Al section = 0339 ohm
As equivalent resistance at Al section is higher then 02934(Equivalent resistance from bridge test) the fault is
located at Aluminium section of cable
Table 1 Impedance data for 635011000 V Cable Aluminium
Table II Impedance data 11000 V Cable Copper
TABLE VI ndash 6001000V (ALUMINIUM)
TABLE VII ndash 38006600V (ALUMINIUM)
TABLE VIII- 635011000 (ALUMINIUM)
TABLE V
610 COMMISSIONING OF UNDERGROUND CABLE
High Voltage DC testing
High voltage testing is carried out in order to determine the electrical strength of cable insulation Site tests are performed by applying a predetermined high voltage to the insulation DC site test voltages regardless of insulation type are used to ensure that the cable cable joints and termination are correctly made and installed
High Voltage Testing (cont-)
IEC Standards specify maintenance tests after installation should be 70 of permissible factory test DC voltages (15 minutes) = 175kV
Insulation Resistance Test (cont-)
Test voltages should be slowly raised to the required value over a period of about 1 minute and the test period starts once the full voltage is reached In this way the capacitive and absorption currents will have decreased and circuit conditions stabilized such that true leakage current may be measured
61 Applications Of Underground Cables
Underground cables are necessary for supply connection in the electrical plants in generating stations transmission system and distribution systems utilization plants and so on List of example of underground cable application for connecting one apparatus with the others for the following
- Supply power to the individual machine apparatus in electrical plants- Connection between switchgear and individual load group load- Connection between auxiliary transformer and switchgear- Subtransmission line between receiving substation and distribution substation
62 Underground Distribution System Vs Overhead Line
Safety Reliability of supply Interference Disturbance Maintenance Environment impact Economics
63 Cable Constructions
A cable consists of three main components-bull Conductorbull Insulationbull Sheath
External protection is provided by the sheath against
mechanical damage chemical reaction moisture an so on
63 Cable Construction (continue)-
bull Conductorndash An element design to
transmit electricityndash A single core has one
conductor while a three-core has 3 conductors
ndash A cable may be has single core 3 core or multiple conductor
04172023
6
ETE503 Underground Cable
63 Cable Construction (continue)-
bull Insulationndash Is a material that reduces
or prevents the transmission of electricity
ndash Each conductor is covered by insulation
ndash Insulation is phase to ground and phase to phase
XLPE
PAPER
04172023
7
ETE503 Underground Cable
63 Cable Construction (continue)-
bull Sheathndash Cable protective covering ndash Metallic or nonmetallic
protective covering over the conductor insulation shield
ndash External protection is provided by the sheath against mechanical damage chemical reaction moisture an so on
04172023
8
ETE503 Underground Cable
64 Types of Underground Cables
bullThe identification of the cable are based on the several items
bull Insulationbull Voltage Systembull Cable Sizing And Corebull Technical Specification Characteristics Of The
Cable
9
64 Types of Underground Cables (continue)-
bullUsually the operating voltage decides the types of insulation and cable placed in various categories depending upon the voltage for which they are designed
ndashLow Voltage Cable (LV) 11kVndashHigh Voltage Cable (HV) 11 kV
10
64 Types of Underground Cables (continue)-
bullPaper Insulationndash3 core belted 11kV PILC cablendashSingle core screened 11 kV PILC cable
bullPolymer Insulationndash3 core XLPE 11 kV cablendashSingle core XLPE 11 kV cable
11
64 Types of Underground Cables (continue)-
bull A = Conductor (Aluminum)
bull B = Strand Screen (carbon black paper )bull C = Insulation (Paper)bull D = Insulation Screen
(carbon black paper)bull E = Sheath (copper
lead)bull F = Jacket
Example of Single core screened 11 kV PILC cable
12
65 Types of Underground Cables (continue)-
bull A = Conductor (Aluminum)
bull B = Strand Screen (extruded
semiconducting)bull C = Insulation (XLPE)bull D = Insulation Screen (extruded
semiconducting)bull E = Shield (copper
tape)bull F = Jacket Example of Single core
XLPE 11 kV cable
13
65 Types of Underground Cables (continue)-
ndash Excellent Electrical amp Physical Properties
ndash Capable Of Carrying Large Current At High Temperaturebull Normal ~ 90ocbull Emergency ~ 130ocbull Short Circuit Conditions
~250ocndash Easy To Install ndash XLPE Easier
To Jointndash No Need For Metallic Sheath
14
66 (CABLE FAULT) INTRODUCTION
1048707 Cable faults are undesirable causes because-
1 Power supply is interrupted
2 Locating fault in a long underground cable is difficult and time consuming
3 Repairing faulty cable is difficult and time consuming
66 Cable Fault Introduction (continue)-
662 CAUSES OF UNDERGROUND CABLE FAILURE
Major factors that cause failure of a cable are-
bull Damaged accidentally by external mechanical means
bull Damage caused as a results of mishandling the cable
during layout
bull Poor workmanship in cable jointing
bull Natural causes due to aging
of cable
bull Damaged caused by movement
of soil and erosion
661 MECHANICAL
663 MISHANDLING
Mishandling of cable may be occurred during installation
Some of the examples are
1 Excessive pull
2 Sharp bend
3 Accident crush
664 Poor workmanship During Cable Jointing
The cable are jointed together with poor workmanship can lead to cable fault after a period of time
665 NATURAL CAUSES DUE TO AGING OF CABLE
CONTINUE-
CONTINUE-
66 TYPES OF CABLE FAULT
GENERAL
bull Series (open circuit) Fault
- Failure of continuity (conductor (s) or cable)
bull Shunt (short circuit) fault
- failure of insulation
66 TYPES OF FAULT (CONTINUE)-
66 TYPES OF FAULT (CONTINUE)-
661 SERIES AND SHUNT FAULT
Are subsided into the following categories
Low Resistance Faultof ZR 10
Where Zo= cable surge impedance
=10 ndash 100 ohm
Usually happens in series fault
High Resistance Fault
Where ZoR f 10
662 INTERMITTENT OR FLASH FAULT
- Usually not apparent to insulation resistance measuring
instrument
- Does not manifest itself at lower voltages or a surge
- Breakdown will appear under application of high voltage
dc or DC pressure test
67 FAULT LOCATION PROCEDURE
The proper sequence of cable fault location are as follows
a) Analysis of fault
b) Pre-location
c) Pin Pointing
d) Confirmation and re-test
671 Analysis of Cable Fault
To analyze a cable fault is to determine and confirm the nature or characteristics of the fault
Objective to select which of the cable fault locator equipment of method is best suited to locate the particular fault
Analysis of cable fault normally consists of insulation resistance test and continuity test carried out using 1000V or 5000 V insulation tester
671 Insulation Resistance test
With all earth connections removed from the cable conductor terminal using IR tester of 100025005000V range measure and record the IR in M-ohms K-ohms or ohms between conductors and between conductors and earth Six measurement are to be taken R-E Y-E B-E R-Y Y-B B-R An IR of 100M-ohm indicates a healthy cable
672 Continuity Test
With the cable conductor looped or shorted at the tail endremote end using IR tester selected at continuity range perform continuity test
This test will determine whether any of the cable is open circuited
672 Continuity test (cont-)
Continuity Test - With the cable conductor shorted or looped at the remote end perform continuity test on the cable
- Measure and record the results in ohm
- Three measurements are to be carried out between R-Y Y-B B-R
- The test will determine whether any of cable is open circuited - The resistance per-conductor per km is provided in Table VI VII VIII and IX (refer appendix A)
672 (cont-)
if the continuity of the cable is sound insulation resistance from one end are sufficient
If continuity is broken IR test should be carried out at both ends of the cable
68 BURNING A FAULT
The continuity and IR test may indicate that burning of fault by means of HT pressure test set is required
-
68 BURNING A FAULT (continue-)
Burning a fault is achieve by passing current from a DC HT test set through the fault
Other conductors not under test should be earthed
HT is applied for about 5 to 10 minutes to burn the fault
HT test is used to determine which fault location equipment is suitable to be used
HT is the last resort often used because it sometimes produce ambiguous and unpredictable results
Therefore fault location equipment should be attempted first
69 Pre- Location of fault
Pre-location is the application of a test at the terminals of a
given cable to give an indication of the distance to the fault
from the test point
Whilst the measurement should be accurate as condition will
allow the primary purpose of pre-location is to give an
indication as quick as possible of the vicinity in which to
commence the final pin-pointing tests
69 Pre- Location of fault (cont)-
Generally there are four pre-location methods which are practised and the are as follows
1Bridge or loop Method2 Pulse Echo Time Domain Reflectometry (TDR) method3 Impulse Current Method4 Arc Reflection and secondary Impulse method
691 Bridge method
6911 Direct reading fault localizer
Conditions required
i) No break in continuity if possible one sound core available
ii) The fault resistance of cable should not be more then 20 k-ohm
iii) Applied voltage not to exceed 600 V (DC)
Case 1 Single conductor to earth with sound core available
Fault distance from front end X= n100 x L
Where n = localiser reading in
L= Cable route length
Case2 Conductor to conductor with sound core available
Fault distance from front end X= n100 x L
Where n = localiser reading in
L= Cable route length
Case 3 Three phase fault
By Open loop test and close loop test
(i) Rf2Rf1 gt 5 and (ii) Rf2 lt 20 k-ohm
Open loop test
Case 3 Three phase fault
Close loop test
Open loop and close loop test (cont)
Fault distance from front end
Lxm
mn
X 2
1001
)200200
(
Where m = open loop bridge reading
n = close loop bridge reading
L = cable route length
High Tension Bridge
Conditions required
i) No break in continuity if possible one sound core available
ii) The fault resistance of cable should be more then 20 k-ohm
iii) Applied voltage not to exceed 20kV (DC)
High Tension Bridge
Case 1Single Conductor to earth with sound core available
Fault Distance from test end X = n100 x 2L
High Tension Bridge
Case 2Conductor to conductor with sound core available
Fault Distance from test end X = n100 x 2L
High Tension Bridge
Case 3Three phase Fault with no sound core available
Rf2Rf1gt5 Rf2gt20 kohm
Lxm
)mn
(X 2
1002
1
100100
Where m = open loop bridge reading
n = close loop bridge reading
L = cable route length
6912 Universal Bridge to measure capacitance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be more then 100 ohm
iii) Earth all the conductors not under test
6912 Universal Bridge to measure capacitance of Cable
Open circuit with sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with sound core available
Fault Distance from test end x = CA x L CAB
CA = Capacitance of open circuited coreCAB = Capacitance of sound coreL = conductor length
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
Fault Distance from test endfront end = ((CA(CA+CB))x L
6913 Universal Bridge to measure Inductance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be less then 30 ohm
iii) All conductors be left free from earth
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
Fault distance X = LA x L LAB
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
Fault distance X = LA x L LA + LB
692 Pulse Echo (PE) Time Domain Reflectometry (TDR)
A simple method which works on travelling wave principles It is applicable to all series and shunt faults with Rf gt=Zo10 (5 ohm) and Rflt=10 Zo (500 ohm) respectively For power cables typical Zo=50 ohm
6921 Travelling wave principles
A pulse injected into a cable is reflected back to the source by any change in characteristics impedance (Zo) of the cable
The waveform generated can be monitored using an oscilloscope as shown in Figure below
6921 Continue-
Waveform interpretion
Shunt Fault
Series Fault
6921 Continue-
Waveform interpretion
Low Resistance Shunt Fault (Rflt=500 Ohm)
-Reflection Negative
-Amplitude depends on fault resistance
Series Fault (Rfgt=5 ohm)
- Reflection Positive
- Amplitude depends on fault resistance
Zo = Characteristics impedance = radic(LC)
L = Inductance of cable C = Capacitance of Cable
69211 Reflection Factor (r)
- Is to determine the degree of reflection
Shunt fault r (V) = -Zo1(2Rf + Zo1)
Series Fault r (V) = Rf(2Zo1 + Rf)
Example shunt fault fault resistance = 5 ohm Zo = 50 ohm
r(V) = -50 (2 x5 +50) = - 833If fault resistance = 500 ohm
r(V) = -50 (2x 500 + 50) = -475
69212 Velocity of Propagation (Vp)
- Vp depends on the cable dielectric and is defined as
Vp = Vs radicEr
- where Vs = Velocity of light in free space
Er = Relative permittivity of dielectric
Vp = 300radicEr
Dielectrics Velocity of PropagationVp (mmicroS)
Impregnated Paper 150 -171
PVC 152-175
PE Approx 200
XLPE 156-176
Typical Vp for different Dielectric
69212 Continue (Vp)
Example
Determine the velocity of propagation Vp2 for a cable length of 2500 meters t1= 3164 S and t2 = 1745 S Calculate the fault distances
6921 Continue-
The distance to the fault Lf (m) is given by
Lf = T x Vp2
6921 Continue
Solutions
Velocity of propagation = Vp2 = 25003164 = 79 mS
Distance of Fault from test end = Vp2 x t2
= 79 x 1745
= 137855
693 Impulse Current Method
Working on Travelling wave principles it is applicable to all types of fault
694 Arc Reflection or Secondary Impulse method
Basically PE and TDR associated with fault treatment It is also applicable to faults of all nature but with easily interpreted breakdown waveforms
610 PIN POINTING
Pin Pointing is the application of a test that positively confirms the exact position of the fault
Before the commencement of pin pointing the prelocated fault distance should mark on the cable route which is measured by means of a trumeter
Pin pointing is normally carried out by the shock wave discharge method The fault can be detected by the use of semisphone
611 Confirmation and Re-Test
After the pin-pointed position of the fault has been marked and exposed check for physical sign of fault If there is the fault is confirm
After confirmation the fault should be cut away IR and continuity test should be carried out on the remaining cable sections to determine the soundeness of these cable sections
The insulation resistance test are again carried out after jointing followed by pressure test before supply can be restored
Flowchart for Fault location Procedure
Example
Find a fault distance using direct reading fault localiser from end test which has a test data as below
Bridge Balance Reading =60 Length of Cable 140 meters
Solutions
Fault Distance= = 60100 x 140 = 84 meterxL
n
100
For cables route with combination of 2 or more different type of cable
Determine the resistance of cable for each section separately from Table 1 and Table 2
Example Cable 11kV Al Length 150 m size = 16mm2
From Table 1= Resistance for 1km =226 ohm
For 150 m = 1501000 x 226 = 0339 ohm
From Table 1 Calculate the equivalent resistance for all the combined section
Example for Al = 0339 ohm for copper = 0150 ohm
Equivalent Resistance = 0339+0150 =0489 ohm
Continue
Calculate equivalent resistance from obtained bridge reading
Example Bridge Reading = 60
Equivalent resistance = 06 x 0489 ohm = 02934 ohm
Determine which section of cable the fault occurs
Total resistance at Al section = 0339 ohm
As equivalent resistance at Al section is higher then 02934(Equivalent resistance from bridge test) the fault is
located at Aluminium section of cable
Table 1 Impedance data for 635011000 V Cable Aluminium
Table II Impedance data 11000 V Cable Copper
TABLE VI ndash 6001000V (ALUMINIUM)
TABLE VII ndash 38006600V (ALUMINIUM)
TABLE VIII- 635011000 (ALUMINIUM)
TABLE V
610 COMMISSIONING OF UNDERGROUND CABLE
High Voltage DC testing
High voltage testing is carried out in order to determine the electrical strength of cable insulation Site tests are performed by applying a predetermined high voltage to the insulation DC site test voltages regardless of insulation type are used to ensure that the cable cable joints and termination are correctly made and installed
High Voltage Testing (cont-)
IEC Standards specify maintenance tests after installation should be 70 of permissible factory test DC voltages (15 minutes) = 175kV
Insulation Resistance Test (cont-)
Test voltages should be slowly raised to the required value over a period of about 1 minute and the test period starts once the full voltage is reached In this way the capacitive and absorption currents will have decreased and circuit conditions stabilized such that true leakage current may be measured
62 Underground Distribution System Vs Overhead Line
Safety Reliability of supply Interference Disturbance Maintenance Environment impact Economics
63 Cable Constructions
A cable consists of three main components-bull Conductorbull Insulationbull Sheath
External protection is provided by the sheath against
mechanical damage chemical reaction moisture an so on
63 Cable Construction (continue)-
bull Conductorndash An element design to
transmit electricityndash A single core has one
conductor while a three-core has 3 conductors
ndash A cable may be has single core 3 core or multiple conductor
04172023
6
ETE503 Underground Cable
63 Cable Construction (continue)-
bull Insulationndash Is a material that reduces
or prevents the transmission of electricity
ndash Each conductor is covered by insulation
ndash Insulation is phase to ground and phase to phase
XLPE
PAPER
04172023
7
ETE503 Underground Cable
63 Cable Construction (continue)-
bull Sheathndash Cable protective covering ndash Metallic or nonmetallic
protective covering over the conductor insulation shield
ndash External protection is provided by the sheath against mechanical damage chemical reaction moisture an so on
04172023
8
ETE503 Underground Cable
64 Types of Underground Cables
bullThe identification of the cable are based on the several items
bull Insulationbull Voltage Systembull Cable Sizing And Corebull Technical Specification Characteristics Of The
Cable
9
64 Types of Underground Cables (continue)-
bullUsually the operating voltage decides the types of insulation and cable placed in various categories depending upon the voltage for which they are designed
ndashLow Voltage Cable (LV) 11kVndashHigh Voltage Cable (HV) 11 kV
10
64 Types of Underground Cables (continue)-
bullPaper Insulationndash3 core belted 11kV PILC cablendashSingle core screened 11 kV PILC cable
bullPolymer Insulationndash3 core XLPE 11 kV cablendashSingle core XLPE 11 kV cable
11
64 Types of Underground Cables (continue)-
bull A = Conductor (Aluminum)
bull B = Strand Screen (carbon black paper )bull C = Insulation (Paper)bull D = Insulation Screen
(carbon black paper)bull E = Sheath (copper
lead)bull F = Jacket
Example of Single core screened 11 kV PILC cable
12
65 Types of Underground Cables (continue)-
bull A = Conductor (Aluminum)
bull B = Strand Screen (extruded
semiconducting)bull C = Insulation (XLPE)bull D = Insulation Screen (extruded
semiconducting)bull E = Shield (copper
tape)bull F = Jacket Example of Single core
XLPE 11 kV cable
13
65 Types of Underground Cables (continue)-
ndash Excellent Electrical amp Physical Properties
ndash Capable Of Carrying Large Current At High Temperaturebull Normal ~ 90ocbull Emergency ~ 130ocbull Short Circuit Conditions
~250ocndash Easy To Install ndash XLPE Easier
To Jointndash No Need For Metallic Sheath
14
66 (CABLE FAULT) INTRODUCTION
1048707 Cable faults are undesirable causes because-
1 Power supply is interrupted
2 Locating fault in a long underground cable is difficult and time consuming
3 Repairing faulty cable is difficult and time consuming
66 Cable Fault Introduction (continue)-
662 CAUSES OF UNDERGROUND CABLE FAILURE
Major factors that cause failure of a cable are-
bull Damaged accidentally by external mechanical means
bull Damage caused as a results of mishandling the cable
during layout
bull Poor workmanship in cable jointing
bull Natural causes due to aging
of cable
bull Damaged caused by movement
of soil and erosion
661 MECHANICAL
663 MISHANDLING
Mishandling of cable may be occurred during installation
Some of the examples are
1 Excessive pull
2 Sharp bend
3 Accident crush
664 Poor workmanship During Cable Jointing
The cable are jointed together with poor workmanship can lead to cable fault after a period of time
665 NATURAL CAUSES DUE TO AGING OF CABLE
CONTINUE-
CONTINUE-
66 TYPES OF CABLE FAULT
GENERAL
bull Series (open circuit) Fault
- Failure of continuity (conductor (s) or cable)
bull Shunt (short circuit) fault
- failure of insulation
66 TYPES OF FAULT (CONTINUE)-
66 TYPES OF FAULT (CONTINUE)-
661 SERIES AND SHUNT FAULT
Are subsided into the following categories
Low Resistance Faultof ZR 10
Where Zo= cable surge impedance
=10 ndash 100 ohm
Usually happens in series fault
High Resistance Fault
Where ZoR f 10
662 INTERMITTENT OR FLASH FAULT
- Usually not apparent to insulation resistance measuring
instrument
- Does not manifest itself at lower voltages or a surge
- Breakdown will appear under application of high voltage
dc or DC pressure test
67 FAULT LOCATION PROCEDURE
The proper sequence of cable fault location are as follows
a) Analysis of fault
b) Pre-location
c) Pin Pointing
d) Confirmation and re-test
671 Analysis of Cable Fault
To analyze a cable fault is to determine and confirm the nature or characteristics of the fault
Objective to select which of the cable fault locator equipment of method is best suited to locate the particular fault
Analysis of cable fault normally consists of insulation resistance test and continuity test carried out using 1000V or 5000 V insulation tester
671 Insulation Resistance test
With all earth connections removed from the cable conductor terminal using IR tester of 100025005000V range measure and record the IR in M-ohms K-ohms or ohms between conductors and between conductors and earth Six measurement are to be taken R-E Y-E B-E R-Y Y-B B-R An IR of 100M-ohm indicates a healthy cable
672 Continuity Test
With the cable conductor looped or shorted at the tail endremote end using IR tester selected at continuity range perform continuity test
This test will determine whether any of the cable is open circuited
672 Continuity test (cont-)
Continuity Test - With the cable conductor shorted or looped at the remote end perform continuity test on the cable
- Measure and record the results in ohm
- Three measurements are to be carried out between R-Y Y-B B-R
- The test will determine whether any of cable is open circuited - The resistance per-conductor per km is provided in Table VI VII VIII and IX (refer appendix A)
672 (cont-)
if the continuity of the cable is sound insulation resistance from one end are sufficient
If continuity is broken IR test should be carried out at both ends of the cable
68 BURNING A FAULT
The continuity and IR test may indicate that burning of fault by means of HT pressure test set is required
-
68 BURNING A FAULT (continue-)
Burning a fault is achieve by passing current from a DC HT test set through the fault
Other conductors not under test should be earthed
HT is applied for about 5 to 10 minutes to burn the fault
HT test is used to determine which fault location equipment is suitable to be used
HT is the last resort often used because it sometimes produce ambiguous and unpredictable results
Therefore fault location equipment should be attempted first
69 Pre- Location of fault
Pre-location is the application of a test at the terminals of a
given cable to give an indication of the distance to the fault
from the test point
Whilst the measurement should be accurate as condition will
allow the primary purpose of pre-location is to give an
indication as quick as possible of the vicinity in which to
commence the final pin-pointing tests
69 Pre- Location of fault (cont)-
Generally there are four pre-location methods which are practised and the are as follows
1Bridge or loop Method2 Pulse Echo Time Domain Reflectometry (TDR) method3 Impulse Current Method4 Arc Reflection and secondary Impulse method
691 Bridge method
6911 Direct reading fault localizer
Conditions required
i) No break in continuity if possible one sound core available
ii) The fault resistance of cable should not be more then 20 k-ohm
iii) Applied voltage not to exceed 600 V (DC)
Case 1 Single conductor to earth with sound core available
Fault distance from front end X= n100 x L
Where n = localiser reading in
L= Cable route length
Case2 Conductor to conductor with sound core available
Fault distance from front end X= n100 x L
Where n = localiser reading in
L= Cable route length
Case 3 Three phase fault
By Open loop test and close loop test
(i) Rf2Rf1 gt 5 and (ii) Rf2 lt 20 k-ohm
Open loop test
Case 3 Three phase fault
Close loop test
Open loop and close loop test (cont)
Fault distance from front end
Lxm
mn
X 2
1001
)200200
(
Where m = open loop bridge reading
n = close loop bridge reading
L = cable route length
High Tension Bridge
Conditions required
i) No break in continuity if possible one sound core available
ii) The fault resistance of cable should be more then 20 k-ohm
iii) Applied voltage not to exceed 20kV (DC)
High Tension Bridge
Case 1Single Conductor to earth with sound core available
Fault Distance from test end X = n100 x 2L
High Tension Bridge
Case 2Conductor to conductor with sound core available
Fault Distance from test end X = n100 x 2L
High Tension Bridge
Case 3Three phase Fault with no sound core available
Rf2Rf1gt5 Rf2gt20 kohm
Lxm
)mn
(X 2
1002
1
100100
Where m = open loop bridge reading
n = close loop bridge reading
L = cable route length
6912 Universal Bridge to measure capacitance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be more then 100 ohm
iii) Earth all the conductors not under test
6912 Universal Bridge to measure capacitance of Cable
Open circuit with sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with sound core available
Fault Distance from test end x = CA x L CAB
CA = Capacitance of open circuited coreCAB = Capacitance of sound coreL = conductor length
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
Fault Distance from test endfront end = ((CA(CA+CB))x L
6913 Universal Bridge to measure Inductance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be less then 30 ohm
iii) All conductors be left free from earth
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
Fault distance X = LA x L LAB
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
Fault distance X = LA x L LA + LB
692 Pulse Echo (PE) Time Domain Reflectometry (TDR)
A simple method which works on travelling wave principles It is applicable to all series and shunt faults with Rf gt=Zo10 (5 ohm) and Rflt=10 Zo (500 ohm) respectively For power cables typical Zo=50 ohm
6921 Travelling wave principles
A pulse injected into a cable is reflected back to the source by any change in characteristics impedance (Zo) of the cable
The waveform generated can be monitored using an oscilloscope as shown in Figure below
6921 Continue-
Waveform interpretion
Shunt Fault
Series Fault
6921 Continue-
Waveform interpretion
Low Resistance Shunt Fault (Rflt=500 Ohm)
-Reflection Negative
-Amplitude depends on fault resistance
Series Fault (Rfgt=5 ohm)
- Reflection Positive
- Amplitude depends on fault resistance
Zo = Characteristics impedance = radic(LC)
L = Inductance of cable C = Capacitance of Cable
69211 Reflection Factor (r)
- Is to determine the degree of reflection
Shunt fault r (V) = -Zo1(2Rf + Zo1)
Series Fault r (V) = Rf(2Zo1 + Rf)
Example shunt fault fault resistance = 5 ohm Zo = 50 ohm
r(V) = -50 (2 x5 +50) = - 833If fault resistance = 500 ohm
r(V) = -50 (2x 500 + 50) = -475
69212 Velocity of Propagation (Vp)
- Vp depends on the cable dielectric and is defined as
Vp = Vs radicEr
- where Vs = Velocity of light in free space
Er = Relative permittivity of dielectric
Vp = 300radicEr
Dielectrics Velocity of PropagationVp (mmicroS)
Impregnated Paper 150 -171
PVC 152-175
PE Approx 200
XLPE 156-176
Typical Vp for different Dielectric
69212 Continue (Vp)
Example
Determine the velocity of propagation Vp2 for a cable length of 2500 meters t1= 3164 S and t2 = 1745 S Calculate the fault distances
6921 Continue-
The distance to the fault Lf (m) is given by
Lf = T x Vp2
6921 Continue
Solutions
Velocity of propagation = Vp2 = 25003164 = 79 mS
Distance of Fault from test end = Vp2 x t2
= 79 x 1745
= 137855
693 Impulse Current Method
Working on Travelling wave principles it is applicable to all types of fault
694 Arc Reflection or Secondary Impulse method
Basically PE and TDR associated with fault treatment It is also applicable to faults of all nature but with easily interpreted breakdown waveforms
610 PIN POINTING
Pin Pointing is the application of a test that positively confirms the exact position of the fault
Before the commencement of pin pointing the prelocated fault distance should mark on the cable route which is measured by means of a trumeter
Pin pointing is normally carried out by the shock wave discharge method The fault can be detected by the use of semisphone
611 Confirmation and Re-Test
After the pin-pointed position of the fault has been marked and exposed check for physical sign of fault If there is the fault is confirm
After confirmation the fault should be cut away IR and continuity test should be carried out on the remaining cable sections to determine the soundeness of these cable sections
The insulation resistance test are again carried out after jointing followed by pressure test before supply can be restored
Flowchart for Fault location Procedure
Example
Find a fault distance using direct reading fault localiser from end test which has a test data as below
Bridge Balance Reading =60 Length of Cable 140 meters
Solutions
Fault Distance= = 60100 x 140 = 84 meterxL
n
100
For cables route with combination of 2 or more different type of cable
Determine the resistance of cable for each section separately from Table 1 and Table 2
Example Cable 11kV Al Length 150 m size = 16mm2
From Table 1= Resistance for 1km =226 ohm
For 150 m = 1501000 x 226 = 0339 ohm
From Table 1 Calculate the equivalent resistance for all the combined section
Example for Al = 0339 ohm for copper = 0150 ohm
Equivalent Resistance = 0339+0150 =0489 ohm
Continue
Calculate equivalent resistance from obtained bridge reading
Example Bridge Reading = 60
Equivalent resistance = 06 x 0489 ohm = 02934 ohm
Determine which section of cable the fault occurs
Total resistance at Al section = 0339 ohm
As equivalent resistance at Al section is higher then 02934(Equivalent resistance from bridge test) the fault is
located at Aluminium section of cable
Table 1 Impedance data for 635011000 V Cable Aluminium
Table II Impedance data 11000 V Cable Copper
TABLE VI ndash 6001000V (ALUMINIUM)
TABLE VII ndash 38006600V (ALUMINIUM)
TABLE VIII- 635011000 (ALUMINIUM)
TABLE V
610 COMMISSIONING OF UNDERGROUND CABLE
High Voltage DC testing
High voltage testing is carried out in order to determine the electrical strength of cable insulation Site tests are performed by applying a predetermined high voltage to the insulation DC site test voltages regardless of insulation type are used to ensure that the cable cable joints and termination are correctly made and installed
High Voltage Testing (cont-)
IEC Standards specify maintenance tests after installation should be 70 of permissible factory test DC voltages (15 minutes) = 175kV
Insulation Resistance Test (cont-)
Test voltages should be slowly raised to the required value over a period of about 1 minute and the test period starts once the full voltage is reached In this way the capacitive and absorption currents will have decreased and circuit conditions stabilized such that true leakage current may be measured
63 Cable Constructions
A cable consists of three main components-bull Conductorbull Insulationbull Sheath
External protection is provided by the sheath against
mechanical damage chemical reaction moisture an so on
63 Cable Construction (continue)-
bull Conductorndash An element design to
transmit electricityndash A single core has one
conductor while a three-core has 3 conductors
ndash A cable may be has single core 3 core or multiple conductor
04172023
6
ETE503 Underground Cable
63 Cable Construction (continue)-
bull Insulationndash Is a material that reduces
or prevents the transmission of electricity
ndash Each conductor is covered by insulation
ndash Insulation is phase to ground and phase to phase
XLPE
PAPER
04172023
7
ETE503 Underground Cable
63 Cable Construction (continue)-
bull Sheathndash Cable protective covering ndash Metallic or nonmetallic
protective covering over the conductor insulation shield
ndash External protection is provided by the sheath against mechanical damage chemical reaction moisture an so on
04172023
8
ETE503 Underground Cable
64 Types of Underground Cables
bullThe identification of the cable are based on the several items
bull Insulationbull Voltage Systembull Cable Sizing And Corebull Technical Specification Characteristics Of The
Cable
9
64 Types of Underground Cables (continue)-
bullUsually the operating voltage decides the types of insulation and cable placed in various categories depending upon the voltage for which they are designed
ndashLow Voltage Cable (LV) 11kVndashHigh Voltage Cable (HV) 11 kV
10
64 Types of Underground Cables (continue)-
bullPaper Insulationndash3 core belted 11kV PILC cablendashSingle core screened 11 kV PILC cable
bullPolymer Insulationndash3 core XLPE 11 kV cablendashSingle core XLPE 11 kV cable
11
64 Types of Underground Cables (continue)-
bull A = Conductor (Aluminum)
bull B = Strand Screen (carbon black paper )bull C = Insulation (Paper)bull D = Insulation Screen
(carbon black paper)bull E = Sheath (copper
lead)bull F = Jacket
Example of Single core screened 11 kV PILC cable
12
65 Types of Underground Cables (continue)-
bull A = Conductor (Aluminum)
bull B = Strand Screen (extruded
semiconducting)bull C = Insulation (XLPE)bull D = Insulation Screen (extruded
semiconducting)bull E = Shield (copper
tape)bull F = Jacket Example of Single core
XLPE 11 kV cable
13
65 Types of Underground Cables (continue)-
ndash Excellent Electrical amp Physical Properties
ndash Capable Of Carrying Large Current At High Temperaturebull Normal ~ 90ocbull Emergency ~ 130ocbull Short Circuit Conditions
~250ocndash Easy To Install ndash XLPE Easier
To Jointndash No Need For Metallic Sheath
14
66 (CABLE FAULT) INTRODUCTION
1048707 Cable faults are undesirable causes because-
1 Power supply is interrupted
2 Locating fault in a long underground cable is difficult and time consuming
3 Repairing faulty cable is difficult and time consuming
66 Cable Fault Introduction (continue)-
662 CAUSES OF UNDERGROUND CABLE FAILURE
Major factors that cause failure of a cable are-
bull Damaged accidentally by external mechanical means
bull Damage caused as a results of mishandling the cable
during layout
bull Poor workmanship in cable jointing
bull Natural causes due to aging
of cable
bull Damaged caused by movement
of soil and erosion
661 MECHANICAL
663 MISHANDLING
Mishandling of cable may be occurred during installation
Some of the examples are
1 Excessive pull
2 Sharp bend
3 Accident crush
664 Poor workmanship During Cable Jointing
The cable are jointed together with poor workmanship can lead to cable fault after a period of time
665 NATURAL CAUSES DUE TO AGING OF CABLE
CONTINUE-
CONTINUE-
66 TYPES OF CABLE FAULT
GENERAL
bull Series (open circuit) Fault
- Failure of continuity (conductor (s) or cable)
bull Shunt (short circuit) fault
- failure of insulation
66 TYPES OF FAULT (CONTINUE)-
66 TYPES OF FAULT (CONTINUE)-
661 SERIES AND SHUNT FAULT
Are subsided into the following categories
Low Resistance Faultof ZR 10
Where Zo= cable surge impedance
=10 ndash 100 ohm
Usually happens in series fault
High Resistance Fault
Where ZoR f 10
662 INTERMITTENT OR FLASH FAULT
- Usually not apparent to insulation resistance measuring
instrument
- Does not manifest itself at lower voltages or a surge
- Breakdown will appear under application of high voltage
dc or DC pressure test
67 FAULT LOCATION PROCEDURE
The proper sequence of cable fault location are as follows
a) Analysis of fault
b) Pre-location
c) Pin Pointing
d) Confirmation and re-test
671 Analysis of Cable Fault
To analyze a cable fault is to determine and confirm the nature or characteristics of the fault
Objective to select which of the cable fault locator equipment of method is best suited to locate the particular fault
Analysis of cable fault normally consists of insulation resistance test and continuity test carried out using 1000V or 5000 V insulation tester
671 Insulation Resistance test
With all earth connections removed from the cable conductor terminal using IR tester of 100025005000V range measure and record the IR in M-ohms K-ohms or ohms between conductors and between conductors and earth Six measurement are to be taken R-E Y-E B-E R-Y Y-B B-R An IR of 100M-ohm indicates a healthy cable
672 Continuity Test
With the cable conductor looped or shorted at the tail endremote end using IR tester selected at continuity range perform continuity test
This test will determine whether any of the cable is open circuited
672 Continuity test (cont-)
Continuity Test - With the cable conductor shorted or looped at the remote end perform continuity test on the cable
- Measure and record the results in ohm
- Three measurements are to be carried out between R-Y Y-B B-R
- The test will determine whether any of cable is open circuited - The resistance per-conductor per km is provided in Table VI VII VIII and IX (refer appendix A)
672 (cont-)
if the continuity of the cable is sound insulation resistance from one end are sufficient
If continuity is broken IR test should be carried out at both ends of the cable
68 BURNING A FAULT
The continuity and IR test may indicate that burning of fault by means of HT pressure test set is required
-
68 BURNING A FAULT (continue-)
Burning a fault is achieve by passing current from a DC HT test set through the fault
Other conductors not under test should be earthed
HT is applied for about 5 to 10 minutes to burn the fault
HT test is used to determine which fault location equipment is suitable to be used
HT is the last resort often used because it sometimes produce ambiguous and unpredictable results
Therefore fault location equipment should be attempted first
69 Pre- Location of fault
Pre-location is the application of a test at the terminals of a
given cable to give an indication of the distance to the fault
from the test point
Whilst the measurement should be accurate as condition will
allow the primary purpose of pre-location is to give an
indication as quick as possible of the vicinity in which to
commence the final pin-pointing tests
69 Pre- Location of fault (cont)-
Generally there are four pre-location methods which are practised and the are as follows
1Bridge or loop Method2 Pulse Echo Time Domain Reflectometry (TDR) method3 Impulse Current Method4 Arc Reflection and secondary Impulse method
691 Bridge method
6911 Direct reading fault localizer
Conditions required
i) No break in continuity if possible one sound core available
ii) The fault resistance of cable should not be more then 20 k-ohm
iii) Applied voltage not to exceed 600 V (DC)
Case 1 Single conductor to earth with sound core available
Fault distance from front end X= n100 x L
Where n = localiser reading in
L= Cable route length
Case2 Conductor to conductor with sound core available
Fault distance from front end X= n100 x L
Where n = localiser reading in
L= Cable route length
Case 3 Three phase fault
By Open loop test and close loop test
(i) Rf2Rf1 gt 5 and (ii) Rf2 lt 20 k-ohm
Open loop test
Case 3 Three phase fault
Close loop test
Open loop and close loop test (cont)
Fault distance from front end
Lxm
mn
X 2
1001
)200200
(
Where m = open loop bridge reading
n = close loop bridge reading
L = cable route length
High Tension Bridge
Conditions required
i) No break in continuity if possible one sound core available
ii) The fault resistance of cable should be more then 20 k-ohm
iii) Applied voltage not to exceed 20kV (DC)
High Tension Bridge
Case 1Single Conductor to earth with sound core available
Fault Distance from test end X = n100 x 2L
High Tension Bridge
Case 2Conductor to conductor with sound core available
Fault Distance from test end X = n100 x 2L
High Tension Bridge
Case 3Three phase Fault with no sound core available
Rf2Rf1gt5 Rf2gt20 kohm
Lxm
)mn
(X 2
1002
1
100100
Where m = open loop bridge reading
n = close loop bridge reading
L = cable route length
6912 Universal Bridge to measure capacitance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be more then 100 ohm
iii) Earth all the conductors not under test
6912 Universal Bridge to measure capacitance of Cable
Open circuit with sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with sound core available
Fault Distance from test end x = CA x L CAB
CA = Capacitance of open circuited coreCAB = Capacitance of sound coreL = conductor length
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
Fault Distance from test endfront end = ((CA(CA+CB))x L
6913 Universal Bridge to measure Inductance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be less then 30 ohm
iii) All conductors be left free from earth
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
Fault distance X = LA x L LAB
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
Fault distance X = LA x L LA + LB
692 Pulse Echo (PE) Time Domain Reflectometry (TDR)
A simple method which works on travelling wave principles It is applicable to all series and shunt faults with Rf gt=Zo10 (5 ohm) and Rflt=10 Zo (500 ohm) respectively For power cables typical Zo=50 ohm
6921 Travelling wave principles
A pulse injected into a cable is reflected back to the source by any change in characteristics impedance (Zo) of the cable
The waveform generated can be monitored using an oscilloscope as shown in Figure below
6921 Continue-
Waveform interpretion
Shunt Fault
Series Fault
6921 Continue-
Waveform interpretion
Low Resistance Shunt Fault (Rflt=500 Ohm)
-Reflection Negative
-Amplitude depends on fault resistance
Series Fault (Rfgt=5 ohm)
- Reflection Positive
- Amplitude depends on fault resistance
Zo = Characteristics impedance = radic(LC)
L = Inductance of cable C = Capacitance of Cable
69211 Reflection Factor (r)
- Is to determine the degree of reflection
Shunt fault r (V) = -Zo1(2Rf + Zo1)
Series Fault r (V) = Rf(2Zo1 + Rf)
Example shunt fault fault resistance = 5 ohm Zo = 50 ohm
r(V) = -50 (2 x5 +50) = - 833If fault resistance = 500 ohm
r(V) = -50 (2x 500 + 50) = -475
69212 Velocity of Propagation (Vp)
- Vp depends on the cable dielectric and is defined as
Vp = Vs radicEr
- where Vs = Velocity of light in free space
Er = Relative permittivity of dielectric
Vp = 300radicEr
Dielectrics Velocity of PropagationVp (mmicroS)
Impregnated Paper 150 -171
PVC 152-175
PE Approx 200
XLPE 156-176
Typical Vp for different Dielectric
69212 Continue (Vp)
Example
Determine the velocity of propagation Vp2 for a cable length of 2500 meters t1= 3164 S and t2 = 1745 S Calculate the fault distances
6921 Continue-
The distance to the fault Lf (m) is given by
Lf = T x Vp2
6921 Continue
Solutions
Velocity of propagation = Vp2 = 25003164 = 79 mS
Distance of Fault from test end = Vp2 x t2
= 79 x 1745
= 137855
693 Impulse Current Method
Working on Travelling wave principles it is applicable to all types of fault
694 Arc Reflection or Secondary Impulse method
Basically PE and TDR associated with fault treatment It is also applicable to faults of all nature but with easily interpreted breakdown waveforms
610 PIN POINTING
Pin Pointing is the application of a test that positively confirms the exact position of the fault
Before the commencement of pin pointing the prelocated fault distance should mark on the cable route which is measured by means of a trumeter
Pin pointing is normally carried out by the shock wave discharge method The fault can be detected by the use of semisphone
611 Confirmation and Re-Test
After the pin-pointed position of the fault has been marked and exposed check for physical sign of fault If there is the fault is confirm
After confirmation the fault should be cut away IR and continuity test should be carried out on the remaining cable sections to determine the soundeness of these cable sections
The insulation resistance test are again carried out after jointing followed by pressure test before supply can be restored
Flowchart for Fault location Procedure
Example
Find a fault distance using direct reading fault localiser from end test which has a test data as below
Bridge Balance Reading =60 Length of Cable 140 meters
Solutions
Fault Distance= = 60100 x 140 = 84 meterxL
n
100
For cables route with combination of 2 or more different type of cable
Determine the resistance of cable for each section separately from Table 1 and Table 2
Example Cable 11kV Al Length 150 m size = 16mm2
From Table 1= Resistance for 1km =226 ohm
For 150 m = 1501000 x 226 = 0339 ohm
From Table 1 Calculate the equivalent resistance for all the combined section
Example for Al = 0339 ohm for copper = 0150 ohm
Equivalent Resistance = 0339+0150 =0489 ohm
Continue
Calculate equivalent resistance from obtained bridge reading
Example Bridge Reading = 60
Equivalent resistance = 06 x 0489 ohm = 02934 ohm
Determine which section of cable the fault occurs
Total resistance at Al section = 0339 ohm
As equivalent resistance at Al section is higher then 02934(Equivalent resistance from bridge test) the fault is
located at Aluminium section of cable
Table 1 Impedance data for 635011000 V Cable Aluminium
Table II Impedance data 11000 V Cable Copper
TABLE VI ndash 6001000V (ALUMINIUM)
TABLE VII ndash 38006600V (ALUMINIUM)
TABLE VIII- 635011000 (ALUMINIUM)
TABLE V
610 COMMISSIONING OF UNDERGROUND CABLE
High Voltage DC testing
High voltage testing is carried out in order to determine the electrical strength of cable insulation Site tests are performed by applying a predetermined high voltage to the insulation DC site test voltages regardless of insulation type are used to ensure that the cable cable joints and termination are correctly made and installed
High Voltage Testing (cont-)
IEC Standards specify maintenance tests after installation should be 70 of permissible factory test DC voltages (15 minutes) = 175kV
Insulation Resistance Test (cont-)
Test voltages should be slowly raised to the required value over a period of about 1 minute and the test period starts once the full voltage is reached In this way the capacitive and absorption currents will have decreased and circuit conditions stabilized such that true leakage current may be measured
63 Cable Construction (continue)-
bull Conductorndash An element design to
transmit electricityndash A single core has one
conductor while a three-core has 3 conductors
ndash A cable may be has single core 3 core or multiple conductor
04172023
6
ETE503 Underground Cable
63 Cable Construction (continue)-
bull Insulationndash Is a material that reduces
or prevents the transmission of electricity
ndash Each conductor is covered by insulation
ndash Insulation is phase to ground and phase to phase
XLPE
PAPER
04172023
7
ETE503 Underground Cable
63 Cable Construction (continue)-
bull Sheathndash Cable protective covering ndash Metallic or nonmetallic
protective covering over the conductor insulation shield
ndash External protection is provided by the sheath against mechanical damage chemical reaction moisture an so on
04172023
8
ETE503 Underground Cable
64 Types of Underground Cables
bullThe identification of the cable are based on the several items
bull Insulationbull Voltage Systembull Cable Sizing And Corebull Technical Specification Characteristics Of The
Cable
9
64 Types of Underground Cables (continue)-
bullUsually the operating voltage decides the types of insulation and cable placed in various categories depending upon the voltage for which they are designed
ndashLow Voltage Cable (LV) 11kVndashHigh Voltage Cable (HV) 11 kV
10
64 Types of Underground Cables (continue)-
bullPaper Insulationndash3 core belted 11kV PILC cablendashSingle core screened 11 kV PILC cable
bullPolymer Insulationndash3 core XLPE 11 kV cablendashSingle core XLPE 11 kV cable
11
64 Types of Underground Cables (continue)-
bull A = Conductor (Aluminum)
bull B = Strand Screen (carbon black paper )bull C = Insulation (Paper)bull D = Insulation Screen
(carbon black paper)bull E = Sheath (copper
lead)bull F = Jacket
Example of Single core screened 11 kV PILC cable
12
65 Types of Underground Cables (continue)-
bull A = Conductor (Aluminum)
bull B = Strand Screen (extruded
semiconducting)bull C = Insulation (XLPE)bull D = Insulation Screen (extruded
semiconducting)bull E = Shield (copper
tape)bull F = Jacket Example of Single core
XLPE 11 kV cable
13
65 Types of Underground Cables (continue)-
ndash Excellent Electrical amp Physical Properties
ndash Capable Of Carrying Large Current At High Temperaturebull Normal ~ 90ocbull Emergency ~ 130ocbull Short Circuit Conditions
~250ocndash Easy To Install ndash XLPE Easier
To Jointndash No Need For Metallic Sheath
14
66 (CABLE FAULT) INTRODUCTION
1048707 Cable faults are undesirable causes because-
1 Power supply is interrupted
2 Locating fault in a long underground cable is difficult and time consuming
3 Repairing faulty cable is difficult and time consuming
66 Cable Fault Introduction (continue)-
662 CAUSES OF UNDERGROUND CABLE FAILURE
Major factors that cause failure of a cable are-
bull Damaged accidentally by external mechanical means
bull Damage caused as a results of mishandling the cable
during layout
bull Poor workmanship in cable jointing
bull Natural causes due to aging
of cable
bull Damaged caused by movement
of soil and erosion
661 MECHANICAL
663 MISHANDLING
Mishandling of cable may be occurred during installation
Some of the examples are
1 Excessive pull
2 Sharp bend
3 Accident crush
664 Poor workmanship During Cable Jointing
The cable are jointed together with poor workmanship can lead to cable fault after a period of time
665 NATURAL CAUSES DUE TO AGING OF CABLE
CONTINUE-
CONTINUE-
66 TYPES OF CABLE FAULT
GENERAL
bull Series (open circuit) Fault
- Failure of continuity (conductor (s) or cable)
bull Shunt (short circuit) fault
- failure of insulation
66 TYPES OF FAULT (CONTINUE)-
66 TYPES OF FAULT (CONTINUE)-
661 SERIES AND SHUNT FAULT
Are subsided into the following categories
Low Resistance Faultof ZR 10
Where Zo= cable surge impedance
=10 ndash 100 ohm
Usually happens in series fault
High Resistance Fault
Where ZoR f 10
662 INTERMITTENT OR FLASH FAULT
- Usually not apparent to insulation resistance measuring
instrument
- Does not manifest itself at lower voltages or a surge
- Breakdown will appear under application of high voltage
dc or DC pressure test
67 FAULT LOCATION PROCEDURE
The proper sequence of cable fault location are as follows
a) Analysis of fault
b) Pre-location
c) Pin Pointing
d) Confirmation and re-test
671 Analysis of Cable Fault
To analyze a cable fault is to determine and confirm the nature or characteristics of the fault
Objective to select which of the cable fault locator equipment of method is best suited to locate the particular fault
Analysis of cable fault normally consists of insulation resistance test and continuity test carried out using 1000V or 5000 V insulation tester
671 Insulation Resistance test
With all earth connections removed from the cable conductor terminal using IR tester of 100025005000V range measure and record the IR in M-ohms K-ohms or ohms between conductors and between conductors and earth Six measurement are to be taken R-E Y-E B-E R-Y Y-B B-R An IR of 100M-ohm indicates a healthy cable
672 Continuity Test
With the cable conductor looped or shorted at the tail endremote end using IR tester selected at continuity range perform continuity test
This test will determine whether any of the cable is open circuited
672 Continuity test (cont-)
Continuity Test - With the cable conductor shorted or looped at the remote end perform continuity test on the cable
- Measure and record the results in ohm
- Three measurements are to be carried out between R-Y Y-B B-R
- The test will determine whether any of cable is open circuited - The resistance per-conductor per km is provided in Table VI VII VIII and IX (refer appendix A)
672 (cont-)
if the continuity of the cable is sound insulation resistance from one end are sufficient
If continuity is broken IR test should be carried out at both ends of the cable
68 BURNING A FAULT
The continuity and IR test may indicate that burning of fault by means of HT pressure test set is required
-
68 BURNING A FAULT (continue-)
Burning a fault is achieve by passing current from a DC HT test set through the fault
Other conductors not under test should be earthed
HT is applied for about 5 to 10 minutes to burn the fault
HT test is used to determine which fault location equipment is suitable to be used
HT is the last resort often used because it sometimes produce ambiguous and unpredictable results
Therefore fault location equipment should be attempted first
69 Pre- Location of fault
Pre-location is the application of a test at the terminals of a
given cable to give an indication of the distance to the fault
from the test point
Whilst the measurement should be accurate as condition will
allow the primary purpose of pre-location is to give an
indication as quick as possible of the vicinity in which to
commence the final pin-pointing tests
69 Pre- Location of fault (cont)-
Generally there are four pre-location methods which are practised and the are as follows
1Bridge or loop Method2 Pulse Echo Time Domain Reflectometry (TDR) method3 Impulse Current Method4 Arc Reflection and secondary Impulse method
691 Bridge method
6911 Direct reading fault localizer
Conditions required
i) No break in continuity if possible one sound core available
ii) The fault resistance of cable should not be more then 20 k-ohm
iii) Applied voltage not to exceed 600 V (DC)
Case 1 Single conductor to earth with sound core available
Fault distance from front end X= n100 x L
Where n = localiser reading in
L= Cable route length
Case2 Conductor to conductor with sound core available
Fault distance from front end X= n100 x L
Where n = localiser reading in
L= Cable route length
Case 3 Three phase fault
By Open loop test and close loop test
(i) Rf2Rf1 gt 5 and (ii) Rf2 lt 20 k-ohm
Open loop test
Case 3 Three phase fault
Close loop test
Open loop and close loop test (cont)
Fault distance from front end
Lxm
mn
X 2
1001
)200200
(
Where m = open loop bridge reading
n = close loop bridge reading
L = cable route length
High Tension Bridge
Conditions required
i) No break in continuity if possible one sound core available
ii) The fault resistance of cable should be more then 20 k-ohm
iii) Applied voltage not to exceed 20kV (DC)
High Tension Bridge
Case 1Single Conductor to earth with sound core available
Fault Distance from test end X = n100 x 2L
High Tension Bridge
Case 2Conductor to conductor with sound core available
Fault Distance from test end X = n100 x 2L
High Tension Bridge
Case 3Three phase Fault with no sound core available
Rf2Rf1gt5 Rf2gt20 kohm
Lxm
)mn
(X 2
1002
1
100100
Where m = open loop bridge reading
n = close loop bridge reading
L = cable route length
6912 Universal Bridge to measure capacitance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be more then 100 ohm
iii) Earth all the conductors not under test
6912 Universal Bridge to measure capacitance of Cable
Open circuit with sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with sound core available
Fault Distance from test end x = CA x L CAB
CA = Capacitance of open circuited coreCAB = Capacitance of sound coreL = conductor length
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
Fault Distance from test endfront end = ((CA(CA+CB))x L
6913 Universal Bridge to measure Inductance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be less then 30 ohm
iii) All conductors be left free from earth
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
Fault distance X = LA x L LAB
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
Fault distance X = LA x L LA + LB
692 Pulse Echo (PE) Time Domain Reflectometry (TDR)
A simple method which works on travelling wave principles It is applicable to all series and shunt faults with Rf gt=Zo10 (5 ohm) and Rflt=10 Zo (500 ohm) respectively For power cables typical Zo=50 ohm
6921 Travelling wave principles
A pulse injected into a cable is reflected back to the source by any change in characteristics impedance (Zo) of the cable
The waveform generated can be monitored using an oscilloscope as shown in Figure below
6921 Continue-
Waveform interpretion
Shunt Fault
Series Fault
6921 Continue-
Waveform interpretion
Low Resistance Shunt Fault (Rflt=500 Ohm)
-Reflection Negative
-Amplitude depends on fault resistance
Series Fault (Rfgt=5 ohm)
- Reflection Positive
- Amplitude depends on fault resistance
Zo = Characteristics impedance = radic(LC)
L = Inductance of cable C = Capacitance of Cable
69211 Reflection Factor (r)
- Is to determine the degree of reflection
Shunt fault r (V) = -Zo1(2Rf + Zo1)
Series Fault r (V) = Rf(2Zo1 + Rf)
Example shunt fault fault resistance = 5 ohm Zo = 50 ohm
r(V) = -50 (2 x5 +50) = - 833If fault resistance = 500 ohm
r(V) = -50 (2x 500 + 50) = -475
69212 Velocity of Propagation (Vp)
- Vp depends on the cable dielectric and is defined as
Vp = Vs radicEr
- where Vs = Velocity of light in free space
Er = Relative permittivity of dielectric
Vp = 300radicEr
Dielectrics Velocity of PropagationVp (mmicroS)
Impregnated Paper 150 -171
PVC 152-175
PE Approx 200
XLPE 156-176
Typical Vp for different Dielectric
69212 Continue (Vp)
Example
Determine the velocity of propagation Vp2 for a cable length of 2500 meters t1= 3164 S and t2 = 1745 S Calculate the fault distances
6921 Continue-
The distance to the fault Lf (m) is given by
Lf = T x Vp2
6921 Continue
Solutions
Velocity of propagation = Vp2 = 25003164 = 79 mS
Distance of Fault from test end = Vp2 x t2
= 79 x 1745
= 137855
693 Impulse Current Method
Working on Travelling wave principles it is applicable to all types of fault
694 Arc Reflection or Secondary Impulse method
Basically PE and TDR associated with fault treatment It is also applicable to faults of all nature but with easily interpreted breakdown waveforms
610 PIN POINTING
Pin Pointing is the application of a test that positively confirms the exact position of the fault
Before the commencement of pin pointing the prelocated fault distance should mark on the cable route which is measured by means of a trumeter
Pin pointing is normally carried out by the shock wave discharge method The fault can be detected by the use of semisphone
611 Confirmation and Re-Test
After the pin-pointed position of the fault has been marked and exposed check for physical sign of fault If there is the fault is confirm
After confirmation the fault should be cut away IR and continuity test should be carried out on the remaining cable sections to determine the soundeness of these cable sections
The insulation resistance test are again carried out after jointing followed by pressure test before supply can be restored
Flowchart for Fault location Procedure
Example
Find a fault distance using direct reading fault localiser from end test which has a test data as below
Bridge Balance Reading =60 Length of Cable 140 meters
Solutions
Fault Distance= = 60100 x 140 = 84 meterxL
n
100
For cables route with combination of 2 or more different type of cable
Determine the resistance of cable for each section separately from Table 1 and Table 2
Example Cable 11kV Al Length 150 m size = 16mm2
From Table 1= Resistance for 1km =226 ohm
For 150 m = 1501000 x 226 = 0339 ohm
From Table 1 Calculate the equivalent resistance for all the combined section
Example for Al = 0339 ohm for copper = 0150 ohm
Equivalent Resistance = 0339+0150 =0489 ohm
Continue
Calculate equivalent resistance from obtained bridge reading
Example Bridge Reading = 60
Equivalent resistance = 06 x 0489 ohm = 02934 ohm
Determine which section of cable the fault occurs
Total resistance at Al section = 0339 ohm
As equivalent resistance at Al section is higher then 02934(Equivalent resistance from bridge test) the fault is
located at Aluminium section of cable
Table 1 Impedance data for 635011000 V Cable Aluminium
Table II Impedance data 11000 V Cable Copper
TABLE VI ndash 6001000V (ALUMINIUM)
TABLE VII ndash 38006600V (ALUMINIUM)
TABLE VIII- 635011000 (ALUMINIUM)
TABLE V
610 COMMISSIONING OF UNDERGROUND CABLE
High Voltage DC testing
High voltage testing is carried out in order to determine the electrical strength of cable insulation Site tests are performed by applying a predetermined high voltage to the insulation DC site test voltages regardless of insulation type are used to ensure that the cable cable joints and termination are correctly made and installed
High Voltage Testing (cont-)
IEC Standards specify maintenance tests after installation should be 70 of permissible factory test DC voltages (15 minutes) = 175kV
Insulation Resistance Test (cont-)
Test voltages should be slowly raised to the required value over a period of about 1 minute and the test period starts once the full voltage is reached In this way the capacitive and absorption currents will have decreased and circuit conditions stabilized such that true leakage current may be measured
63 Cable Construction (continue)-
bull Insulationndash Is a material that reduces
or prevents the transmission of electricity
ndash Each conductor is covered by insulation
ndash Insulation is phase to ground and phase to phase
XLPE
PAPER
04172023
7
ETE503 Underground Cable
63 Cable Construction (continue)-
bull Sheathndash Cable protective covering ndash Metallic or nonmetallic
protective covering over the conductor insulation shield
ndash External protection is provided by the sheath against mechanical damage chemical reaction moisture an so on
04172023
8
ETE503 Underground Cable
64 Types of Underground Cables
bullThe identification of the cable are based on the several items
bull Insulationbull Voltage Systembull Cable Sizing And Corebull Technical Specification Characteristics Of The
Cable
9
64 Types of Underground Cables (continue)-
bullUsually the operating voltage decides the types of insulation and cable placed in various categories depending upon the voltage for which they are designed
ndashLow Voltage Cable (LV) 11kVndashHigh Voltage Cable (HV) 11 kV
10
64 Types of Underground Cables (continue)-
bullPaper Insulationndash3 core belted 11kV PILC cablendashSingle core screened 11 kV PILC cable
bullPolymer Insulationndash3 core XLPE 11 kV cablendashSingle core XLPE 11 kV cable
11
64 Types of Underground Cables (continue)-
bull A = Conductor (Aluminum)
bull B = Strand Screen (carbon black paper )bull C = Insulation (Paper)bull D = Insulation Screen
(carbon black paper)bull E = Sheath (copper
lead)bull F = Jacket
Example of Single core screened 11 kV PILC cable
12
65 Types of Underground Cables (continue)-
bull A = Conductor (Aluminum)
bull B = Strand Screen (extruded
semiconducting)bull C = Insulation (XLPE)bull D = Insulation Screen (extruded
semiconducting)bull E = Shield (copper
tape)bull F = Jacket Example of Single core
XLPE 11 kV cable
13
65 Types of Underground Cables (continue)-
ndash Excellent Electrical amp Physical Properties
ndash Capable Of Carrying Large Current At High Temperaturebull Normal ~ 90ocbull Emergency ~ 130ocbull Short Circuit Conditions
~250ocndash Easy To Install ndash XLPE Easier
To Jointndash No Need For Metallic Sheath
14
66 (CABLE FAULT) INTRODUCTION
1048707 Cable faults are undesirable causes because-
1 Power supply is interrupted
2 Locating fault in a long underground cable is difficult and time consuming
3 Repairing faulty cable is difficult and time consuming
66 Cable Fault Introduction (continue)-
662 CAUSES OF UNDERGROUND CABLE FAILURE
Major factors that cause failure of a cable are-
bull Damaged accidentally by external mechanical means
bull Damage caused as a results of mishandling the cable
during layout
bull Poor workmanship in cable jointing
bull Natural causes due to aging
of cable
bull Damaged caused by movement
of soil and erosion
661 MECHANICAL
663 MISHANDLING
Mishandling of cable may be occurred during installation
Some of the examples are
1 Excessive pull
2 Sharp bend
3 Accident crush
664 Poor workmanship During Cable Jointing
The cable are jointed together with poor workmanship can lead to cable fault after a period of time
665 NATURAL CAUSES DUE TO AGING OF CABLE
CONTINUE-
CONTINUE-
66 TYPES OF CABLE FAULT
GENERAL
bull Series (open circuit) Fault
- Failure of continuity (conductor (s) or cable)
bull Shunt (short circuit) fault
- failure of insulation
66 TYPES OF FAULT (CONTINUE)-
66 TYPES OF FAULT (CONTINUE)-
661 SERIES AND SHUNT FAULT
Are subsided into the following categories
Low Resistance Faultof ZR 10
Where Zo= cable surge impedance
=10 ndash 100 ohm
Usually happens in series fault
High Resistance Fault
Where ZoR f 10
662 INTERMITTENT OR FLASH FAULT
- Usually not apparent to insulation resistance measuring
instrument
- Does not manifest itself at lower voltages or a surge
- Breakdown will appear under application of high voltage
dc or DC pressure test
67 FAULT LOCATION PROCEDURE
The proper sequence of cable fault location are as follows
a) Analysis of fault
b) Pre-location
c) Pin Pointing
d) Confirmation and re-test
671 Analysis of Cable Fault
To analyze a cable fault is to determine and confirm the nature or characteristics of the fault
Objective to select which of the cable fault locator equipment of method is best suited to locate the particular fault
Analysis of cable fault normally consists of insulation resistance test and continuity test carried out using 1000V or 5000 V insulation tester
671 Insulation Resistance test
With all earth connections removed from the cable conductor terminal using IR tester of 100025005000V range measure and record the IR in M-ohms K-ohms or ohms between conductors and between conductors and earth Six measurement are to be taken R-E Y-E B-E R-Y Y-B B-R An IR of 100M-ohm indicates a healthy cable
672 Continuity Test
With the cable conductor looped or shorted at the tail endremote end using IR tester selected at continuity range perform continuity test
This test will determine whether any of the cable is open circuited
672 Continuity test (cont-)
Continuity Test - With the cable conductor shorted or looped at the remote end perform continuity test on the cable
- Measure and record the results in ohm
- Three measurements are to be carried out between R-Y Y-B B-R
- The test will determine whether any of cable is open circuited - The resistance per-conductor per km is provided in Table VI VII VIII and IX (refer appendix A)
672 (cont-)
if the continuity of the cable is sound insulation resistance from one end are sufficient
If continuity is broken IR test should be carried out at both ends of the cable
68 BURNING A FAULT
The continuity and IR test may indicate that burning of fault by means of HT pressure test set is required
-
68 BURNING A FAULT (continue-)
Burning a fault is achieve by passing current from a DC HT test set through the fault
Other conductors not under test should be earthed
HT is applied for about 5 to 10 minutes to burn the fault
HT test is used to determine which fault location equipment is suitable to be used
HT is the last resort often used because it sometimes produce ambiguous and unpredictable results
Therefore fault location equipment should be attempted first
69 Pre- Location of fault
Pre-location is the application of a test at the terminals of a
given cable to give an indication of the distance to the fault
from the test point
Whilst the measurement should be accurate as condition will
allow the primary purpose of pre-location is to give an
indication as quick as possible of the vicinity in which to
commence the final pin-pointing tests
69 Pre- Location of fault (cont)-
Generally there are four pre-location methods which are practised and the are as follows
1Bridge or loop Method2 Pulse Echo Time Domain Reflectometry (TDR) method3 Impulse Current Method4 Arc Reflection and secondary Impulse method
691 Bridge method
6911 Direct reading fault localizer
Conditions required
i) No break in continuity if possible one sound core available
ii) The fault resistance of cable should not be more then 20 k-ohm
iii) Applied voltage not to exceed 600 V (DC)
Case 1 Single conductor to earth with sound core available
Fault distance from front end X= n100 x L
Where n = localiser reading in
L= Cable route length
Case2 Conductor to conductor with sound core available
Fault distance from front end X= n100 x L
Where n = localiser reading in
L= Cable route length
Case 3 Three phase fault
By Open loop test and close loop test
(i) Rf2Rf1 gt 5 and (ii) Rf2 lt 20 k-ohm
Open loop test
Case 3 Three phase fault
Close loop test
Open loop and close loop test (cont)
Fault distance from front end
Lxm
mn
X 2
1001
)200200
(
Where m = open loop bridge reading
n = close loop bridge reading
L = cable route length
High Tension Bridge
Conditions required
i) No break in continuity if possible one sound core available
ii) The fault resistance of cable should be more then 20 k-ohm
iii) Applied voltage not to exceed 20kV (DC)
High Tension Bridge
Case 1Single Conductor to earth with sound core available
Fault Distance from test end X = n100 x 2L
High Tension Bridge
Case 2Conductor to conductor with sound core available
Fault Distance from test end X = n100 x 2L
High Tension Bridge
Case 3Three phase Fault with no sound core available
Rf2Rf1gt5 Rf2gt20 kohm
Lxm
)mn
(X 2
1002
1
100100
Where m = open loop bridge reading
n = close loop bridge reading
L = cable route length
6912 Universal Bridge to measure capacitance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be more then 100 ohm
iii) Earth all the conductors not under test
6912 Universal Bridge to measure capacitance of Cable
Open circuit with sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with sound core available
Fault Distance from test end x = CA x L CAB
CA = Capacitance of open circuited coreCAB = Capacitance of sound coreL = conductor length
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
Fault Distance from test endfront end = ((CA(CA+CB))x L
6913 Universal Bridge to measure Inductance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be less then 30 ohm
iii) All conductors be left free from earth
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
Fault distance X = LA x L LAB
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
Fault distance X = LA x L LA + LB
692 Pulse Echo (PE) Time Domain Reflectometry (TDR)
A simple method which works on travelling wave principles It is applicable to all series and shunt faults with Rf gt=Zo10 (5 ohm) and Rflt=10 Zo (500 ohm) respectively For power cables typical Zo=50 ohm
6921 Travelling wave principles
A pulse injected into a cable is reflected back to the source by any change in characteristics impedance (Zo) of the cable
The waveform generated can be monitored using an oscilloscope as shown in Figure below
6921 Continue-
Waveform interpretion
Shunt Fault
Series Fault
6921 Continue-
Waveform interpretion
Low Resistance Shunt Fault (Rflt=500 Ohm)
-Reflection Negative
-Amplitude depends on fault resistance
Series Fault (Rfgt=5 ohm)
- Reflection Positive
- Amplitude depends on fault resistance
Zo = Characteristics impedance = radic(LC)
L = Inductance of cable C = Capacitance of Cable
69211 Reflection Factor (r)
- Is to determine the degree of reflection
Shunt fault r (V) = -Zo1(2Rf + Zo1)
Series Fault r (V) = Rf(2Zo1 + Rf)
Example shunt fault fault resistance = 5 ohm Zo = 50 ohm
r(V) = -50 (2 x5 +50) = - 833If fault resistance = 500 ohm
r(V) = -50 (2x 500 + 50) = -475
69212 Velocity of Propagation (Vp)
- Vp depends on the cable dielectric and is defined as
Vp = Vs radicEr
- where Vs = Velocity of light in free space
Er = Relative permittivity of dielectric
Vp = 300radicEr
Dielectrics Velocity of PropagationVp (mmicroS)
Impregnated Paper 150 -171
PVC 152-175
PE Approx 200
XLPE 156-176
Typical Vp for different Dielectric
69212 Continue (Vp)
Example
Determine the velocity of propagation Vp2 for a cable length of 2500 meters t1= 3164 S and t2 = 1745 S Calculate the fault distances
6921 Continue-
The distance to the fault Lf (m) is given by
Lf = T x Vp2
6921 Continue
Solutions
Velocity of propagation = Vp2 = 25003164 = 79 mS
Distance of Fault from test end = Vp2 x t2
= 79 x 1745
= 137855
693 Impulse Current Method
Working on Travelling wave principles it is applicable to all types of fault
694 Arc Reflection or Secondary Impulse method
Basically PE and TDR associated with fault treatment It is also applicable to faults of all nature but with easily interpreted breakdown waveforms
610 PIN POINTING
Pin Pointing is the application of a test that positively confirms the exact position of the fault
Before the commencement of pin pointing the prelocated fault distance should mark on the cable route which is measured by means of a trumeter
Pin pointing is normally carried out by the shock wave discharge method The fault can be detected by the use of semisphone
611 Confirmation and Re-Test
After the pin-pointed position of the fault has been marked and exposed check for physical sign of fault If there is the fault is confirm
After confirmation the fault should be cut away IR and continuity test should be carried out on the remaining cable sections to determine the soundeness of these cable sections
The insulation resistance test are again carried out after jointing followed by pressure test before supply can be restored
Flowchart for Fault location Procedure
Example
Find a fault distance using direct reading fault localiser from end test which has a test data as below
Bridge Balance Reading =60 Length of Cable 140 meters
Solutions
Fault Distance= = 60100 x 140 = 84 meterxL
n
100
For cables route with combination of 2 or more different type of cable
Determine the resistance of cable for each section separately from Table 1 and Table 2
Example Cable 11kV Al Length 150 m size = 16mm2
From Table 1= Resistance for 1km =226 ohm
For 150 m = 1501000 x 226 = 0339 ohm
From Table 1 Calculate the equivalent resistance for all the combined section
Example for Al = 0339 ohm for copper = 0150 ohm
Equivalent Resistance = 0339+0150 =0489 ohm
Continue
Calculate equivalent resistance from obtained bridge reading
Example Bridge Reading = 60
Equivalent resistance = 06 x 0489 ohm = 02934 ohm
Determine which section of cable the fault occurs
Total resistance at Al section = 0339 ohm
As equivalent resistance at Al section is higher then 02934(Equivalent resistance from bridge test) the fault is
located at Aluminium section of cable
Table 1 Impedance data for 635011000 V Cable Aluminium
Table II Impedance data 11000 V Cable Copper
TABLE VI ndash 6001000V (ALUMINIUM)
TABLE VII ndash 38006600V (ALUMINIUM)
TABLE VIII- 635011000 (ALUMINIUM)
TABLE V
610 COMMISSIONING OF UNDERGROUND CABLE
High Voltage DC testing
High voltage testing is carried out in order to determine the electrical strength of cable insulation Site tests are performed by applying a predetermined high voltage to the insulation DC site test voltages regardless of insulation type are used to ensure that the cable cable joints and termination are correctly made and installed
High Voltage Testing (cont-)
IEC Standards specify maintenance tests after installation should be 70 of permissible factory test DC voltages (15 minutes) = 175kV
Insulation Resistance Test (cont-)
Test voltages should be slowly raised to the required value over a period of about 1 minute and the test period starts once the full voltage is reached In this way the capacitive and absorption currents will have decreased and circuit conditions stabilized such that true leakage current may be measured
63 Cable Construction (continue)-
bull Sheathndash Cable protective covering ndash Metallic or nonmetallic
protective covering over the conductor insulation shield
ndash External protection is provided by the sheath against mechanical damage chemical reaction moisture an so on
04172023
8
ETE503 Underground Cable
64 Types of Underground Cables
bullThe identification of the cable are based on the several items
bull Insulationbull Voltage Systembull Cable Sizing And Corebull Technical Specification Characteristics Of The
Cable
9
64 Types of Underground Cables (continue)-
bullUsually the operating voltage decides the types of insulation and cable placed in various categories depending upon the voltage for which they are designed
ndashLow Voltage Cable (LV) 11kVndashHigh Voltage Cable (HV) 11 kV
10
64 Types of Underground Cables (continue)-
bullPaper Insulationndash3 core belted 11kV PILC cablendashSingle core screened 11 kV PILC cable
bullPolymer Insulationndash3 core XLPE 11 kV cablendashSingle core XLPE 11 kV cable
11
64 Types of Underground Cables (continue)-
bull A = Conductor (Aluminum)
bull B = Strand Screen (carbon black paper )bull C = Insulation (Paper)bull D = Insulation Screen
(carbon black paper)bull E = Sheath (copper
lead)bull F = Jacket
Example of Single core screened 11 kV PILC cable
12
65 Types of Underground Cables (continue)-
bull A = Conductor (Aluminum)
bull B = Strand Screen (extruded
semiconducting)bull C = Insulation (XLPE)bull D = Insulation Screen (extruded
semiconducting)bull E = Shield (copper
tape)bull F = Jacket Example of Single core
XLPE 11 kV cable
13
65 Types of Underground Cables (continue)-
ndash Excellent Electrical amp Physical Properties
ndash Capable Of Carrying Large Current At High Temperaturebull Normal ~ 90ocbull Emergency ~ 130ocbull Short Circuit Conditions
~250ocndash Easy To Install ndash XLPE Easier
To Jointndash No Need For Metallic Sheath
14
66 (CABLE FAULT) INTRODUCTION
1048707 Cable faults are undesirable causes because-
1 Power supply is interrupted
2 Locating fault in a long underground cable is difficult and time consuming
3 Repairing faulty cable is difficult and time consuming
66 Cable Fault Introduction (continue)-
662 CAUSES OF UNDERGROUND CABLE FAILURE
Major factors that cause failure of a cable are-
bull Damaged accidentally by external mechanical means
bull Damage caused as a results of mishandling the cable
during layout
bull Poor workmanship in cable jointing
bull Natural causes due to aging
of cable
bull Damaged caused by movement
of soil and erosion
661 MECHANICAL
663 MISHANDLING
Mishandling of cable may be occurred during installation
Some of the examples are
1 Excessive pull
2 Sharp bend
3 Accident crush
664 Poor workmanship During Cable Jointing
The cable are jointed together with poor workmanship can lead to cable fault after a period of time
665 NATURAL CAUSES DUE TO AGING OF CABLE
CONTINUE-
CONTINUE-
66 TYPES OF CABLE FAULT
GENERAL
bull Series (open circuit) Fault
- Failure of continuity (conductor (s) or cable)
bull Shunt (short circuit) fault
- failure of insulation
66 TYPES OF FAULT (CONTINUE)-
66 TYPES OF FAULT (CONTINUE)-
661 SERIES AND SHUNT FAULT
Are subsided into the following categories
Low Resistance Faultof ZR 10
Where Zo= cable surge impedance
=10 ndash 100 ohm
Usually happens in series fault
High Resistance Fault
Where ZoR f 10
662 INTERMITTENT OR FLASH FAULT
- Usually not apparent to insulation resistance measuring
instrument
- Does not manifest itself at lower voltages or a surge
- Breakdown will appear under application of high voltage
dc or DC pressure test
67 FAULT LOCATION PROCEDURE
The proper sequence of cable fault location are as follows
a) Analysis of fault
b) Pre-location
c) Pin Pointing
d) Confirmation and re-test
671 Analysis of Cable Fault
To analyze a cable fault is to determine and confirm the nature or characteristics of the fault
Objective to select which of the cable fault locator equipment of method is best suited to locate the particular fault
Analysis of cable fault normally consists of insulation resistance test and continuity test carried out using 1000V or 5000 V insulation tester
671 Insulation Resistance test
With all earth connections removed from the cable conductor terminal using IR tester of 100025005000V range measure and record the IR in M-ohms K-ohms or ohms between conductors and between conductors and earth Six measurement are to be taken R-E Y-E B-E R-Y Y-B B-R An IR of 100M-ohm indicates a healthy cable
672 Continuity Test
With the cable conductor looped or shorted at the tail endremote end using IR tester selected at continuity range perform continuity test
This test will determine whether any of the cable is open circuited
672 Continuity test (cont-)
Continuity Test - With the cable conductor shorted or looped at the remote end perform continuity test on the cable
- Measure and record the results in ohm
- Three measurements are to be carried out between R-Y Y-B B-R
- The test will determine whether any of cable is open circuited - The resistance per-conductor per km is provided in Table VI VII VIII and IX (refer appendix A)
672 (cont-)
if the continuity of the cable is sound insulation resistance from one end are sufficient
If continuity is broken IR test should be carried out at both ends of the cable
68 BURNING A FAULT
The continuity and IR test may indicate that burning of fault by means of HT pressure test set is required
-
68 BURNING A FAULT (continue-)
Burning a fault is achieve by passing current from a DC HT test set through the fault
Other conductors not under test should be earthed
HT is applied for about 5 to 10 minutes to burn the fault
HT test is used to determine which fault location equipment is suitable to be used
HT is the last resort often used because it sometimes produce ambiguous and unpredictable results
Therefore fault location equipment should be attempted first
69 Pre- Location of fault
Pre-location is the application of a test at the terminals of a
given cable to give an indication of the distance to the fault
from the test point
Whilst the measurement should be accurate as condition will
allow the primary purpose of pre-location is to give an
indication as quick as possible of the vicinity in which to
commence the final pin-pointing tests
69 Pre- Location of fault (cont)-
Generally there are four pre-location methods which are practised and the are as follows
1Bridge or loop Method2 Pulse Echo Time Domain Reflectometry (TDR) method3 Impulse Current Method4 Arc Reflection and secondary Impulse method
691 Bridge method
6911 Direct reading fault localizer
Conditions required
i) No break in continuity if possible one sound core available
ii) The fault resistance of cable should not be more then 20 k-ohm
iii) Applied voltage not to exceed 600 V (DC)
Case 1 Single conductor to earth with sound core available
Fault distance from front end X= n100 x L
Where n = localiser reading in
L= Cable route length
Case2 Conductor to conductor with sound core available
Fault distance from front end X= n100 x L
Where n = localiser reading in
L= Cable route length
Case 3 Three phase fault
By Open loop test and close loop test
(i) Rf2Rf1 gt 5 and (ii) Rf2 lt 20 k-ohm
Open loop test
Case 3 Three phase fault
Close loop test
Open loop and close loop test (cont)
Fault distance from front end
Lxm
mn
X 2
1001
)200200
(
Where m = open loop bridge reading
n = close loop bridge reading
L = cable route length
High Tension Bridge
Conditions required
i) No break in continuity if possible one sound core available
ii) The fault resistance of cable should be more then 20 k-ohm
iii) Applied voltage not to exceed 20kV (DC)
High Tension Bridge
Case 1Single Conductor to earth with sound core available
Fault Distance from test end X = n100 x 2L
High Tension Bridge
Case 2Conductor to conductor with sound core available
Fault Distance from test end X = n100 x 2L
High Tension Bridge
Case 3Three phase Fault with no sound core available
Rf2Rf1gt5 Rf2gt20 kohm
Lxm
)mn
(X 2
1002
1
100100
Where m = open loop bridge reading
n = close loop bridge reading
L = cable route length
6912 Universal Bridge to measure capacitance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be more then 100 ohm
iii) Earth all the conductors not under test
6912 Universal Bridge to measure capacitance of Cable
Open circuit with sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with sound core available
Fault Distance from test end x = CA x L CAB
CA = Capacitance of open circuited coreCAB = Capacitance of sound coreL = conductor length
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
Fault Distance from test endfront end = ((CA(CA+CB))x L
6913 Universal Bridge to measure Inductance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be less then 30 ohm
iii) All conductors be left free from earth
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
Fault distance X = LA x L LAB
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
Fault distance X = LA x L LA + LB
692 Pulse Echo (PE) Time Domain Reflectometry (TDR)
A simple method which works on travelling wave principles It is applicable to all series and shunt faults with Rf gt=Zo10 (5 ohm) and Rflt=10 Zo (500 ohm) respectively For power cables typical Zo=50 ohm
6921 Travelling wave principles
A pulse injected into a cable is reflected back to the source by any change in characteristics impedance (Zo) of the cable
The waveform generated can be monitored using an oscilloscope as shown in Figure below
6921 Continue-
Waveform interpretion
Shunt Fault
Series Fault
6921 Continue-
Waveform interpretion
Low Resistance Shunt Fault (Rflt=500 Ohm)
-Reflection Negative
-Amplitude depends on fault resistance
Series Fault (Rfgt=5 ohm)
- Reflection Positive
- Amplitude depends on fault resistance
Zo = Characteristics impedance = radic(LC)
L = Inductance of cable C = Capacitance of Cable
69211 Reflection Factor (r)
- Is to determine the degree of reflection
Shunt fault r (V) = -Zo1(2Rf + Zo1)
Series Fault r (V) = Rf(2Zo1 + Rf)
Example shunt fault fault resistance = 5 ohm Zo = 50 ohm
r(V) = -50 (2 x5 +50) = - 833If fault resistance = 500 ohm
r(V) = -50 (2x 500 + 50) = -475
69212 Velocity of Propagation (Vp)
- Vp depends on the cable dielectric and is defined as
Vp = Vs radicEr
- where Vs = Velocity of light in free space
Er = Relative permittivity of dielectric
Vp = 300radicEr
Dielectrics Velocity of PropagationVp (mmicroS)
Impregnated Paper 150 -171
PVC 152-175
PE Approx 200
XLPE 156-176
Typical Vp for different Dielectric
69212 Continue (Vp)
Example
Determine the velocity of propagation Vp2 for a cable length of 2500 meters t1= 3164 S and t2 = 1745 S Calculate the fault distances
6921 Continue-
The distance to the fault Lf (m) is given by
Lf = T x Vp2
6921 Continue
Solutions
Velocity of propagation = Vp2 = 25003164 = 79 mS
Distance of Fault from test end = Vp2 x t2
= 79 x 1745
= 137855
693 Impulse Current Method
Working on Travelling wave principles it is applicable to all types of fault
694 Arc Reflection or Secondary Impulse method
Basically PE and TDR associated with fault treatment It is also applicable to faults of all nature but with easily interpreted breakdown waveforms
610 PIN POINTING
Pin Pointing is the application of a test that positively confirms the exact position of the fault
Before the commencement of pin pointing the prelocated fault distance should mark on the cable route which is measured by means of a trumeter
Pin pointing is normally carried out by the shock wave discharge method The fault can be detected by the use of semisphone
611 Confirmation and Re-Test
After the pin-pointed position of the fault has been marked and exposed check for physical sign of fault If there is the fault is confirm
After confirmation the fault should be cut away IR and continuity test should be carried out on the remaining cable sections to determine the soundeness of these cable sections
The insulation resistance test are again carried out after jointing followed by pressure test before supply can be restored
Flowchart for Fault location Procedure
Example
Find a fault distance using direct reading fault localiser from end test which has a test data as below
Bridge Balance Reading =60 Length of Cable 140 meters
Solutions
Fault Distance= = 60100 x 140 = 84 meterxL
n
100
For cables route with combination of 2 or more different type of cable
Determine the resistance of cable for each section separately from Table 1 and Table 2
Example Cable 11kV Al Length 150 m size = 16mm2
From Table 1= Resistance for 1km =226 ohm
For 150 m = 1501000 x 226 = 0339 ohm
From Table 1 Calculate the equivalent resistance for all the combined section
Example for Al = 0339 ohm for copper = 0150 ohm
Equivalent Resistance = 0339+0150 =0489 ohm
Continue
Calculate equivalent resistance from obtained bridge reading
Example Bridge Reading = 60
Equivalent resistance = 06 x 0489 ohm = 02934 ohm
Determine which section of cable the fault occurs
Total resistance at Al section = 0339 ohm
As equivalent resistance at Al section is higher then 02934(Equivalent resistance from bridge test) the fault is
located at Aluminium section of cable
Table 1 Impedance data for 635011000 V Cable Aluminium
Table II Impedance data 11000 V Cable Copper
TABLE VI ndash 6001000V (ALUMINIUM)
TABLE VII ndash 38006600V (ALUMINIUM)
TABLE VIII- 635011000 (ALUMINIUM)
TABLE V
610 COMMISSIONING OF UNDERGROUND CABLE
High Voltage DC testing
High voltage testing is carried out in order to determine the electrical strength of cable insulation Site tests are performed by applying a predetermined high voltage to the insulation DC site test voltages regardless of insulation type are used to ensure that the cable cable joints and termination are correctly made and installed
High Voltage Testing (cont-)
IEC Standards specify maintenance tests after installation should be 70 of permissible factory test DC voltages (15 minutes) = 175kV
Insulation Resistance Test (cont-)
Test voltages should be slowly raised to the required value over a period of about 1 minute and the test period starts once the full voltage is reached In this way the capacitive and absorption currents will have decreased and circuit conditions stabilized such that true leakage current may be measured
64 Types of Underground Cables
bullThe identification of the cable are based on the several items
bull Insulationbull Voltage Systembull Cable Sizing And Corebull Technical Specification Characteristics Of The
Cable
9
64 Types of Underground Cables (continue)-
bullUsually the operating voltage decides the types of insulation and cable placed in various categories depending upon the voltage for which they are designed
ndashLow Voltage Cable (LV) 11kVndashHigh Voltage Cable (HV) 11 kV
10
64 Types of Underground Cables (continue)-
bullPaper Insulationndash3 core belted 11kV PILC cablendashSingle core screened 11 kV PILC cable
bullPolymer Insulationndash3 core XLPE 11 kV cablendashSingle core XLPE 11 kV cable
11
64 Types of Underground Cables (continue)-
bull A = Conductor (Aluminum)
bull B = Strand Screen (carbon black paper )bull C = Insulation (Paper)bull D = Insulation Screen
(carbon black paper)bull E = Sheath (copper
lead)bull F = Jacket
Example of Single core screened 11 kV PILC cable
12
65 Types of Underground Cables (continue)-
bull A = Conductor (Aluminum)
bull B = Strand Screen (extruded
semiconducting)bull C = Insulation (XLPE)bull D = Insulation Screen (extruded
semiconducting)bull E = Shield (copper
tape)bull F = Jacket Example of Single core
XLPE 11 kV cable
13
65 Types of Underground Cables (continue)-
ndash Excellent Electrical amp Physical Properties
ndash Capable Of Carrying Large Current At High Temperaturebull Normal ~ 90ocbull Emergency ~ 130ocbull Short Circuit Conditions
~250ocndash Easy To Install ndash XLPE Easier
To Jointndash No Need For Metallic Sheath
14
66 (CABLE FAULT) INTRODUCTION
1048707 Cable faults are undesirable causes because-
1 Power supply is interrupted
2 Locating fault in a long underground cable is difficult and time consuming
3 Repairing faulty cable is difficult and time consuming
66 Cable Fault Introduction (continue)-
662 CAUSES OF UNDERGROUND CABLE FAILURE
Major factors that cause failure of a cable are-
bull Damaged accidentally by external mechanical means
bull Damage caused as a results of mishandling the cable
during layout
bull Poor workmanship in cable jointing
bull Natural causes due to aging
of cable
bull Damaged caused by movement
of soil and erosion
661 MECHANICAL
663 MISHANDLING
Mishandling of cable may be occurred during installation
Some of the examples are
1 Excessive pull
2 Sharp bend
3 Accident crush
664 Poor workmanship During Cable Jointing
The cable are jointed together with poor workmanship can lead to cable fault after a period of time
665 NATURAL CAUSES DUE TO AGING OF CABLE
CONTINUE-
CONTINUE-
66 TYPES OF CABLE FAULT
GENERAL
bull Series (open circuit) Fault
- Failure of continuity (conductor (s) or cable)
bull Shunt (short circuit) fault
- failure of insulation
66 TYPES OF FAULT (CONTINUE)-
66 TYPES OF FAULT (CONTINUE)-
661 SERIES AND SHUNT FAULT
Are subsided into the following categories
Low Resistance Faultof ZR 10
Where Zo= cable surge impedance
=10 ndash 100 ohm
Usually happens in series fault
High Resistance Fault
Where ZoR f 10
662 INTERMITTENT OR FLASH FAULT
- Usually not apparent to insulation resistance measuring
instrument
- Does not manifest itself at lower voltages or a surge
- Breakdown will appear under application of high voltage
dc or DC pressure test
67 FAULT LOCATION PROCEDURE
The proper sequence of cable fault location are as follows
a) Analysis of fault
b) Pre-location
c) Pin Pointing
d) Confirmation and re-test
671 Analysis of Cable Fault
To analyze a cable fault is to determine and confirm the nature or characteristics of the fault
Objective to select which of the cable fault locator equipment of method is best suited to locate the particular fault
Analysis of cable fault normally consists of insulation resistance test and continuity test carried out using 1000V or 5000 V insulation tester
671 Insulation Resistance test
With all earth connections removed from the cable conductor terminal using IR tester of 100025005000V range measure and record the IR in M-ohms K-ohms or ohms between conductors and between conductors and earth Six measurement are to be taken R-E Y-E B-E R-Y Y-B B-R An IR of 100M-ohm indicates a healthy cable
672 Continuity Test
With the cable conductor looped or shorted at the tail endremote end using IR tester selected at continuity range perform continuity test
This test will determine whether any of the cable is open circuited
672 Continuity test (cont-)
Continuity Test - With the cable conductor shorted or looped at the remote end perform continuity test on the cable
- Measure and record the results in ohm
- Three measurements are to be carried out between R-Y Y-B B-R
- The test will determine whether any of cable is open circuited - The resistance per-conductor per km is provided in Table VI VII VIII and IX (refer appendix A)
672 (cont-)
if the continuity of the cable is sound insulation resistance from one end are sufficient
If continuity is broken IR test should be carried out at both ends of the cable
68 BURNING A FAULT
The continuity and IR test may indicate that burning of fault by means of HT pressure test set is required
-
68 BURNING A FAULT (continue-)
Burning a fault is achieve by passing current from a DC HT test set through the fault
Other conductors not under test should be earthed
HT is applied for about 5 to 10 minutes to burn the fault
HT test is used to determine which fault location equipment is suitable to be used
HT is the last resort often used because it sometimes produce ambiguous and unpredictable results
Therefore fault location equipment should be attempted first
69 Pre- Location of fault
Pre-location is the application of a test at the terminals of a
given cable to give an indication of the distance to the fault
from the test point
Whilst the measurement should be accurate as condition will
allow the primary purpose of pre-location is to give an
indication as quick as possible of the vicinity in which to
commence the final pin-pointing tests
69 Pre- Location of fault (cont)-
Generally there are four pre-location methods which are practised and the are as follows
1Bridge or loop Method2 Pulse Echo Time Domain Reflectometry (TDR) method3 Impulse Current Method4 Arc Reflection and secondary Impulse method
691 Bridge method
6911 Direct reading fault localizer
Conditions required
i) No break in continuity if possible one sound core available
ii) The fault resistance of cable should not be more then 20 k-ohm
iii) Applied voltage not to exceed 600 V (DC)
Case 1 Single conductor to earth with sound core available
Fault distance from front end X= n100 x L
Where n = localiser reading in
L= Cable route length
Case2 Conductor to conductor with sound core available
Fault distance from front end X= n100 x L
Where n = localiser reading in
L= Cable route length
Case 3 Three phase fault
By Open loop test and close loop test
(i) Rf2Rf1 gt 5 and (ii) Rf2 lt 20 k-ohm
Open loop test
Case 3 Three phase fault
Close loop test
Open loop and close loop test (cont)
Fault distance from front end
Lxm
mn
X 2
1001
)200200
(
Where m = open loop bridge reading
n = close loop bridge reading
L = cable route length
High Tension Bridge
Conditions required
i) No break in continuity if possible one sound core available
ii) The fault resistance of cable should be more then 20 k-ohm
iii) Applied voltage not to exceed 20kV (DC)
High Tension Bridge
Case 1Single Conductor to earth with sound core available
Fault Distance from test end X = n100 x 2L
High Tension Bridge
Case 2Conductor to conductor with sound core available
Fault Distance from test end X = n100 x 2L
High Tension Bridge
Case 3Three phase Fault with no sound core available
Rf2Rf1gt5 Rf2gt20 kohm
Lxm
)mn
(X 2
1002
1
100100
Where m = open loop bridge reading
n = close loop bridge reading
L = cable route length
6912 Universal Bridge to measure capacitance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be more then 100 ohm
iii) Earth all the conductors not under test
6912 Universal Bridge to measure capacitance of Cable
Open circuit with sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with sound core available
Fault Distance from test end x = CA x L CAB
CA = Capacitance of open circuited coreCAB = Capacitance of sound coreL = conductor length
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
Fault Distance from test endfront end = ((CA(CA+CB))x L
6913 Universal Bridge to measure Inductance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be less then 30 ohm
iii) All conductors be left free from earth
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
Fault distance X = LA x L LAB
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
Fault distance X = LA x L LA + LB
692 Pulse Echo (PE) Time Domain Reflectometry (TDR)
A simple method which works on travelling wave principles It is applicable to all series and shunt faults with Rf gt=Zo10 (5 ohm) and Rflt=10 Zo (500 ohm) respectively For power cables typical Zo=50 ohm
6921 Travelling wave principles
A pulse injected into a cable is reflected back to the source by any change in characteristics impedance (Zo) of the cable
The waveform generated can be monitored using an oscilloscope as shown in Figure below
6921 Continue-
Waveform interpretion
Shunt Fault
Series Fault
6921 Continue-
Waveform interpretion
Low Resistance Shunt Fault (Rflt=500 Ohm)
-Reflection Negative
-Amplitude depends on fault resistance
Series Fault (Rfgt=5 ohm)
- Reflection Positive
- Amplitude depends on fault resistance
Zo = Characteristics impedance = radic(LC)
L = Inductance of cable C = Capacitance of Cable
69211 Reflection Factor (r)
- Is to determine the degree of reflection
Shunt fault r (V) = -Zo1(2Rf + Zo1)
Series Fault r (V) = Rf(2Zo1 + Rf)
Example shunt fault fault resistance = 5 ohm Zo = 50 ohm
r(V) = -50 (2 x5 +50) = - 833If fault resistance = 500 ohm
r(V) = -50 (2x 500 + 50) = -475
69212 Velocity of Propagation (Vp)
- Vp depends on the cable dielectric and is defined as
Vp = Vs radicEr
- where Vs = Velocity of light in free space
Er = Relative permittivity of dielectric
Vp = 300radicEr
Dielectrics Velocity of PropagationVp (mmicroS)
Impregnated Paper 150 -171
PVC 152-175
PE Approx 200
XLPE 156-176
Typical Vp for different Dielectric
69212 Continue (Vp)
Example
Determine the velocity of propagation Vp2 for a cable length of 2500 meters t1= 3164 S and t2 = 1745 S Calculate the fault distances
6921 Continue-
The distance to the fault Lf (m) is given by
Lf = T x Vp2
6921 Continue
Solutions
Velocity of propagation = Vp2 = 25003164 = 79 mS
Distance of Fault from test end = Vp2 x t2
= 79 x 1745
= 137855
693 Impulse Current Method
Working on Travelling wave principles it is applicable to all types of fault
694 Arc Reflection or Secondary Impulse method
Basically PE and TDR associated with fault treatment It is also applicable to faults of all nature but with easily interpreted breakdown waveforms
610 PIN POINTING
Pin Pointing is the application of a test that positively confirms the exact position of the fault
Before the commencement of pin pointing the prelocated fault distance should mark on the cable route which is measured by means of a trumeter
Pin pointing is normally carried out by the shock wave discharge method The fault can be detected by the use of semisphone
611 Confirmation and Re-Test
After the pin-pointed position of the fault has been marked and exposed check for physical sign of fault If there is the fault is confirm
After confirmation the fault should be cut away IR and continuity test should be carried out on the remaining cable sections to determine the soundeness of these cable sections
The insulation resistance test are again carried out after jointing followed by pressure test before supply can be restored
Flowchart for Fault location Procedure
Example
Find a fault distance using direct reading fault localiser from end test which has a test data as below
Bridge Balance Reading =60 Length of Cable 140 meters
Solutions
Fault Distance= = 60100 x 140 = 84 meterxL
n
100
For cables route with combination of 2 or more different type of cable
Determine the resistance of cable for each section separately from Table 1 and Table 2
Example Cable 11kV Al Length 150 m size = 16mm2
From Table 1= Resistance for 1km =226 ohm
For 150 m = 1501000 x 226 = 0339 ohm
From Table 1 Calculate the equivalent resistance for all the combined section
Example for Al = 0339 ohm for copper = 0150 ohm
Equivalent Resistance = 0339+0150 =0489 ohm
Continue
Calculate equivalent resistance from obtained bridge reading
Example Bridge Reading = 60
Equivalent resistance = 06 x 0489 ohm = 02934 ohm
Determine which section of cable the fault occurs
Total resistance at Al section = 0339 ohm
As equivalent resistance at Al section is higher then 02934(Equivalent resistance from bridge test) the fault is
located at Aluminium section of cable
Table 1 Impedance data for 635011000 V Cable Aluminium
Table II Impedance data 11000 V Cable Copper
TABLE VI ndash 6001000V (ALUMINIUM)
TABLE VII ndash 38006600V (ALUMINIUM)
TABLE VIII- 635011000 (ALUMINIUM)
TABLE V
610 COMMISSIONING OF UNDERGROUND CABLE
High Voltage DC testing
High voltage testing is carried out in order to determine the electrical strength of cable insulation Site tests are performed by applying a predetermined high voltage to the insulation DC site test voltages regardless of insulation type are used to ensure that the cable cable joints and termination are correctly made and installed
High Voltage Testing (cont-)
IEC Standards specify maintenance tests after installation should be 70 of permissible factory test DC voltages (15 minutes) = 175kV
Insulation Resistance Test (cont-)
Test voltages should be slowly raised to the required value over a period of about 1 minute and the test period starts once the full voltage is reached In this way the capacitive and absorption currents will have decreased and circuit conditions stabilized such that true leakage current may be measured
64 Types of Underground Cables (continue)-
bullUsually the operating voltage decides the types of insulation and cable placed in various categories depending upon the voltage for which they are designed
ndashLow Voltage Cable (LV) 11kVndashHigh Voltage Cable (HV) 11 kV
10
64 Types of Underground Cables (continue)-
bullPaper Insulationndash3 core belted 11kV PILC cablendashSingle core screened 11 kV PILC cable
bullPolymer Insulationndash3 core XLPE 11 kV cablendashSingle core XLPE 11 kV cable
11
64 Types of Underground Cables (continue)-
bull A = Conductor (Aluminum)
bull B = Strand Screen (carbon black paper )bull C = Insulation (Paper)bull D = Insulation Screen
(carbon black paper)bull E = Sheath (copper
lead)bull F = Jacket
Example of Single core screened 11 kV PILC cable
12
65 Types of Underground Cables (continue)-
bull A = Conductor (Aluminum)
bull B = Strand Screen (extruded
semiconducting)bull C = Insulation (XLPE)bull D = Insulation Screen (extruded
semiconducting)bull E = Shield (copper
tape)bull F = Jacket Example of Single core
XLPE 11 kV cable
13
65 Types of Underground Cables (continue)-
ndash Excellent Electrical amp Physical Properties
ndash Capable Of Carrying Large Current At High Temperaturebull Normal ~ 90ocbull Emergency ~ 130ocbull Short Circuit Conditions
~250ocndash Easy To Install ndash XLPE Easier
To Jointndash No Need For Metallic Sheath
14
66 (CABLE FAULT) INTRODUCTION
1048707 Cable faults are undesirable causes because-
1 Power supply is interrupted
2 Locating fault in a long underground cable is difficult and time consuming
3 Repairing faulty cable is difficult and time consuming
66 Cable Fault Introduction (continue)-
662 CAUSES OF UNDERGROUND CABLE FAILURE
Major factors that cause failure of a cable are-
bull Damaged accidentally by external mechanical means
bull Damage caused as a results of mishandling the cable
during layout
bull Poor workmanship in cable jointing
bull Natural causes due to aging
of cable
bull Damaged caused by movement
of soil and erosion
661 MECHANICAL
663 MISHANDLING
Mishandling of cable may be occurred during installation
Some of the examples are
1 Excessive pull
2 Sharp bend
3 Accident crush
664 Poor workmanship During Cable Jointing
The cable are jointed together with poor workmanship can lead to cable fault after a period of time
665 NATURAL CAUSES DUE TO AGING OF CABLE
CONTINUE-
CONTINUE-
66 TYPES OF CABLE FAULT
GENERAL
bull Series (open circuit) Fault
- Failure of continuity (conductor (s) or cable)
bull Shunt (short circuit) fault
- failure of insulation
66 TYPES OF FAULT (CONTINUE)-
66 TYPES OF FAULT (CONTINUE)-
661 SERIES AND SHUNT FAULT
Are subsided into the following categories
Low Resistance Faultof ZR 10
Where Zo= cable surge impedance
=10 ndash 100 ohm
Usually happens in series fault
High Resistance Fault
Where ZoR f 10
662 INTERMITTENT OR FLASH FAULT
- Usually not apparent to insulation resistance measuring
instrument
- Does not manifest itself at lower voltages or a surge
- Breakdown will appear under application of high voltage
dc or DC pressure test
67 FAULT LOCATION PROCEDURE
The proper sequence of cable fault location are as follows
a) Analysis of fault
b) Pre-location
c) Pin Pointing
d) Confirmation and re-test
671 Analysis of Cable Fault
To analyze a cable fault is to determine and confirm the nature or characteristics of the fault
Objective to select which of the cable fault locator equipment of method is best suited to locate the particular fault
Analysis of cable fault normally consists of insulation resistance test and continuity test carried out using 1000V or 5000 V insulation tester
671 Insulation Resistance test
With all earth connections removed from the cable conductor terminal using IR tester of 100025005000V range measure and record the IR in M-ohms K-ohms or ohms between conductors and between conductors and earth Six measurement are to be taken R-E Y-E B-E R-Y Y-B B-R An IR of 100M-ohm indicates a healthy cable
672 Continuity Test
With the cable conductor looped or shorted at the tail endremote end using IR tester selected at continuity range perform continuity test
This test will determine whether any of the cable is open circuited
672 Continuity test (cont-)
Continuity Test - With the cable conductor shorted or looped at the remote end perform continuity test on the cable
- Measure and record the results in ohm
- Three measurements are to be carried out between R-Y Y-B B-R
- The test will determine whether any of cable is open circuited - The resistance per-conductor per km is provided in Table VI VII VIII and IX (refer appendix A)
672 (cont-)
if the continuity of the cable is sound insulation resistance from one end are sufficient
If continuity is broken IR test should be carried out at both ends of the cable
68 BURNING A FAULT
The continuity and IR test may indicate that burning of fault by means of HT pressure test set is required
-
68 BURNING A FAULT (continue-)
Burning a fault is achieve by passing current from a DC HT test set through the fault
Other conductors not under test should be earthed
HT is applied for about 5 to 10 minutes to burn the fault
HT test is used to determine which fault location equipment is suitable to be used
HT is the last resort often used because it sometimes produce ambiguous and unpredictable results
Therefore fault location equipment should be attempted first
69 Pre- Location of fault
Pre-location is the application of a test at the terminals of a
given cable to give an indication of the distance to the fault
from the test point
Whilst the measurement should be accurate as condition will
allow the primary purpose of pre-location is to give an
indication as quick as possible of the vicinity in which to
commence the final pin-pointing tests
69 Pre- Location of fault (cont)-
Generally there are four pre-location methods which are practised and the are as follows
1Bridge or loop Method2 Pulse Echo Time Domain Reflectometry (TDR) method3 Impulse Current Method4 Arc Reflection and secondary Impulse method
691 Bridge method
6911 Direct reading fault localizer
Conditions required
i) No break in continuity if possible one sound core available
ii) The fault resistance of cable should not be more then 20 k-ohm
iii) Applied voltage not to exceed 600 V (DC)
Case 1 Single conductor to earth with sound core available
Fault distance from front end X= n100 x L
Where n = localiser reading in
L= Cable route length
Case2 Conductor to conductor with sound core available
Fault distance from front end X= n100 x L
Where n = localiser reading in
L= Cable route length
Case 3 Three phase fault
By Open loop test and close loop test
(i) Rf2Rf1 gt 5 and (ii) Rf2 lt 20 k-ohm
Open loop test
Case 3 Three phase fault
Close loop test
Open loop and close loop test (cont)
Fault distance from front end
Lxm
mn
X 2
1001
)200200
(
Where m = open loop bridge reading
n = close loop bridge reading
L = cable route length
High Tension Bridge
Conditions required
i) No break in continuity if possible one sound core available
ii) The fault resistance of cable should be more then 20 k-ohm
iii) Applied voltage not to exceed 20kV (DC)
High Tension Bridge
Case 1Single Conductor to earth with sound core available
Fault Distance from test end X = n100 x 2L
High Tension Bridge
Case 2Conductor to conductor with sound core available
Fault Distance from test end X = n100 x 2L
High Tension Bridge
Case 3Three phase Fault with no sound core available
Rf2Rf1gt5 Rf2gt20 kohm
Lxm
)mn
(X 2
1002
1
100100
Where m = open loop bridge reading
n = close loop bridge reading
L = cable route length
6912 Universal Bridge to measure capacitance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be more then 100 ohm
iii) Earth all the conductors not under test
6912 Universal Bridge to measure capacitance of Cable
Open circuit with sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with sound core available
Fault Distance from test end x = CA x L CAB
CA = Capacitance of open circuited coreCAB = Capacitance of sound coreL = conductor length
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
Fault Distance from test endfront end = ((CA(CA+CB))x L
6913 Universal Bridge to measure Inductance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be less then 30 ohm
iii) All conductors be left free from earth
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
Fault distance X = LA x L LAB
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
Fault distance X = LA x L LA + LB
692 Pulse Echo (PE) Time Domain Reflectometry (TDR)
A simple method which works on travelling wave principles It is applicable to all series and shunt faults with Rf gt=Zo10 (5 ohm) and Rflt=10 Zo (500 ohm) respectively For power cables typical Zo=50 ohm
6921 Travelling wave principles
A pulse injected into a cable is reflected back to the source by any change in characteristics impedance (Zo) of the cable
The waveform generated can be monitored using an oscilloscope as shown in Figure below
6921 Continue-
Waveform interpretion
Shunt Fault
Series Fault
6921 Continue-
Waveform interpretion
Low Resistance Shunt Fault (Rflt=500 Ohm)
-Reflection Negative
-Amplitude depends on fault resistance
Series Fault (Rfgt=5 ohm)
- Reflection Positive
- Amplitude depends on fault resistance
Zo = Characteristics impedance = radic(LC)
L = Inductance of cable C = Capacitance of Cable
69211 Reflection Factor (r)
- Is to determine the degree of reflection
Shunt fault r (V) = -Zo1(2Rf + Zo1)
Series Fault r (V) = Rf(2Zo1 + Rf)
Example shunt fault fault resistance = 5 ohm Zo = 50 ohm
r(V) = -50 (2 x5 +50) = - 833If fault resistance = 500 ohm
r(V) = -50 (2x 500 + 50) = -475
69212 Velocity of Propagation (Vp)
- Vp depends on the cable dielectric and is defined as
Vp = Vs radicEr
- where Vs = Velocity of light in free space
Er = Relative permittivity of dielectric
Vp = 300radicEr
Dielectrics Velocity of PropagationVp (mmicroS)
Impregnated Paper 150 -171
PVC 152-175
PE Approx 200
XLPE 156-176
Typical Vp for different Dielectric
69212 Continue (Vp)
Example
Determine the velocity of propagation Vp2 for a cable length of 2500 meters t1= 3164 S and t2 = 1745 S Calculate the fault distances
6921 Continue-
The distance to the fault Lf (m) is given by
Lf = T x Vp2
6921 Continue
Solutions
Velocity of propagation = Vp2 = 25003164 = 79 mS
Distance of Fault from test end = Vp2 x t2
= 79 x 1745
= 137855
693 Impulse Current Method
Working on Travelling wave principles it is applicable to all types of fault
694 Arc Reflection or Secondary Impulse method
Basically PE and TDR associated with fault treatment It is also applicable to faults of all nature but with easily interpreted breakdown waveforms
610 PIN POINTING
Pin Pointing is the application of a test that positively confirms the exact position of the fault
Before the commencement of pin pointing the prelocated fault distance should mark on the cable route which is measured by means of a trumeter
Pin pointing is normally carried out by the shock wave discharge method The fault can be detected by the use of semisphone
611 Confirmation and Re-Test
After the pin-pointed position of the fault has been marked and exposed check for physical sign of fault If there is the fault is confirm
After confirmation the fault should be cut away IR and continuity test should be carried out on the remaining cable sections to determine the soundeness of these cable sections
The insulation resistance test are again carried out after jointing followed by pressure test before supply can be restored
Flowchart for Fault location Procedure
Example
Find a fault distance using direct reading fault localiser from end test which has a test data as below
Bridge Balance Reading =60 Length of Cable 140 meters
Solutions
Fault Distance= = 60100 x 140 = 84 meterxL
n
100
For cables route with combination of 2 or more different type of cable
Determine the resistance of cable for each section separately from Table 1 and Table 2
Example Cable 11kV Al Length 150 m size = 16mm2
From Table 1= Resistance for 1km =226 ohm
For 150 m = 1501000 x 226 = 0339 ohm
From Table 1 Calculate the equivalent resistance for all the combined section
Example for Al = 0339 ohm for copper = 0150 ohm
Equivalent Resistance = 0339+0150 =0489 ohm
Continue
Calculate equivalent resistance from obtained bridge reading
Example Bridge Reading = 60
Equivalent resistance = 06 x 0489 ohm = 02934 ohm
Determine which section of cable the fault occurs
Total resistance at Al section = 0339 ohm
As equivalent resistance at Al section is higher then 02934(Equivalent resistance from bridge test) the fault is
located at Aluminium section of cable
Table 1 Impedance data for 635011000 V Cable Aluminium
Table II Impedance data 11000 V Cable Copper
TABLE VI ndash 6001000V (ALUMINIUM)
TABLE VII ndash 38006600V (ALUMINIUM)
TABLE VIII- 635011000 (ALUMINIUM)
TABLE V
610 COMMISSIONING OF UNDERGROUND CABLE
High Voltage DC testing
High voltage testing is carried out in order to determine the electrical strength of cable insulation Site tests are performed by applying a predetermined high voltage to the insulation DC site test voltages regardless of insulation type are used to ensure that the cable cable joints and termination are correctly made and installed
High Voltage Testing (cont-)
IEC Standards specify maintenance tests after installation should be 70 of permissible factory test DC voltages (15 minutes) = 175kV
Insulation Resistance Test (cont-)
Test voltages should be slowly raised to the required value over a period of about 1 minute and the test period starts once the full voltage is reached In this way the capacitive and absorption currents will have decreased and circuit conditions stabilized such that true leakage current may be measured
64 Types of Underground Cables (continue)-
bullPaper Insulationndash3 core belted 11kV PILC cablendashSingle core screened 11 kV PILC cable
bullPolymer Insulationndash3 core XLPE 11 kV cablendashSingle core XLPE 11 kV cable
11
64 Types of Underground Cables (continue)-
bull A = Conductor (Aluminum)
bull B = Strand Screen (carbon black paper )bull C = Insulation (Paper)bull D = Insulation Screen
(carbon black paper)bull E = Sheath (copper
lead)bull F = Jacket
Example of Single core screened 11 kV PILC cable
12
65 Types of Underground Cables (continue)-
bull A = Conductor (Aluminum)
bull B = Strand Screen (extruded
semiconducting)bull C = Insulation (XLPE)bull D = Insulation Screen (extruded
semiconducting)bull E = Shield (copper
tape)bull F = Jacket Example of Single core
XLPE 11 kV cable
13
65 Types of Underground Cables (continue)-
ndash Excellent Electrical amp Physical Properties
ndash Capable Of Carrying Large Current At High Temperaturebull Normal ~ 90ocbull Emergency ~ 130ocbull Short Circuit Conditions
~250ocndash Easy To Install ndash XLPE Easier
To Jointndash No Need For Metallic Sheath
14
66 (CABLE FAULT) INTRODUCTION
1048707 Cable faults are undesirable causes because-
1 Power supply is interrupted
2 Locating fault in a long underground cable is difficult and time consuming
3 Repairing faulty cable is difficult and time consuming
66 Cable Fault Introduction (continue)-
662 CAUSES OF UNDERGROUND CABLE FAILURE
Major factors that cause failure of a cable are-
bull Damaged accidentally by external mechanical means
bull Damage caused as a results of mishandling the cable
during layout
bull Poor workmanship in cable jointing
bull Natural causes due to aging
of cable
bull Damaged caused by movement
of soil and erosion
661 MECHANICAL
663 MISHANDLING
Mishandling of cable may be occurred during installation
Some of the examples are
1 Excessive pull
2 Sharp bend
3 Accident crush
664 Poor workmanship During Cable Jointing
The cable are jointed together with poor workmanship can lead to cable fault after a period of time
665 NATURAL CAUSES DUE TO AGING OF CABLE
CONTINUE-
CONTINUE-
66 TYPES OF CABLE FAULT
GENERAL
bull Series (open circuit) Fault
- Failure of continuity (conductor (s) or cable)
bull Shunt (short circuit) fault
- failure of insulation
66 TYPES OF FAULT (CONTINUE)-
66 TYPES OF FAULT (CONTINUE)-
661 SERIES AND SHUNT FAULT
Are subsided into the following categories
Low Resistance Faultof ZR 10
Where Zo= cable surge impedance
=10 ndash 100 ohm
Usually happens in series fault
High Resistance Fault
Where ZoR f 10
662 INTERMITTENT OR FLASH FAULT
- Usually not apparent to insulation resistance measuring
instrument
- Does not manifest itself at lower voltages or a surge
- Breakdown will appear under application of high voltage
dc or DC pressure test
67 FAULT LOCATION PROCEDURE
The proper sequence of cable fault location are as follows
a) Analysis of fault
b) Pre-location
c) Pin Pointing
d) Confirmation and re-test
671 Analysis of Cable Fault
To analyze a cable fault is to determine and confirm the nature or characteristics of the fault
Objective to select which of the cable fault locator equipment of method is best suited to locate the particular fault
Analysis of cable fault normally consists of insulation resistance test and continuity test carried out using 1000V or 5000 V insulation tester
671 Insulation Resistance test
With all earth connections removed from the cable conductor terminal using IR tester of 100025005000V range measure and record the IR in M-ohms K-ohms or ohms between conductors and between conductors and earth Six measurement are to be taken R-E Y-E B-E R-Y Y-B B-R An IR of 100M-ohm indicates a healthy cable
672 Continuity Test
With the cable conductor looped or shorted at the tail endremote end using IR tester selected at continuity range perform continuity test
This test will determine whether any of the cable is open circuited
672 Continuity test (cont-)
Continuity Test - With the cable conductor shorted or looped at the remote end perform continuity test on the cable
- Measure and record the results in ohm
- Three measurements are to be carried out between R-Y Y-B B-R
- The test will determine whether any of cable is open circuited - The resistance per-conductor per km is provided in Table VI VII VIII and IX (refer appendix A)
672 (cont-)
if the continuity of the cable is sound insulation resistance from one end are sufficient
If continuity is broken IR test should be carried out at both ends of the cable
68 BURNING A FAULT
The continuity and IR test may indicate that burning of fault by means of HT pressure test set is required
-
68 BURNING A FAULT (continue-)
Burning a fault is achieve by passing current from a DC HT test set through the fault
Other conductors not under test should be earthed
HT is applied for about 5 to 10 minutes to burn the fault
HT test is used to determine which fault location equipment is suitable to be used
HT is the last resort often used because it sometimes produce ambiguous and unpredictable results
Therefore fault location equipment should be attempted first
69 Pre- Location of fault
Pre-location is the application of a test at the terminals of a
given cable to give an indication of the distance to the fault
from the test point
Whilst the measurement should be accurate as condition will
allow the primary purpose of pre-location is to give an
indication as quick as possible of the vicinity in which to
commence the final pin-pointing tests
69 Pre- Location of fault (cont)-
Generally there are four pre-location methods which are practised and the are as follows
1Bridge or loop Method2 Pulse Echo Time Domain Reflectometry (TDR) method3 Impulse Current Method4 Arc Reflection and secondary Impulse method
691 Bridge method
6911 Direct reading fault localizer
Conditions required
i) No break in continuity if possible one sound core available
ii) The fault resistance of cable should not be more then 20 k-ohm
iii) Applied voltage not to exceed 600 V (DC)
Case 1 Single conductor to earth with sound core available
Fault distance from front end X= n100 x L
Where n = localiser reading in
L= Cable route length
Case2 Conductor to conductor with sound core available
Fault distance from front end X= n100 x L
Where n = localiser reading in
L= Cable route length
Case 3 Three phase fault
By Open loop test and close loop test
(i) Rf2Rf1 gt 5 and (ii) Rf2 lt 20 k-ohm
Open loop test
Case 3 Three phase fault
Close loop test
Open loop and close loop test (cont)
Fault distance from front end
Lxm
mn
X 2
1001
)200200
(
Where m = open loop bridge reading
n = close loop bridge reading
L = cable route length
High Tension Bridge
Conditions required
i) No break in continuity if possible one sound core available
ii) The fault resistance of cable should be more then 20 k-ohm
iii) Applied voltage not to exceed 20kV (DC)
High Tension Bridge
Case 1Single Conductor to earth with sound core available
Fault Distance from test end X = n100 x 2L
High Tension Bridge
Case 2Conductor to conductor with sound core available
Fault Distance from test end X = n100 x 2L
High Tension Bridge
Case 3Three phase Fault with no sound core available
Rf2Rf1gt5 Rf2gt20 kohm
Lxm
)mn
(X 2
1002
1
100100
Where m = open loop bridge reading
n = close loop bridge reading
L = cable route length
6912 Universal Bridge to measure capacitance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be more then 100 ohm
iii) Earth all the conductors not under test
6912 Universal Bridge to measure capacitance of Cable
Open circuit with sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with sound core available
Fault Distance from test end x = CA x L CAB
CA = Capacitance of open circuited coreCAB = Capacitance of sound coreL = conductor length
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
Fault Distance from test endfront end = ((CA(CA+CB))x L
6913 Universal Bridge to measure Inductance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be less then 30 ohm
iii) All conductors be left free from earth
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
Fault distance X = LA x L LAB
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
Fault distance X = LA x L LA + LB
692 Pulse Echo (PE) Time Domain Reflectometry (TDR)
A simple method which works on travelling wave principles It is applicable to all series and shunt faults with Rf gt=Zo10 (5 ohm) and Rflt=10 Zo (500 ohm) respectively For power cables typical Zo=50 ohm
6921 Travelling wave principles
A pulse injected into a cable is reflected back to the source by any change in characteristics impedance (Zo) of the cable
The waveform generated can be monitored using an oscilloscope as shown in Figure below
6921 Continue-
Waveform interpretion
Shunt Fault
Series Fault
6921 Continue-
Waveform interpretion
Low Resistance Shunt Fault (Rflt=500 Ohm)
-Reflection Negative
-Amplitude depends on fault resistance
Series Fault (Rfgt=5 ohm)
- Reflection Positive
- Amplitude depends on fault resistance
Zo = Characteristics impedance = radic(LC)
L = Inductance of cable C = Capacitance of Cable
69211 Reflection Factor (r)
- Is to determine the degree of reflection
Shunt fault r (V) = -Zo1(2Rf + Zo1)
Series Fault r (V) = Rf(2Zo1 + Rf)
Example shunt fault fault resistance = 5 ohm Zo = 50 ohm
r(V) = -50 (2 x5 +50) = - 833If fault resistance = 500 ohm
r(V) = -50 (2x 500 + 50) = -475
69212 Velocity of Propagation (Vp)
- Vp depends on the cable dielectric and is defined as
Vp = Vs radicEr
- where Vs = Velocity of light in free space
Er = Relative permittivity of dielectric
Vp = 300radicEr
Dielectrics Velocity of PropagationVp (mmicroS)
Impregnated Paper 150 -171
PVC 152-175
PE Approx 200
XLPE 156-176
Typical Vp for different Dielectric
69212 Continue (Vp)
Example
Determine the velocity of propagation Vp2 for a cable length of 2500 meters t1= 3164 S and t2 = 1745 S Calculate the fault distances
6921 Continue-
The distance to the fault Lf (m) is given by
Lf = T x Vp2
6921 Continue
Solutions
Velocity of propagation = Vp2 = 25003164 = 79 mS
Distance of Fault from test end = Vp2 x t2
= 79 x 1745
= 137855
693 Impulse Current Method
Working on Travelling wave principles it is applicable to all types of fault
694 Arc Reflection or Secondary Impulse method
Basically PE and TDR associated with fault treatment It is also applicable to faults of all nature but with easily interpreted breakdown waveforms
610 PIN POINTING
Pin Pointing is the application of a test that positively confirms the exact position of the fault
Before the commencement of pin pointing the prelocated fault distance should mark on the cable route which is measured by means of a trumeter
Pin pointing is normally carried out by the shock wave discharge method The fault can be detected by the use of semisphone
611 Confirmation and Re-Test
After the pin-pointed position of the fault has been marked and exposed check for physical sign of fault If there is the fault is confirm
After confirmation the fault should be cut away IR and continuity test should be carried out on the remaining cable sections to determine the soundeness of these cable sections
The insulation resistance test are again carried out after jointing followed by pressure test before supply can be restored
Flowchart for Fault location Procedure
Example
Find a fault distance using direct reading fault localiser from end test which has a test data as below
Bridge Balance Reading =60 Length of Cable 140 meters
Solutions
Fault Distance= = 60100 x 140 = 84 meterxL
n
100
For cables route with combination of 2 or more different type of cable
Determine the resistance of cable for each section separately from Table 1 and Table 2
Example Cable 11kV Al Length 150 m size = 16mm2
From Table 1= Resistance for 1km =226 ohm
For 150 m = 1501000 x 226 = 0339 ohm
From Table 1 Calculate the equivalent resistance for all the combined section
Example for Al = 0339 ohm for copper = 0150 ohm
Equivalent Resistance = 0339+0150 =0489 ohm
Continue
Calculate equivalent resistance from obtained bridge reading
Example Bridge Reading = 60
Equivalent resistance = 06 x 0489 ohm = 02934 ohm
Determine which section of cable the fault occurs
Total resistance at Al section = 0339 ohm
As equivalent resistance at Al section is higher then 02934(Equivalent resistance from bridge test) the fault is
located at Aluminium section of cable
Table 1 Impedance data for 635011000 V Cable Aluminium
Table II Impedance data 11000 V Cable Copper
TABLE VI ndash 6001000V (ALUMINIUM)
TABLE VII ndash 38006600V (ALUMINIUM)
TABLE VIII- 635011000 (ALUMINIUM)
TABLE V
610 COMMISSIONING OF UNDERGROUND CABLE
High Voltage DC testing
High voltage testing is carried out in order to determine the electrical strength of cable insulation Site tests are performed by applying a predetermined high voltage to the insulation DC site test voltages regardless of insulation type are used to ensure that the cable cable joints and termination are correctly made and installed
High Voltage Testing (cont-)
IEC Standards specify maintenance tests after installation should be 70 of permissible factory test DC voltages (15 minutes) = 175kV
Insulation Resistance Test (cont-)
Test voltages should be slowly raised to the required value over a period of about 1 minute and the test period starts once the full voltage is reached In this way the capacitive and absorption currents will have decreased and circuit conditions stabilized such that true leakage current may be measured
64 Types of Underground Cables (continue)-
bull A = Conductor (Aluminum)
bull B = Strand Screen (carbon black paper )bull C = Insulation (Paper)bull D = Insulation Screen
(carbon black paper)bull E = Sheath (copper
lead)bull F = Jacket
Example of Single core screened 11 kV PILC cable
12
65 Types of Underground Cables (continue)-
bull A = Conductor (Aluminum)
bull B = Strand Screen (extruded
semiconducting)bull C = Insulation (XLPE)bull D = Insulation Screen (extruded
semiconducting)bull E = Shield (copper
tape)bull F = Jacket Example of Single core
XLPE 11 kV cable
13
65 Types of Underground Cables (continue)-
ndash Excellent Electrical amp Physical Properties
ndash Capable Of Carrying Large Current At High Temperaturebull Normal ~ 90ocbull Emergency ~ 130ocbull Short Circuit Conditions
~250ocndash Easy To Install ndash XLPE Easier
To Jointndash No Need For Metallic Sheath
14
66 (CABLE FAULT) INTRODUCTION
1048707 Cable faults are undesirable causes because-
1 Power supply is interrupted
2 Locating fault in a long underground cable is difficult and time consuming
3 Repairing faulty cable is difficult and time consuming
66 Cable Fault Introduction (continue)-
662 CAUSES OF UNDERGROUND CABLE FAILURE
Major factors that cause failure of a cable are-
bull Damaged accidentally by external mechanical means
bull Damage caused as a results of mishandling the cable
during layout
bull Poor workmanship in cable jointing
bull Natural causes due to aging
of cable
bull Damaged caused by movement
of soil and erosion
661 MECHANICAL
663 MISHANDLING
Mishandling of cable may be occurred during installation
Some of the examples are
1 Excessive pull
2 Sharp bend
3 Accident crush
664 Poor workmanship During Cable Jointing
The cable are jointed together with poor workmanship can lead to cable fault after a period of time
665 NATURAL CAUSES DUE TO AGING OF CABLE
CONTINUE-
CONTINUE-
66 TYPES OF CABLE FAULT
GENERAL
bull Series (open circuit) Fault
- Failure of continuity (conductor (s) or cable)
bull Shunt (short circuit) fault
- failure of insulation
66 TYPES OF FAULT (CONTINUE)-
66 TYPES OF FAULT (CONTINUE)-
661 SERIES AND SHUNT FAULT
Are subsided into the following categories
Low Resistance Faultof ZR 10
Where Zo= cable surge impedance
=10 ndash 100 ohm
Usually happens in series fault
High Resistance Fault
Where ZoR f 10
662 INTERMITTENT OR FLASH FAULT
- Usually not apparent to insulation resistance measuring
instrument
- Does not manifest itself at lower voltages or a surge
- Breakdown will appear under application of high voltage
dc or DC pressure test
67 FAULT LOCATION PROCEDURE
The proper sequence of cable fault location are as follows
a) Analysis of fault
b) Pre-location
c) Pin Pointing
d) Confirmation and re-test
671 Analysis of Cable Fault
To analyze a cable fault is to determine and confirm the nature or characteristics of the fault
Objective to select which of the cable fault locator equipment of method is best suited to locate the particular fault
Analysis of cable fault normally consists of insulation resistance test and continuity test carried out using 1000V or 5000 V insulation tester
671 Insulation Resistance test
With all earth connections removed from the cable conductor terminal using IR tester of 100025005000V range measure and record the IR in M-ohms K-ohms or ohms between conductors and between conductors and earth Six measurement are to be taken R-E Y-E B-E R-Y Y-B B-R An IR of 100M-ohm indicates a healthy cable
672 Continuity Test
With the cable conductor looped or shorted at the tail endremote end using IR tester selected at continuity range perform continuity test
This test will determine whether any of the cable is open circuited
672 Continuity test (cont-)
Continuity Test - With the cable conductor shorted or looped at the remote end perform continuity test on the cable
- Measure and record the results in ohm
- Three measurements are to be carried out between R-Y Y-B B-R
- The test will determine whether any of cable is open circuited - The resistance per-conductor per km is provided in Table VI VII VIII and IX (refer appendix A)
672 (cont-)
if the continuity of the cable is sound insulation resistance from one end are sufficient
If continuity is broken IR test should be carried out at both ends of the cable
68 BURNING A FAULT
The continuity and IR test may indicate that burning of fault by means of HT pressure test set is required
-
68 BURNING A FAULT (continue-)
Burning a fault is achieve by passing current from a DC HT test set through the fault
Other conductors not under test should be earthed
HT is applied for about 5 to 10 minutes to burn the fault
HT test is used to determine which fault location equipment is suitable to be used
HT is the last resort often used because it sometimes produce ambiguous and unpredictable results
Therefore fault location equipment should be attempted first
69 Pre- Location of fault
Pre-location is the application of a test at the terminals of a
given cable to give an indication of the distance to the fault
from the test point
Whilst the measurement should be accurate as condition will
allow the primary purpose of pre-location is to give an
indication as quick as possible of the vicinity in which to
commence the final pin-pointing tests
69 Pre- Location of fault (cont)-
Generally there are four pre-location methods which are practised and the are as follows
1Bridge or loop Method2 Pulse Echo Time Domain Reflectometry (TDR) method3 Impulse Current Method4 Arc Reflection and secondary Impulse method
691 Bridge method
6911 Direct reading fault localizer
Conditions required
i) No break in continuity if possible one sound core available
ii) The fault resistance of cable should not be more then 20 k-ohm
iii) Applied voltage not to exceed 600 V (DC)
Case 1 Single conductor to earth with sound core available
Fault distance from front end X= n100 x L
Where n = localiser reading in
L= Cable route length
Case2 Conductor to conductor with sound core available
Fault distance from front end X= n100 x L
Where n = localiser reading in
L= Cable route length
Case 3 Three phase fault
By Open loop test and close loop test
(i) Rf2Rf1 gt 5 and (ii) Rf2 lt 20 k-ohm
Open loop test
Case 3 Three phase fault
Close loop test
Open loop and close loop test (cont)
Fault distance from front end
Lxm
mn
X 2
1001
)200200
(
Where m = open loop bridge reading
n = close loop bridge reading
L = cable route length
High Tension Bridge
Conditions required
i) No break in continuity if possible one sound core available
ii) The fault resistance of cable should be more then 20 k-ohm
iii) Applied voltage not to exceed 20kV (DC)
High Tension Bridge
Case 1Single Conductor to earth with sound core available
Fault Distance from test end X = n100 x 2L
High Tension Bridge
Case 2Conductor to conductor with sound core available
Fault Distance from test end X = n100 x 2L
High Tension Bridge
Case 3Three phase Fault with no sound core available
Rf2Rf1gt5 Rf2gt20 kohm
Lxm
)mn
(X 2
1002
1
100100
Where m = open loop bridge reading
n = close loop bridge reading
L = cable route length
6912 Universal Bridge to measure capacitance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be more then 100 ohm
iii) Earth all the conductors not under test
6912 Universal Bridge to measure capacitance of Cable
Open circuit with sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with sound core available
Fault Distance from test end x = CA x L CAB
CA = Capacitance of open circuited coreCAB = Capacitance of sound coreL = conductor length
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
Fault Distance from test endfront end = ((CA(CA+CB))x L
6913 Universal Bridge to measure Inductance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be less then 30 ohm
iii) All conductors be left free from earth
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
Fault distance X = LA x L LAB
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
Fault distance X = LA x L LA + LB
692 Pulse Echo (PE) Time Domain Reflectometry (TDR)
A simple method which works on travelling wave principles It is applicable to all series and shunt faults with Rf gt=Zo10 (5 ohm) and Rflt=10 Zo (500 ohm) respectively For power cables typical Zo=50 ohm
6921 Travelling wave principles
A pulse injected into a cable is reflected back to the source by any change in characteristics impedance (Zo) of the cable
The waveform generated can be monitored using an oscilloscope as shown in Figure below
6921 Continue-
Waveform interpretion
Shunt Fault
Series Fault
6921 Continue-
Waveform interpretion
Low Resistance Shunt Fault (Rflt=500 Ohm)
-Reflection Negative
-Amplitude depends on fault resistance
Series Fault (Rfgt=5 ohm)
- Reflection Positive
- Amplitude depends on fault resistance
Zo = Characteristics impedance = radic(LC)
L = Inductance of cable C = Capacitance of Cable
69211 Reflection Factor (r)
- Is to determine the degree of reflection
Shunt fault r (V) = -Zo1(2Rf + Zo1)
Series Fault r (V) = Rf(2Zo1 + Rf)
Example shunt fault fault resistance = 5 ohm Zo = 50 ohm
r(V) = -50 (2 x5 +50) = - 833If fault resistance = 500 ohm
r(V) = -50 (2x 500 + 50) = -475
69212 Velocity of Propagation (Vp)
- Vp depends on the cable dielectric and is defined as
Vp = Vs radicEr
- where Vs = Velocity of light in free space
Er = Relative permittivity of dielectric
Vp = 300radicEr
Dielectrics Velocity of PropagationVp (mmicroS)
Impregnated Paper 150 -171
PVC 152-175
PE Approx 200
XLPE 156-176
Typical Vp for different Dielectric
69212 Continue (Vp)
Example
Determine the velocity of propagation Vp2 for a cable length of 2500 meters t1= 3164 S and t2 = 1745 S Calculate the fault distances
6921 Continue-
The distance to the fault Lf (m) is given by
Lf = T x Vp2
6921 Continue
Solutions
Velocity of propagation = Vp2 = 25003164 = 79 mS
Distance of Fault from test end = Vp2 x t2
= 79 x 1745
= 137855
693 Impulse Current Method
Working on Travelling wave principles it is applicable to all types of fault
694 Arc Reflection or Secondary Impulse method
Basically PE and TDR associated with fault treatment It is also applicable to faults of all nature but with easily interpreted breakdown waveforms
610 PIN POINTING
Pin Pointing is the application of a test that positively confirms the exact position of the fault
Before the commencement of pin pointing the prelocated fault distance should mark on the cable route which is measured by means of a trumeter
Pin pointing is normally carried out by the shock wave discharge method The fault can be detected by the use of semisphone
611 Confirmation and Re-Test
After the pin-pointed position of the fault has been marked and exposed check for physical sign of fault If there is the fault is confirm
After confirmation the fault should be cut away IR and continuity test should be carried out on the remaining cable sections to determine the soundeness of these cable sections
The insulation resistance test are again carried out after jointing followed by pressure test before supply can be restored
Flowchart for Fault location Procedure
Example
Find a fault distance using direct reading fault localiser from end test which has a test data as below
Bridge Balance Reading =60 Length of Cable 140 meters
Solutions
Fault Distance= = 60100 x 140 = 84 meterxL
n
100
For cables route with combination of 2 or more different type of cable
Determine the resistance of cable for each section separately from Table 1 and Table 2
Example Cable 11kV Al Length 150 m size = 16mm2
From Table 1= Resistance for 1km =226 ohm
For 150 m = 1501000 x 226 = 0339 ohm
From Table 1 Calculate the equivalent resistance for all the combined section
Example for Al = 0339 ohm for copper = 0150 ohm
Equivalent Resistance = 0339+0150 =0489 ohm
Continue
Calculate equivalent resistance from obtained bridge reading
Example Bridge Reading = 60
Equivalent resistance = 06 x 0489 ohm = 02934 ohm
Determine which section of cable the fault occurs
Total resistance at Al section = 0339 ohm
As equivalent resistance at Al section is higher then 02934(Equivalent resistance from bridge test) the fault is
located at Aluminium section of cable
Table 1 Impedance data for 635011000 V Cable Aluminium
Table II Impedance data 11000 V Cable Copper
TABLE VI ndash 6001000V (ALUMINIUM)
TABLE VII ndash 38006600V (ALUMINIUM)
TABLE VIII- 635011000 (ALUMINIUM)
TABLE V
610 COMMISSIONING OF UNDERGROUND CABLE
High Voltage DC testing
High voltage testing is carried out in order to determine the electrical strength of cable insulation Site tests are performed by applying a predetermined high voltage to the insulation DC site test voltages regardless of insulation type are used to ensure that the cable cable joints and termination are correctly made and installed
High Voltage Testing (cont-)
IEC Standards specify maintenance tests after installation should be 70 of permissible factory test DC voltages (15 minutes) = 175kV
Insulation Resistance Test (cont-)
Test voltages should be slowly raised to the required value over a period of about 1 minute and the test period starts once the full voltage is reached In this way the capacitive and absorption currents will have decreased and circuit conditions stabilized such that true leakage current may be measured
65 Types of Underground Cables (continue)-
bull A = Conductor (Aluminum)
bull B = Strand Screen (extruded
semiconducting)bull C = Insulation (XLPE)bull D = Insulation Screen (extruded
semiconducting)bull E = Shield (copper
tape)bull F = Jacket Example of Single core
XLPE 11 kV cable
13
65 Types of Underground Cables (continue)-
ndash Excellent Electrical amp Physical Properties
ndash Capable Of Carrying Large Current At High Temperaturebull Normal ~ 90ocbull Emergency ~ 130ocbull Short Circuit Conditions
~250ocndash Easy To Install ndash XLPE Easier
To Jointndash No Need For Metallic Sheath
14
66 (CABLE FAULT) INTRODUCTION
1048707 Cable faults are undesirable causes because-
1 Power supply is interrupted
2 Locating fault in a long underground cable is difficult and time consuming
3 Repairing faulty cable is difficult and time consuming
66 Cable Fault Introduction (continue)-
662 CAUSES OF UNDERGROUND CABLE FAILURE
Major factors that cause failure of a cable are-
bull Damaged accidentally by external mechanical means
bull Damage caused as a results of mishandling the cable
during layout
bull Poor workmanship in cable jointing
bull Natural causes due to aging
of cable
bull Damaged caused by movement
of soil and erosion
661 MECHANICAL
663 MISHANDLING
Mishandling of cable may be occurred during installation
Some of the examples are
1 Excessive pull
2 Sharp bend
3 Accident crush
664 Poor workmanship During Cable Jointing
The cable are jointed together with poor workmanship can lead to cable fault after a period of time
665 NATURAL CAUSES DUE TO AGING OF CABLE
CONTINUE-
CONTINUE-
66 TYPES OF CABLE FAULT
GENERAL
bull Series (open circuit) Fault
- Failure of continuity (conductor (s) or cable)
bull Shunt (short circuit) fault
- failure of insulation
66 TYPES OF FAULT (CONTINUE)-
66 TYPES OF FAULT (CONTINUE)-
661 SERIES AND SHUNT FAULT
Are subsided into the following categories
Low Resistance Faultof ZR 10
Where Zo= cable surge impedance
=10 ndash 100 ohm
Usually happens in series fault
High Resistance Fault
Where ZoR f 10
662 INTERMITTENT OR FLASH FAULT
- Usually not apparent to insulation resistance measuring
instrument
- Does not manifest itself at lower voltages or a surge
- Breakdown will appear under application of high voltage
dc or DC pressure test
67 FAULT LOCATION PROCEDURE
The proper sequence of cable fault location are as follows
a) Analysis of fault
b) Pre-location
c) Pin Pointing
d) Confirmation and re-test
671 Analysis of Cable Fault
To analyze a cable fault is to determine and confirm the nature or characteristics of the fault
Objective to select which of the cable fault locator equipment of method is best suited to locate the particular fault
Analysis of cable fault normally consists of insulation resistance test and continuity test carried out using 1000V or 5000 V insulation tester
671 Insulation Resistance test
With all earth connections removed from the cable conductor terminal using IR tester of 100025005000V range measure and record the IR in M-ohms K-ohms or ohms between conductors and between conductors and earth Six measurement are to be taken R-E Y-E B-E R-Y Y-B B-R An IR of 100M-ohm indicates a healthy cable
672 Continuity Test
With the cable conductor looped or shorted at the tail endremote end using IR tester selected at continuity range perform continuity test
This test will determine whether any of the cable is open circuited
672 Continuity test (cont-)
Continuity Test - With the cable conductor shorted or looped at the remote end perform continuity test on the cable
- Measure and record the results in ohm
- Three measurements are to be carried out between R-Y Y-B B-R
- The test will determine whether any of cable is open circuited - The resistance per-conductor per km is provided in Table VI VII VIII and IX (refer appendix A)
672 (cont-)
if the continuity of the cable is sound insulation resistance from one end are sufficient
If continuity is broken IR test should be carried out at both ends of the cable
68 BURNING A FAULT
The continuity and IR test may indicate that burning of fault by means of HT pressure test set is required
-
68 BURNING A FAULT (continue-)
Burning a fault is achieve by passing current from a DC HT test set through the fault
Other conductors not under test should be earthed
HT is applied for about 5 to 10 minutes to burn the fault
HT test is used to determine which fault location equipment is suitable to be used
HT is the last resort often used because it sometimes produce ambiguous and unpredictable results
Therefore fault location equipment should be attempted first
69 Pre- Location of fault
Pre-location is the application of a test at the terminals of a
given cable to give an indication of the distance to the fault
from the test point
Whilst the measurement should be accurate as condition will
allow the primary purpose of pre-location is to give an
indication as quick as possible of the vicinity in which to
commence the final pin-pointing tests
69 Pre- Location of fault (cont)-
Generally there are four pre-location methods which are practised and the are as follows
1Bridge or loop Method2 Pulse Echo Time Domain Reflectometry (TDR) method3 Impulse Current Method4 Arc Reflection and secondary Impulse method
691 Bridge method
6911 Direct reading fault localizer
Conditions required
i) No break in continuity if possible one sound core available
ii) The fault resistance of cable should not be more then 20 k-ohm
iii) Applied voltage not to exceed 600 V (DC)
Case 1 Single conductor to earth with sound core available
Fault distance from front end X= n100 x L
Where n = localiser reading in
L= Cable route length
Case2 Conductor to conductor with sound core available
Fault distance from front end X= n100 x L
Where n = localiser reading in
L= Cable route length
Case 3 Three phase fault
By Open loop test and close loop test
(i) Rf2Rf1 gt 5 and (ii) Rf2 lt 20 k-ohm
Open loop test
Case 3 Three phase fault
Close loop test
Open loop and close loop test (cont)
Fault distance from front end
Lxm
mn
X 2
1001
)200200
(
Where m = open loop bridge reading
n = close loop bridge reading
L = cable route length
High Tension Bridge
Conditions required
i) No break in continuity if possible one sound core available
ii) The fault resistance of cable should be more then 20 k-ohm
iii) Applied voltage not to exceed 20kV (DC)
High Tension Bridge
Case 1Single Conductor to earth with sound core available
Fault Distance from test end X = n100 x 2L
High Tension Bridge
Case 2Conductor to conductor with sound core available
Fault Distance from test end X = n100 x 2L
High Tension Bridge
Case 3Three phase Fault with no sound core available
Rf2Rf1gt5 Rf2gt20 kohm
Lxm
)mn
(X 2
1002
1
100100
Where m = open loop bridge reading
n = close loop bridge reading
L = cable route length
6912 Universal Bridge to measure capacitance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be more then 100 ohm
iii) Earth all the conductors not under test
6912 Universal Bridge to measure capacitance of Cable
Open circuit with sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with sound core available
Fault Distance from test end x = CA x L CAB
CA = Capacitance of open circuited coreCAB = Capacitance of sound coreL = conductor length
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
Fault Distance from test endfront end = ((CA(CA+CB))x L
6913 Universal Bridge to measure Inductance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be less then 30 ohm
iii) All conductors be left free from earth
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
Fault distance X = LA x L LAB
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
Fault distance X = LA x L LA + LB
692 Pulse Echo (PE) Time Domain Reflectometry (TDR)
A simple method which works on travelling wave principles It is applicable to all series and shunt faults with Rf gt=Zo10 (5 ohm) and Rflt=10 Zo (500 ohm) respectively For power cables typical Zo=50 ohm
6921 Travelling wave principles
A pulse injected into a cable is reflected back to the source by any change in characteristics impedance (Zo) of the cable
The waveform generated can be monitored using an oscilloscope as shown in Figure below
6921 Continue-
Waveform interpretion
Shunt Fault
Series Fault
6921 Continue-
Waveform interpretion
Low Resistance Shunt Fault (Rflt=500 Ohm)
-Reflection Negative
-Amplitude depends on fault resistance
Series Fault (Rfgt=5 ohm)
- Reflection Positive
- Amplitude depends on fault resistance
Zo = Characteristics impedance = radic(LC)
L = Inductance of cable C = Capacitance of Cable
69211 Reflection Factor (r)
- Is to determine the degree of reflection
Shunt fault r (V) = -Zo1(2Rf + Zo1)
Series Fault r (V) = Rf(2Zo1 + Rf)
Example shunt fault fault resistance = 5 ohm Zo = 50 ohm
r(V) = -50 (2 x5 +50) = - 833If fault resistance = 500 ohm
r(V) = -50 (2x 500 + 50) = -475
69212 Velocity of Propagation (Vp)
- Vp depends on the cable dielectric and is defined as
Vp = Vs radicEr
- where Vs = Velocity of light in free space
Er = Relative permittivity of dielectric
Vp = 300radicEr
Dielectrics Velocity of PropagationVp (mmicroS)
Impregnated Paper 150 -171
PVC 152-175
PE Approx 200
XLPE 156-176
Typical Vp for different Dielectric
69212 Continue (Vp)
Example
Determine the velocity of propagation Vp2 for a cable length of 2500 meters t1= 3164 S and t2 = 1745 S Calculate the fault distances
6921 Continue-
The distance to the fault Lf (m) is given by
Lf = T x Vp2
6921 Continue
Solutions
Velocity of propagation = Vp2 = 25003164 = 79 mS
Distance of Fault from test end = Vp2 x t2
= 79 x 1745
= 137855
693 Impulse Current Method
Working on Travelling wave principles it is applicable to all types of fault
694 Arc Reflection or Secondary Impulse method
Basically PE and TDR associated with fault treatment It is also applicable to faults of all nature but with easily interpreted breakdown waveforms
610 PIN POINTING
Pin Pointing is the application of a test that positively confirms the exact position of the fault
Before the commencement of pin pointing the prelocated fault distance should mark on the cable route which is measured by means of a trumeter
Pin pointing is normally carried out by the shock wave discharge method The fault can be detected by the use of semisphone
611 Confirmation and Re-Test
After the pin-pointed position of the fault has been marked and exposed check for physical sign of fault If there is the fault is confirm
After confirmation the fault should be cut away IR and continuity test should be carried out on the remaining cable sections to determine the soundeness of these cable sections
The insulation resistance test are again carried out after jointing followed by pressure test before supply can be restored
Flowchart for Fault location Procedure
Example
Find a fault distance using direct reading fault localiser from end test which has a test data as below
Bridge Balance Reading =60 Length of Cable 140 meters
Solutions
Fault Distance= = 60100 x 140 = 84 meterxL
n
100
For cables route with combination of 2 or more different type of cable
Determine the resistance of cable for each section separately from Table 1 and Table 2
Example Cable 11kV Al Length 150 m size = 16mm2
From Table 1= Resistance for 1km =226 ohm
For 150 m = 1501000 x 226 = 0339 ohm
From Table 1 Calculate the equivalent resistance for all the combined section
Example for Al = 0339 ohm for copper = 0150 ohm
Equivalent Resistance = 0339+0150 =0489 ohm
Continue
Calculate equivalent resistance from obtained bridge reading
Example Bridge Reading = 60
Equivalent resistance = 06 x 0489 ohm = 02934 ohm
Determine which section of cable the fault occurs
Total resistance at Al section = 0339 ohm
As equivalent resistance at Al section is higher then 02934(Equivalent resistance from bridge test) the fault is
located at Aluminium section of cable
Table 1 Impedance data for 635011000 V Cable Aluminium
Table II Impedance data 11000 V Cable Copper
TABLE VI ndash 6001000V (ALUMINIUM)
TABLE VII ndash 38006600V (ALUMINIUM)
TABLE VIII- 635011000 (ALUMINIUM)
TABLE V
610 COMMISSIONING OF UNDERGROUND CABLE
High Voltage DC testing
High voltage testing is carried out in order to determine the electrical strength of cable insulation Site tests are performed by applying a predetermined high voltage to the insulation DC site test voltages regardless of insulation type are used to ensure that the cable cable joints and termination are correctly made and installed
High Voltage Testing (cont-)
IEC Standards specify maintenance tests after installation should be 70 of permissible factory test DC voltages (15 minutes) = 175kV
Insulation Resistance Test (cont-)
Test voltages should be slowly raised to the required value over a period of about 1 minute and the test period starts once the full voltage is reached In this way the capacitive and absorption currents will have decreased and circuit conditions stabilized such that true leakage current may be measured
65 Types of Underground Cables (continue)-
ndash Excellent Electrical amp Physical Properties
ndash Capable Of Carrying Large Current At High Temperaturebull Normal ~ 90ocbull Emergency ~ 130ocbull Short Circuit Conditions
~250ocndash Easy To Install ndash XLPE Easier
To Jointndash No Need For Metallic Sheath
14
66 (CABLE FAULT) INTRODUCTION
1048707 Cable faults are undesirable causes because-
1 Power supply is interrupted
2 Locating fault in a long underground cable is difficult and time consuming
3 Repairing faulty cable is difficult and time consuming
66 Cable Fault Introduction (continue)-
662 CAUSES OF UNDERGROUND CABLE FAILURE
Major factors that cause failure of a cable are-
bull Damaged accidentally by external mechanical means
bull Damage caused as a results of mishandling the cable
during layout
bull Poor workmanship in cable jointing
bull Natural causes due to aging
of cable
bull Damaged caused by movement
of soil and erosion
661 MECHANICAL
663 MISHANDLING
Mishandling of cable may be occurred during installation
Some of the examples are
1 Excessive pull
2 Sharp bend
3 Accident crush
664 Poor workmanship During Cable Jointing
The cable are jointed together with poor workmanship can lead to cable fault after a period of time
665 NATURAL CAUSES DUE TO AGING OF CABLE
CONTINUE-
CONTINUE-
66 TYPES OF CABLE FAULT
GENERAL
bull Series (open circuit) Fault
- Failure of continuity (conductor (s) or cable)
bull Shunt (short circuit) fault
- failure of insulation
66 TYPES OF FAULT (CONTINUE)-
66 TYPES OF FAULT (CONTINUE)-
661 SERIES AND SHUNT FAULT
Are subsided into the following categories
Low Resistance Faultof ZR 10
Where Zo= cable surge impedance
=10 ndash 100 ohm
Usually happens in series fault
High Resistance Fault
Where ZoR f 10
662 INTERMITTENT OR FLASH FAULT
- Usually not apparent to insulation resistance measuring
instrument
- Does not manifest itself at lower voltages or a surge
- Breakdown will appear under application of high voltage
dc or DC pressure test
67 FAULT LOCATION PROCEDURE
The proper sequence of cable fault location are as follows
a) Analysis of fault
b) Pre-location
c) Pin Pointing
d) Confirmation and re-test
671 Analysis of Cable Fault
To analyze a cable fault is to determine and confirm the nature or characteristics of the fault
Objective to select which of the cable fault locator equipment of method is best suited to locate the particular fault
Analysis of cable fault normally consists of insulation resistance test and continuity test carried out using 1000V or 5000 V insulation tester
671 Insulation Resistance test
With all earth connections removed from the cable conductor terminal using IR tester of 100025005000V range measure and record the IR in M-ohms K-ohms or ohms between conductors and between conductors and earth Six measurement are to be taken R-E Y-E B-E R-Y Y-B B-R An IR of 100M-ohm indicates a healthy cable
672 Continuity Test
With the cable conductor looped or shorted at the tail endremote end using IR tester selected at continuity range perform continuity test
This test will determine whether any of the cable is open circuited
672 Continuity test (cont-)
Continuity Test - With the cable conductor shorted or looped at the remote end perform continuity test on the cable
- Measure and record the results in ohm
- Three measurements are to be carried out between R-Y Y-B B-R
- The test will determine whether any of cable is open circuited - The resistance per-conductor per km is provided in Table VI VII VIII and IX (refer appendix A)
672 (cont-)
if the continuity of the cable is sound insulation resistance from one end are sufficient
If continuity is broken IR test should be carried out at both ends of the cable
68 BURNING A FAULT
The continuity and IR test may indicate that burning of fault by means of HT pressure test set is required
-
68 BURNING A FAULT (continue-)
Burning a fault is achieve by passing current from a DC HT test set through the fault
Other conductors not under test should be earthed
HT is applied for about 5 to 10 minutes to burn the fault
HT test is used to determine which fault location equipment is suitable to be used
HT is the last resort often used because it sometimes produce ambiguous and unpredictable results
Therefore fault location equipment should be attempted first
69 Pre- Location of fault
Pre-location is the application of a test at the terminals of a
given cable to give an indication of the distance to the fault
from the test point
Whilst the measurement should be accurate as condition will
allow the primary purpose of pre-location is to give an
indication as quick as possible of the vicinity in which to
commence the final pin-pointing tests
69 Pre- Location of fault (cont)-
Generally there are four pre-location methods which are practised and the are as follows
1Bridge or loop Method2 Pulse Echo Time Domain Reflectometry (TDR) method3 Impulse Current Method4 Arc Reflection and secondary Impulse method
691 Bridge method
6911 Direct reading fault localizer
Conditions required
i) No break in continuity if possible one sound core available
ii) The fault resistance of cable should not be more then 20 k-ohm
iii) Applied voltage not to exceed 600 V (DC)
Case 1 Single conductor to earth with sound core available
Fault distance from front end X= n100 x L
Where n = localiser reading in
L= Cable route length
Case2 Conductor to conductor with sound core available
Fault distance from front end X= n100 x L
Where n = localiser reading in
L= Cable route length
Case 3 Three phase fault
By Open loop test and close loop test
(i) Rf2Rf1 gt 5 and (ii) Rf2 lt 20 k-ohm
Open loop test
Case 3 Three phase fault
Close loop test
Open loop and close loop test (cont)
Fault distance from front end
Lxm
mn
X 2
1001
)200200
(
Where m = open loop bridge reading
n = close loop bridge reading
L = cable route length
High Tension Bridge
Conditions required
i) No break in continuity if possible one sound core available
ii) The fault resistance of cable should be more then 20 k-ohm
iii) Applied voltage not to exceed 20kV (DC)
High Tension Bridge
Case 1Single Conductor to earth with sound core available
Fault Distance from test end X = n100 x 2L
High Tension Bridge
Case 2Conductor to conductor with sound core available
Fault Distance from test end X = n100 x 2L
High Tension Bridge
Case 3Three phase Fault with no sound core available
Rf2Rf1gt5 Rf2gt20 kohm
Lxm
)mn
(X 2
1002
1
100100
Where m = open loop bridge reading
n = close loop bridge reading
L = cable route length
6912 Universal Bridge to measure capacitance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be more then 100 ohm
iii) Earth all the conductors not under test
6912 Universal Bridge to measure capacitance of Cable
Open circuit with sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with sound core available
Fault Distance from test end x = CA x L CAB
CA = Capacitance of open circuited coreCAB = Capacitance of sound coreL = conductor length
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
Fault Distance from test endfront end = ((CA(CA+CB))x L
6913 Universal Bridge to measure Inductance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be less then 30 ohm
iii) All conductors be left free from earth
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
Fault distance X = LA x L LAB
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
Fault distance X = LA x L LA + LB
692 Pulse Echo (PE) Time Domain Reflectometry (TDR)
A simple method which works on travelling wave principles It is applicable to all series and shunt faults with Rf gt=Zo10 (5 ohm) and Rflt=10 Zo (500 ohm) respectively For power cables typical Zo=50 ohm
6921 Travelling wave principles
A pulse injected into a cable is reflected back to the source by any change in characteristics impedance (Zo) of the cable
The waveform generated can be monitored using an oscilloscope as shown in Figure below
6921 Continue-
Waveform interpretion
Shunt Fault
Series Fault
6921 Continue-
Waveform interpretion
Low Resistance Shunt Fault (Rflt=500 Ohm)
-Reflection Negative
-Amplitude depends on fault resistance
Series Fault (Rfgt=5 ohm)
- Reflection Positive
- Amplitude depends on fault resistance
Zo = Characteristics impedance = radic(LC)
L = Inductance of cable C = Capacitance of Cable
69211 Reflection Factor (r)
- Is to determine the degree of reflection
Shunt fault r (V) = -Zo1(2Rf + Zo1)
Series Fault r (V) = Rf(2Zo1 + Rf)
Example shunt fault fault resistance = 5 ohm Zo = 50 ohm
r(V) = -50 (2 x5 +50) = - 833If fault resistance = 500 ohm
r(V) = -50 (2x 500 + 50) = -475
69212 Velocity of Propagation (Vp)
- Vp depends on the cable dielectric and is defined as
Vp = Vs radicEr
- where Vs = Velocity of light in free space
Er = Relative permittivity of dielectric
Vp = 300radicEr
Dielectrics Velocity of PropagationVp (mmicroS)
Impregnated Paper 150 -171
PVC 152-175
PE Approx 200
XLPE 156-176
Typical Vp for different Dielectric
69212 Continue (Vp)
Example
Determine the velocity of propagation Vp2 for a cable length of 2500 meters t1= 3164 S and t2 = 1745 S Calculate the fault distances
6921 Continue-
The distance to the fault Lf (m) is given by
Lf = T x Vp2
6921 Continue
Solutions
Velocity of propagation = Vp2 = 25003164 = 79 mS
Distance of Fault from test end = Vp2 x t2
= 79 x 1745
= 137855
693 Impulse Current Method
Working on Travelling wave principles it is applicable to all types of fault
694 Arc Reflection or Secondary Impulse method
Basically PE and TDR associated with fault treatment It is also applicable to faults of all nature but with easily interpreted breakdown waveforms
610 PIN POINTING
Pin Pointing is the application of a test that positively confirms the exact position of the fault
Before the commencement of pin pointing the prelocated fault distance should mark on the cable route which is measured by means of a trumeter
Pin pointing is normally carried out by the shock wave discharge method The fault can be detected by the use of semisphone
611 Confirmation and Re-Test
After the pin-pointed position of the fault has been marked and exposed check for physical sign of fault If there is the fault is confirm
After confirmation the fault should be cut away IR and continuity test should be carried out on the remaining cable sections to determine the soundeness of these cable sections
The insulation resistance test are again carried out after jointing followed by pressure test before supply can be restored
Flowchart for Fault location Procedure
Example
Find a fault distance using direct reading fault localiser from end test which has a test data as below
Bridge Balance Reading =60 Length of Cable 140 meters
Solutions
Fault Distance= = 60100 x 140 = 84 meterxL
n
100
For cables route with combination of 2 or more different type of cable
Determine the resistance of cable for each section separately from Table 1 and Table 2
Example Cable 11kV Al Length 150 m size = 16mm2
From Table 1= Resistance for 1km =226 ohm
For 150 m = 1501000 x 226 = 0339 ohm
From Table 1 Calculate the equivalent resistance for all the combined section
Example for Al = 0339 ohm for copper = 0150 ohm
Equivalent Resistance = 0339+0150 =0489 ohm
Continue
Calculate equivalent resistance from obtained bridge reading
Example Bridge Reading = 60
Equivalent resistance = 06 x 0489 ohm = 02934 ohm
Determine which section of cable the fault occurs
Total resistance at Al section = 0339 ohm
As equivalent resistance at Al section is higher then 02934(Equivalent resistance from bridge test) the fault is
located at Aluminium section of cable
Table 1 Impedance data for 635011000 V Cable Aluminium
Table II Impedance data 11000 V Cable Copper
TABLE VI ndash 6001000V (ALUMINIUM)
TABLE VII ndash 38006600V (ALUMINIUM)
TABLE VIII- 635011000 (ALUMINIUM)
TABLE V
610 COMMISSIONING OF UNDERGROUND CABLE
High Voltage DC testing
High voltage testing is carried out in order to determine the electrical strength of cable insulation Site tests are performed by applying a predetermined high voltage to the insulation DC site test voltages regardless of insulation type are used to ensure that the cable cable joints and termination are correctly made and installed
High Voltage Testing (cont-)
IEC Standards specify maintenance tests after installation should be 70 of permissible factory test DC voltages (15 minutes) = 175kV
Insulation Resistance Test (cont-)
Test voltages should be slowly raised to the required value over a period of about 1 minute and the test period starts once the full voltage is reached In this way the capacitive and absorption currents will have decreased and circuit conditions stabilized such that true leakage current may be measured
66 (CABLE FAULT) INTRODUCTION
1048707 Cable faults are undesirable causes because-
1 Power supply is interrupted
2 Locating fault in a long underground cable is difficult and time consuming
3 Repairing faulty cable is difficult and time consuming
66 Cable Fault Introduction (continue)-
662 CAUSES OF UNDERGROUND CABLE FAILURE
Major factors that cause failure of a cable are-
bull Damaged accidentally by external mechanical means
bull Damage caused as a results of mishandling the cable
during layout
bull Poor workmanship in cable jointing
bull Natural causes due to aging
of cable
bull Damaged caused by movement
of soil and erosion
661 MECHANICAL
663 MISHANDLING
Mishandling of cable may be occurred during installation
Some of the examples are
1 Excessive pull
2 Sharp bend
3 Accident crush
664 Poor workmanship During Cable Jointing
The cable are jointed together with poor workmanship can lead to cable fault after a period of time
665 NATURAL CAUSES DUE TO AGING OF CABLE
CONTINUE-
CONTINUE-
66 TYPES OF CABLE FAULT
GENERAL
bull Series (open circuit) Fault
- Failure of continuity (conductor (s) or cable)
bull Shunt (short circuit) fault
- failure of insulation
66 TYPES OF FAULT (CONTINUE)-
66 TYPES OF FAULT (CONTINUE)-
661 SERIES AND SHUNT FAULT
Are subsided into the following categories
Low Resistance Faultof ZR 10
Where Zo= cable surge impedance
=10 ndash 100 ohm
Usually happens in series fault
High Resistance Fault
Where ZoR f 10
662 INTERMITTENT OR FLASH FAULT
- Usually not apparent to insulation resistance measuring
instrument
- Does not manifest itself at lower voltages or a surge
- Breakdown will appear under application of high voltage
dc or DC pressure test
67 FAULT LOCATION PROCEDURE
The proper sequence of cable fault location are as follows
a) Analysis of fault
b) Pre-location
c) Pin Pointing
d) Confirmation and re-test
671 Analysis of Cable Fault
To analyze a cable fault is to determine and confirm the nature or characteristics of the fault
Objective to select which of the cable fault locator equipment of method is best suited to locate the particular fault
Analysis of cable fault normally consists of insulation resistance test and continuity test carried out using 1000V or 5000 V insulation tester
671 Insulation Resistance test
With all earth connections removed from the cable conductor terminal using IR tester of 100025005000V range measure and record the IR in M-ohms K-ohms or ohms between conductors and between conductors and earth Six measurement are to be taken R-E Y-E B-E R-Y Y-B B-R An IR of 100M-ohm indicates a healthy cable
672 Continuity Test
With the cable conductor looped or shorted at the tail endremote end using IR tester selected at continuity range perform continuity test
This test will determine whether any of the cable is open circuited
672 Continuity test (cont-)
Continuity Test - With the cable conductor shorted or looped at the remote end perform continuity test on the cable
- Measure and record the results in ohm
- Three measurements are to be carried out between R-Y Y-B B-R
- The test will determine whether any of cable is open circuited - The resistance per-conductor per km is provided in Table VI VII VIII and IX (refer appendix A)
672 (cont-)
if the continuity of the cable is sound insulation resistance from one end are sufficient
If continuity is broken IR test should be carried out at both ends of the cable
68 BURNING A FAULT
The continuity and IR test may indicate that burning of fault by means of HT pressure test set is required
-
68 BURNING A FAULT (continue-)
Burning a fault is achieve by passing current from a DC HT test set through the fault
Other conductors not under test should be earthed
HT is applied for about 5 to 10 minutes to burn the fault
HT test is used to determine which fault location equipment is suitable to be used
HT is the last resort often used because it sometimes produce ambiguous and unpredictable results
Therefore fault location equipment should be attempted first
69 Pre- Location of fault
Pre-location is the application of a test at the terminals of a
given cable to give an indication of the distance to the fault
from the test point
Whilst the measurement should be accurate as condition will
allow the primary purpose of pre-location is to give an
indication as quick as possible of the vicinity in which to
commence the final pin-pointing tests
69 Pre- Location of fault (cont)-
Generally there are four pre-location methods which are practised and the are as follows
1Bridge or loop Method2 Pulse Echo Time Domain Reflectometry (TDR) method3 Impulse Current Method4 Arc Reflection and secondary Impulse method
691 Bridge method
6911 Direct reading fault localizer
Conditions required
i) No break in continuity if possible one sound core available
ii) The fault resistance of cable should not be more then 20 k-ohm
iii) Applied voltage not to exceed 600 V (DC)
Case 1 Single conductor to earth with sound core available
Fault distance from front end X= n100 x L
Where n = localiser reading in
L= Cable route length
Case2 Conductor to conductor with sound core available
Fault distance from front end X= n100 x L
Where n = localiser reading in
L= Cable route length
Case 3 Three phase fault
By Open loop test and close loop test
(i) Rf2Rf1 gt 5 and (ii) Rf2 lt 20 k-ohm
Open loop test
Case 3 Three phase fault
Close loop test
Open loop and close loop test (cont)
Fault distance from front end
Lxm
mn
X 2
1001
)200200
(
Where m = open loop bridge reading
n = close loop bridge reading
L = cable route length
High Tension Bridge
Conditions required
i) No break in continuity if possible one sound core available
ii) The fault resistance of cable should be more then 20 k-ohm
iii) Applied voltage not to exceed 20kV (DC)
High Tension Bridge
Case 1Single Conductor to earth with sound core available
Fault Distance from test end X = n100 x 2L
High Tension Bridge
Case 2Conductor to conductor with sound core available
Fault Distance from test end X = n100 x 2L
High Tension Bridge
Case 3Three phase Fault with no sound core available
Rf2Rf1gt5 Rf2gt20 kohm
Lxm
)mn
(X 2
1002
1
100100
Where m = open loop bridge reading
n = close loop bridge reading
L = cable route length
6912 Universal Bridge to measure capacitance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be more then 100 ohm
iii) Earth all the conductors not under test
6912 Universal Bridge to measure capacitance of Cable
Open circuit with sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with sound core available
Fault Distance from test end x = CA x L CAB
CA = Capacitance of open circuited coreCAB = Capacitance of sound coreL = conductor length
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
Fault Distance from test endfront end = ((CA(CA+CB))x L
6913 Universal Bridge to measure Inductance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be less then 30 ohm
iii) All conductors be left free from earth
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
Fault distance X = LA x L LAB
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
Fault distance X = LA x L LA + LB
692 Pulse Echo (PE) Time Domain Reflectometry (TDR)
A simple method which works on travelling wave principles It is applicable to all series and shunt faults with Rf gt=Zo10 (5 ohm) and Rflt=10 Zo (500 ohm) respectively For power cables typical Zo=50 ohm
6921 Travelling wave principles
A pulse injected into a cable is reflected back to the source by any change in characteristics impedance (Zo) of the cable
The waveform generated can be monitored using an oscilloscope as shown in Figure below
6921 Continue-
Waveform interpretion
Shunt Fault
Series Fault
6921 Continue-
Waveform interpretion
Low Resistance Shunt Fault (Rflt=500 Ohm)
-Reflection Negative
-Amplitude depends on fault resistance
Series Fault (Rfgt=5 ohm)
- Reflection Positive
- Amplitude depends on fault resistance
Zo = Characteristics impedance = radic(LC)
L = Inductance of cable C = Capacitance of Cable
69211 Reflection Factor (r)
- Is to determine the degree of reflection
Shunt fault r (V) = -Zo1(2Rf + Zo1)
Series Fault r (V) = Rf(2Zo1 + Rf)
Example shunt fault fault resistance = 5 ohm Zo = 50 ohm
r(V) = -50 (2 x5 +50) = - 833If fault resistance = 500 ohm
r(V) = -50 (2x 500 + 50) = -475
69212 Velocity of Propagation (Vp)
- Vp depends on the cable dielectric and is defined as
Vp = Vs radicEr
- where Vs = Velocity of light in free space
Er = Relative permittivity of dielectric
Vp = 300radicEr
Dielectrics Velocity of PropagationVp (mmicroS)
Impregnated Paper 150 -171
PVC 152-175
PE Approx 200
XLPE 156-176
Typical Vp for different Dielectric
69212 Continue (Vp)
Example
Determine the velocity of propagation Vp2 for a cable length of 2500 meters t1= 3164 S and t2 = 1745 S Calculate the fault distances
6921 Continue-
The distance to the fault Lf (m) is given by
Lf = T x Vp2
6921 Continue
Solutions
Velocity of propagation = Vp2 = 25003164 = 79 mS
Distance of Fault from test end = Vp2 x t2
= 79 x 1745
= 137855
693 Impulse Current Method
Working on Travelling wave principles it is applicable to all types of fault
694 Arc Reflection or Secondary Impulse method
Basically PE and TDR associated with fault treatment It is also applicable to faults of all nature but with easily interpreted breakdown waveforms
610 PIN POINTING
Pin Pointing is the application of a test that positively confirms the exact position of the fault
Before the commencement of pin pointing the prelocated fault distance should mark on the cable route which is measured by means of a trumeter
Pin pointing is normally carried out by the shock wave discharge method The fault can be detected by the use of semisphone
611 Confirmation and Re-Test
After the pin-pointed position of the fault has been marked and exposed check for physical sign of fault If there is the fault is confirm
After confirmation the fault should be cut away IR and continuity test should be carried out on the remaining cable sections to determine the soundeness of these cable sections
The insulation resistance test are again carried out after jointing followed by pressure test before supply can be restored
Flowchart for Fault location Procedure
Example
Find a fault distance using direct reading fault localiser from end test which has a test data as below
Bridge Balance Reading =60 Length of Cable 140 meters
Solutions
Fault Distance= = 60100 x 140 = 84 meterxL
n
100
For cables route with combination of 2 or more different type of cable
Determine the resistance of cable for each section separately from Table 1 and Table 2
Example Cable 11kV Al Length 150 m size = 16mm2
From Table 1= Resistance for 1km =226 ohm
For 150 m = 1501000 x 226 = 0339 ohm
From Table 1 Calculate the equivalent resistance for all the combined section
Example for Al = 0339 ohm for copper = 0150 ohm
Equivalent Resistance = 0339+0150 =0489 ohm
Continue
Calculate equivalent resistance from obtained bridge reading
Example Bridge Reading = 60
Equivalent resistance = 06 x 0489 ohm = 02934 ohm
Determine which section of cable the fault occurs
Total resistance at Al section = 0339 ohm
As equivalent resistance at Al section is higher then 02934(Equivalent resistance from bridge test) the fault is
located at Aluminium section of cable
Table 1 Impedance data for 635011000 V Cable Aluminium
Table II Impedance data 11000 V Cable Copper
TABLE VI ndash 6001000V (ALUMINIUM)
TABLE VII ndash 38006600V (ALUMINIUM)
TABLE VIII- 635011000 (ALUMINIUM)
TABLE V
610 COMMISSIONING OF UNDERGROUND CABLE
High Voltage DC testing
High voltage testing is carried out in order to determine the electrical strength of cable insulation Site tests are performed by applying a predetermined high voltage to the insulation DC site test voltages regardless of insulation type are used to ensure that the cable cable joints and termination are correctly made and installed
High Voltage Testing (cont-)
IEC Standards specify maintenance tests after installation should be 70 of permissible factory test DC voltages (15 minutes) = 175kV
Insulation Resistance Test (cont-)
Test voltages should be slowly raised to the required value over a period of about 1 minute and the test period starts once the full voltage is reached In this way the capacitive and absorption currents will have decreased and circuit conditions stabilized such that true leakage current may be measured
1048707 Cable faults are undesirable causes because-
1 Power supply is interrupted
2 Locating fault in a long underground cable is difficult and time consuming
3 Repairing faulty cable is difficult and time consuming
66 Cable Fault Introduction (continue)-
662 CAUSES OF UNDERGROUND CABLE FAILURE
Major factors that cause failure of a cable are-
bull Damaged accidentally by external mechanical means
bull Damage caused as a results of mishandling the cable
during layout
bull Poor workmanship in cable jointing
bull Natural causes due to aging
of cable
bull Damaged caused by movement
of soil and erosion
661 MECHANICAL
663 MISHANDLING
Mishandling of cable may be occurred during installation
Some of the examples are
1 Excessive pull
2 Sharp bend
3 Accident crush
664 Poor workmanship During Cable Jointing
The cable are jointed together with poor workmanship can lead to cable fault after a period of time
665 NATURAL CAUSES DUE TO AGING OF CABLE
CONTINUE-
CONTINUE-
66 TYPES OF CABLE FAULT
GENERAL
bull Series (open circuit) Fault
- Failure of continuity (conductor (s) or cable)
bull Shunt (short circuit) fault
- failure of insulation
66 TYPES OF FAULT (CONTINUE)-
66 TYPES OF FAULT (CONTINUE)-
661 SERIES AND SHUNT FAULT
Are subsided into the following categories
Low Resistance Faultof ZR 10
Where Zo= cable surge impedance
=10 ndash 100 ohm
Usually happens in series fault
High Resistance Fault
Where ZoR f 10
662 INTERMITTENT OR FLASH FAULT
- Usually not apparent to insulation resistance measuring
instrument
- Does not manifest itself at lower voltages or a surge
- Breakdown will appear under application of high voltage
dc or DC pressure test
67 FAULT LOCATION PROCEDURE
The proper sequence of cable fault location are as follows
a) Analysis of fault
b) Pre-location
c) Pin Pointing
d) Confirmation and re-test
671 Analysis of Cable Fault
To analyze a cable fault is to determine and confirm the nature or characteristics of the fault
Objective to select which of the cable fault locator equipment of method is best suited to locate the particular fault
Analysis of cable fault normally consists of insulation resistance test and continuity test carried out using 1000V or 5000 V insulation tester
671 Insulation Resistance test
With all earth connections removed from the cable conductor terminal using IR tester of 100025005000V range measure and record the IR in M-ohms K-ohms or ohms between conductors and between conductors and earth Six measurement are to be taken R-E Y-E B-E R-Y Y-B B-R An IR of 100M-ohm indicates a healthy cable
672 Continuity Test
With the cable conductor looped or shorted at the tail endremote end using IR tester selected at continuity range perform continuity test
This test will determine whether any of the cable is open circuited
672 Continuity test (cont-)
Continuity Test - With the cable conductor shorted or looped at the remote end perform continuity test on the cable
- Measure and record the results in ohm
- Three measurements are to be carried out between R-Y Y-B B-R
- The test will determine whether any of cable is open circuited - The resistance per-conductor per km is provided in Table VI VII VIII and IX (refer appendix A)
672 (cont-)
if the continuity of the cable is sound insulation resistance from one end are sufficient
If continuity is broken IR test should be carried out at both ends of the cable
68 BURNING A FAULT
The continuity and IR test may indicate that burning of fault by means of HT pressure test set is required
-
68 BURNING A FAULT (continue-)
Burning a fault is achieve by passing current from a DC HT test set through the fault
Other conductors not under test should be earthed
HT is applied for about 5 to 10 minutes to burn the fault
HT test is used to determine which fault location equipment is suitable to be used
HT is the last resort often used because it sometimes produce ambiguous and unpredictable results
Therefore fault location equipment should be attempted first
69 Pre- Location of fault
Pre-location is the application of a test at the terminals of a
given cable to give an indication of the distance to the fault
from the test point
Whilst the measurement should be accurate as condition will
allow the primary purpose of pre-location is to give an
indication as quick as possible of the vicinity in which to
commence the final pin-pointing tests
69 Pre- Location of fault (cont)-
Generally there are four pre-location methods which are practised and the are as follows
1Bridge or loop Method2 Pulse Echo Time Domain Reflectometry (TDR) method3 Impulse Current Method4 Arc Reflection and secondary Impulse method
691 Bridge method
6911 Direct reading fault localizer
Conditions required
i) No break in continuity if possible one sound core available
ii) The fault resistance of cable should not be more then 20 k-ohm
iii) Applied voltage not to exceed 600 V (DC)
Case 1 Single conductor to earth with sound core available
Fault distance from front end X= n100 x L
Where n = localiser reading in
L= Cable route length
Case2 Conductor to conductor with sound core available
Fault distance from front end X= n100 x L
Where n = localiser reading in
L= Cable route length
Case 3 Three phase fault
By Open loop test and close loop test
(i) Rf2Rf1 gt 5 and (ii) Rf2 lt 20 k-ohm
Open loop test
Case 3 Three phase fault
Close loop test
Open loop and close loop test (cont)
Fault distance from front end
Lxm
mn
X 2
1001
)200200
(
Where m = open loop bridge reading
n = close loop bridge reading
L = cable route length
High Tension Bridge
Conditions required
i) No break in continuity if possible one sound core available
ii) The fault resistance of cable should be more then 20 k-ohm
iii) Applied voltage not to exceed 20kV (DC)
High Tension Bridge
Case 1Single Conductor to earth with sound core available
Fault Distance from test end X = n100 x 2L
High Tension Bridge
Case 2Conductor to conductor with sound core available
Fault Distance from test end X = n100 x 2L
High Tension Bridge
Case 3Three phase Fault with no sound core available
Rf2Rf1gt5 Rf2gt20 kohm
Lxm
)mn
(X 2
1002
1
100100
Where m = open loop bridge reading
n = close loop bridge reading
L = cable route length
6912 Universal Bridge to measure capacitance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be more then 100 ohm
iii) Earth all the conductors not under test
6912 Universal Bridge to measure capacitance of Cable
Open circuit with sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with sound core available
Fault Distance from test end x = CA x L CAB
CA = Capacitance of open circuited coreCAB = Capacitance of sound coreL = conductor length
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
Fault Distance from test endfront end = ((CA(CA+CB))x L
6913 Universal Bridge to measure Inductance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be less then 30 ohm
iii) All conductors be left free from earth
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
Fault distance X = LA x L LAB
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
Fault distance X = LA x L LA + LB
692 Pulse Echo (PE) Time Domain Reflectometry (TDR)
A simple method which works on travelling wave principles It is applicable to all series and shunt faults with Rf gt=Zo10 (5 ohm) and Rflt=10 Zo (500 ohm) respectively For power cables typical Zo=50 ohm
6921 Travelling wave principles
A pulse injected into a cable is reflected back to the source by any change in characteristics impedance (Zo) of the cable
The waveform generated can be monitored using an oscilloscope as shown in Figure below
6921 Continue-
Waveform interpretion
Shunt Fault
Series Fault
6921 Continue-
Waveform interpretion
Low Resistance Shunt Fault (Rflt=500 Ohm)
-Reflection Negative
-Amplitude depends on fault resistance
Series Fault (Rfgt=5 ohm)
- Reflection Positive
- Amplitude depends on fault resistance
Zo = Characteristics impedance = radic(LC)
L = Inductance of cable C = Capacitance of Cable
69211 Reflection Factor (r)
- Is to determine the degree of reflection
Shunt fault r (V) = -Zo1(2Rf + Zo1)
Series Fault r (V) = Rf(2Zo1 + Rf)
Example shunt fault fault resistance = 5 ohm Zo = 50 ohm
r(V) = -50 (2 x5 +50) = - 833If fault resistance = 500 ohm
r(V) = -50 (2x 500 + 50) = -475
69212 Velocity of Propagation (Vp)
- Vp depends on the cable dielectric and is defined as
Vp = Vs radicEr
- where Vs = Velocity of light in free space
Er = Relative permittivity of dielectric
Vp = 300radicEr
Dielectrics Velocity of PropagationVp (mmicroS)
Impregnated Paper 150 -171
PVC 152-175
PE Approx 200
XLPE 156-176
Typical Vp for different Dielectric
69212 Continue (Vp)
Example
Determine the velocity of propagation Vp2 for a cable length of 2500 meters t1= 3164 S and t2 = 1745 S Calculate the fault distances
6921 Continue-
The distance to the fault Lf (m) is given by
Lf = T x Vp2
6921 Continue
Solutions
Velocity of propagation = Vp2 = 25003164 = 79 mS
Distance of Fault from test end = Vp2 x t2
= 79 x 1745
= 137855
693 Impulse Current Method
Working on Travelling wave principles it is applicable to all types of fault
694 Arc Reflection or Secondary Impulse method
Basically PE and TDR associated with fault treatment It is also applicable to faults of all nature but with easily interpreted breakdown waveforms
610 PIN POINTING
Pin Pointing is the application of a test that positively confirms the exact position of the fault
Before the commencement of pin pointing the prelocated fault distance should mark on the cable route which is measured by means of a trumeter
Pin pointing is normally carried out by the shock wave discharge method The fault can be detected by the use of semisphone
611 Confirmation and Re-Test
After the pin-pointed position of the fault has been marked and exposed check for physical sign of fault If there is the fault is confirm
After confirmation the fault should be cut away IR and continuity test should be carried out on the remaining cable sections to determine the soundeness of these cable sections
The insulation resistance test are again carried out after jointing followed by pressure test before supply can be restored
Flowchart for Fault location Procedure
Example
Find a fault distance using direct reading fault localiser from end test which has a test data as below
Bridge Balance Reading =60 Length of Cable 140 meters
Solutions
Fault Distance= = 60100 x 140 = 84 meterxL
n
100
For cables route with combination of 2 or more different type of cable
Determine the resistance of cable for each section separately from Table 1 and Table 2
Example Cable 11kV Al Length 150 m size = 16mm2
From Table 1= Resistance for 1km =226 ohm
For 150 m = 1501000 x 226 = 0339 ohm
From Table 1 Calculate the equivalent resistance for all the combined section
Example for Al = 0339 ohm for copper = 0150 ohm
Equivalent Resistance = 0339+0150 =0489 ohm
Continue
Calculate equivalent resistance from obtained bridge reading
Example Bridge Reading = 60
Equivalent resistance = 06 x 0489 ohm = 02934 ohm
Determine which section of cable the fault occurs
Total resistance at Al section = 0339 ohm
As equivalent resistance at Al section is higher then 02934(Equivalent resistance from bridge test) the fault is
located at Aluminium section of cable
Table 1 Impedance data for 635011000 V Cable Aluminium
Table II Impedance data 11000 V Cable Copper
TABLE VI ndash 6001000V (ALUMINIUM)
TABLE VII ndash 38006600V (ALUMINIUM)
TABLE VIII- 635011000 (ALUMINIUM)
TABLE V
610 COMMISSIONING OF UNDERGROUND CABLE
High Voltage DC testing
High voltage testing is carried out in order to determine the electrical strength of cable insulation Site tests are performed by applying a predetermined high voltage to the insulation DC site test voltages regardless of insulation type are used to ensure that the cable cable joints and termination are correctly made and installed
High Voltage Testing (cont-)
IEC Standards specify maintenance tests after installation should be 70 of permissible factory test DC voltages (15 minutes) = 175kV
Insulation Resistance Test (cont-)
Test voltages should be slowly raised to the required value over a period of about 1 minute and the test period starts once the full voltage is reached In this way the capacitive and absorption currents will have decreased and circuit conditions stabilized such that true leakage current may be measured
662 CAUSES OF UNDERGROUND CABLE FAILURE
Major factors that cause failure of a cable are-
bull Damaged accidentally by external mechanical means
bull Damage caused as a results of mishandling the cable
during layout
bull Poor workmanship in cable jointing
bull Natural causes due to aging
of cable
bull Damaged caused by movement
of soil and erosion
661 MECHANICAL
663 MISHANDLING
Mishandling of cable may be occurred during installation
Some of the examples are
1 Excessive pull
2 Sharp bend
3 Accident crush
664 Poor workmanship During Cable Jointing
The cable are jointed together with poor workmanship can lead to cable fault after a period of time
665 NATURAL CAUSES DUE TO AGING OF CABLE
CONTINUE-
CONTINUE-
66 TYPES OF CABLE FAULT
GENERAL
bull Series (open circuit) Fault
- Failure of continuity (conductor (s) or cable)
bull Shunt (short circuit) fault
- failure of insulation
66 TYPES OF FAULT (CONTINUE)-
66 TYPES OF FAULT (CONTINUE)-
661 SERIES AND SHUNT FAULT
Are subsided into the following categories
Low Resistance Faultof ZR 10
Where Zo= cable surge impedance
=10 ndash 100 ohm
Usually happens in series fault
High Resistance Fault
Where ZoR f 10
662 INTERMITTENT OR FLASH FAULT
- Usually not apparent to insulation resistance measuring
instrument
- Does not manifest itself at lower voltages or a surge
- Breakdown will appear under application of high voltage
dc or DC pressure test
67 FAULT LOCATION PROCEDURE
The proper sequence of cable fault location are as follows
a) Analysis of fault
b) Pre-location
c) Pin Pointing
d) Confirmation and re-test
671 Analysis of Cable Fault
To analyze a cable fault is to determine and confirm the nature or characteristics of the fault
Objective to select which of the cable fault locator equipment of method is best suited to locate the particular fault
Analysis of cable fault normally consists of insulation resistance test and continuity test carried out using 1000V or 5000 V insulation tester
671 Insulation Resistance test
With all earth connections removed from the cable conductor terminal using IR tester of 100025005000V range measure and record the IR in M-ohms K-ohms or ohms between conductors and between conductors and earth Six measurement are to be taken R-E Y-E B-E R-Y Y-B B-R An IR of 100M-ohm indicates a healthy cable
672 Continuity Test
With the cable conductor looped or shorted at the tail endremote end using IR tester selected at continuity range perform continuity test
This test will determine whether any of the cable is open circuited
672 Continuity test (cont-)
Continuity Test - With the cable conductor shorted or looped at the remote end perform continuity test on the cable
- Measure and record the results in ohm
- Three measurements are to be carried out between R-Y Y-B B-R
- The test will determine whether any of cable is open circuited - The resistance per-conductor per km is provided in Table VI VII VIII and IX (refer appendix A)
672 (cont-)
if the continuity of the cable is sound insulation resistance from one end are sufficient
If continuity is broken IR test should be carried out at both ends of the cable
68 BURNING A FAULT
The continuity and IR test may indicate that burning of fault by means of HT pressure test set is required
-
68 BURNING A FAULT (continue-)
Burning a fault is achieve by passing current from a DC HT test set through the fault
Other conductors not under test should be earthed
HT is applied for about 5 to 10 minutes to burn the fault
HT test is used to determine which fault location equipment is suitable to be used
HT is the last resort often used because it sometimes produce ambiguous and unpredictable results
Therefore fault location equipment should be attempted first
69 Pre- Location of fault
Pre-location is the application of a test at the terminals of a
given cable to give an indication of the distance to the fault
from the test point
Whilst the measurement should be accurate as condition will
allow the primary purpose of pre-location is to give an
indication as quick as possible of the vicinity in which to
commence the final pin-pointing tests
69 Pre- Location of fault (cont)-
Generally there are four pre-location methods which are practised and the are as follows
1Bridge or loop Method2 Pulse Echo Time Domain Reflectometry (TDR) method3 Impulse Current Method4 Arc Reflection and secondary Impulse method
691 Bridge method
6911 Direct reading fault localizer
Conditions required
i) No break in continuity if possible one sound core available
ii) The fault resistance of cable should not be more then 20 k-ohm
iii) Applied voltage not to exceed 600 V (DC)
Case 1 Single conductor to earth with sound core available
Fault distance from front end X= n100 x L
Where n = localiser reading in
L= Cable route length
Case2 Conductor to conductor with sound core available
Fault distance from front end X= n100 x L
Where n = localiser reading in
L= Cable route length
Case 3 Three phase fault
By Open loop test and close loop test
(i) Rf2Rf1 gt 5 and (ii) Rf2 lt 20 k-ohm
Open loop test
Case 3 Three phase fault
Close loop test
Open loop and close loop test (cont)
Fault distance from front end
Lxm
mn
X 2
1001
)200200
(
Where m = open loop bridge reading
n = close loop bridge reading
L = cable route length
High Tension Bridge
Conditions required
i) No break in continuity if possible one sound core available
ii) The fault resistance of cable should be more then 20 k-ohm
iii) Applied voltage not to exceed 20kV (DC)
High Tension Bridge
Case 1Single Conductor to earth with sound core available
Fault Distance from test end X = n100 x 2L
High Tension Bridge
Case 2Conductor to conductor with sound core available
Fault Distance from test end X = n100 x 2L
High Tension Bridge
Case 3Three phase Fault with no sound core available
Rf2Rf1gt5 Rf2gt20 kohm
Lxm
)mn
(X 2
1002
1
100100
Where m = open loop bridge reading
n = close loop bridge reading
L = cable route length
6912 Universal Bridge to measure capacitance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be more then 100 ohm
iii) Earth all the conductors not under test
6912 Universal Bridge to measure capacitance of Cable
Open circuit with sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with sound core available
Fault Distance from test end x = CA x L CAB
CA = Capacitance of open circuited coreCAB = Capacitance of sound coreL = conductor length
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
Fault Distance from test endfront end = ((CA(CA+CB))x L
6913 Universal Bridge to measure Inductance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be less then 30 ohm
iii) All conductors be left free from earth
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
Fault distance X = LA x L LAB
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
Fault distance X = LA x L LA + LB
692 Pulse Echo (PE) Time Domain Reflectometry (TDR)
A simple method which works on travelling wave principles It is applicable to all series and shunt faults with Rf gt=Zo10 (5 ohm) and Rflt=10 Zo (500 ohm) respectively For power cables typical Zo=50 ohm
6921 Travelling wave principles
A pulse injected into a cable is reflected back to the source by any change in characteristics impedance (Zo) of the cable
The waveform generated can be monitored using an oscilloscope as shown in Figure below
6921 Continue-
Waveform interpretion
Shunt Fault
Series Fault
6921 Continue-
Waveform interpretion
Low Resistance Shunt Fault (Rflt=500 Ohm)
-Reflection Negative
-Amplitude depends on fault resistance
Series Fault (Rfgt=5 ohm)
- Reflection Positive
- Amplitude depends on fault resistance
Zo = Characteristics impedance = radic(LC)
L = Inductance of cable C = Capacitance of Cable
69211 Reflection Factor (r)
- Is to determine the degree of reflection
Shunt fault r (V) = -Zo1(2Rf + Zo1)
Series Fault r (V) = Rf(2Zo1 + Rf)
Example shunt fault fault resistance = 5 ohm Zo = 50 ohm
r(V) = -50 (2 x5 +50) = - 833If fault resistance = 500 ohm
r(V) = -50 (2x 500 + 50) = -475
69212 Velocity of Propagation (Vp)
- Vp depends on the cable dielectric and is defined as
Vp = Vs radicEr
- where Vs = Velocity of light in free space
Er = Relative permittivity of dielectric
Vp = 300radicEr
Dielectrics Velocity of PropagationVp (mmicroS)
Impregnated Paper 150 -171
PVC 152-175
PE Approx 200
XLPE 156-176
Typical Vp for different Dielectric
69212 Continue (Vp)
Example
Determine the velocity of propagation Vp2 for a cable length of 2500 meters t1= 3164 S and t2 = 1745 S Calculate the fault distances
6921 Continue-
The distance to the fault Lf (m) is given by
Lf = T x Vp2
6921 Continue
Solutions
Velocity of propagation = Vp2 = 25003164 = 79 mS
Distance of Fault from test end = Vp2 x t2
= 79 x 1745
= 137855
693 Impulse Current Method
Working on Travelling wave principles it is applicable to all types of fault
694 Arc Reflection or Secondary Impulse method
Basically PE and TDR associated with fault treatment It is also applicable to faults of all nature but with easily interpreted breakdown waveforms
610 PIN POINTING
Pin Pointing is the application of a test that positively confirms the exact position of the fault
Before the commencement of pin pointing the prelocated fault distance should mark on the cable route which is measured by means of a trumeter
Pin pointing is normally carried out by the shock wave discharge method The fault can be detected by the use of semisphone
611 Confirmation and Re-Test
After the pin-pointed position of the fault has been marked and exposed check for physical sign of fault If there is the fault is confirm
After confirmation the fault should be cut away IR and continuity test should be carried out on the remaining cable sections to determine the soundeness of these cable sections
The insulation resistance test are again carried out after jointing followed by pressure test before supply can be restored
Flowchart for Fault location Procedure
Example
Find a fault distance using direct reading fault localiser from end test which has a test data as below
Bridge Balance Reading =60 Length of Cable 140 meters
Solutions
Fault Distance= = 60100 x 140 = 84 meterxL
n
100
For cables route with combination of 2 or more different type of cable
Determine the resistance of cable for each section separately from Table 1 and Table 2
Example Cable 11kV Al Length 150 m size = 16mm2
From Table 1= Resistance for 1km =226 ohm
For 150 m = 1501000 x 226 = 0339 ohm
From Table 1 Calculate the equivalent resistance for all the combined section
Example for Al = 0339 ohm for copper = 0150 ohm
Equivalent Resistance = 0339+0150 =0489 ohm
Continue
Calculate equivalent resistance from obtained bridge reading
Example Bridge Reading = 60
Equivalent resistance = 06 x 0489 ohm = 02934 ohm
Determine which section of cable the fault occurs
Total resistance at Al section = 0339 ohm
As equivalent resistance at Al section is higher then 02934(Equivalent resistance from bridge test) the fault is
located at Aluminium section of cable
Table 1 Impedance data for 635011000 V Cable Aluminium
Table II Impedance data 11000 V Cable Copper
TABLE VI ndash 6001000V (ALUMINIUM)
TABLE VII ndash 38006600V (ALUMINIUM)
TABLE VIII- 635011000 (ALUMINIUM)
TABLE V
610 COMMISSIONING OF UNDERGROUND CABLE
High Voltage DC testing
High voltage testing is carried out in order to determine the electrical strength of cable insulation Site tests are performed by applying a predetermined high voltage to the insulation DC site test voltages regardless of insulation type are used to ensure that the cable cable joints and termination are correctly made and installed
High Voltage Testing (cont-)
IEC Standards specify maintenance tests after installation should be 70 of permissible factory test DC voltages (15 minutes) = 175kV
Insulation Resistance Test (cont-)
Test voltages should be slowly raised to the required value over a period of about 1 minute and the test period starts once the full voltage is reached In this way the capacitive and absorption currents will have decreased and circuit conditions stabilized such that true leakage current may be measured
661 MECHANICAL
663 MISHANDLING
Mishandling of cable may be occurred during installation
Some of the examples are
1 Excessive pull
2 Sharp bend
3 Accident crush
664 Poor workmanship During Cable Jointing
The cable are jointed together with poor workmanship can lead to cable fault after a period of time
665 NATURAL CAUSES DUE TO AGING OF CABLE
CONTINUE-
CONTINUE-
66 TYPES OF CABLE FAULT
GENERAL
bull Series (open circuit) Fault
- Failure of continuity (conductor (s) or cable)
bull Shunt (short circuit) fault
- failure of insulation
66 TYPES OF FAULT (CONTINUE)-
66 TYPES OF FAULT (CONTINUE)-
661 SERIES AND SHUNT FAULT
Are subsided into the following categories
Low Resistance Faultof ZR 10
Where Zo= cable surge impedance
=10 ndash 100 ohm
Usually happens in series fault
High Resistance Fault
Where ZoR f 10
662 INTERMITTENT OR FLASH FAULT
- Usually not apparent to insulation resistance measuring
instrument
- Does not manifest itself at lower voltages or a surge
- Breakdown will appear under application of high voltage
dc or DC pressure test
67 FAULT LOCATION PROCEDURE
The proper sequence of cable fault location are as follows
a) Analysis of fault
b) Pre-location
c) Pin Pointing
d) Confirmation and re-test
671 Analysis of Cable Fault
To analyze a cable fault is to determine and confirm the nature or characteristics of the fault
Objective to select which of the cable fault locator equipment of method is best suited to locate the particular fault
Analysis of cable fault normally consists of insulation resistance test and continuity test carried out using 1000V or 5000 V insulation tester
671 Insulation Resistance test
With all earth connections removed from the cable conductor terminal using IR tester of 100025005000V range measure and record the IR in M-ohms K-ohms or ohms between conductors and between conductors and earth Six measurement are to be taken R-E Y-E B-E R-Y Y-B B-R An IR of 100M-ohm indicates a healthy cable
672 Continuity Test
With the cable conductor looped or shorted at the tail endremote end using IR tester selected at continuity range perform continuity test
This test will determine whether any of the cable is open circuited
672 Continuity test (cont-)
Continuity Test - With the cable conductor shorted or looped at the remote end perform continuity test on the cable
- Measure and record the results in ohm
- Three measurements are to be carried out between R-Y Y-B B-R
- The test will determine whether any of cable is open circuited - The resistance per-conductor per km is provided in Table VI VII VIII and IX (refer appendix A)
672 (cont-)
if the continuity of the cable is sound insulation resistance from one end are sufficient
If continuity is broken IR test should be carried out at both ends of the cable
68 BURNING A FAULT
The continuity and IR test may indicate that burning of fault by means of HT pressure test set is required
-
68 BURNING A FAULT (continue-)
Burning a fault is achieve by passing current from a DC HT test set through the fault
Other conductors not under test should be earthed
HT is applied for about 5 to 10 minutes to burn the fault
HT test is used to determine which fault location equipment is suitable to be used
HT is the last resort often used because it sometimes produce ambiguous and unpredictable results
Therefore fault location equipment should be attempted first
69 Pre- Location of fault
Pre-location is the application of a test at the terminals of a
given cable to give an indication of the distance to the fault
from the test point
Whilst the measurement should be accurate as condition will
allow the primary purpose of pre-location is to give an
indication as quick as possible of the vicinity in which to
commence the final pin-pointing tests
69 Pre- Location of fault (cont)-
Generally there are four pre-location methods which are practised and the are as follows
1Bridge or loop Method2 Pulse Echo Time Domain Reflectometry (TDR) method3 Impulse Current Method4 Arc Reflection and secondary Impulse method
691 Bridge method
6911 Direct reading fault localizer
Conditions required
i) No break in continuity if possible one sound core available
ii) The fault resistance of cable should not be more then 20 k-ohm
iii) Applied voltage not to exceed 600 V (DC)
Case 1 Single conductor to earth with sound core available
Fault distance from front end X= n100 x L
Where n = localiser reading in
L= Cable route length
Case2 Conductor to conductor with sound core available
Fault distance from front end X= n100 x L
Where n = localiser reading in
L= Cable route length
Case 3 Three phase fault
By Open loop test and close loop test
(i) Rf2Rf1 gt 5 and (ii) Rf2 lt 20 k-ohm
Open loop test
Case 3 Three phase fault
Close loop test
Open loop and close loop test (cont)
Fault distance from front end
Lxm
mn
X 2
1001
)200200
(
Where m = open loop bridge reading
n = close loop bridge reading
L = cable route length
High Tension Bridge
Conditions required
i) No break in continuity if possible one sound core available
ii) The fault resistance of cable should be more then 20 k-ohm
iii) Applied voltage not to exceed 20kV (DC)
High Tension Bridge
Case 1Single Conductor to earth with sound core available
Fault Distance from test end X = n100 x 2L
High Tension Bridge
Case 2Conductor to conductor with sound core available
Fault Distance from test end X = n100 x 2L
High Tension Bridge
Case 3Three phase Fault with no sound core available
Rf2Rf1gt5 Rf2gt20 kohm
Lxm
)mn
(X 2
1002
1
100100
Where m = open loop bridge reading
n = close loop bridge reading
L = cable route length
6912 Universal Bridge to measure capacitance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be more then 100 ohm
iii) Earth all the conductors not under test
6912 Universal Bridge to measure capacitance of Cable
Open circuit with sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with sound core available
Fault Distance from test end x = CA x L CAB
CA = Capacitance of open circuited coreCAB = Capacitance of sound coreL = conductor length
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
Fault Distance from test endfront end = ((CA(CA+CB))x L
6913 Universal Bridge to measure Inductance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be less then 30 ohm
iii) All conductors be left free from earth
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
Fault distance X = LA x L LAB
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
Fault distance X = LA x L LA + LB
692 Pulse Echo (PE) Time Domain Reflectometry (TDR)
A simple method which works on travelling wave principles It is applicable to all series and shunt faults with Rf gt=Zo10 (5 ohm) and Rflt=10 Zo (500 ohm) respectively For power cables typical Zo=50 ohm
6921 Travelling wave principles
A pulse injected into a cable is reflected back to the source by any change in characteristics impedance (Zo) of the cable
The waveform generated can be monitored using an oscilloscope as shown in Figure below
6921 Continue-
Waveform interpretion
Shunt Fault
Series Fault
6921 Continue-
Waveform interpretion
Low Resistance Shunt Fault (Rflt=500 Ohm)
-Reflection Negative
-Amplitude depends on fault resistance
Series Fault (Rfgt=5 ohm)
- Reflection Positive
- Amplitude depends on fault resistance
Zo = Characteristics impedance = radic(LC)
L = Inductance of cable C = Capacitance of Cable
69211 Reflection Factor (r)
- Is to determine the degree of reflection
Shunt fault r (V) = -Zo1(2Rf + Zo1)
Series Fault r (V) = Rf(2Zo1 + Rf)
Example shunt fault fault resistance = 5 ohm Zo = 50 ohm
r(V) = -50 (2 x5 +50) = - 833If fault resistance = 500 ohm
r(V) = -50 (2x 500 + 50) = -475
69212 Velocity of Propagation (Vp)
- Vp depends on the cable dielectric and is defined as
Vp = Vs radicEr
- where Vs = Velocity of light in free space
Er = Relative permittivity of dielectric
Vp = 300radicEr
Dielectrics Velocity of PropagationVp (mmicroS)
Impregnated Paper 150 -171
PVC 152-175
PE Approx 200
XLPE 156-176
Typical Vp for different Dielectric
69212 Continue (Vp)
Example
Determine the velocity of propagation Vp2 for a cable length of 2500 meters t1= 3164 S and t2 = 1745 S Calculate the fault distances
6921 Continue-
The distance to the fault Lf (m) is given by
Lf = T x Vp2
6921 Continue
Solutions
Velocity of propagation = Vp2 = 25003164 = 79 mS
Distance of Fault from test end = Vp2 x t2
= 79 x 1745
= 137855
693 Impulse Current Method
Working on Travelling wave principles it is applicable to all types of fault
694 Arc Reflection or Secondary Impulse method
Basically PE and TDR associated with fault treatment It is also applicable to faults of all nature but with easily interpreted breakdown waveforms
610 PIN POINTING
Pin Pointing is the application of a test that positively confirms the exact position of the fault
Before the commencement of pin pointing the prelocated fault distance should mark on the cable route which is measured by means of a trumeter
Pin pointing is normally carried out by the shock wave discharge method The fault can be detected by the use of semisphone
611 Confirmation and Re-Test
After the pin-pointed position of the fault has been marked and exposed check for physical sign of fault If there is the fault is confirm
After confirmation the fault should be cut away IR and continuity test should be carried out on the remaining cable sections to determine the soundeness of these cable sections
The insulation resistance test are again carried out after jointing followed by pressure test before supply can be restored
Flowchart for Fault location Procedure
Example
Find a fault distance using direct reading fault localiser from end test which has a test data as below
Bridge Balance Reading =60 Length of Cable 140 meters
Solutions
Fault Distance= = 60100 x 140 = 84 meterxL
n
100
For cables route with combination of 2 or more different type of cable
Determine the resistance of cable for each section separately from Table 1 and Table 2
Example Cable 11kV Al Length 150 m size = 16mm2
From Table 1= Resistance for 1km =226 ohm
For 150 m = 1501000 x 226 = 0339 ohm
From Table 1 Calculate the equivalent resistance for all the combined section
Example for Al = 0339 ohm for copper = 0150 ohm
Equivalent Resistance = 0339+0150 =0489 ohm
Continue
Calculate equivalent resistance from obtained bridge reading
Example Bridge Reading = 60
Equivalent resistance = 06 x 0489 ohm = 02934 ohm
Determine which section of cable the fault occurs
Total resistance at Al section = 0339 ohm
As equivalent resistance at Al section is higher then 02934(Equivalent resistance from bridge test) the fault is
located at Aluminium section of cable
Table 1 Impedance data for 635011000 V Cable Aluminium
Table II Impedance data 11000 V Cable Copper
TABLE VI ndash 6001000V (ALUMINIUM)
TABLE VII ndash 38006600V (ALUMINIUM)
TABLE VIII- 635011000 (ALUMINIUM)
TABLE V
610 COMMISSIONING OF UNDERGROUND CABLE
High Voltage DC testing
High voltage testing is carried out in order to determine the electrical strength of cable insulation Site tests are performed by applying a predetermined high voltage to the insulation DC site test voltages regardless of insulation type are used to ensure that the cable cable joints and termination are correctly made and installed
High Voltage Testing (cont-)
IEC Standards specify maintenance tests after installation should be 70 of permissible factory test DC voltages (15 minutes) = 175kV
Insulation Resistance Test (cont-)
Test voltages should be slowly raised to the required value over a period of about 1 minute and the test period starts once the full voltage is reached In this way the capacitive and absorption currents will have decreased and circuit conditions stabilized such that true leakage current may be measured
663 MISHANDLING
Mishandling of cable may be occurred during installation
Some of the examples are
1 Excessive pull
2 Sharp bend
3 Accident crush
664 Poor workmanship During Cable Jointing
The cable are jointed together with poor workmanship can lead to cable fault after a period of time
665 NATURAL CAUSES DUE TO AGING OF CABLE
CONTINUE-
CONTINUE-
66 TYPES OF CABLE FAULT
GENERAL
bull Series (open circuit) Fault
- Failure of continuity (conductor (s) or cable)
bull Shunt (short circuit) fault
- failure of insulation
66 TYPES OF FAULT (CONTINUE)-
66 TYPES OF FAULT (CONTINUE)-
661 SERIES AND SHUNT FAULT
Are subsided into the following categories
Low Resistance Faultof ZR 10
Where Zo= cable surge impedance
=10 ndash 100 ohm
Usually happens in series fault
High Resistance Fault
Where ZoR f 10
662 INTERMITTENT OR FLASH FAULT
- Usually not apparent to insulation resistance measuring
instrument
- Does not manifest itself at lower voltages or a surge
- Breakdown will appear under application of high voltage
dc or DC pressure test
67 FAULT LOCATION PROCEDURE
The proper sequence of cable fault location are as follows
a) Analysis of fault
b) Pre-location
c) Pin Pointing
d) Confirmation and re-test
671 Analysis of Cable Fault
To analyze a cable fault is to determine and confirm the nature or characteristics of the fault
Objective to select which of the cable fault locator equipment of method is best suited to locate the particular fault
Analysis of cable fault normally consists of insulation resistance test and continuity test carried out using 1000V or 5000 V insulation tester
671 Insulation Resistance test
With all earth connections removed from the cable conductor terminal using IR tester of 100025005000V range measure and record the IR in M-ohms K-ohms or ohms between conductors and between conductors and earth Six measurement are to be taken R-E Y-E B-E R-Y Y-B B-R An IR of 100M-ohm indicates a healthy cable
672 Continuity Test
With the cable conductor looped or shorted at the tail endremote end using IR tester selected at continuity range perform continuity test
This test will determine whether any of the cable is open circuited
672 Continuity test (cont-)
Continuity Test - With the cable conductor shorted or looped at the remote end perform continuity test on the cable
- Measure and record the results in ohm
- Three measurements are to be carried out between R-Y Y-B B-R
- The test will determine whether any of cable is open circuited - The resistance per-conductor per km is provided in Table VI VII VIII and IX (refer appendix A)
672 (cont-)
if the continuity of the cable is sound insulation resistance from one end are sufficient
If continuity is broken IR test should be carried out at both ends of the cable
68 BURNING A FAULT
The continuity and IR test may indicate that burning of fault by means of HT pressure test set is required
-
68 BURNING A FAULT (continue-)
Burning a fault is achieve by passing current from a DC HT test set through the fault
Other conductors not under test should be earthed
HT is applied for about 5 to 10 minutes to burn the fault
HT test is used to determine which fault location equipment is suitable to be used
HT is the last resort often used because it sometimes produce ambiguous and unpredictable results
Therefore fault location equipment should be attempted first
69 Pre- Location of fault
Pre-location is the application of a test at the terminals of a
given cable to give an indication of the distance to the fault
from the test point
Whilst the measurement should be accurate as condition will
allow the primary purpose of pre-location is to give an
indication as quick as possible of the vicinity in which to
commence the final pin-pointing tests
69 Pre- Location of fault (cont)-
Generally there are four pre-location methods which are practised and the are as follows
1Bridge or loop Method2 Pulse Echo Time Domain Reflectometry (TDR) method3 Impulse Current Method4 Arc Reflection and secondary Impulse method
691 Bridge method
6911 Direct reading fault localizer
Conditions required
i) No break in continuity if possible one sound core available
ii) The fault resistance of cable should not be more then 20 k-ohm
iii) Applied voltage not to exceed 600 V (DC)
Case 1 Single conductor to earth with sound core available
Fault distance from front end X= n100 x L
Where n = localiser reading in
L= Cable route length
Case2 Conductor to conductor with sound core available
Fault distance from front end X= n100 x L
Where n = localiser reading in
L= Cable route length
Case 3 Three phase fault
By Open loop test and close loop test
(i) Rf2Rf1 gt 5 and (ii) Rf2 lt 20 k-ohm
Open loop test
Case 3 Three phase fault
Close loop test
Open loop and close loop test (cont)
Fault distance from front end
Lxm
mn
X 2
1001
)200200
(
Where m = open loop bridge reading
n = close loop bridge reading
L = cable route length
High Tension Bridge
Conditions required
i) No break in continuity if possible one sound core available
ii) The fault resistance of cable should be more then 20 k-ohm
iii) Applied voltage not to exceed 20kV (DC)
High Tension Bridge
Case 1Single Conductor to earth with sound core available
Fault Distance from test end X = n100 x 2L
High Tension Bridge
Case 2Conductor to conductor with sound core available
Fault Distance from test end X = n100 x 2L
High Tension Bridge
Case 3Three phase Fault with no sound core available
Rf2Rf1gt5 Rf2gt20 kohm
Lxm
)mn
(X 2
1002
1
100100
Where m = open loop bridge reading
n = close loop bridge reading
L = cable route length
6912 Universal Bridge to measure capacitance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be more then 100 ohm
iii) Earth all the conductors not under test
6912 Universal Bridge to measure capacitance of Cable
Open circuit with sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with sound core available
Fault Distance from test end x = CA x L CAB
CA = Capacitance of open circuited coreCAB = Capacitance of sound coreL = conductor length
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
Fault Distance from test endfront end = ((CA(CA+CB))x L
6913 Universal Bridge to measure Inductance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be less then 30 ohm
iii) All conductors be left free from earth
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
Fault distance X = LA x L LAB
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
Fault distance X = LA x L LA + LB
692 Pulse Echo (PE) Time Domain Reflectometry (TDR)
A simple method which works on travelling wave principles It is applicable to all series and shunt faults with Rf gt=Zo10 (5 ohm) and Rflt=10 Zo (500 ohm) respectively For power cables typical Zo=50 ohm
6921 Travelling wave principles
A pulse injected into a cable is reflected back to the source by any change in characteristics impedance (Zo) of the cable
The waveform generated can be monitored using an oscilloscope as shown in Figure below
6921 Continue-
Waveform interpretion
Shunt Fault
Series Fault
6921 Continue-
Waveform interpretion
Low Resistance Shunt Fault (Rflt=500 Ohm)
-Reflection Negative
-Amplitude depends on fault resistance
Series Fault (Rfgt=5 ohm)
- Reflection Positive
- Amplitude depends on fault resistance
Zo = Characteristics impedance = radic(LC)
L = Inductance of cable C = Capacitance of Cable
69211 Reflection Factor (r)
- Is to determine the degree of reflection
Shunt fault r (V) = -Zo1(2Rf + Zo1)
Series Fault r (V) = Rf(2Zo1 + Rf)
Example shunt fault fault resistance = 5 ohm Zo = 50 ohm
r(V) = -50 (2 x5 +50) = - 833If fault resistance = 500 ohm
r(V) = -50 (2x 500 + 50) = -475
69212 Velocity of Propagation (Vp)
- Vp depends on the cable dielectric and is defined as
Vp = Vs radicEr
- where Vs = Velocity of light in free space
Er = Relative permittivity of dielectric
Vp = 300radicEr
Dielectrics Velocity of PropagationVp (mmicroS)
Impregnated Paper 150 -171
PVC 152-175
PE Approx 200
XLPE 156-176
Typical Vp for different Dielectric
69212 Continue (Vp)
Example
Determine the velocity of propagation Vp2 for a cable length of 2500 meters t1= 3164 S and t2 = 1745 S Calculate the fault distances
6921 Continue-
The distance to the fault Lf (m) is given by
Lf = T x Vp2
6921 Continue
Solutions
Velocity of propagation = Vp2 = 25003164 = 79 mS
Distance of Fault from test end = Vp2 x t2
= 79 x 1745
= 137855
693 Impulse Current Method
Working on Travelling wave principles it is applicable to all types of fault
694 Arc Reflection or Secondary Impulse method
Basically PE and TDR associated with fault treatment It is also applicable to faults of all nature but with easily interpreted breakdown waveforms
610 PIN POINTING
Pin Pointing is the application of a test that positively confirms the exact position of the fault
Before the commencement of pin pointing the prelocated fault distance should mark on the cable route which is measured by means of a trumeter
Pin pointing is normally carried out by the shock wave discharge method The fault can be detected by the use of semisphone
611 Confirmation and Re-Test
After the pin-pointed position of the fault has been marked and exposed check for physical sign of fault If there is the fault is confirm
After confirmation the fault should be cut away IR and continuity test should be carried out on the remaining cable sections to determine the soundeness of these cable sections
The insulation resistance test are again carried out after jointing followed by pressure test before supply can be restored
Flowchart for Fault location Procedure
Example
Find a fault distance using direct reading fault localiser from end test which has a test data as below
Bridge Balance Reading =60 Length of Cable 140 meters
Solutions
Fault Distance= = 60100 x 140 = 84 meterxL
n
100
For cables route with combination of 2 or more different type of cable
Determine the resistance of cable for each section separately from Table 1 and Table 2
Example Cable 11kV Al Length 150 m size = 16mm2
From Table 1= Resistance for 1km =226 ohm
For 150 m = 1501000 x 226 = 0339 ohm
From Table 1 Calculate the equivalent resistance for all the combined section
Example for Al = 0339 ohm for copper = 0150 ohm
Equivalent Resistance = 0339+0150 =0489 ohm
Continue
Calculate equivalent resistance from obtained bridge reading
Example Bridge Reading = 60
Equivalent resistance = 06 x 0489 ohm = 02934 ohm
Determine which section of cable the fault occurs
Total resistance at Al section = 0339 ohm
As equivalent resistance at Al section is higher then 02934(Equivalent resistance from bridge test) the fault is
located at Aluminium section of cable
Table 1 Impedance data for 635011000 V Cable Aluminium
Table II Impedance data 11000 V Cable Copper
TABLE VI ndash 6001000V (ALUMINIUM)
TABLE VII ndash 38006600V (ALUMINIUM)
TABLE VIII- 635011000 (ALUMINIUM)
TABLE V
610 COMMISSIONING OF UNDERGROUND CABLE
High Voltage DC testing
High voltage testing is carried out in order to determine the electrical strength of cable insulation Site tests are performed by applying a predetermined high voltage to the insulation DC site test voltages regardless of insulation type are used to ensure that the cable cable joints and termination are correctly made and installed
High Voltage Testing (cont-)
IEC Standards specify maintenance tests after installation should be 70 of permissible factory test DC voltages (15 minutes) = 175kV
Insulation Resistance Test (cont-)
Test voltages should be slowly raised to the required value over a period of about 1 minute and the test period starts once the full voltage is reached In this way the capacitive and absorption currents will have decreased and circuit conditions stabilized such that true leakage current may be measured
664 Poor workmanship During Cable Jointing
The cable are jointed together with poor workmanship can lead to cable fault after a period of time
665 NATURAL CAUSES DUE TO AGING OF CABLE
CONTINUE-
CONTINUE-
66 TYPES OF CABLE FAULT
GENERAL
bull Series (open circuit) Fault
- Failure of continuity (conductor (s) or cable)
bull Shunt (short circuit) fault
- failure of insulation
66 TYPES OF FAULT (CONTINUE)-
66 TYPES OF FAULT (CONTINUE)-
661 SERIES AND SHUNT FAULT
Are subsided into the following categories
Low Resistance Faultof ZR 10
Where Zo= cable surge impedance
=10 ndash 100 ohm
Usually happens in series fault
High Resistance Fault
Where ZoR f 10
662 INTERMITTENT OR FLASH FAULT
- Usually not apparent to insulation resistance measuring
instrument
- Does not manifest itself at lower voltages or a surge
- Breakdown will appear under application of high voltage
dc or DC pressure test
67 FAULT LOCATION PROCEDURE
The proper sequence of cable fault location are as follows
a) Analysis of fault
b) Pre-location
c) Pin Pointing
d) Confirmation and re-test
671 Analysis of Cable Fault
To analyze a cable fault is to determine and confirm the nature or characteristics of the fault
Objective to select which of the cable fault locator equipment of method is best suited to locate the particular fault
Analysis of cable fault normally consists of insulation resistance test and continuity test carried out using 1000V or 5000 V insulation tester
671 Insulation Resistance test
With all earth connections removed from the cable conductor terminal using IR tester of 100025005000V range measure and record the IR in M-ohms K-ohms or ohms between conductors and between conductors and earth Six measurement are to be taken R-E Y-E B-E R-Y Y-B B-R An IR of 100M-ohm indicates a healthy cable
672 Continuity Test
With the cable conductor looped or shorted at the tail endremote end using IR tester selected at continuity range perform continuity test
This test will determine whether any of the cable is open circuited
672 Continuity test (cont-)
Continuity Test - With the cable conductor shorted or looped at the remote end perform continuity test on the cable
- Measure and record the results in ohm
- Three measurements are to be carried out between R-Y Y-B B-R
- The test will determine whether any of cable is open circuited - The resistance per-conductor per km is provided in Table VI VII VIII and IX (refer appendix A)
672 (cont-)
if the continuity of the cable is sound insulation resistance from one end are sufficient
If continuity is broken IR test should be carried out at both ends of the cable
68 BURNING A FAULT
The continuity and IR test may indicate that burning of fault by means of HT pressure test set is required
-
68 BURNING A FAULT (continue-)
Burning a fault is achieve by passing current from a DC HT test set through the fault
Other conductors not under test should be earthed
HT is applied for about 5 to 10 minutes to burn the fault
HT test is used to determine which fault location equipment is suitable to be used
HT is the last resort often used because it sometimes produce ambiguous and unpredictable results
Therefore fault location equipment should be attempted first
69 Pre- Location of fault
Pre-location is the application of a test at the terminals of a
given cable to give an indication of the distance to the fault
from the test point
Whilst the measurement should be accurate as condition will
allow the primary purpose of pre-location is to give an
indication as quick as possible of the vicinity in which to
commence the final pin-pointing tests
69 Pre- Location of fault (cont)-
Generally there are four pre-location methods which are practised and the are as follows
1Bridge or loop Method2 Pulse Echo Time Domain Reflectometry (TDR) method3 Impulse Current Method4 Arc Reflection and secondary Impulse method
691 Bridge method
6911 Direct reading fault localizer
Conditions required
i) No break in continuity if possible one sound core available
ii) The fault resistance of cable should not be more then 20 k-ohm
iii) Applied voltage not to exceed 600 V (DC)
Case 1 Single conductor to earth with sound core available
Fault distance from front end X= n100 x L
Where n = localiser reading in
L= Cable route length
Case2 Conductor to conductor with sound core available
Fault distance from front end X= n100 x L
Where n = localiser reading in
L= Cable route length
Case 3 Three phase fault
By Open loop test and close loop test
(i) Rf2Rf1 gt 5 and (ii) Rf2 lt 20 k-ohm
Open loop test
Case 3 Three phase fault
Close loop test
Open loop and close loop test (cont)
Fault distance from front end
Lxm
mn
X 2
1001
)200200
(
Where m = open loop bridge reading
n = close loop bridge reading
L = cable route length
High Tension Bridge
Conditions required
i) No break in continuity if possible one sound core available
ii) The fault resistance of cable should be more then 20 k-ohm
iii) Applied voltage not to exceed 20kV (DC)
High Tension Bridge
Case 1Single Conductor to earth with sound core available
Fault Distance from test end X = n100 x 2L
High Tension Bridge
Case 2Conductor to conductor with sound core available
Fault Distance from test end X = n100 x 2L
High Tension Bridge
Case 3Three phase Fault with no sound core available
Rf2Rf1gt5 Rf2gt20 kohm
Lxm
)mn
(X 2
1002
1
100100
Where m = open loop bridge reading
n = close loop bridge reading
L = cable route length
6912 Universal Bridge to measure capacitance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be more then 100 ohm
iii) Earth all the conductors not under test
6912 Universal Bridge to measure capacitance of Cable
Open circuit with sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with sound core available
Fault Distance from test end x = CA x L CAB
CA = Capacitance of open circuited coreCAB = Capacitance of sound coreL = conductor length
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
Fault Distance from test endfront end = ((CA(CA+CB))x L
6913 Universal Bridge to measure Inductance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be less then 30 ohm
iii) All conductors be left free from earth
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
Fault distance X = LA x L LAB
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
Fault distance X = LA x L LA + LB
692 Pulse Echo (PE) Time Domain Reflectometry (TDR)
A simple method which works on travelling wave principles It is applicable to all series and shunt faults with Rf gt=Zo10 (5 ohm) and Rflt=10 Zo (500 ohm) respectively For power cables typical Zo=50 ohm
6921 Travelling wave principles
A pulse injected into a cable is reflected back to the source by any change in characteristics impedance (Zo) of the cable
The waveform generated can be monitored using an oscilloscope as shown in Figure below
6921 Continue-
Waveform interpretion
Shunt Fault
Series Fault
6921 Continue-
Waveform interpretion
Low Resistance Shunt Fault (Rflt=500 Ohm)
-Reflection Negative
-Amplitude depends on fault resistance
Series Fault (Rfgt=5 ohm)
- Reflection Positive
- Amplitude depends on fault resistance
Zo = Characteristics impedance = radic(LC)
L = Inductance of cable C = Capacitance of Cable
69211 Reflection Factor (r)
- Is to determine the degree of reflection
Shunt fault r (V) = -Zo1(2Rf + Zo1)
Series Fault r (V) = Rf(2Zo1 + Rf)
Example shunt fault fault resistance = 5 ohm Zo = 50 ohm
r(V) = -50 (2 x5 +50) = - 833If fault resistance = 500 ohm
r(V) = -50 (2x 500 + 50) = -475
69212 Velocity of Propagation (Vp)
- Vp depends on the cable dielectric and is defined as
Vp = Vs radicEr
- where Vs = Velocity of light in free space
Er = Relative permittivity of dielectric
Vp = 300radicEr
Dielectrics Velocity of PropagationVp (mmicroS)
Impregnated Paper 150 -171
PVC 152-175
PE Approx 200
XLPE 156-176
Typical Vp for different Dielectric
69212 Continue (Vp)
Example
Determine the velocity of propagation Vp2 for a cable length of 2500 meters t1= 3164 S and t2 = 1745 S Calculate the fault distances
6921 Continue-
The distance to the fault Lf (m) is given by
Lf = T x Vp2
6921 Continue
Solutions
Velocity of propagation = Vp2 = 25003164 = 79 mS
Distance of Fault from test end = Vp2 x t2
= 79 x 1745
= 137855
693 Impulse Current Method
Working on Travelling wave principles it is applicable to all types of fault
694 Arc Reflection or Secondary Impulse method
Basically PE and TDR associated with fault treatment It is also applicable to faults of all nature but with easily interpreted breakdown waveforms
610 PIN POINTING
Pin Pointing is the application of a test that positively confirms the exact position of the fault
Before the commencement of pin pointing the prelocated fault distance should mark on the cable route which is measured by means of a trumeter
Pin pointing is normally carried out by the shock wave discharge method The fault can be detected by the use of semisphone
611 Confirmation and Re-Test
After the pin-pointed position of the fault has been marked and exposed check for physical sign of fault If there is the fault is confirm
After confirmation the fault should be cut away IR and continuity test should be carried out on the remaining cable sections to determine the soundeness of these cable sections
The insulation resistance test are again carried out after jointing followed by pressure test before supply can be restored
Flowchart for Fault location Procedure
Example
Find a fault distance using direct reading fault localiser from end test which has a test data as below
Bridge Balance Reading =60 Length of Cable 140 meters
Solutions
Fault Distance= = 60100 x 140 = 84 meterxL
n
100
For cables route with combination of 2 or more different type of cable
Determine the resistance of cable for each section separately from Table 1 and Table 2
Example Cable 11kV Al Length 150 m size = 16mm2
From Table 1= Resistance for 1km =226 ohm
For 150 m = 1501000 x 226 = 0339 ohm
From Table 1 Calculate the equivalent resistance for all the combined section
Example for Al = 0339 ohm for copper = 0150 ohm
Equivalent Resistance = 0339+0150 =0489 ohm
Continue
Calculate equivalent resistance from obtained bridge reading
Example Bridge Reading = 60
Equivalent resistance = 06 x 0489 ohm = 02934 ohm
Determine which section of cable the fault occurs
Total resistance at Al section = 0339 ohm
As equivalent resistance at Al section is higher then 02934(Equivalent resistance from bridge test) the fault is
located at Aluminium section of cable
Table 1 Impedance data for 635011000 V Cable Aluminium
Table II Impedance data 11000 V Cable Copper
TABLE VI ndash 6001000V (ALUMINIUM)
TABLE VII ndash 38006600V (ALUMINIUM)
TABLE VIII- 635011000 (ALUMINIUM)
TABLE V
610 COMMISSIONING OF UNDERGROUND CABLE
High Voltage DC testing
High voltage testing is carried out in order to determine the electrical strength of cable insulation Site tests are performed by applying a predetermined high voltage to the insulation DC site test voltages regardless of insulation type are used to ensure that the cable cable joints and termination are correctly made and installed
High Voltage Testing (cont-)
IEC Standards specify maintenance tests after installation should be 70 of permissible factory test DC voltages (15 minutes) = 175kV
Insulation Resistance Test (cont-)
Test voltages should be slowly raised to the required value over a period of about 1 minute and the test period starts once the full voltage is reached In this way the capacitive and absorption currents will have decreased and circuit conditions stabilized such that true leakage current may be measured
665 NATURAL CAUSES DUE TO AGING OF CABLE
CONTINUE-
CONTINUE-
66 TYPES OF CABLE FAULT
GENERAL
bull Series (open circuit) Fault
- Failure of continuity (conductor (s) or cable)
bull Shunt (short circuit) fault
- failure of insulation
66 TYPES OF FAULT (CONTINUE)-
66 TYPES OF FAULT (CONTINUE)-
661 SERIES AND SHUNT FAULT
Are subsided into the following categories
Low Resistance Faultof ZR 10
Where Zo= cable surge impedance
=10 ndash 100 ohm
Usually happens in series fault
High Resistance Fault
Where ZoR f 10
662 INTERMITTENT OR FLASH FAULT
- Usually not apparent to insulation resistance measuring
instrument
- Does not manifest itself at lower voltages or a surge
- Breakdown will appear under application of high voltage
dc or DC pressure test
67 FAULT LOCATION PROCEDURE
The proper sequence of cable fault location are as follows
a) Analysis of fault
b) Pre-location
c) Pin Pointing
d) Confirmation and re-test
671 Analysis of Cable Fault
To analyze a cable fault is to determine and confirm the nature or characteristics of the fault
Objective to select which of the cable fault locator equipment of method is best suited to locate the particular fault
Analysis of cable fault normally consists of insulation resistance test and continuity test carried out using 1000V or 5000 V insulation tester
671 Insulation Resistance test
With all earth connections removed from the cable conductor terminal using IR tester of 100025005000V range measure and record the IR in M-ohms K-ohms or ohms between conductors and between conductors and earth Six measurement are to be taken R-E Y-E B-E R-Y Y-B B-R An IR of 100M-ohm indicates a healthy cable
672 Continuity Test
With the cable conductor looped or shorted at the tail endremote end using IR tester selected at continuity range perform continuity test
This test will determine whether any of the cable is open circuited
672 Continuity test (cont-)
Continuity Test - With the cable conductor shorted or looped at the remote end perform continuity test on the cable
- Measure and record the results in ohm
- Three measurements are to be carried out between R-Y Y-B B-R
- The test will determine whether any of cable is open circuited - The resistance per-conductor per km is provided in Table VI VII VIII and IX (refer appendix A)
672 (cont-)
if the continuity of the cable is sound insulation resistance from one end are sufficient
If continuity is broken IR test should be carried out at both ends of the cable
68 BURNING A FAULT
The continuity and IR test may indicate that burning of fault by means of HT pressure test set is required
-
68 BURNING A FAULT (continue-)
Burning a fault is achieve by passing current from a DC HT test set through the fault
Other conductors not under test should be earthed
HT is applied for about 5 to 10 minutes to burn the fault
HT test is used to determine which fault location equipment is suitable to be used
HT is the last resort often used because it sometimes produce ambiguous and unpredictable results
Therefore fault location equipment should be attempted first
69 Pre- Location of fault
Pre-location is the application of a test at the terminals of a
given cable to give an indication of the distance to the fault
from the test point
Whilst the measurement should be accurate as condition will
allow the primary purpose of pre-location is to give an
indication as quick as possible of the vicinity in which to
commence the final pin-pointing tests
69 Pre- Location of fault (cont)-
Generally there are four pre-location methods which are practised and the are as follows
1Bridge or loop Method2 Pulse Echo Time Domain Reflectometry (TDR) method3 Impulse Current Method4 Arc Reflection and secondary Impulse method
691 Bridge method
6911 Direct reading fault localizer
Conditions required
i) No break in continuity if possible one sound core available
ii) The fault resistance of cable should not be more then 20 k-ohm
iii) Applied voltage not to exceed 600 V (DC)
Case 1 Single conductor to earth with sound core available
Fault distance from front end X= n100 x L
Where n = localiser reading in
L= Cable route length
Case2 Conductor to conductor with sound core available
Fault distance from front end X= n100 x L
Where n = localiser reading in
L= Cable route length
Case 3 Three phase fault
By Open loop test and close loop test
(i) Rf2Rf1 gt 5 and (ii) Rf2 lt 20 k-ohm
Open loop test
Case 3 Three phase fault
Close loop test
Open loop and close loop test (cont)
Fault distance from front end
Lxm
mn
X 2
1001
)200200
(
Where m = open loop bridge reading
n = close loop bridge reading
L = cable route length
High Tension Bridge
Conditions required
i) No break in continuity if possible one sound core available
ii) The fault resistance of cable should be more then 20 k-ohm
iii) Applied voltage not to exceed 20kV (DC)
High Tension Bridge
Case 1Single Conductor to earth with sound core available
Fault Distance from test end X = n100 x 2L
High Tension Bridge
Case 2Conductor to conductor with sound core available
Fault Distance from test end X = n100 x 2L
High Tension Bridge
Case 3Three phase Fault with no sound core available
Rf2Rf1gt5 Rf2gt20 kohm
Lxm
)mn
(X 2
1002
1
100100
Where m = open loop bridge reading
n = close loop bridge reading
L = cable route length
6912 Universal Bridge to measure capacitance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be more then 100 ohm
iii) Earth all the conductors not under test
6912 Universal Bridge to measure capacitance of Cable
Open circuit with sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with sound core available
Fault Distance from test end x = CA x L CAB
CA = Capacitance of open circuited coreCAB = Capacitance of sound coreL = conductor length
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
Fault Distance from test endfront end = ((CA(CA+CB))x L
6913 Universal Bridge to measure Inductance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be less then 30 ohm
iii) All conductors be left free from earth
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
Fault distance X = LA x L LAB
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
Fault distance X = LA x L LA + LB
692 Pulse Echo (PE) Time Domain Reflectometry (TDR)
A simple method which works on travelling wave principles It is applicable to all series and shunt faults with Rf gt=Zo10 (5 ohm) and Rflt=10 Zo (500 ohm) respectively For power cables typical Zo=50 ohm
6921 Travelling wave principles
A pulse injected into a cable is reflected back to the source by any change in characteristics impedance (Zo) of the cable
The waveform generated can be monitored using an oscilloscope as shown in Figure below
6921 Continue-
Waveform interpretion
Shunt Fault
Series Fault
6921 Continue-
Waveform interpretion
Low Resistance Shunt Fault (Rflt=500 Ohm)
-Reflection Negative
-Amplitude depends on fault resistance
Series Fault (Rfgt=5 ohm)
- Reflection Positive
- Amplitude depends on fault resistance
Zo = Characteristics impedance = radic(LC)
L = Inductance of cable C = Capacitance of Cable
69211 Reflection Factor (r)
- Is to determine the degree of reflection
Shunt fault r (V) = -Zo1(2Rf + Zo1)
Series Fault r (V) = Rf(2Zo1 + Rf)
Example shunt fault fault resistance = 5 ohm Zo = 50 ohm
r(V) = -50 (2 x5 +50) = - 833If fault resistance = 500 ohm
r(V) = -50 (2x 500 + 50) = -475
69212 Velocity of Propagation (Vp)
- Vp depends on the cable dielectric and is defined as
Vp = Vs radicEr
- where Vs = Velocity of light in free space
Er = Relative permittivity of dielectric
Vp = 300radicEr
Dielectrics Velocity of PropagationVp (mmicroS)
Impregnated Paper 150 -171
PVC 152-175
PE Approx 200
XLPE 156-176
Typical Vp for different Dielectric
69212 Continue (Vp)
Example
Determine the velocity of propagation Vp2 for a cable length of 2500 meters t1= 3164 S and t2 = 1745 S Calculate the fault distances
6921 Continue-
The distance to the fault Lf (m) is given by
Lf = T x Vp2
6921 Continue
Solutions
Velocity of propagation = Vp2 = 25003164 = 79 mS
Distance of Fault from test end = Vp2 x t2
= 79 x 1745
= 137855
693 Impulse Current Method
Working on Travelling wave principles it is applicable to all types of fault
694 Arc Reflection or Secondary Impulse method
Basically PE and TDR associated with fault treatment It is also applicable to faults of all nature but with easily interpreted breakdown waveforms
610 PIN POINTING
Pin Pointing is the application of a test that positively confirms the exact position of the fault
Before the commencement of pin pointing the prelocated fault distance should mark on the cable route which is measured by means of a trumeter
Pin pointing is normally carried out by the shock wave discharge method The fault can be detected by the use of semisphone
611 Confirmation and Re-Test
After the pin-pointed position of the fault has been marked and exposed check for physical sign of fault If there is the fault is confirm
After confirmation the fault should be cut away IR and continuity test should be carried out on the remaining cable sections to determine the soundeness of these cable sections
The insulation resistance test are again carried out after jointing followed by pressure test before supply can be restored
Flowchart for Fault location Procedure
Example
Find a fault distance using direct reading fault localiser from end test which has a test data as below
Bridge Balance Reading =60 Length of Cable 140 meters
Solutions
Fault Distance= = 60100 x 140 = 84 meterxL
n
100
For cables route with combination of 2 or more different type of cable
Determine the resistance of cable for each section separately from Table 1 and Table 2
Example Cable 11kV Al Length 150 m size = 16mm2
From Table 1= Resistance for 1km =226 ohm
For 150 m = 1501000 x 226 = 0339 ohm
From Table 1 Calculate the equivalent resistance for all the combined section
Example for Al = 0339 ohm for copper = 0150 ohm
Equivalent Resistance = 0339+0150 =0489 ohm
Continue
Calculate equivalent resistance from obtained bridge reading
Example Bridge Reading = 60
Equivalent resistance = 06 x 0489 ohm = 02934 ohm
Determine which section of cable the fault occurs
Total resistance at Al section = 0339 ohm
As equivalent resistance at Al section is higher then 02934(Equivalent resistance from bridge test) the fault is
located at Aluminium section of cable
Table 1 Impedance data for 635011000 V Cable Aluminium
Table II Impedance data 11000 V Cable Copper
TABLE VI ndash 6001000V (ALUMINIUM)
TABLE VII ndash 38006600V (ALUMINIUM)
TABLE VIII- 635011000 (ALUMINIUM)
TABLE V
610 COMMISSIONING OF UNDERGROUND CABLE
High Voltage DC testing
High voltage testing is carried out in order to determine the electrical strength of cable insulation Site tests are performed by applying a predetermined high voltage to the insulation DC site test voltages regardless of insulation type are used to ensure that the cable cable joints and termination are correctly made and installed
High Voltage Testing (cont-)
IEC Standards specify maintenance tests after installation should be 70 of permissible factory test DC voltages (15 minutes) = 175kV
Insulation Resistance Test (cont-)
Test voltages should be slowly raised to the required value over a period of about 1 minute and the test period starts once the full voltage is reached In this way the capacitive and absorption currents will have decreased and circuit conditions stabilized such that true leakage current may be measured
CONTINUE-
CONTINUE-
66 TYPES OF CABLE FAULT
GENERAL
bull Series (open circuit) Fault
- Failure of continuity (conductor (s) or cable)
bull Shunt (short circuit) fault
- failure of insulation
66 TYPES OF FAULT (CONTINUE)-
66 TYPES OF FAULT (CONTINUE)-
661 SERIES AND SHUNT FAULT
Are subsided into the following categories
Low Resistance Faultof ZR 10
Where Zo= cable surge impedance
=10 ndash 100 ohm
Usually happens in series fault
High Resistance Fault
Where ZoR f 10
662 INTERMITTENT OR FLASH FAULT
- Usually not apparent to insulation resistance measuring
instrument
- Does not manifest itself at lower voltages or a surge
- Breakdown will appear under application of high voltage
dc or DC pressure test
67 FAULT LOCATION PROCEDURE
The proper sequence of cable fault location are as follows
a) Analysis of fault
b) Pre-location
c) Pin Pointing
d) Confirmation and re-test
671 Analysis of Cable Fault
To analyze a cable fault is to determine and confirm the nature or characteristics of the fault
Objective to select which of the cable fault locator equipment of method is best suited to locate the particular fault
Analysis of cable fault normally consists of insulation resistance test and continuity test carried out using 1000V or 5000 V insulation tester
671 Insulation Resistance test
With all earth connections removed from the cable conductor terminal using IR tester of 100025005000V range measure and record the IR in M-ohms K-ohms or ohms between conductors and between conductors and earth Six measurement are to be taken R-E Y-E B-E R-Y Y-B B-R An IR of 100M-ohm indicates a healthy cable
672 Continuity Test
With the cable conductor looped or shorted at the tail endremote end using IR tester selected at continuity range perform continuity test
This test will determine whether any of the cable is open circuited
672 Continuity test (cont-)
Continuity Test - With the cable conductor shorted or looped at the remote end perform continuity test on the cable
- Measure and record the results in ohm
- Three measurements are to be carried out between R-Y Y-B B-R
- The test will determine whether any of cable is open circuited - The resistance per-conductor per km is provided in Table VI VII VIII and IX (refer appendix A)
672 (cont-)
if the continuity of the cable is sound insulation resistance from one end are sufficient
If continuity is broken IR test should be carried out at both ends of the cable
68 BURNING A FAULT
The continuity and IR test may indicate that burning of fault by means of HT pressure test set is required
-
68 BURNING A FAULT (continue-)
Burning a fault is achieve by passing current from a DC HT test set through the fault
Other conductors not under test should be earthed
HT is applied for about 5 to 10 minutes to burn the fault
HT test is used to determine which fault location equipment is suitable to be used
HT is the last resort often used because it sometimes produce ambiguous and unpredictable results
Therefore fault location equipment should be attempted first
69 Pre- Location of fault
Pre-location is the application of a test at the terminals of a
given cable to give an indication of the distance to the fault
from the test point
Whilst the measurement should be accurate as condition will
allow the primary purpose of pre-location is to give an
indication as quick as possible of the vicinity in which to
commence the final pin-pointing tests
69 Pre- Location of fault (cont)-
Generally there are four pre-location methods which are practised and the are as follows
1Bridge or loop Method2 Pulse Echo Time Domain Reflectometry (TDR) method3 Impulse Current Method4 Arc Reflection and secondary Impulse method
691 Bridge method
6911 Direct reading fault localizer
Conditions required
i) No break in continuity if possible one sound core available
ii) The fault resistance of cable should not be more then 20 k-ohm
iii) Applied voltage not to exceed 600 V (DC)
Case 1 Single conductor to earth with sound core available
Fault distance from front end X= n100 x L
Where n = localiser reading in
L= Cable route length
Case2 Conductor to conductor with sound core available
Fault distance from front end X= n100 x L
Where n = localiser reading in
L= Cable route length
Case 3 Three phase fault
By Open loop test and close loop test
(i) Rf2Rf1 gt 5 and (ii) Rf2 lt 20 k-ohm
Open loop test
Case 3 Three phase fault
Close loop test
Open loop and close loop test (cont)
Fault distance from front end
Lxm
mn
X 2
1001
)200200
(
Where m = open loop bridge reading
n = close loop bridge reading
L = cable route length
High Tension Bridge
Conditions required
i) No break in continuity if possible one sound core available
ii) The fault resistance of cable should be more then 20 k-ohm
iii) Applied voltage not to exceed 20kV (DC)
High Tension Bridge
Case 1Single Conductor to earth with sound core available
Fault Distance from test end X = n100 x 2L
High Tension Bridge
Case 2Conductor to conductor with sound core available
Fault Distance from test end X = n100 x 2L
High Tension Bridge
Case 3Three phase Fault with no sound core available
Rf2Rf1gt5 Rf2gt20 kohm
Lxm
)mn
(X 2
1002
1
100100
Where m = open loop bridge reading
n = close loop bridge reading
L = cable route length
6912 Universal Bridge to measure capacitance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be more then 100 ohm
iii) Earth all the conductors not under test
6912 Universal Bridge to measure capacitance of Cable
Open circuit with sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with sound core available
Fault Distance from test end x = CA x L CAB
CA = Capacitance of open circuited coreCAB = Capacitance of sound coreL = conductor length
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
Fault Distance from test endfront end = ((CA(CA+CB))x L
6913 Universal Bridge to measure Inductance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be less then 30 ohm
iii) All conductors be left free from earth
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
Fault distance X = LA x L LAB
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
Fault distance X = LA x L LA + LB
692 Pulse Echo (PE) Time Domain Reflectometry (TDR)
A simple method which works on travelling wave principles It is applicable to all series and shunt faults with Rf gt=Zo10 (5 ohm) and Rflt=10 Zo (500 ohm) respectively For power cables typical Zo=50 ohm
6921 Travelling wave principles
A pulse injected into a cable is reflected back to the source by any change in characteristics impedance (Zo) of the cable
The waveform generated can be monitored using an oscilloscope as shown in Figure below
6921 Continue-
Waveform interpretion
Shunt Fault
Series Fault
6921 Continue-
Waveform interpretion
Low Resistance Shunt Fault (Rflt=500 Ohm)
-Reflection Negative
-Amplitude depends on fault resistance
Series Fault (Rfgt=5 ohm)
- Reflection Positive
- Amplitude depends on fault resistance
Zo = Characteristics impedance = radic(LC)
L = Inductance of cable C = Capacitance of Cable
69211 Reflection Factor (r)
- Is to determine the degree of reflection
Shunt fault r (V) = -Zo1(2Rf + Zo1)
Series Fault r (V) = Rf(2Zo1 + Rf)
Example shunt fault fault resistance = 5 ohm Zo = 50 ohm
r(V) = -50 (2 x5 +50) = - 833If fault resistance = 500 ohm
r(V) = -50 (2x 500 + 50) = -475
69212 Velocity of Propagation (Vp)
- Vp depends on the cable dielectric and is defined as
Vp = Vs radicEr
- where Vs = Velocity of light in free space
Er = Relative permittivity of dielectric
Vp = 300radicEr
Dielectrics Velocity of PropagationVp (mmicroS)
Impregnated Paper 150 -171
PVC 152-175
PE Approx 200
XLPE 156-176
Typical Vp for different Dielectric
69212 Continue (Vp)
Example
Determine the velocity of propagation Vp2 for a cable length of 2500 meters t1= 3164 S and t2 = 1745 S Calculate the fault distances
6921 Continue-
The distance to the fault Lf (m) is given by
Lf = T x Vp2
6921 Continue
Solutions
Velocity of propagation = Vp2 = 25003164 = 79 mS
Distance of Fault from test end = Vp2 x t2
= 79 x 1745
= 137855
693 Impulse Current Method
Working on Travelling wave principles it is applicable to all types of fault
694 Arc Reflection or Secondary Impulse method
Basically PE and TDR associated with fault treatment It is also applicable to faults of all nature but with easily interpreted breakdown waveforms
610 PIN POINTING
Pin Pointing is the application of a test that positively confirms the exact position of the fault
Before the commencement of pin pointing the prelocated fault distance should mark on the cable route which is measured by means of a trumeter
Pin pointing is normally carried out by the shock wave discharge method The fault can be detected by the use of semisphone
611 Confirmation and Re-Test
After the pin-pointed position of the fault has been marked and exposed check for physical sign of fault If there is the fault is confirm
After confirmation the fault should be cut away IR and continuity test should be carried out on the remaining cable sections to determine the soundeness of these cable sections
The insulation resistance test are again carried out after jointing followed by pressure test before supply can be restored
Flowchart for Fault location Procedure
Example
Find a fault distance using direct reading fault localiser from end test which has a test data as below
Bridge Balance Reading =60 Length of Cable 140 meters
Solutions
Fault Distance= = 60100 x 140 = 84 meterxL
n
100
For cables route with combination of 2 or more different type of cable
Determine the resistance of cable for each section separately from Table 1 and Table 2
Example Cable 11kV Al Length 150 m size = 16mm2
From Table 1= Resistance for 1km =226 ohm
For 150 m = 1501000 x 226 = 0339 ohm
From Table 1 Calculate the equivalent resistance for all the combined section
Example for Al = 0339 ohm for copper = 0150 ohm
Equivalent Resistance = 0339+0150 =0489 ohm
Continue
Calculate equivalent resistance from obtained bridge reading
Example Bridge Reading = 60
Equivalent resistance = 06 x 0489 ohm = 02934 ohm
Determine which section of cable the fault occurs
Total resistance at Al section = 0339 ohm
As equivalent resistance at Al section is higher then 02934(Equivalent resistance from bridge test) the fault is
located at Aluminium section of cable
Table 1 Impedance data for 635011000 V Cable Aluminium
Table II Impedance data 11000 V Cable Copper
TABLE VI ndash 6001000V (ALUMINIUM)
TABLE VII ndash 38006600V (ALUMINIUM)
TABLE VIII- 635011000 (ALUMINIUM)
TABLE V
610 COMMISSIONING OF UNDERGROUND CABLE
High Voltage DC testing
High voltage testing is carried out in order to determine the electrical strength of cable insulation Site tests are performed by applying a predetermined high voltage to the insulation DC site test voltages regardless of insulation type are used to ensure that the cable cable joints and termination are correctly made and installed
High Voltage Testing (cont-)
IEC Standards specify maintenance tests after installation should be 70 of permissible factory test DC voltages (15 minutes) = 175kV
Insulation Resistance Test (cont-)
Test voltages should be slowly raised to the required value over a period of about 1 minute and the test period starts once the full voltage is reached In this way the capacitive and absorption currents will have decreased and circuit conditions stabilized such that true leakage current may be measured
CONTINUE-
66 TYPES OF CABLE FAULT
GENERAL
bull Series (open circuit) Fault
- Failure of continuity (conductor (s) or cable)
bull Shunt (short circuit) fault
- failure of insulation
66 TYPES OF FAULT (CONTINUE)-
66 TYPES OF FAULT (CONTINUE)-
661 SERIES AND SHUNT FAULT
Are subsided into the following categories
Low Resistance Faultof ZR 10
Where Zo= cable surge impedance
=10 ndash 100 ohm
Usually happens in series fault
High Resistance Fault
Where ZoR f 10
662 INTERMITTENT OR FLASH FAULT
- Usually not apparent to insulation resistance measuring
instrument
- Does not manifest itself at lower voltages or a surge
- Breakdown will appear under application of high voltage
dc or DC pressure test
67 FAULT LOCATION PROCEDURE
The proper sequence of cable fault location are as follows
a) Analysis of fault
b) Pre-location
c) Pin Pointing
d) Confirmation and re-test
671 Analysis of Cable Fault
To analyze a cable fault is to determine and confirm the nature or characteristics of the fault
Objective to select which of the cable fault locator equipment of method is best suited to locate the particular fault
Analysis of cable fault normally consists of insulation resistance test and continuity test carried out using 1000V or 5000 V insulation tester
671 Insulation Resistance test
With all earth connections removed from the cable conductor terminal using IR tester of 100025005000V range measure and record the IR in M-ohms K-ohms or ohms between conductors and between conductors and earth Six measurement are to be taken R-E Y-E B-E R-Y Y-B B-R An IR of 100M-ohm indicates a healthy cable
672 Continuity Test
With the cable conductor looped or shorted at the tail endremote end using IR tester selected at continuity range perform continuity test
This test will determine whether any of the cable is open circuited
672 Continuity test (cont-)
Continuity Test - With the cable conductor shorted or looped at the remote end perform continuity test on the cable
- Measure and record the results in ohm
- Three measurements are to be carried out between R-Y Y-B B-R
- The test will determine whether any of cable is open circuited - The resistance per-conductor per km is provided in Table VI VII VIII and IX (refer appendix A)
672 (cont-)
if the continuity of the cable is sound insulation resistance from one end are sufficient
If continuity is broken IR test should be carried out at both ends of the cable
68 BURNING A FAULT
The continuity and IR test may indicate that burning of fault by means of HT pressure test set is required
-
68 BURNING A FAULT (continue-)
Burning a fault is achieve by passing current from a DC HT test set through the fault
Other conductors not under test should be earthed
HT is applied for about 5 to 10 minutes to burn the fault
HT test is used to determine which fault location equipment is suitable to be used
HT is the last resort often used because it sometimes produce ambiguous and unpredictable results
Therefore fault location equipment should be attempted first
69 Pre- Location of fault
Pre-location is the application of a test at the terminals of a
given cable to give an indication of the distance to the fault
from the test point
Whilst the measurement should be accurate as condition will
allow the primary purpose of pre-location is to give an
indication as quick as possible of the vicinity in which to
commence the final pin-pointing tests
69 Pre- Location of fault (cont)-
Generally there are four pre-location methods which are practised and the are as follows
1Bridge or loop Method2 Pulse Echo Time Domain Reflectometry (TDR) method3 Impulse Current Method4 Arc Reflection and secondary Impulse method
691 Bridge method
6911 Direct reading fault localizer
Conditions required
i) No break in continuity if possible one sound core available
ii) The fault resistance of cable should not be more then 20 k-ohm
iii) Applied voltage not to exceed 600 V (DC)
Case 1 Single conductor to earth with sound core available
Fault distance from front end X= n100 x L
Where n = localiser reading in
L= Cable route length
Case2 Conductor to conductor with sound core available
Fault distance from front end X= n100 x L
Where n = localiser reading in
L= Cable route length
Case 3 Three phase fault
By Open loop test and close loop test
(i) Rf2Rf1 gt 5 and (ii) Rf2 lt 20 k-ohm
Open loop test
Case 3 Three phase fault
Close loop test
Open loop and close loop test (cont)
Fault distance from front end
Lxm
mn
X 2
1001
)200200
(
Where m = open loop bridge reading
n = close loop bridge reading
L = cable route length
High Tension Bridge
Conditions required
i) No break in continuity if possible one sound core available
ii) The fault resistance of cable should be more then 20 k-ohm
iii) Applied voltage not to exceed 20kV (DC)
High Tension Bridge
Case 1Single Conductor to earth with sound core available
Fault Distance from test end X = n100 x 2L
High Tension Bridge
Case 2Conductor to conductor with sound core available
Fault Distance from test end X = n100 x 2L
High Tension Bridge
Case 3Three phase Fault with no sound core available
Rf2Rf1gt5 Rf2gt20 kohm
Lxm
)mn
(X 2
1002
1
100100
Where m = open loop bridge reading
n = close loop bridge reading
L = cable route length
6912 Universal Bridge to measure capacitance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be more then 100 ohm
iii) Earth all the conductors not under test
6912 Universal Bridge to measure capacitance of Cable
Open circuit with sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with sound core available
Fault Distance from test end x = CA x L CAB
CA = Capacitance of open circuited coreCAB = Capacitance of sound coreL = conductor length
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
Fault Distance from test endfront end = ((CA(CA+CB))x L
6913 Universal Bridge to measure Inductance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be less then 30 ohm
iii) All conductors be left free from earth
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
Fault distance X = LA x L LAB
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
Fault distance X = LA x L LA + LB
692 Pulse Echo (PE) Time Domain Reflectometry (TDR)
A simple method which works on travelling wave principles It is applicable to all series and shunt faults with Rf gt=Zo10 (5 ohm) and Rflt=10 Zo (500 ohm) respectively For power cables typical Zo=50 ohm
6921 Travelling wave principles
A pulse injected into a cable is reflected back to the source by any change in characteristics impedance (Zo) of the cable
The waveform generated can be monitored using an oscilloscope as shown in Figure below
6921 Continue-
Waveform interpretion
Shunt Fault
Series Fault
6921 Continue-
Waveform interpretion
Low Resistance Shunt Fault (Rflt=500 Ohm)
-Reflection Negative
-Amplitude depends on fault resistance
Series Fault (Rfgt=5 ohm)
- Reflection Positive
- Amplitude depends on fault resistance
Zo = Characteristics impedance = radic(LC)
L = Inductance of cable C = Capacitance of Cable
69211 Reflection Factor (r)
- Is to determine the degree of reflection
Shunt fault r (V) = -Zo1(2Rf + Zo1)
Series Fault r (V) = Rf(2Zo1 + Rf)
Example shunt fault fault resistance = 5 ohm Zo = 50 ohm
r(V) = -50 (2 x5 +50) = - 833If fault resistance = 500 ohm
r(V) = -50 (2x 500 + 50) = -475
69212 Velocity of Propagation (Vp)
- Vp depends on the cable dielectric and is defined as
Vp = Vs radicEr
- where Vs = Velocity of light in free space
Er = Relative permittivity of dielectric
Vp = 300radicEr
Dielectrics Velocity of PropagationVp (mmicroS)
Impregnated Paper 150 -171
PVC 152-175
PE Approx 200
XLPE 156-176
Typical Vp for different Dielectric
69212 Continue (Vp)
Example
Determine the velocity of propagation Vp2 for a cable length of 2500 meters t1= 3164 S and t2 = 1745 S Calculate the fault distances
6921 Continue-
The distance to the fault Lf (m) is given by
Lf = T x Vp2
6921 Continue
Solutions
Velocity of propagation = Vp2 = 25003164 = 79 mS
Distance of Fault from test end = Vp2 x t2
= 79 x 1745
= 137855
693 Impulse Current Method
Working on Travelling wave principles it is applicable to all types of fault
694 Arc Reflection or Secondary Impulse method
Basically PE and TDR associated with fault treatment It is also applicable to faults of all nature but with easily interpreted breakdown waveforms
610 PIN POINTING
Pin Pointing is the application of a test that positively confirms the exact position of the fault
Before the commencement of pin pointing the prelocated fault distance should mark on the cable route which is measured by means of a trumeter
Pin pointing is normally carried out by the shock wave discharge method The fault can be detected by the use of semisphone
611 Confirmation and Re-Test
After the pin-pointed position of the fault has been marked and exposed check for physical sign of fault If there is the fault is confirm
After confirmation the fault should be cut away IR and continuity test should be carried out on the remaining cable sections to determine the soundeness of these cable sections
The insulation resistance test are again carried out after jointing followed by pressure test before supply can be restored
Flowchart for Fault location Procedure
Example
Find a fault distance using direct reading fault localiser from end test which has a test data as below
Bridge Balance Reading =60 Length of Cable 140 meters
Solutions
Fault Distance= = 60100 x 140 = 84 meterxL
n
100
For cables route with combination of 2 or more different type of cable
Determine the resistance of cable for each section separately from Table 1 and Table 2
Example Cable 11kV Al Length 150 m size = 16mm2
From Table 1= Resistance for 1km =226 ohm
For 150 m = 1501000 x 226 = 0339 ohm
From Table 1 Calculate the equivalent resistance for all the combined section
Example for Al = 0339 ohm for copper = 0150 ohm
Equivalent Resistance = 0339+0150 =0489 ohm
Continue
Calculate equivalent resistance from obtained bridge reading
Example Bridge Reading = 60
Equivalent resistance = 06 x 0489 ohm = 02934 ohm
Determine which section of cable the fault occurs
Total resistance at Al section = 0339 ohm
As equivalent resistance at Al section is higher then 02934(Equivalent resistance from bridge test) the fault is
located at Aluminium section of cable
Table 1 Impedance data for 635011000 V Cable Aluminium
Table II Impedance data 11000 V Cable Copper
TABLE VI ndash 6001000V (ALUMINIUM)
TABLE VII ndash 38006600V (ALUMINIUM)
TABLE VIII- 635011000 (ALUMINIUM)
TABLE V
610 COMMISSIONING OF UNDERGROUND CABLE
High Voltage DC testing
High voltage testing is carried out in order to determine the electrical strength of cable insulation Site tests are performed by applying a predetermined high voltage to the insulation DC site test voltages regardless of insulation type are used to ensure that the cable cable joints and termination are correctly made and installed
High Voltage Testing (cont-)
IEC Standards specify maintenance tests after installation should be 70 of permissible factory test DC voltages (15 minutes) = 175kV
Insulation Resistance Test (cont-)
Test voltages should be slowly raised to the required value over a period of about 1 minute and the test period starts once the full voltage is reached In this way the capacitive and absorption currents will have decreased and circuit conditions stabilized such that true leakage current may be measured
66 TYPES OF CABLE FAULT
GENERAL
bull Series (open circuit) Fault
- Failure of continuity (conductor (s) or cable)
bull Shunt (short circuit) fault
- failure of insulation
66 TYPES OF FAULT (CONTINUE)-
66 TYPES OF FAULT (CONTINUE)-
661 SERIES AND SHUNT FAULT
Are subsided into the following categories
Low Resistance Faultof ZR 10
Where Zo= cable surge impedance
=10 ndash 100 ohm
Usually happens in series fault
High Resistance Fault
Where ZoR f 10
662 INTERMITTENT OR FLASH FAULT
- Usually not apparent to insulation resistance measuring
instrument
- Does not manifest itself at lower voltages or a surge
- Breakdown will appear under application of high voltage
dc or DC pressure test
67 FAULT LOCATION PROCEDURE
The proper sequence of cable fault location are as follows
a) Analysis of fault
b) Pre-location
c) Pin Pointing
d) Confirmation and re-test
671 Analysis of Cable Fault
To analyze a cable fault is to determine and confirm the nature or characteristics of the fault
Objective to select which of the cable fault locator equipment of method is best suited to locate the particular fault
Analysis of cable fault normally consists of insulation resistance test and continuity test carried out using 1000V or 5000 V insulation tester
671 Insulation Resistance test
With all earth connections removed from the cable conductor terminal using IR tester of 100025005000V range measure and record the IR in M-ohms K-ohms or ohms between conductors and between conductors and earth Six measurement are to be taken R-E Y-E B-E R-Y Y-B B-R An IR of 100M-ohm indicates a healthy cable
672 Continuity Test
With the cable conductor looped or shorted at the tail endremote end using IR tester selected at continuity range perform continuity test
This test will determine whether any of the cable is open circuited
672 Continuity test (cont-)
Continuity Test - With the cable conductor shorted or looped at the remote end perform continuity test on the cable
- Measure and record the results in ohm
- Three measurements are to be carried out between R-Y Y-B B-R
- The test will determine whether any of cable is open circuited - The resistance per-conductor per km is provided in Table VI VII VIII and IX (refer appendix A)
672 (cont-)
if the continuity of the cable is sound insulation resistance from one end are sufficient
If continuity is broken IR test should be carried out at both ends of the cable
68 BURNING A FAULT
The continuity and IR test may indicate that burning of fault by means of HT pressure test set is required
-
68 BURNING A FAULT (continue-)
Burning a fault is achieve by passing current from a DC HT test set through the fault
Other conductors not under test should be earthed
HT is applied for about 5 to 10 minutes to burn the fault
HT test is used to determine which fault location equipment is suitable to be used
HT is the last resort often used because it sometimes produce ambiguous and unpredictable results
Therefore fault location equipment should be attempted first
69 Pre- Location of fault
Pre-location is the application of a test at the terminals of a
given cable to give an indication of the distance to the fault
from the test point
Whilst the measurement should be accurate as condition will
allow the primary purpose of pre-location is to give an
indication as quick as possible of the vicinity in which to
commence the final pin-pointing tests
69 Pre- Location of fault (cont)-
Generally there are four pre-location methods which are practised and the are as follows
1Bridge or loop Method2 Pulse Echo Time Domain Reflectometry (TDR) method3 Impulse Current Method4 Arc Reflection and secondary Impulse method
691 Bridge method
6911 Direct reading fault localizer
Conditions required
i) No break in continuity if possible one sound core available
ii) The fault resistance of cable should not be more then 20 k-ohm
iii) Applied voltage not to exceed 600 V (DC)
Case 1 Single conductor to earth with sound core available
Fault distance from front end X= n100 x L
Where n = localiser reading in
L= Cable route length
Case2 Conductor to conductor with sound core available
Fault distance from front end X= n100 x L
Where n = localiser reading in
L= Cable route length
Case 3 Three phase fault
By Open loop test and close loop test
(i) Rf2Rf1 gt 5 and (ii) Rf2 lt 20 k-ohm
Open loop test
Case 3 Three phase fault
Close loop test
Open loop and close loop test (cont)
Fault distance from front end
Lxm
mn
X 2
1001
)200200
(
Where m = open loop bridge reading
n = close loop bridge reading
L = cable route length
High Tension Bridge
Conditions required
i) No break in continuity if possible one sound core available
ii) The fault resistance of cable should be more then 20 k-ohm
iii) Applied voltage not to exceed 20kV (DC)
High Tension Bridge
Case 1Single Conductor to earth with sound core available
Fault Distance from test end X = n100 x 2L
High Tension Bridge
Case 2Conductor to conductor with sound core available
Fault Distance from test end X = n100 x 2L
High Tension Bridge
Case 3Three phase Fault with no sound core available
Rf2Rf1gt5 Rf2gt20 kohm
Lxm
)mn
(X 2
1002
1
100100
Where m = open loop bridge reading
n = close loop bridge reading
L = cable route length
6912 Universal Bridge to measure capacitance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be more then 100 ohm
iii) Earth all the conductors not under test
6912 Universal Bridge to measure capacitance of Cable
Open circuit with sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with sound core available
Fault Distance from test end x = CA x L CAB
CA = Capacitance of open circuited coreCAB = Capacitance of sound coreL = conductor length
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
Fault Distance from test endfront end = ((CA(CA+CB))x L
6913 Universal Bridge to measure Inductance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be less then 30 ohm
iii) All conductors be left free from earth
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
Fault distance X = LA x L LAB
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
Fault distance X = LA x L LA + LB
692 Pulse Echo (PE) Time Domain Reflectometry (TDR)
A simple method which works on travelling wave principles It is applicable to all series and shunt faults with Rf gt=Zo10 (5 ohm) and Rflt=10 Zo (500 ohm) respectively For power cables typical Zo=50 ohm
6921 Travelling wave principles
A pulse injected into a cable is reflected back to the source by any change in characteristics impedance (Zo) of the cable
The waveform generated can be monitored using an oscilloscope as shown in Figure below
6921 Continue-
Waveform interpretion
Shunt Fault
Series Fault
6921 Continue-
Waveform interpretion
Low Resistance Shunt Fault (Rflt=500 Ohm)
-Reflection Negative
-Amplitude depends on fault resistance
Series Fault (Rfgt=5 ohm)
- Reflection Positive
- Amplitude depends on fault resistance
Zo = Characteristics impedance = radic(LC)
L = Inductance of cable C = Capacitance of Cable
69211 Reflection Factor (r)
- Is to determine the degree of reflection
Shunt fault r (V) = -Zo1(2Rf + Zo1)
Series Fault r (V) = Rf(2Zo1 + Rf)
Example shunt fault fault resistance = 5 ohm Zo = 50 ohm
r(V) = -50 (2 x5 +50) = - 833If fault resistance = 500 ohm
r(V) = -50 (2x 500 + 50) = -475
69212 Velocity of Propagation (Vp)
- Vp depends on the cable dielectric and is defined as
Vp = Vs radicEr
- where Vs = Velocity of light in free space
Er = Relative permittivity of dielectric
Vp = 300radicEr
Dielectrics Velocity of PropagationVp (mmicroS)
Impregnated Paper 150 -171
PVC 152-175
PE Approx 200
XLPE 156-176
Typical Vp for different Dielectric
69212 Continue (Vp)
Example
Determine the velocity of propagation Vp2 for a cable length of 2500 meters t1= 3164 S and t2 = 1745 S Calculate the fault distances
6921 Continue-
The distance to the fault Lf (m) is given by
Lf = T x Vp2
6921 Continue
Solutions
Velocity of propagation = Vp2 = 25003164 = 79 mS
Distance of Fault from test end = Vp2 x t2
= 79 x 1745
= 137855
693 Impulse Current Method
Working on Travelling wave principles it is applicable to all types of fault
694 Arc Reflection or Secondary Impulse method
Basically PE and TDR associated with fault treatment It is also applicable to faults of all nature but with easily interpreted breakdown waveforms
610 PIN POINTING
Pin Pointing is the application of a test that positively confirms the exact position of the fault
Before the commencement of pin pointing the prelocated fault distance should mark on the cable route which is measured by means of a trumeter
Pin pointing is normally carried out by the shock wave discharge method The fault can be detected by the use of semisphone
611 Confirmation and Re-Test
After the pin-pointed position of the fault has been marked and exposed check for physical sign of fault If there is the fault is confirm
After confirmation the fault should be cut away IR and continuity test should be carried out on the remaining cable sections to determine the soundeness of these cable sections
The insulation resistance test are again carried out after jointing followed by pressure test before supply can be restored
Flowchart for Fault location Procedure
Example
Find a fault distance using direct reading fault localiser from end test which has a test data as below
Bridge Balance Reading =60 Length of Cable 140 meters
Solutions
Fault Distance= = 60100 x 140 = 84 meterxL
n
100
For cables route with combination of 2 or more different type of cable
Determine the resistance of cable for each section separately from Table 1 and Table 2
Example Cable 11kV Al Length 150 m size = 16mm2
From Table 1= Resistance for 1km =226 ohm
For 150 m = 1501000 x 226 = 0339 ohm
From Table 1 Calculate the equivalent resistance for all the combined section
Example for Al = 0339 ohm for copper = 0150 ohm
Equivalent Resistance = 0339+0150 =0489 ohm
Continue
Calculate equivalent resistance from obtained bridge reading
Example Bridge Reading = 60
Equivalent resistance = 06 x 0489 ohm = 02934 ohm
Determine which section of cable the fault occurs
Total resistance at Al section = 0339 ohm
As equivalent resistance at Al section is higher then 02934(Equivalent resistance from bridge test) the fault is
located at Aluminium section of cable
Table 1 Impedance data for 635011000 V Cable Aluminium
Table II Impedance data 11000 V Cable Copper
TABLE VI ndash 6001000V (ALUMINIUM)
TABLE VII ndash 38006600V (ALUMINIUM)
TABLE VIII- 635011000 (ALUMINIUM)
TABLE V
610 COMMISSIONING OF UNDERGROUND CABLE
High Voltage DC testing
High voltage testing is carried out in order to determine the electrical strength of cable insulation Site tests are performed by applying a predetermined high voltage to the insulation DC site test voltages regardless of insulation type are used to ensure that the cable cable joints and termination are correctly made and installed
High Voltage Testing (cont-)
IEC Standards specify maintenance tests after installation should be 70 of permissible factory test DC voltages (15 minutes) = 175kV
Insulation Resistance Test (cont-)
Test voltages should be slowly raised to the required value over a period of about 1 minute and the test period starts once the full voltage is reached In this way the capacitive and absorption currents will have decreased and circuit conditions stabilized such that true leakage current may be measured
66 TYPES OF FAULT (CONTINUE)-
66 TYPES OF FAULT (CONTINUE)-
661 SERIES AND SHUNT FAULT
Are subsided into the following categories
Low Resistance Faultof ZR 10
Where Zo= cable surge impedance
=10 ndash 100 ohm
Usually happens in series fault
High Resistance Fault
Where ZoR f 10
662 INTERMITTENT OR FLASH FAULT
- Usually not apparent to insulation resistance measuring
instrument
- Does not manifest itself at lower voltages or a surge
- Breakdown will appear under application of high voltage
dc or DC pressure test
67 FAULT LOCATION PROCEDURE
The proper sequence of cable fault location are as follows
a) Analysis of fault
b) Pre-location
c) Pin Pointing
d) Confirmation and re-test
671 Analysis of Cable Fault
To analyze a cable fault is to determine and confirm the nature or characteristics of the fault
Objective to select which of the cable fault locator equipment of method is best suited to locate the particular fault
Analysis of cable fault normally consists of insulation resistance test and continuity test carried out using 1000V or 5000 V insulation tester
671 Insulation Resistance test
With all earth connections removed from the cable conductor terminal using IR tester of 100025005000V range measure and record the IR in M-ohms K-ohms or ohms between conductors and between conductors and earth Six measurement are to be taken R-E Y-E B-E R-Y Y-B B-R An IR of 100M-ohm indicates a healthy cable
672 Continuity Test
With the cable conductor looped or shorted at the tail endremote end using IR tester selected at continuity range perform continuity test
This test will determine whether any of the cable is open circuited
672 Continuity test (cont-)
Continuity Test - With the cable conductor shorted or looped at the remote end perform continuity test on the cable
- Measure and record the results in ohm
- Three measurements are to be carried out between R-Y Y-B B-R
- The test will determine whether any of cable is open circuited - The resistance per-conductor per km is provided in Table VI VII VIII and IX (refer appendix A)
672 (cont-)
if the continuity of the cable is sound insulation resistance from one end are sufficient
If continuity is broken IR test should be carried out at both ends of the cable
68 BURNING A FAULT
The continuity and IR test may indicate that burning of fault by means of HT pressure test set is required
-
68 BURNING A FAULT (continue-)
Burning a fault is achieve by passing current from a DC HT test set through the fault
Other conductors not under test should be earthed
HT is applied for about 5 to 10 minutes to burn the fault
HT test is used to determine which fault location equipment is suitable to be used
HT is the last resort often used because it sometimes produce ambiguous and unpredictable results
Therefore fault location equipment should be attempted first
69 Pre- Location of fault
Pre-location is the application of a test at the terminals of a
given cable to give an indication of the distance to the fault
from the test point
Whilst the measurement should be accurate as condition will
allow the primary purpose of pre-location is to give an
indication as quick as possible of the vicinity in which to
commence the final pin-pointing tests
69 Pre- Location of fault (cont)-
Generally there are four pre-location methods which are practised and the are as follows
1Bridge or loop Method2 Pulse Echo Time Domain Reflectometry (TDR) method3 Impulse Current Method4 Arc Reflection and secondary Impulse method
691 Bridge method
6911 Direct reading fault localizer
Conditions required
i) No break in continuity if possible one sound core available
ii) The fault resistance of cable should not be more then 20 k-ohm
iii) Applied voltage not to exceed 600 V (DC)
Case 1 Single conductor to earth with sound core available
Fault distance from front end X= n100 x L
Where n = localiser reading in
L= Cable route length
Case2 Conductor to conductor with sound core available
Fault distance from front end X= n100 x L
Where n = localiser reading in
L= Cable route length
Case 3 Three phase fault
By Open loop test and close loop test
(i) Rf2Rf1 gt 5 and (ii) Rf2 lt 20 k-ohm
Open loop test
Case 3 Three phase fault
Close loop test
Open loop and close loop test (cont)
Fault distance from front end
Lxm
mn
X 2
1001
)200200
(
Where m = open loop bridge reading
n = close loop bridge reading
L = cable route length
High Tension Bridge
Conditions required
i) No break in continuity if possible one sound core available
ii) The fault resistance of cable should be more then 20 k-ohm
iii) Applied voltage not to exceed 20kV (DC)
High Tension Bridge
Case 1Single Conductor to earth with sound core available
Fault Distance from test end X = n100 x 2L
High Tension Bridge
Case 2Conductor to conductor with sound core available
Fault Distance from test end X = n100 x 2L
High Tension Bridge
Case 3Three phase Fault with no sound core available
Rf2Rf1gt5 Rf2gt20 kohm
Lxm
)mn
(X 2
1002
1
100100
Where m = open loop bridge reading
n = close loop bridge reading
L = cable route length
6912 Universal Bridge to measure capacitance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be more then 100 ohm
iii) Earth all the conductors not under test
6912 Universal Bridge to measure capacitance of Cable
Open circuit with sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with sound core available
Fault Distance from test end x = CA x L CAB
CA = Capacitance of open circuited coreCAB = Capacitance of sound coreL = conductor length
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
Fault Distance from test endfront end = ((CA(CA+CB))x L
6913 Universal Bridge to measure Inductance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be less then 30 ohm
iii) All conductors be left free from earth
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
Fault distance X = LA x L LAB
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
Fault distance X = LA x L LA + LB
692 Pulse Echo (PE) Time Domain Reflectometry (TDR)
A simple method which works on travelling wave principles It is applicable to all series and shunt faults with Rf gt=Zo10 (5 ohm) and Rflt=10 Zo (500 ohm) respectively For power cables typical Zo=50 ohm
6921 Travelling wave principles
A pulse injected into a cable is reflected back to the source by any change in characteristics impedance (Zo) of the cable
The waveform generated can be monitored using an oscilloscope as shown in Figure below
6921 Continue-
Waveform interpretion
Shunt Fault
Series Fault
6921 Continue-
Waveform interpretion
Low Resistance Shunt Fault (Rflt=500 Ohm)
-Reflection Negative
-Amplitude depends on fault resistance
Series Fault (Rfgt=5 ohm)
- Reflection Positive
- Amplitude depends on fault resistance
Zo = Characteristics impedance = radic(LC)
L = Inductance of cable C = Capacitance of Cable
69211 Reflection Factor (r)
- Is to determine the degree of reflection
Shunt fault r (V) = -Zo1(2Rf + Zo1)
Series Fault r (V) = Rf(2Zo1 + Rf)
Example shunt fault fault resistance = 5 ohm Zo = 50 ohm
r(V) = -50 (2 x5 +50) = - 833If fault resistance = 500 ohm
r(V) = -50 (2x 500 + 50) = -475
69212 Velocity of Propagation (Vp)
- Vp depends on the cable dielectric and is defined as
Vp = Vs radicEr
- where Vs = Velocity of light in free space
Er = Relative permittivity of dielectric
Vp = 300radicEr
Dielectrics Velocity of PropagationVp (mmicroS)
Impregnated Paper 150 -171
PVC 152-175
PE Approx 200
XLPE 156-176
Typical Vp for different Dielectric
69212 Continue (Vp)
Example
Determine the velocity of propagation Vp2 for a cable length of 2500 meters t1= 3164 S and t2 = 1745 S Calculate the fault distances
6921 Continue-
The distance to the fault Lf (m) is given by
Lf = T x Vp2
6921 Continue
Solutions
Velocity of propagation = Vp2 = 25003164 = 79 mS
Distance of Fault from test end = Vp2 x t2
= 79 x 1745
= 137855
693 Impulse Current Method
Working on Travelling wave principles it is applicable to all types of fault
694 Arc Reflection or Secondary Impulse method
Basically PE and TDR associated with fault treatment It is also applicable to faults of all nature but with easily interpreted breakdown waveforms
610 PIN POINTING
Pin Pointing is the application of a test that positively confirms the exact position of the fault
Before the commencement of pin pointing the prelocated fault distance should mark on the cable route which is measured by means of a trumeter
Pin pointing is normally carried out by the shock wave discharge method The fault can be detected by the use of semisphone
611 Confirmation and Re-Test
After the pin-pointed position of the fault has been marked and exposed check for physical sign of fault If there is the fault is confirm
After confirmation the fault should be cut away IR and continuity test should be carried out on the remaining cable sections to determine the soundeness of these cable sections
The insulation resistance test are again carried out after jointing followed by pressure test before supply can be restored
Flowchart for Fault location Procedure
Example
Find a fault distance using direct reading fault localiser from end test which has a test data as below
Bridge Balance Reading =60 Length of Cable 140 meters
Solutions
Fault Distance= = 60100 x 140 = 84 meterxL
n
100
For cables route with combination of 2 or more different type of cable
Determine the resistance of cable for each section separately from Table 1 and Table 2
Example Cable 11kV Al Length 150 m size = 16mm2
From Table 1= Resistance for 1km =226 ohm
For 150 m = 1501000 x 226 = 0339 ohm
From Table 1 Calculate the equivalent resistance for all the combined section
Example for Al = 0339 ohm for copper = 0150 ohm
Equivalent Resistance = 0339+0150 =0489 ohm
Continue
Calculate equivalent resistance from obtained bridge reading
Example Bridge Reading = 60
Equivalent resistance = 06 x 0489 ohm = 02934 ohm
Determine which section of cable the fault occurs
Total resistance at Al section = 0339 ohm
As equivalent resistance at Al section is higher then 02934(Equivalent resistance from bridge test) the fault is
located at Aluminium section of cable
Table 1 Impedance data for 635011000 V Cable Aluminium
Table II Impedance data 11000 V Cable Copper
TABLE VI ndash 6001000V (ALUMINIUM)
TABLE VII ndash 38006600V (ALUMINIUM)
TABLE VIII- 635011000 (ALUMINIUM)
TABLE V
610 COMMISSIONING OF UNDERGROUND CABLE
High Voltage DC testing
High voltage testing is carried out in order to determine the electrical strength of cable insulation Site tests are performed by applying a predetermined high voltage to the insulation DC site test voltages regardless of insulation type are used to ensure that the cable cable joints and termination are correctly made and installed
High Voltage Testing (cont-)
IEC Standards specify maintenance tests after installation should be 70 of permissible factory test DC voltages (15 minutes) = 175kV
Insulation Resistance Test (cont-)
Test voltages should be slowly raised to the required value over a period of about 1 minute and the test period starts once the full voltage is reached In this way the capacitive and absorption currents will have decreased and circuit conditions stabilized such that true leakage current may be measured
66 TYPES OF FAULT (CONTINUE)-
661 SERIES AND SHUNT FAULT
Are subsided into the following categories
Low Resistance Faultof ZR 10
Where Zo= cable surge impedance
=10 ndash 100 ohm
Usually happens in series fault
High Resistance Fault
Where ZoR f 10
662 INTERMITTENT OR FLASH FAULT
- Usually not apparent to insulation resistance measuring
instrument
- Does not manifest itself at lower voltages or a surge
- Breakdown will appear under application of high voltage
dc or DC pressure test
67 FAULT LOCATION PROCEDURE
The proper sequence of cable fault location are as follows
a) Analysis of fault
b) Pre-location
c) Pin Pointing
d) Confirmation and re-test
671 Analysis of Cable Fault
To analyze a cable fault is to determine and confirm the nature or characteristics of the fault
Objective to select which of the cable fault locator equipment of method is best suited to locate the particular fault
Analysis of cable fault normally consists of insulation resistance test and continuity test carried out using 1000V or 5000 V insulation tester
671 Insulation Resistance test
With all earth connections removed from the cable conductor terminal using IR tester of 100025005000V range measure and record the IR in M-ohms K-ohms or ohms between conductors and between conductors and earth Six measurement are to be taken R-E Y-E B-E R-Y Y-B B-R An IR of 100M-ohm indicates a healthy cable
672 Continuity Test
With the cable conductor looped or shorted at the tail endremote end using IR tester selected at continuity range perform continuity test
This test will determine whether any of the cable is open circuited
672 Continuity test (cont-)
Continuity Test - With the cable conductor shorted or looped at the remote end perform continuity test on the cable
- Measure and record the results in ohm
- Three measurements are to be carried out between R-Y Y-B B-R
- The test will determine whether any of cable is open circuited - The resistance per-conductor per km is provided in Table VI VII VIII and IX (refer appendix A)
672 (cont-)
if the continuity of the cable is sound insulation resistance from one end are sufficient
If continuity is broken IR test should be carried out at both ends of the cable
68 BURNING A FAULT
The continuity and IR test may indicate that burning of fault by means of HT pressure test set is required
-
68 BURNING A FAULT (continue-)
Burning a fault is achieve by passing current from a DC HT test set through the fault
Other conductors not under test should be earthed
HT is applied for about 5 to 10 minutes to burn the fault
HT test is used to determine which fault location equipment is suitable to be used
HT is the last resort often used because it sometimes produce ambiguous and unpredictable results
Therefore fault location equipment should be attempted first
69 Pre- Location of fault
Pre-location is the application of a test at the terminals of a
given cable to give an indication of the distance to the fault
from the test point
Whilst the measurement should be accurate as condition will
allow the primary purpose of pre-location is to give an
indication as quick as possible of the vicinity in which to
commence the final pin-pointing tests
69 Pre- Location of fault (cont)-
Generally there are four pre-location methods which are practised and the are as follows
1Bridge or loop Method2 Pulse Echo Time Domain Reflectometry (TDR) method3 Impulse Current Method4 Arc Reflection and secondary Impulse method
691 Bridge method
6911 Direct reading fault localizer
Conditions required
i) No break in continuity if possible one sound core available
ii) The fault resistance of cable should not be more then 20 k-ohm
iii) Applied voltage not to exceed 600 V (DC)
Case 1 Single conductor to earth with sound core available
Fault distance from front end X= n100 x L
Where n = localiser reading in
L= Cable route length
Case2 Conductor to conductor with sound core available
Fault distance from front end X= n100 x L
Where n = localiser reading in
L= Cable route length
Case 3 Three phase fault
By Open loop test and close loop test
(i) Rf2Rf1 gt 5 and (ii) Rf2 lt 20 k-ohm
Open loop test
Case 3 Three phase fault
Close loop test
Open loop and close loop test (cont)
Fault distance from front end
Lxm
mn
X 2
1001
)200200
(
Where m = open loop bridge reading
n = close loop bridge reading
L = cable route length
High Tension Bridge
Conditions required
i) No break in continuity if possible one sound core available
ii) The fault resistance of cable should be more then 20 k-ohm
iii) Applied voltage not to exceed 20kV (DC)
High Tension Bridge
Case 1Single Conductor to earth with sound core available
Fault Distance from test end X = n100 x 2L
High Tension Bridge
Case 2Conductor to conductor with sound core available
Fault Distance from test end X = n100 x 2L
High Tension Bridge
Case 3Three phase Fault with no sound core available
Rf2Rf1gt5 Rf2gt20 kohm
Lxm
)mn
(X 2
1002
1
100100
Where m = open loop bridge reading
n = close loop bridge reading
L = cable route length
6912 Universal Bridge to measure capacitance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be more then 100 ohm
iii) Earth all the conductors not under test
6912 Universal Bridge to measure capacitance of Cable
Open circuit with sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with sound core available
Fault Distance from test end x = CA x L CAB
CA = Capacitance of open circuited coreCAB = Capacitance of sound coreL = conductor length
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
Fault Distance from test endfront end = ((CA(CA+CB))x L
6913 Universal Bridge to measure Inductance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be less then 30 ohm
iii) All conductors be left free from earth
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
Fault distance X = LA x L LAB
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
Fault distance X = LA x L LA + LB
692 Pulse Echo (PE) Time Domain Reflectometry (TDR)
A simple method which works on travelling wave principles It is applicable to all series and shunt faults with Rf gt=Zo10 (5 ohm) and Rflt=10 Zo (500 ohm) respectively For power cables typical Zo=50 ohm
6921 Travelling wave principles
A pulse injected into a cable is reflected back to the source by any change in characteristics impedance (Zo) of the cable
The waveform generated can be monitored using an oscilloscope as shown in Figure below
6921 Continue-
Waveform interpretion
Shunt Fault
Series Fault
6921 Continue-
Waveform interpretion
Low Resistance Shunt Fault (Rflt=500 Ohm)
-Reflection Negative
-Amplitude depends on fault resistance
Series Fault (Rfgt=5 ohm)
- Reflection Positive
- Amplitude depends on fault resistance
Zo = Characteristics impedance = radic(LC)
L = Inductance of cable C = Capacitance of Cable
69211 Reflection Factor (r)
- Is to determine the degree of reflection
Shunt fault r (V) = -Zo1(2Rf + Zo1)
Series Fault r (V) = Rf(2Zo1 + Rf)
Example shunt fault fault resistance = 5 ohm Zo = 50 ohm
r(V) = -50 (2 x5 +50) = - 833If fault resistance = 500 ohm
r(V) = -50 (2x 500 + 50) = -475
69212 Velocity of Propagation (Vp)
- Vp depends on the cable dielectric and is defined as
Vp = Vs radicEr
- where Vs = Velocity of light in free space
Er = Relative permittivity of dielectric
Vp = 300radicEr
Dielectrics Velocity of PropagationVp (mmicroS)
Impregnated Paper 150 -171
PVC 152-175
PE Approx 200
XLPE 156-176
Typical Vp for different Dielectric
69212 Continue (Vp)
Example
Determine the velocity of propagation Vp2 for a cable length of 2500 meters t1= 3164 S and t2 = 1745 S Calculate the fault distances
6921 Continue-
The distance to the fault Lf (m) is given by
Lf = T x Vp2
6921 Continue
Solutions
Velocity of propagation = Vp2 = 25003164 = 79 mS
Distance of Fault from test end = Vp2 x t2
= 79 x 1745
= 137855
693 Impulse Current Method
Working on Travelling wave principles it is applicable to all types of fault
694 Arc Reflection or Secondary Impulse method
Basically PE and TDR associated with fault treatment It is also applicable to faults of all nature but with easily interpreted breakdown waveforms
610 PIN POINTING
Pin Pointing is the application of a test that positively confirms the exact position of the fault
Before the commencement of pin pointing the prelocated fault distance should mark on the cable route which is measured by means of a trumeter
Pin pointing is normally carried out by the shock wave discharge method The fault can be detected by the use of semisphone
611 Confirmation and Re-Test
After the pin-pointed position of the fault has been marked and exposed check for physical sign of fault If there is the fault is confirm
After confirmation the fault should be cut away IR and continuity test should be carried out on the remaining cable sections to determine the soundeness of these cable sections
The insulation resistance test are again carried out after jointing followed by pressure test before supply can be restored
Flowchart for Fault location Procedure
Example
Find a fault distance using direct reading fault localiser from end test which has a test data as below
Bridge Balance Reading =60 Length of Cable 140 meters
Solutions
Fault Distance= = 60100 x 140 = 84 meterxL
n
100
For cables route with combination of 2 or more different type of cable
Determine the resistance of cable for each section separately from Table 1 and Table 2
Example Cable 11kV Al Length 150 m size = 16mm2
From Table 1= Resistance for 1km =226 ohm
For 150 m = 1501000 x 226 = 0339 ohm
From Table 1 Calculate the equivalent resistance for all the combined section
Example for Al = 0339 ohm for copper = 0150 ohm
Equivalent Resistance = 0339+0150 =0489 ohm
Continue
Calculate equivalent resistance from obtained bridge reading
Example Bridge Reading = 60
Equivalent resistance = 06 x 0489 ohm = 02934 ohm
Determine which section of cable the fault occurs
Total resistance at Al section = 0339 ohm
As equivalent resistance at Al section is higher then 02934(Equivalent resistance from bridge test) the fault is
located at Aluminium section of cable
Table 1 Impedance data for 635011000 V Cable Aluminium
Table II Impedance data 11000 V Cable Copper
TABLE VI ndash 6001000V (ALUMINIUM)
TABLE VII ndash 38006600V (ALUMINIUM)
TABLE VIII- 635011000 (ALUMINIUM)
TABLE V
610 COMMISSIONING OF UNDERGROUND CABLE
High Voltage DC testing
High voltage testing is carried out in order to determine the electrical strength of cable insulation Site tests are performed by applying a predetermined high voltage to the insulation DC site test voltages regardless of insulation type are used to ensure that the cable cable joints and termination are correctly made and installed
High Voltage Testing (cont-)
IEC Standards specify maintenance tests after installation should be 70 of permissible factory test DC voltages (15 minutes) = 175kV
Insulation Resistance Test (cont-)
Test voltages should be slowly raised to the required value over a period of about 1 minute and the test period starts once the full voltage is reached In this way the capacitive and absorption currents will have decreased and circuit conditions stabilized such that true leakage current may be measured
661 SERIES AND SHUNT FAULT
Are subsided into the following categories
Low Resistance Faultof ZR 10
Where Zo= cable surge impedance
=10 ndash 100 ohm
Usually happens in series fault
High Resistance Fault
Where ZoR f 10
662 INTERMITTENT OR FLASH FAULT
- Usually not apparent to insulation resistance measuring
instrument
- Does not manifest itself at lower voltages or a surge
- Breakdown will appear under application of high voltage
dc or DC pressure test
67 FAULT LOCATION PROCEDURE
The proper sequence of cable fault location are as follows
a) Analysis of fault
b) Pre-location
c) Pin Pointing
d) Confirmation and re-test
671 Analysis of Cable Fault
To analyze a cable fault is to determine and confirm the nature or characteristics of the fault
Objective to select which of the cable fault locator equipment of method is best suited to locate the particular fault
Analysis of cable fault normally consists of insulation resistance test and continuity test carried out using 1000V or 5000 V insulation tester
671 Insulation Resistance test
With all earth connections removed from the cable conductor terminal using IR tester of 100025005000V range measure and record the IR in M-ohms K-ohms or ohms between conductors and between conductors and earth Six measurement are to be taken R-E Y-E B-E R-Y Y-B B-R An IR of 100M-ohm indicates a healthy cable
672 Continuity Test
With the cable conductor looped or shorted at the tail endremote end using IR tester selected at continuity range perform continuity test
This test will determine whether any of the cable is open circuited
672 Continuity test (cont-)
Continuity Test - With the cable conductor shorted or looped at the remote end perform continuity test on the cable
- Measure and record the results in ohm
- Three measurements are to be carried out between R-Y Y-B B-R
- The test will determine whether any of cable is open circuited - The resistance per-conductor per km is provided in Table VI VII VIII and IX (refer appendix A)
672 (cont-)
if the continuity of the cable is sound insulation resistance from one end are sufficient
If continuity is broken IR test should be carried out at both ends of the cable
68 BURNING A FAULT
The continuity and IR test may indicate that burning of fault by means of HT pressure test set is required
-
68 BURNING A FAULT (continue-)
Burning a fault is achieve by passing current from a DC HT test set through the fault
Other conductors not under test should be earthed
HT is applied for about 5 to 10 minutes to burn the fault
HT test is used to determine which fault location equipment is suitable to be used
HT is the last resort often used because it sometimes produce ambiguous and unpredictable results
Therefore fault location equipment should be attempted first
69 Pre- Location of fault
Pre-location is the application of a test at the terminals of a
given cable to give an indication of the distance to the fault
from the test point
Whilst the measurement should be accurate as condition will
allow the primary purpose of pre-location is to give an
indication as quick as possible of the vicinity in which to
commence the final pin-pointing tests
69 Pre- Location of fault (cont)-
Generally there are four pre-location methods which are practised and the are as follows
1Bridge or loop Method2 Pulse Echo Time Domain Reflectometry (TDR) method3 Impulse Current Method4 Arc Reflection and secondary Impulse method
691 Bridge method
6911 Direct reading fault localizer
Conditions required
i) No break in continuity if possible one sound core available
ii) The fault resistance of cable should not be more then 20 k-ohm
iii) Applied voltage not to exceed 600 V (DC)
Case 1 Single conductor to earth with sound core available
Fault distance from front end X= n100 x L
Where n = localiser reading in
L= Cable route length
Case2 Conductor to conductor with sound core available
Fault distance from front end X= n100 x L
Where n = localiser reading in
L= Cable route length
Case 3 Three phase fault
By Open loop test and close loop test
(i) Rf2Rf1 gt 5 and (ii) Rf2 lt 20 k-ohm
Open loop test
Case 3 Three phase fault
Close loop test
Open loop and close loop test (cont)
Fault distance from front end
Lxm
mn
X 2
1001
)200200
(
Where m = open loop bridge reading
n = close loop bridge reading
L = cable route length
High Tension Bridge
Conditions required
i) No break in continuity if possible one sound core available
ii) The fault resistance of cable should be more then 20 k-ohm
iii) Applied voltage not to exceed 20kV (DC)
High Tension Bridge
Case 1Single Conductor to earth with sound core available
Fault Distance from test end X = n100 x 2L
High Tension Bridge
Case 2Conductor to conductor with sound core available
Fault Distance from test end X = n100 x 2L
High Tension Bridge
Case 3Three phase Fault with no sound core available
Rf2Rf1gt5 Rf2gt20 kohm
Lxm
)mn
(X 2
1002
1
100100
Where m = open loop bridge reading
n = close loop bridge reading
L = cable route length
6912 Universal Bridge to measure capacitance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be more then 100 ohm
iii) Earth all the conductors not under test
6912 Universal Bridge to measure capacitance of Cable
Open circuit with sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with sound core available
Fault Distance from test end x = CA x L CAB
CA = Capacitance of open circuited coreCAB = Capacitance of sound coreL = conductor length
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
Fault Distance from test endfront end = ((CA(CA+CB))x L
6913 Universal Bridge to measure Inductance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be less then 30 ohm
iii) All conductors be left free from earth
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
Fault distance X = LA x L LAB
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
Fault distance X = LA x L LA + LB
692 Pulse Echo (PE) Time Domain Reflectometry (TDR)
A simple method which works on travelling wave principles It is applicable to all series and shunt faults with Rf gt=Zo10 (5 ohm) and Rflt=10 Zo (500 ohm) respectively For power cables typical Zo=50 ohm
6921 Travelling wave principles
A pulse injected into a cable is reflected back to the source by any change in characteristics impedance (Zo) of the cable
The waveform generated can be monitored using an oscilloscope as shown in Figure below
6921 Continue-
Waveform interpretion
Shunt Fault
Series Fault
6921 Continue-
Waveform interpretion
Low Resistance Shunt Fault (Rflt=500 Ohm)
-Reflection Negative
-Amplitude depends on fault resistance
Series Fault (Rfgt=5 ohm)
- Reflection Positive
- Amplitude depends on fault resistance
Zo = Characteristics impedance = radic(LC)
L = Inductance of cable C = Capacitance of Cable
69211 Reflection Factor (r)
- Is to determine the degree of reflection
Shunt fault r (V) = -Zo1(2Rf + Zo1)
Series Fault r (V) = Rf(2Zo1 + Rf)
Example shunt fault fault resistance = 5 ohm Zo = 50 ohm
r(V) = -50 (2 x5 +50) = - 833If fault resistance = 500 ohm
r(V) = -50 (2x 500 + 50) = -475
69212 Velocity of Propagation (Vp)
- Vp depends on the cable dielectric and is defined as
Vp = Vs radicEr
- where Vs = Velocity of light in free space
Er = Relative permittivity of dielectric
Vp = 300radicEr
Dielectrics Velocity of PropagationVp (mmicroS)
Impregnated Paper 150 -171
PVC 152-175
PE Approx 200
XLPE 156-176
Typical Vp for different Dielectric
69212 Continue (Vp)
Example
Determine the velocity of propagation Vp2 for a cable length of 2500 meters t1= 3164 S and t2 = 1745 S Calculate the fault distances
6921 Continue-
The distance to the fault Lf (m) is given by
Lf = T x Vp2
6921 Continue
Solutions
Velocity of propagation = Vp2 = 25003164 = 79 mS
Distance of Fault from test end = Vp2 x t2
= 79 x 1745
= 137855
693 Impulse Current Method
Working on Travelling wave principles it is applicable to all types of fault
694 Arc Reflection or Secondary Impulse method
Basically PE and TDR associated with fault treatment It is also applicable to faults of all nature but with easily interpreted breakdown waveforms
610 PIN POINTING
Pin Pointing is the application of a test that positively confirms the exact position of the fault
Before the commencement of pin pointing the prelocated fault distance should mark on the cable route which is measured by means of a trumeter
Pin pointing is normally carried out by the shock wave discharge method The fault can be detected by the use of semisphone
611 Confirmation and Re-Test
After the pin-pointed position of the fault has been marked and exposed check for physical sign of fault If there is the fault is confirm
After confirmation the fault should be cut away IR and continuity test should be carried out on the remaining cable sections to determine the soundeness of these cable sections
The insulation resistance test are again carried out after jointing followed by pressure test before supply can be restored
Flowchart for Fault location Procedure
Example
Find a fault distance using direct reading fault localiser from end test which has a test data as below
Bridge Balance Reading =60 Length of Cable 140 meters
Solutions
Fault Distance= = 60100 x 140 = 84 meterxL
n
100
For cables route with combination of 2 or more different type of cable
Determine the resistance of cable for each section separately from Table 1 and Table 2
Example Cable 11kV Al Length 150 m size = 16mm2
From Table 1= Resistance for 1km =226 ohm
For 150 m = 1501000 x 226 = 0339 ohm
From Table 1 Calculate the equivalent resistance for all the combined section
Example for Al = 0339 ohm for copper = 0150 ohm
Equivalent Resistance = 0339+0150 =0489 ohm
Continue
Calculate equivalent resistance from obtained bridge reading
Example Bridge Reading = 60
Equivalent resistance = 06 x 0489 ohm = 02934 ohm
Determine which section of cable the fault occurs
Total resistance at Al section = 0339 ohm
As equivalent resistance at Al section is higher then 02934(Equivalent resistance from bridge test) the fault is
located at Aluminium section of cable
Table 1 Impedance data for 635011000 V Cable Aluminium
Table II Impedance data 11000 V Cable Copper
TABLE VI ndash 6001000V (ALUMINIUM)
TABLE VII ndash 38006600V (ALUMINIUM)
TABLE VIII- 635011000 (ALUMINIUM)
TABLE V
610 COMMISSIONING OF UNDERGROUND CABLE
High Voltage DC testing
High voltage testing is carried out in order to determine the electrical strength of cable insulation Site tests are performed by applying a predetermined high voltage to the insulation DC site test voltages regardless of insulation type are used to ensure that the cable cable joints and termination are correctly made and installed
High Voltage Testing (cont-)
IEC Standards specify maintenance tests after installation should be 70 of permissible factory test DC voltages (15 minutes) = 175kV
Insulation Resistance Test (cont-)
Test voltages should be slowly raised to the required value over a period of about 1 minute and the test period starts once the full voltage is reached In this way the capacitive and absorption currents will have decreased and circuit conditions stabilized such that true leakage current may be measured
662 INTERMITTENT OR FLASH FAULT
- Usually not apparent to insulation resistance measuring
instrument
- Does not manifest itself at lower voltages or a surge
- Breakdown will appear under application of high voltage
dc or DC pressure test
67 FAULT LOCATION PROCEDURE
The proper sequence of cable fault location are as follows
a) Analysis of fault
b) Pre-location
c) Pin Pointing
d) Confirmation and re-test
671 Analysis of Cable Fault
To analyze a cable fault is to determine and confirm the nature or characteristics of the fault
Objective to select which of the cable fault locator equipment of method is best suited to locate the particular fault
Analysis of cable fault normally consists of insulation resistance test and continuity test carried out using 1000V or 5000 V insulation tester
671 Insulation Resistance test
With all earth connections removed from the cable conductor terminal using IR tester of 100025005000V range measure and record the IR in M-ohms K-ohms or ohms between conductors and between conductors and earth Six measurement are to be taken R-E Y-E B-E R-Y Y-B B-R An IR of 100M-ohm indicates a healthy cable
672 Continuity Test
With the cable conductor looped or shorted at the tail endremote end using IR tester selected at continuity range perform continuity test
This test will determine whether any of the cable is open circuited
672 Continuity test (cont-)
Continuity Test - With the cable conductor shorted or looped at the remote end perform continuity test on the cable
- Measure and record the results in ohm
- Three measurements are to be carried out between R-Y Y-B B-R
- The test will determine whether any of cable is open circuited - The resistance per-conductor per km is provided in Table VI VII VIII and IX (refer appendix A)
672 (cont-)
if the continuity of the cable is sound insulation resistance from one end are sufficient
If continuity is broken IR test should be carried out at both ends of the cable
68 BURNING A FAULT
The continuity and IR test may indicate that burning of fault by means of HT pressure test set is required
-
68 BURNING A FAULT (continue-)
Burning a fault is achieve by passing current from a DC HT test set through the fault
Other conductors not under test should be earthed
HT is applied for about 5 to 10 minutes to burn the fault
HT test is used to determine which fault location equipment is suitable to be used
HT is the last resort often used because it sometimes produce ambiguous and unpredictable results
Therefore fault location equipment should be attempted first
69 Pre- Location of fault
Pre-location is the application of a test at the terminals of a
given cable to give an indication of the distance to the fault
from the test point
Whilst the measurement should be accurate as condition will
allow the primary purpose of pre-location is to give an
indication as quick as possible of the vicinity in which to
commence the final pin-pointing tests
69 Pre- Location of fault (cont)-
Generally there are four pre-location methods which are practised and the are as follows
1Bridge or loop Method2 Pulse Echo Time Domain Reflectometry (TDR) method3 Impulse Current Method4 Arc Reflection and secondary Impulse method
691 Bridge method
6911 Direct reading fault localizer
Conditions required
i) No break in continuity if possible one sound core available
ii) The fault resistance of cable should not be more then 20 k-ohm
iii) Applied voltage not to exceed 600 V (DC)
Case 1 Single conductor to earth with sound core available
Fault distance from front end X= n100 x L
Where n = localiser reading in
L= Cable route length
Case2 Conductor to conductor with sound core available
Fault distance from front end X= n100 x L
Where n = localiser reading in
L= Cable route length
Case 3 Three phase fault
By Open loop test and close loop test
(i) Rf2Rf1 gt 5 and (ii) Rf2 lt 20 k-ohm
Open loop test
Case 3 Three phase fault
Close loop test
Open loop and close loop test (cont)
Fault distance from front end
Lxm
mn
X 2
1001
)200200
(
Where m = open loop bridge reading
n = close loop bridge reading
L = cable route length
High Tension Bridge
Conditions required
i) No break in continuity if possible one sound core available
ii) The fault resistance of cable should be more then 20 k-ohm
iii) Applied voltage not to exceed 20kV (DC)
High Tension Bridge
Case 1Single Conductor to earth with sound core available
Fault Distance from test end X = n100 x 2L
High Tension Bridge
Case 2Conductor to conductor with sound core available
Fault Distance from test end X = n100 x 2L
High Tension Bridge
Case 3Three phase Fault with no sound core available
Rf2Rf1gt5 Rf2gt20 kohm
Lxm
)mn
(X 2
1002
1
100100
Where m = open loop bridge reading
n = close loop bridge reading
L = cable route length
6912 Universal Bridge to measure capacitance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be more then 100 ohm
iii) Earth all the conductors not under test
6912 Universal Bridge to measure capacitance of Cable
Open circuit with sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with sound core available
Fault Distance from test end x = CA x L CAB
CA = Capacitance of open circuited coreCAB = Capacitance of sound coreL = conductor length
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
Fault Distance from test endfront end = ((CA(CA+CB))x L
6913 Universal Bridge to measure Inductance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be less then 30 ohm
iii) All conductors be left free from earth
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
Fault distance X = LA x L LAB
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
Fault distance X = LA x L LA + LB
692 Pulse Echo (PE) Time Domain Reflectometry (TDR)
A simple method which works on travelling wave principles It is applicable to all series and shunt faults with Rf gt=Zo10 (5 ohm) and Rflt=10 Zo (500 ohm) respectively For power cables typical Zo=50 ohm
6921 Travelling wave principles
A pulse injected into a cable is reflected back to the source by any change in characteristics impedance (Zo) of the cable
The waveform generated can be monitored using an oscilloscope as shown in Figure below
6921 Continue-
Waveform interpretion
Shunt Fault
Series Fault
6921 Continue-
Waveform interpretion
Low Resistance Shunt Fault (Rflt=500 Ohm)
-Reflection Negative
-Amplitude depends on fault resistance
Series Fault (Rfgt=5 ohm)
- Reflection Positive
- Amplitude depends on fault resistance
Zo = Characteristics impedance = radic(LC)
L = Inductance of cable C = Capacitance of Cable
69211 Reflection Factor (r)
- Is to determine the degree of reflection
Shunt fault r (V) = -Zo1(2Rf + Zo1)
Series Fault r (V) = Rf(2Zo1 + Rf)
Example shunt fault fault resistance = 5 ohm Zo = 50 ohm
r(V) = -50 (2 x5 +50) = - 833If fault resistance = 500 ohm
r(V) = -50 (2x 500 + 50) = -475
69212 Velocity of Propagation (Vp)
- Vp depends on the cable dielectric and is defined as
Vp = Vs radicEr
- where Vs = Velocity of light in free space
Er = Relative permittivity of dielectric
Vp = 300radicEr
Dielectrics Velocity of PropagationVp (mmicroS)
Impregnated Paper 150 -171
PVC 152-175
PE Approx 200
XLPE 156-176
Typical Vp for different Dielectric
69212 Continue (Vp)
Example
Determine the velocity of propagation Vp2 for a cable length of 2500 meters t1= 3164 S and t2 = 1745 S Calculate the fault distances
6921 Continue-
The distance to the fault Lf (m) is given by
Lf = T x Vp2
6921 Continue
Solutions
Velocity of propagation = Vp2 = 25003164 = 79 mS
Distance of Fault from test end = Vp2 x t2
= 79 x 1745
= 137855
693 Impulse Current Method
Working on Travelling wave principles it is applicable to all types of fault
694 Arc Reflection or Secondary Impulse method
Basically PE and TDR associated with fault treatment It is also applicable to faults of all nature but with easily interpreted breakdown waveforms
610 PIN POINTING
Pin Pointing is the application of a test that positively confirms the exact position of the fault
Before the commencement of pin pointing the prelocated fault distance should mark on the cable route which is measured by means of a trumeter
Pin pointing is normally carried out by the shock wave discharge method The fault can be detected by the use of semisphone
611 Confirmation and Re-Test
After the pin-pointed position of the fault has been marked and exposed check for physical sign of fault If there is the fault is confirm
After confirmation the fault should be cut away IR and continuity test should be carried out on the remaining cable sections to determine the soundeness of these cable sections
The insulation resistance test are again carried out after jointing followed by pressure test before supply can be restored
Flowchart for Fault location Procedure
Example
Find a fault distance using direct reading fault localiser from end test which has a test data as below
Bridge Balance Reading =60 Length of Cable 140 meters
Solutions
Fault Distance= = 60100 x 140 = 84 meterxL
n
100
For cables route with combination of 2 or more different type of cable
Determine the resistance of cable for each section separately from Table 1 and Table 2
Example Cable 11kV Al Length 150 m size = 16mm2
From Table 1= Resistance for 1km =226 ohm
For 150 m = 1501000 x 226 = 0339 ohm
From Table 1 Calculate the equivalent resistance for all the combined section
Example for Al = 0339 ohm for copper = 0150 ohm
Equivalent Resistance = 0339+0150 =0489 ohm
Continue
Calculate equivalent resistance from obtained bridge reading
Example Bridge Reading = 60
Equivalent resistance = 06 x 0489 ohm = 02934 ohm
Determine which section of cable the fault occurs
Total resistance at Al section = 0339 ohm
As equivalent resistance at Al section is higher then 02934(Equivalent resistance from bridge test) the fault is
located at Aluminium section of cable
Table 1 Impedance data for 635011000 V Cable Aluminium
Table II Impedance data 11000 V Cable Copper
TABLE VI ndash 6001000V (ALUMINIUM)
TABLE VII ndash 38006600V (ALUMINIUM)
TABLE VIII- 635011000 (ALUMINIUM)
TABLE V
610 COMMISSIONING OF UNDERGROUND CABLE
High Voltage DC testing
High voltage testing is carried out in order to determine the electrical strength of cable insulation Site tests are performed by applying a predetermined high voltage to the insulation DC site test voltages regardless of insulation type are used to ensure that the cable cable joints and termination are correctly made and installed
High Voltage Testing (cont-)
IEC Standards specify maintenance tests after installation should be 70 of permissible factory test DC voltages (15 minutes) = 175kV
Insulation Resistance Test (cont-)
Test voltages should be slowly raised to the required value over a period of about 1 minute and the test period starts once the full voltage is reached In this way the capacitive and absorption currents will have decreased and circuit conditions stabilized such that true leakage current may be measured
67 FAULT LOCATION PROCEDURE
The proper sequence of cable fault location are as follows
a) Analysis of fault
b) Pre-location
c) Pin Pointing
d) Confirmation and re-test
671 Analysis of Cable Fault
To analyze a cable fault is to determine and confirm the nature or characteristics of the fault
Objective to select which of the cable fault locator equipment of method is best suited to locate the particular fault
Analysis of cable fault normally consists of insulation resistance test and continuity test carried out using 1000V or 5000 V insulation tester
671 Insulation Resistance test
With all earth connections removed from the cable conductor terminal using IR tester of 100025005000V range measure and record the IR in M-ohms K-ohms or ohms between conductors and between conductors and earth Six measurement are to be taken R-E Y-E B-E R-Y Y-B B-R An IR of 100M-ohm indicates a healthy cable
672 Continuity Test
With the cable conductor looped or shorted at the tail endremote end using IR tester selected at continuity range perform continuity test
This test will determine whether any of the cable is open circuited
672 Continuity test (cont-)
Continuity Test - With the cable conductor shorted or looped at the remote end perform continuity test on the cable
- Measure and record the results in ohm
- Three measurements are to be carried out between R-Y Y-B B-R
- The test will determine whether any of cable is open circuited - The resistance per-conductor per km is provided in Table VI VII VIII and IX (refer appendix A)
672 (cont-)
if the continuity of the cable is sound insulation resistance from one end are sufficient
If continuity is broken IR test should be carried out at both ends of the cable
68 BURNING A FAULT
The continuity and IR test may indicate that burning of fault by means of HT pressure test set is required
-
68 BURNING A FAULT (continue-)
Burning a fault is achieve by passing current from a DC HT test set through the fault
Other conductors not under test should be earthed
HT is applied for about 5 to 10 minutes to burn the fault
HT test is used to determine which fault location equipment is suitable to be used
HT is the last resort often used because it sometimes produce ambiguous and unpredictable results
Therefore fault location equipment should be attempted first
69 Pre- Location of fault
Pre-location is the application of a test at the terminals of a
given cable to give an indication of the distance to the fault
from the test point
Whilst the measurement should be accurate as condition will
allow the primary purpose of pre-location is to give an
indication as quick as possible of the vicinity in which to
commence the final pin-pointing tests
69 Pre- Location of fault (cont)-
Generally there are four pre-location methods which are practised and the are as follows
1Bridge or loop Method2 Pulse Echo Time Domain Reflectometry (TDR) method3 Impulse Current Method4 Arc Reflection and secondary Impulse method
691 Bridge method
6911 Direct reading fault localizer
Conditions required
i) No break in continuity if possible one sound core available
ii) The fault resistance of cable should not be more then 20 k-ohm
iii) Applied voltage not to exceed 600 V (DC)
Case 1 Single conductor to earth with sound core available
Fault distance from front end X= n100 x L
Where n = localiser reading in
L= Cable route length
Case2 Conductor to conductor with sound core available
Fault distance from front end X= n100 x L
Where n = localiser reading in
L= Cable route length
Case 3 Three phase fault
By Open loop test and close loop test
(i) Rf2Rf1 gt 5 and (ii) Rf2 lt 20 k-ohm
Open loop test
Case 3 Three phase fault
Close loop test
Open loop and close loop test (cont)
Fault distance from front end
Lxm
mn
X 2
1001
)200200
(
Where m = open loop bridge reading
n = close loop bridge reading
L = cable route length
High Tension Bridge
Conditions required
i) No break in continuity if possible one sound core available
ii) The fault resistance of cable should be more then 20 k-ohm
iii) Applied voltage not to exceed 20kV (DC)
High Tension Bridge
Case 1Single Conductor to earth with sound core available
Fault Distance from test end X = n100 x 2L
High Tension Bridge
Case 2Conductor to conductor with sound core available
Fault Distance from test end X = n100 x 2L
High Tension Bridge
Case 3Three phase Fault with no sound core available
Rf2Rf1gt5 Rf2gt20 kohm
Lxm
)mn
(X 2
1002
1
100100
Where m = open loop bridge reading
n = close loop bridge reading
L = cable route length
6912 Universal Bridge to measure capacitance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be more then 100 ohm
iii) Earth all the conductors not under test
6912 Universal Bridge to measure capacitance of Cable
Open circuit with sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with sound core available
Fault Distance from test end x = CA x L CAB
CA = Capacitance of open circuited coreCAB = Capacitance of sound coreL = conductor length
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
Fault Distance from test endfront end = ((CA(CA+CB))x L
6913 Universal Bridge to measure Inductance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be less then 30 ohm
iii) All conductors be left free from earth
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
Fault distance X = LA x L LAB
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
Fault distance X = LA x L LA + LB
692 Pulse Echo (PE) Time Domain Reflectometry (TDR)
A simple method which works on travelling wave principles It is applicable to all series and shunt faults with Rf gt=Zo10 (5 ohm) and Rflt=10 Zo (500 ohm) respectively For power cables typical Zo=50 ohm
6921 Travelling wave principles
A pulse injected into a cable is reflected back to the source by any change in characteristics impedance (Zo) of the cable
The waveform generated can be monitored using an oscilloscope as shown in Figure below
6921 Continue-
Waveform interpretion
Shunt Fault
Series Fault
6921 Continue-
Waveform interpretion
Low Resistance Shunt Fault (Rflt=500 Ohm)
-Reflection Negative
-Amplitude depends on fault resistance
Series Fault (Rfgt=5 ohm)
- Reflection Positive
- Amplitude depends on fault resistance
Zo = Characteristics impedance = radic(LC)
L = Inductance of cable C = Capacitance of Cable
69211 Reflection Factor (r)
- Is to determine the degree of reflection
Shunt fault r (V) = -Zo1(2Rf + Zo1)
Series Fault r (V) = Rf(2Zo1 + Rf)
Example shunt fault fault resistance = 5 ohm Zo = 50 ohm
r(V) = -50 (2 x5 +50) = - 833If fault resistance = 500 ohm
r(V) = -50 (2x 500 + 50) = -475
69212 Velocity of Propagation (Vp)
- Vp depends on the cable dielectric and is defined as
Vp = Vs radicEr
- where Vs = Velocity of light in free space
Er = Relative permittivity of dielectric
Vp = 300radicEr
Dielectrics Velocity of PropagationVp (mmicroS)
Impregnated Paper 150 -171
PVC 152-175
PE Approx 200
XLPE 156-176
Typical Vp for different Dielectric
69212 Continue (Vp)
Example
Determine the velocity of propagation Vp2 for a cable length of 2500 meters t1= 3164 S and t2 = 1745 S Calculate the fault distances
6921 Continue-
The distance to the fault Lf (m) is given by
Lf = T x Vp2
6921 Continue
Solutions
Velocity of propagation = Vp2 = 25003164 = 79 mS
Distance of Fault from test end = Vp2 x t2
= 79 x 1745
= 137855
693 Impulse Current Method
Working on Travelling wave principles it is applicable to all types of fault
694 Arc Reflection or Secondary Impulse method
Basically PE and TDR associated with fault treatment It is also applicable to faults of all nature but with easily interpreted breakdown waveforms
610 PIN POINTING
Pin Pointing is the application of a test that positively confirms the exact position of the fault
Before the commencement of pin pointing the prelocated fault distance should mark on the cable route which is measured by means of a trumeter
Pin pointing is normally carried out by the shock wave discharge method The fault can be detected by the use of semisphone
611 Confirmation and Re-Test
After the pin-pointed position of the fault has been marked and exposed check for physical sign of fault If there is the fault is confirm
After confirmation the fault should be cut away IR and continuity test should be carried out on the remaining cable sections to determine the soundeness of these cable sections
The insulation resistance test are again carried out after jointing followed by pressure test before supply can be restored
Flowchart for Fault location Procedure
Example
Find a fault distance using direct reading fault localiser from end test which has a test data as below
Bridge Balance Reading =60 Length of Cable 140 meters
Solutions
Fault Distance= = 60100 x 140 = 84 meterxL
n
100
For cables route with combination of 2 or more different type of cable
Determine the resistance of cable for each section separately from Table 1 and Table 2
Example Cable 11kV Al Length 150 m size = 16mm2
From Table 1= Resistance for 1km =226 ohm
For 150 m = 1501000 x 226 = 0339 ohm
From Table 1 Calculate the equivalent resistance for all the combined section
Example for Al = 0339 ohm for copper = 0150 ohm
Equivalent Resistance = 0339+0150 =0489 ohm
Continue
Calculate equivalent resistance from obtained bridge reading
Example Bridge Reading = 60
Equivalent resistance = 06 x 0489 ohm = 02934 ohm
Determine which section of cable the fault occurs
Total resistance at Al section = 0339 ohm
As equivalent resistance at Al section is higher then 02934(Equivalent resistance from bridge test) the fault is
located at Aluminium section of cable
Table 1 Impedance data for 635011000 V Cable Aluminium
Table II Impedance data 11000 V Cable Copper
TABLE VI ndash 6001000V (ALUMINIUM)
TABLE VII ndash 38006600V (ALUMINIUM)
TABLE VIII- 635011000 (ALUMINIUM)
TABLE V
610 COMMISSIONING OF UNDERGROUND CABLE
High Voltage DC testing
High voltage testing is carried out in order to determine the electrical strength of cable insulation Site tests are performed by applying a predetermined high voltage to the insulation DC site test voltages regardless of insulation type are used to ensure that the cable cable joints and termination are correctly made and installed
High Voltage Testing (cont-)
IEC Standards specify maintenance tests after installation should be 70 of permissible factory test DC voltages (15 minutes) = 175kV
Insulation Resistance Test (cont-)
Test voltages should be slowly raised to the required value over a period of about 1 minute and the test period starts once the full voltage is reached In this way the capacitive and absorption currents will have decreased and circuit conditions stabilized such that true leakage current may be measured
671 Analysis of Cable Fault
To analyze a cable fault is to determine and confirm the nature or characteristics of the fault
Objective to select which of the cable fault locator equipment of method is best suited to locate the particular fault
Analysis of cable fault normally consists of insulation resistance test and continuity test carried out using 1000V or 5000 V insulation tester
671 Insulation Resistance test
With all earth connections removed from the cable conductor terminal using IR tester of 100025005000V range measure and record the IR in M-ohms K-ohms or ohms between conductors and between conductors and earth Six measurement are to be taken R-E Y-E B-E R-Y Y-B B-R An IR of 100M-ohm indicates a healthy cable
672 Continuity Test
With the cable conductor looped or shorted at the tail endremote end using IR tester selected at continuity range perform continuity test
This test will determine whether any of the cable is open circuited
672 Continuity test (cont-)
Continuity Test - With the cable conductor shorted or looped at the remote end perform continuity test on the cable
- Measure and record the results in ohm
- Three measurements are to be carried out between R-Y Y-B B-R
- The test will determine whether any of cable is open circuited - The resistance per-conductor per km is provided in Table VI VII VIII and IX (refer appendix A)
672 (cont-)
if the continuity of the cable is sound insulation resistance from one end are sufficient
If continuity is broken IR test should be carried out at both ends of the cable
68 BURNING A FAULT
The continuity and IR test may indicate that burning of fault by means of HT pressure test set is required
-
68 BURNING A FAULT (continue-)
Burning a fault is achieve by passing current from a DC HT test set through the fault
Other conductors not under test should be earthed
HT is applied for about 5 to 10 minutes to burn the fault
HT test is used to determine which fault location equipment is suitable to be used
HT is the last resort often used because it sometimes produce ambiguous and unpredictable results
Therefore fault location equipment should be attempted first
69 Pre- Location of fault
Pre-location is the application of a test at the terminals of a
given cable to give an indication of the distance to the fault
from the test point
Whilst the measurement should be accurate as condition will
allow the primary purpose of pre-location is to give an
indication as quick as possible of the vicinity in which to
commence the final pin-pointing tests
69 Pre- Location of fault (cont)-
Generally there are four pre-location methods which are practised and the are as follows
1Bridge or loop Method2 Pulse Echo Time Domain Reflectometry (TDR) method3 Impulse Current Method4 Arc Reflection and secondary Impulse method
691 Bridge method
6911 Direct reading fault localizer
Conditions required
i) No break in continuity if possible one sound core available
ii) The fault resistance of cable should not be more then 20 k-ohm
iii) Applied voltage not to exceed 600 V (DC)
Case 1 Single conductor to earth with sound core available
Fault distance from front end X= n100 x L
Where n = localiser reading in
L= Cable route length
Case2 Conductor to conductor with sound core available
Fault distance from front end X= n100 x L
Where n = localiser reading in
L= Cable route length
Case 3 Three phase fault
By Open loop test and close loop test
(i) Rf2Rf1 gt 5 and (ii) Rf2 lt 20 k-ohm
Open loop test
Case 3 Three phase fault
Close loop test
Open loop and close loop test (cont)
Fault distance from front end
Lxm
mn
X 2
1001
)200200
(
Where m = open loop bridge reading
n = close loop bridge reading
L = cable route length
High Tension Bridge
Conditions required
i) No break in continuity if possible one sound core available
ii) The fault resistance of cable should be more then 20 k-ohm
iii) Applied voltage not to exceed 20kV (DC)
High Tension Bridge
Case 1Single Conductor to earth with sound core available
Fault Distance from test end X = n100 x 2L
High Tension Bridge
Case 2Conductor to conductor with sound core available
Fault Distance from test end X = n100 x 2L
High Tension Bridge
Case 3Three phase Fault with no sound core available
Rf2Rf1gt5 Rf2gt20 kohm
Lxm
)mn
(X 2
1002
1
100100
Where m = open loop bridge reading
n = close loop bridge reading
L = cable route length
6912 Universal Bridge to measure capacitance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be more then 100 ohm
iii) Earth all the conductors not under test
6912 Universal Bridge to measure capacitance of Cable
Open circuit with sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with sound core available
Fault Distance from test end x = CA x L CAB
CA = Capacitance of open circuited coreCAB = Capacitance of sound coreL = conductor length
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
Fault Distance from test endfront end = ((CA(CA+CB))x L
6913 Universal Bridge to measure Inductance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be less then 30 ohm
iii) All conductors be left free from earth
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
Fault distance X = LA x L LAB
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
Fault distance X = LA x L LA + LB
692 Pulse Echo (PE) Time Domain Reflectometry (TDR)
A simple method which works on travelling wave principles It is applicable to all series and shunt faults with Rf gt=Zo10 (5 ohm) and Rflt=10 Zo (500 ohm) respectively For power cables typical Zo=50 ohm
6921 Travelling wave principles
A pulse injected into a cable is reflected back to the source by any change in characteristics impedance (Zo) of the cable
The waveform generated can be monitored using an oscilloscope as shown in Figure below
6921 Continue-
Waveform interpretion
Shunt Fault
Series Fault
6921 Continue-
Waveform interpretion
Low Resistance Shunt Fault (Rflt=500 Ohm)
-Reflection Negative
-Amplitude depends on fault resistance
Series Fault (Rfgt=5 ohm)
- Reflection Positive
- Amplitude depends on fault resistance
Zo = Characteristics impedance = radic(LC)
L = Inductance of cable C = Capacitance of Cable
69211 Reflection Factor (r)
- Is to determine the degree of reflection
Shunt fault r (V) = -Zo1(2Rf + Zo1)
Series Fault r (V) = Rf(2Zo1 + Rf)
Example shunt fault fault resistance = 5 ohm Zo = 50 ohm
r(V) = -50 (2 x5 +50) = - 833If fault resistance = 500 ohm
r(V) = -50 (2x 500 + 50) = -475
69212 Velocity of Propagation (Vp)
- Vp depends on the cable dielectric and is defined as
Vp = Vs radicEr
- where Vs = Velocity of light in free space
Er = Relative permittivity of dielectric
Vp = 300radicEr
Dielectrics Velocity of PropagationVp (mmicroS)
Impregnated Paper 150 -171
PVC 152-175
PE Approx 200
XLPE 156-176
Typical Vp for different Dielectric
69212 Continue (Vp)
Example
Determine the velocity of propagation Vp2 for a cable length of 2500 meters t1= 3164 S and t2 = 1745 S Calculate the fault distances
6921 Continue-
The distance to the fault Lf (m) is given by
Lf = T x Vp2
6921 Continue
Solutions
Velocity of propagation = Vp2 = 25003164 = 79 mS
Distance of Fault from test end = Vp2 x t2
= 79 x 1745
= 137855
693 Impulse Current Method
Working on Travelling wave principles it is applicable to all types of fault
694 Arc Reflection or Secondary Impulse method
Basically PE and TDR associated with fault treatment It is also applicable to faults of all nature but with easily interpreted breakdown waveforms
610 PIN POINTING
Pin Pointing is the application of a test that positively confirms the exact position of the fault
Before the commencement of pin pointing the prelocated fault distance should mark on the cable route which is measured by means of a trumeter
Pin pointing is normally carried out by the shock wave discharge method The fault can be detected by the use of semisphone
611 Confirmation and Re-Test
After the pin-pointed position of the fault has been marked and exposed check for physical sign of fault If there is the fault is confirm
After confirmation the fault should be cut away IR and continuity test should be carried out on the remaining cable sections to determine the soundeness of these cable sections
The insulation resistance test are again carried out after jointing followed by pressure test before supply can be restored
Flowchart for Fault location Procedure
Example
Find a fault distance using direct reading fault localiser from end test which has a test data as below
Bridge Balance Reading =60 Length of Cable 140 meters
Solutions
Fault Distance= = 60100 x 140 = 84 meterxL
n
100
For cables route with combination of 2 or more different type of cable
Determine the resistance of cable for each section separately from Table 1 and Table 2
Example Cable 11kV Al Length 150 m size = 16mm2
From Table 1= Resistance for 1km =226 ohm
For 150 m = 1501000 x 226 = 0339 ohm
From Table 1 Calculate the equivalent resistance for all the combined section
Example for Al = 0339 ohm for copper = 0150 ohm
Equivalent Resistance = 0339+0150 =0489 ohm
Continue
Calculate equivalent resistance from obtained bridge reading
Example Bridge Reading = 60
Equivalent resistance = 06 x 0489 ohm = 02934 ohm
Determine which section of cable the fault occurs
Total resistance at Al section = 0339 ohm
As equivalent resistance at Al section is higher then 02934(Equivalent resistance from bridge test) the fault is
located at Aluminium section of cable
Table 1 Impedance data for 635011000 V Cable Aluminium
Table II Impedance data 11000 V Cable Copper
TABLE VI ndash 6001000V (ALUMINIUM)
TABLE VII ndash 38006600V (ALUMINIUM)
TABLE VIII- 635011000 (ALUMINIUM)
TABLE V
610 COMMISSIONING OF UNDERGROUND CABLE
High Voltage DC testing
High voltage testing is carried out in order to determine the electrical strength of cable insulation Site tests are performed by applying a predetermined high voltage to the insulation DC site test voltages regardless of insulation type are used to ensure that the cable cable joints and termination are correctly made and installed
High Voltage Testing (cont-)
IEC Standards specify maintenance tests after installation should be 70 of permissible factory test DC voltages (15 minutes) = 175kV
Insulation Resistance Test (cont-)
Test voltages should be slowly raised to the required value over a period of about 1 minute and the test period starts once the full voltage is reached In this way the capacitive and absorption currents will have decreased and circuit conditions stabilized such that true leakage current may be measured
671 Insulation Resistance test
With all earth connections removed from the cable conductor terminal using IR tester of 100025005000V range measure and record the IR in M-ohms K-ohms or ohms between conductors and between conductors and earth Six measurement are to be taken R-E Y-E B-E R-Y Y-B B-R An IR of 100M-ohm indicates a healthy cable
672 Continuity Test
With the cable conductor looped or shorted at the tail endremote end using IR tester selected at continuity range perform continuity test
This test will determine whether any of the cable is open circuited
672 Continuity test (cont-)
Continuity Test - With the cable conductor shorted or looped at the remote end perform continuity test on the cable
- Measure and record the results in ohm
- Three measurements are to be carried out between R-Y Y-B B-R
- The test will determine whether any of cable is open circuited - The resistance per-conductor per km is provided in Table VI VII VIII and IX (refer appendix A)
672 (cont-)
if the continuity of the cable is sound insulation resistance from one end are sufficient
If continuity is broken IR test should be carried out at both ends of the cable
68 BURNING A FAULT
The continuity and IR test may indicate that burning of fault by means of HT pressure test set is required
-
68 BURNING A FAULT (continue-)
Burning a fault is achieve by passing current from a DC HT test set through the fault
Other conductors not under test should be earthed
HT is applied for about 5 to 10 minutes to burn the fault
HT test is used to determine which fault location equipment is suitable to be used
HT is the last resort often used because it sometimes produce ambiguous and unpredictable results
Therefore fault location equipment should be attempted first
69 Pre- Location of fault
Pre-location is the application of a test at the terminals of a
given cable to give an indication of the distance to the fault
from the test point
Whilst the measurement should be accurate as condition will
allow the primary purpose of pre-location is to give an
indication as quick as possible of the vicinity in which to
commence the final pin-pointing tests
69 Pre- Location of fault (cont)-
Generally there are four pre-location methods which are practised and the are as follows
1Bridge or loop Method2 Pulse Echo Time Domain Reflectometry (TDR) method3 Impulse Current Method4 Arc Reflection and secondary Impulse method
691 Bridge method
6911 Direct reading fault localizer
Conditions required
i) No break in continuity if possible one sound core available
ii) The fault resistance of cable should not be more then 20 k-ohm
iii) Applied voltage not to exceed 600 V (DC)
Case 1 Single conductor to earth with sound core available
Fault distance from front end X= n100 x L
Where n = localiser reading in
L= Cable route length
Case2 Conductor to conductor with sound core available
Fault distance from front end X= n100 x L
Where n = localiser reading in
L= Cable route length
Case 3 Three phase fault
By Open loop test and close loop test
(i) Rf2Rf1 gt 5 and (ii) Rf2 lt 20 k-ohm
Open loop test
Case 3 Three phase fault
Close loop test
Open loop and close loop test (cont)
Fault distance from front end
Lxm
mn
X 2
1001
)200200
(
Where m = open loop bridge reading
n = close loop bridge reading
L = cable route length
High Tension Bridge
Conditions required
i) No break in continuity if possible one sound core available
ii) The fault resistance of cable should be more then 20 k-ohm
iii) Applied voltage not to exceed 20kV (DC)
High Tension Bridge
Case 1Single Conductor to earth with sound core available
Fault Distance from test end X = n100 x 2L
High Tension Bridge
Case 2Conductor to conductor with sound core available
Fault Distance from test end X = n100 x 2L
High Tension Bridge
Case 3Three phase Fault with no sound core available
Rf2Rf1gt5 Rf2gt20 kohm
Lxm
)mn
(X 2
1002
1
100100
Where m = open loop bridge reading
n = close loop bridge reading
L = cable route length
6912 Universal Bridge to measure capacitance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be more then 100 ohm
iii) Earth all the conductors not under test
6912 Universal Bridge to measure capacitance of Cable
Open circuit with sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with sound core available
Fault Distance from test end x = CA x L CAB
CA = Capacitance of open circuited coreCAB = Capacitance of sound coreL = conductor length
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
Fault Distance from test endfront end = ((CA(CA+CB))x L
6913 Universal Bridge to measure Inductance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be less then 30 ohm
iii) All conductors be left free from earth
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
Fault distance X = LA x L LAB
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
Fault distance X = LA x L LA + LB
692 Pulse Echo (PE) Time Domain Reflectometry (TDR)
A simple method which works on travelling wave principles It is applicable to all series and shunt faults with Rf gt=Zo10 (5 ohm) and Rflt=10 Zo (500 ohm) respectively For power cables typical Zo=50 ohm
6921 Travelling wave principles
A pulse injected into a cable is reflected back to the source by any change in characteristics impedance (Zo) of the cable
The waveform generated can be monitored using an oscilloscope as shown in Figure below
6921 Continue-
Waveform interpretion
Shunt Fault
Series Fault
6921 Continue-
Waveform interpretion
Low Resistance Shunt Fault (Rflt=500 Ohm)
-Reflection Negative
-Amplitude depends on fault resistance
Series Fault (Rfgt=5 ohm)
- Reflection Positive
- Amplitude depends on fault resistance
Zo = Characteristics impedance = radic(LC)
L = Inductance of cable C = Capacitance of Cable
69211 Reflection Factor (r)
- Is to determine the degree of reflection
Shunt fault r (V) = -Zo1(2Rf + Zo1)
Series Fault r (V) = Rf(2Zo1 + Rf)
Example shunt fault fault resistance = 5 ohm Zo = 50 ohm
r(V) = -50 (2 x5 +50) = - 833If fault resistance = 500 ohm
r(V) = -50 (2x 500 + 50) = -475
69212 Velocity of Propagation (Vp)
- Vp depends on the cable dielectric and is defined as
Vp = Vs radicEr
- where Vs = Velocity of light in free space
Er = Relative permittivity of dielectric
Vp = 300radicEr
Dielectrics Velocity of PropagationVp (mmicroS)
Impregnated Paper 150 -171
PVC 152-175
PE Approx 200
XLPE 156-176
Typical Vp for different Dielectric
69212 Continue (Vp)
Example
Determine the velocity of propagation Vp2 for a cable length of 2500 meters t1= 3164 S and t2 = 1745 S Calculate the fault distances
6921 Continue-
The distance to the fault Lf (m) is given by
Lf = T x Vp2
6921 Continue
Solutions
Velocity of propagation = Vp2 = 25003164 = 79 mS
Distance of Fault from test end = Vp2 x t2
= 79 x 1745
= 137855
693 Impulse Current Method
Working on Travelling wave principles it is applicable to all types of fault
694 Arc Reflection or Secondary Impulse method
Basically PE and TDR associated with fault treatment It is also applicable to faults of all nature but with easily interpreted breakdown waveforms
610 PIN POINTING
Pin Pointing is the application of a test that positively confirms the exact position of the fault
Before the commencement of pin pointing the prelocated fault distance should mark on the cable route which is measured by means of a trumeter
Pin pointing is normally carried out by the shock wave discharge method The fault can be detected by the use of semisphone
611 Confirmation and Re-Test
After the pin-pointed position of the fault has been marked and exposed check for physical sign of fault If there is the fault is confirm
After confirmation the fault should be cut away IR and continuity test should be carried out on the remaining cable sections to determine the soundeness of these cable sections
The insulation resistance test are again carried out after jointing followed by pressure test before supply can be restored
Flowchart for Fault location Procedure
Example
Find a fault distance using direct reading fault localiser from end test which has a test data as below
Bridge Balance Reading =60 Length of Cable 140 meters
Solutions
Fault Distance= = 60100 x 140 = 84 meterxL
n
100
For cables route with combination of 2 or more different type of cable
Determine the resistance of cable for each section separately from Table 1 and Table 2
Example Cable 11kV Al Length 150 m size = 16mm2
From Table 1= Resistance for 1km =226 ohm
For 150 m = 1501000 x 226 = 0339 ohm
From Table 1 Calculate the equivalent resistance for all the combined section
Example for Al = 0339 ohm for copper = 0150 ohm
Equivalent Resistance = 0339+0150 =0489 ohm
Continue
Calculate equivalent resistance from obtained bridge reading
Example Bridge Reading = 60
Equivalent resistance = 06 x 0489 ohm = 02934 ohm
Determine which section of cable the fault occurs
Total resistance at Al section = 0339 ohm
As equivalent resistance at Al section is higher then 02934(Equivalent resistance from bridge test) the fault is
located at Aluminium section of cable
Table 1 Impedance data for 635011000 V Cable Aluminium
Table II Impedance data 11000 V Cable Copper
TABLE VI ndash 6001000V (ALUMINIUM)
TABLE VII ndash 38006600V (ALUMINIUM)
TABLE VIII- 635011000 (ALUMINIUM)
TABLE V
610 COMMISSIONING OF UNDERGROUND CABLE
High Voltage DC testing
High voltage testing is carried out in order to determine the electrical strength of cable insulation Site tests are performed by applying a predetermined high voltage to the insulation DC site test voltages regardless of insulation type are used to ensure that the cable cable joints and termination are correctly made and installed
High Voltage Testing (cont-)
IEC Standards specify maintenance tests after installation should be 70 of permissible factory test DC voltages (15 minutes) = 175kV
Insulation Resistance Test (cont-)
Test voltages should be slowly raised to the required value over a period of about 1 minute and the test period starts once the full voltage is reached In this way the capacitive and absorption currents will have decreased and circuit conditions stabilized such that true leakage current may be measured
672 Continuity Test
With the cable conductor looped or shorted at the tail endremote end using IR tester selected at continuity range perform continuity test
This test will determine whether any of the cable is open circuited
672 Continuity test (cont-)
Continuity Test - With the cable conductor shorted or looped at the remote end perform continuity test on the cable
- Measure and record the results in ohm
- Three measurements are to be carried out between R-Y Y-B B-R
- The test will determine whether any of cable is open circuited - The resistance per-conductor per km is provided in Table VI VII VIII and IX (refer appendix A)
672 (cont-)
if the continuity of the cable is sound insulation resistance from one end are sufficient
If continuity is broken IR test should be carried out at both ends of the cable
68 BURNING A FAULT
The continuity and IR test may indicate that burning of fault by means of HT pressure test set is required
-
68 BURNING A FAULT (continue-)
Burning a fault is achieve by passing current from a DC HT test set through the fault
Other conductors not under test should be earthed
HT is applied for about 5 to 10 minutes to burn the fault
HT test is used to determine which fault location equipment is suitable to be used
HT is the last resort often used because it sometimes produce ambiguous and unpredictable results
Therefore fault location equipment should be attempted first
69 Pre- Location of fault
Pre-location is the application of a test at the terminals of a
given cable to give an indication of the distance to the fault
from the test point
Whilst the measurement should be accurate as condition will
allow the primary purpose of pre-location is to give an
indication as quick as possible of the vicinity in which to
commence the final pin-pointing tests
69 Pre- Location of fault (cont)-
Generally there are four pre-location methods which are practised and the are as follows
1Bridge or loop Method2 Pulse Echo Time Domain Reflectometry (TDR) method3 Impulse Current Method4 Arc Reflection and secondary Impulse method
691 Bridge method
6911 Direct reading fault localizer
Conditions required
i) No break in continuity if possible one sound core available
ii) The fault resistance of cable should not be more then 20 k-ohm
iii) Applied voltage not to exceed 600 V (DC)
Case 1 Single conductor to earth with sound core available
Fault distance from front end X= n100 x L
Where n = localiser reading in
L= Cable route length
Case2 Conductor to conductor with sound core available
Fault distance from front end X= n100 x L
Where n = localiser reading in
L= Cable route length
Case 3 Three phase fault
By Open loop test and close loop test
(i) Rf2Rf1 gt 5 and (ii) Rf2 lt 20 k-ohm
Open loop test
Case 3 Three phase fault
Close loop test
Open loop and close loop test (cont)
Fault distance from front end
Lxm
mn
X 2
1001
)200200
(
Where m = open loop bridge reading
n = close loop bridge reading
L = cable route length
High Tension Bridge
Conditions required
i) No break in continuity if possible one sound core available
ii) The fault resistance of cable should be more then 20 k-ohm
iii) Applied voltage not to exceed 20kV (DC)
High Tension Bridge
Case 1Single Conductor to earth with sound core available
Fault Distance from test end X = n100 x 2L
High Tension Bridge
Case 2Conductor to conductor with sound core available
Fault Distance from test end X = n100 x 2L
High Tension Bridge
Case 3Three phase Fault with no sound core available
Rf2Rf1gt5 Rf2gt20 kohm
Lxm
)mn
(X 2
1002
1
100100
Where m = open loop bridge reading
n = close loop bridge reading
L = cable route length
6912 Universal Bridge to measure capacitance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be more then 100 ohm
iii) Earth all the conductors not under test
6912 Universal Bridge to measure capacitance of Cable
Open circuit with sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with sound core available
Fault Distance from test end x = CA x L CAB
CA = Capacitance of open circuited coreCAB = Capacitance of sound coreL = conductor length
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
Fault Distance from test endfront end = ((CA(CA+CB))x L
6913 Universal Bridge to measure Inductance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be less then 30 ohm
iii) All conductors be left free from earth
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
Fault distance X = LA x L LAB
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
Fault distance X = LA x L LA + LB
692 Pulse Echo (PE) Time Domain Reflectometry (TDR)
A simple method which works on travelling wave principles It is applicable to all series and shunt faults with Rf gt=Zo10 (5 ohm) and Rflt=10 Zo (500 ohm) respectively For power cables typical Zo=50 ohm
6921 Travelling wave principles
A pulse injected into a cable is reflected back to the source by any change in characteristics impedance (Zo) of the cable
The waveform generated can be monitored using an oscilloscope as shown in Figure below
6921 Continue-
Waveform interpretion
Shunt Fault
Series Fault
6921 Continue-
Waveform interpretion
Low Resistance Shunt Fault (Rflt=500 Ohm)
-Reflection Negative
-Amplitude depends on fault resistance
Series Fault (Rfgt=5 ohm)
- Reflection Positive
- Amplitude depends on fault resistance
Zo = Characteristics impedance = radic(LC)
L = Inductance of cable C = Capacitance of Cable
69211 Reflection Factor (r)
- Is to determine the degree of reflection
Shunt fault r (V) = -Zo1(2Rf + Zo1)
Series Fault r (V) = Rf(2Zo1 + Rf)
Example shunt fault fault resistance = 5 ohm Zo = 50 ohm
r(V) = -50 (2 x5 +50) = - 833If fault resistance = 500 ohm
r(V) = -50 (2x 500 + 50) = -475
69212 Velocity of Propagation (Vp)
- Vp depends on the cable dielectric and is defined as
Vp = Vs radicEr
- where Vs = Velocity of light in free space
Er = Relative permittivity of dielectric
Vp = 300radicEr
Dielectrics Velocity of PropagationVp (mmicroS)
Impregnated Paper 150 -171
PVC 152-175
PE Approx 200
XLPE 156-176
Typical Vp for different Dielectric
69212 Continue (Vp)
Example
Determine the velocity of propagation Vp2 for a cable length of 2500 meters t1= 3164 S and t2 = 1745 S Calculate the fault distances
6921 Continue-
The distance to the fault Lf (m) is given by
Lf = T x Vp2
6921 Continue
Solutions
Velocity of propagation = Vp2 = 25003164 = 79 mS
Distance of Fault from test end = Vp2 x t2
= 79 x 1745
= 137855
693 Impulse Current Method
Working on Travelling wave principles it is applicable to all types of fault
694 Arc Reflection or Secondary Impulse method
Basically PE and TDR associated with fault treatment It is also applicable to faults of all nature but with easily interpreted breakdown waveforms
610 PIN POINTING
Pin Pointing is the application of a test that positively confirms the exact position of the fault
Before the commencement of pin pointing the prelocated fault distance should mark on the cable route which is measured by means of a trumeter
Pin pointing is normally carried out by the shock wave discharge method The fault can be detected by the use of semisphone
611 Confirmation and Re-Test
After the pin-pointed position of the fault has been marked and exposed check for physical sign of fault If there is the fault is confirm
After confirmation the fault should be cut away IR and continuity test should be carried out on the remaining cable sections to determine the soundeness of these cable sections
The insulation resistance test are again carried out after jointing followed by pressure test before supply can be restored
Flowchart for Fault location Procedure
Example
Find a fault distance using direct reading fault localiser from end test which has a test data as below
Bridge Balance Reading =60 Length of Cable 140 meters
Solutions
Fault Distance= = 60100 x 140 = 84 meterxL
n
100
For cables route with combination of 2 or more different type of cable
Determine the resistance of cable for each section separately from Table 1 and Table 2
Example Cable 11kV Al Length 150 m size = 16mm2
From Table 1= Resistance for 1km =226 ohm
For 150 m = 1501000 x 226 = 0339 ohm
From Table 1 Calculate the equivalent resistance for all the combined section
Example for Al = 0339 ohm for copper = 0150 ohm
Equivalent Resistance = 0339+0150 =0489 ohm
Continue
Calculate equivalent resistance from obtained bridge reading
Example Bridge Reading = 60
Equivalent resistance = 06 x 0489 ohm = 02934 ohm
Determine which section of cable the fault occurs
Total resistance at Al section = 0339 ohm
As equivalent resistance at Al section is higher then 02934(Equivalent resistance from bridge test) the fault is
located at Aluminium section of cable
Table 1 Impedance data for 635011000 V Cable Aluminium
Table II Impedance data 11000 V Cable Copper
TABLE VI ndash 6001000V (ALUMINIUM)
TABLE VII ndash 38006600V (ALUMINIUM)
TABLE VIII- 635011000 (ALUMINIUM)
TABLE V
610 COMMISSIONING OF UNDERGROUND CABLE
High Voltage DC testing
High voltage testing is carried out in order to determine the electrical strength of cable insulation Site tests are performed by applying a predetermined high voltage to the insulation DC site test voltages regardless of insulation type are used to ensure that the cable cable joints and termination are correctly made and installed
High Voltage Testing (cont-)
IEC Standards specify maintenance tests after installation should be 70 of permissible factory test DC voltages (15 minutes) = 175kV
Insulation Resistance Test (cont-)
Test voltages should be slowly raised to the required value over a period of about 1 minute and the test period starts once the full voltage is reached In this way the capacitive and absorption currents will have decreased and circuit conditions stabilized such that true leakage current may be measured
672 Continuity test (cont-)
Continuity Test - With the cable conductor shorted or looped at the remote end perform continuity test on the cable
- Measure and record the results in ohm
- Three measurements are to be carried out between R-Y Y-B B-R
- The test will determine whether any of cable is open circuited - The resistance per-conductor per km is provided in Table VI VII VIII and IX (refer appendix A)
672 (cont-)
if the continuity of the cable is sound insulation resistance from one end are sufficient
If continuity is broken IR test should be carried out at both ends of the cable
68 BURNING A FAULT
The continuity and IR test may indicate that burning of fault by means of HT pressure test set is required
-
68 BURNING A FAULT (continue-)
Burning a fault is achieve by passing current from a DC HT test set through the fault
Other conductors not under test should be earthed
HT is applied for about 5 to 10 minutes to burn the fault
HT test is used to determine which fault location equipment is suitable to be used
HT is the last resort often used because it sometimes produce ambiguous and unpredictable results
Therefore fault location equipment should be attempted first
69 Pre- Location of fault
Pre-location is the application of a test at the terminals of a
given cable to give an indication of the distance to the fault
from the test point
Whilst the measurement should be accurate as condition will
allow the primary purpose of pre-location is to give an
indication as quick as possible of the vicinity in which to
commence the final pin-pointing tests
69 Pre- Location of fault (cont)-
Generally there are four pre-location methods which are practised and the are as follows
1Bridge or loop Method2 Pulse Echo Time Domain Reflectometry (TDR) method3 Impulse Current Method4 Arc Reflection and secondary Impulse method
691 Bridge method
6911 Direct reading fault localizer
Conditions required
i) No break in continuity if possible one sound core available
ii) The fault resistance of cable should not be more then 20 k-ohm
iii) Applied voltage not to exceed 600 V (DC)
Case 1 Single conductor to earth with sound core available
Fault distance from front end X= n100 x L
Where n = localiser reading in
L= Cable route length
Case2 Conductor to conductor with sound core available
Fault distance from front end X= n100 x L
Where n = localiser reading in
L= Cable route length
Case 3 Three phase fault
By Open loop test and close loop test
(i) Rf2Rf1 gt 5 and (ii) Rf2 lt 20 k-ohm
Open loop test
Case 3 Three phase fault
Close loop test
Open loop and close loop test (cont)
Fault distance from front end
Lxm
mn
X 2
1001
)200200
(
Where m = open loop bridge reading
n = close loop bridge reading
L = cable route length
High Tension Bridge
Conditions required
i) No break in continuity if possible one sound core available
ii) The fault resistance of cable should be more then 20 k-ohm
iii) Applied voltage not to exceed 20kV (DC)
High Tension Bridge
Case 1Single Conductor to earth with sound core available
Fault Distance from test end X = n100 x 2L
High Tension Bridge
Case 2Conductor to conductor with sound core available
Fault Distance from test end X = n100 x 2L
High Tension Bridge
Case 3Three phase Fault with no sound core available
Rf2Rf1gt5 Rf2gt20 kohm
Lxm
)mn
(X 2
1002
1
100100
Where m = open loop bridge reading
n = close loop bridge reading
L = cable route length
6912 Universal Bridge to measure capacitance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be more then 100 ohm
iii) Earth all the conductors not under test
6912 Universal Bridge to measure capacitance of Cable
Open circuit with sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with sound core available
Fault Distance from test end x = CA x L CAB
CA = Capacitance of open circuited coreCAB = Capacitance of sound coreL = conductor length
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
Fault Distance from test endfront end = ((CA(CA+CB))x L
6913 Universal Bridge to measure Inductance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be less then 30 ohm
iii) All conductors be left free from earth
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
Fault distance X = LA x L LAB
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
Fault distance X = LA x L LA + LB
692 Pulse Echo (PE) Time Domain Reflectometry (TDR)
A simple method which works on travelling wave principles It is applicable to all series and shunt faults with Rf gt=Zo10 (5 ohm) and Rflt=10 Zo (500 ohm) respectively For power cables typical Zo=50 ohm
6921 Travelling wave principles
A pulse injected into a cable is reflected back to the source by any change in characteristics impedance (Zo) of the cable
The waveform generated can be monitored using an oscilloscope as shown in Figure below
6921 Continue-
Waveform interpretion
Shunt Fault
Series Fault
6921 Continue-
Waveform interpretion
Low Resistance Shunt Fault (Rflt=500 Ohm)
-Reflection Negative
-Amplitude depends on fault resistance
Series Fault (Rfgt=5 ohm)
- Reflection Positive
- Amplitude depends on fault resistance
Zo = Characteristics impedance = radic(LC)
L = Inductance of cable C = Capacitance of Cable
69211 Reflection Factor (r)
- Is to determine the degree of reflection
Shunt fault r (V) = -Zo1(2Rf + Zo1)
Series Fault r (V) = Rf(2Zo1 + Rf)
Example shunt fault fault resistance = 5 ohm Zo = 50 ohm
r(V) = -50 (2 x5 +50) = - 833If fault resistance = 500 ohm
r(V) = -50 (2x 500 + 50) = -475
69212 Velocity of Propagation (Vp)
- Vp depends on the cable dielectric and is defined as
Vp = Vs radicEr
- where Vs = Velocity of light in free space
Er = Relative permittivity of dielectric
Vp = 300radicEr
Dielectrics Velocity of PropagationVp (mmicroS)
Impregnated Paper 150 -171
PVC 152-175
PE Approx 200
XLPE 156-176
Typical Vp for different Dielectric
69212 Continue (Vp)
Example
Determine the velocity of propagation Vp2 for a cable length of 2500 meters t1= 3164 S and t2 = 1745 S Calculate the fault distances
6921 Continue-
The distance to the fault Lf (m) is given by
Lf = T x Vp2
6921 Continue
Solutions
Velocity of propagation = Vp2 = 25003164 = 79 mS
Distance of Fault from test end = Vp2 x t2
= 79 x 1745
= 137855
693 Impulse Current Method
Working on Travelling wave principles it is applicable to all types of fault
694 Arc Reflection or Secondary Impulse method
Basically PE and TDR associated with fault treatment It is also applicable to faults of all nature but with easily interpreted breakdown waveforms
610 PIN POINTING
Pin Pointing is the application of a test that positively confirms the exact position of the fault
Before the commencement of pin pointing the prelocated fault distance should mark on the cable route which is measured by means of a trumeter
Pin pointing is normally carried out by the shock wave discharge method The fault can be detected by the use of semisphone
611 Confirmation and Re-Test
After the pin-pointed position of the fault has been marked and exposed check for physical sign of fault If there is the fault is confirm
After confirmation the fault should be cut away IR and continuity test should be carried out on the remaining cable sections to determine the soundeness of these cable sections
The insulation resistance test are again carried out after jointing followed by pressure test before supply can be restored
Flowchart for Fault location Procedure
Example
Find a fault distance using direct reading fault localiser from end test which has a test data as below
Bridge Balance Reading =60 Length of Cable 140 meters
Solutions
Fault Distance= = 60100 x 140 = 84 meterxL
n
100
For cables route with combination of 2 or more different type of cable
Determine the resistance of cable for each section separately from Table 1 and Table 2
Example Cable 11kV Al Length 150 m size = 16mm2
From Table 1= Resistance for 1km =226 ohm
For 150 m = 1501000 x 226 = 0339 ohm
From Table 1 Calculate the equivalent resistance for all the combined section
Example for Al = 0339 ohm for copper = 0150 ohm
Equivalent Resistance = 0339+0150 =0489 ohm
Continue
Calculate equivalent resistance from obtained bridge reading
Example Bridge Reading = 60
Equivalent resistance = 06 x 0489 ohm = 02934 ohm
Determine which section of cable the fault occurs
Total resistance at Al section = 0339 ohm
As equivalent resistance at Al section is higher then 02934(Equivalent resistance from bridge test) the fault is
located at Aluminium section of cable
Table 1 Impedance data for 635011000 V Cable Aluminium
Table II Impedance data 11000 V Cable Copper
TABLE VI ndash 6001000V (ALUMINIUM)
TABLE VII ndash 38006600V (ALUMINIUM)
TABLE VIII- 635011000 (ALUMINIUM)
TABLE V
610 COMMISSIONING OF UNDERGROUND CABLE
High Voltage DC testing
High voltage testing is carried out in order to determine the electrical strength of cable insulation Site tests are performed by applying a predetermined high voltage to the insulation DC site test voltages regardless of insulation type are used to ensure that the cable cable joints and termination are correctly made and installed
High Voltage Testing (cont-)
IEC Standards specify maintenance tests after installation should be 70 of permissible factory test DC voltages (15 minutes) = 175kV
Insulation Resistance Test (cont-)
Test voltages should be slowly raised to the required value over a period of about 1 minute and the test period starts once the full voltage is reached In this way the capacitive and absorption currents will have decreased and circuit conditions stabilized such that true leakage current may be measured
672 (cont-)
if the continuity of the cable is sound insulation resistance from one end are sufficient
If continuity is broken IR test should be carried out at both ends of the cable
68 BURNING A FAULT
The continuity and IR test may indicate that burning of fault by means of HT pressure test set is required
-
68 BURNING A FAULT (continue-)
Burning a fault is achieve by passing current from a DC HT test set through the fault
Other conductors not under test should be earthed
HT is applied for about 5 to 10 minutes to burn the fault
HT test is used to determine which fault location equipment is suitable to be used
HT is the last resort often used because it sometimes produce ambiguous and unpredictable results
Therefore fault location equipment should be attempted first
69 Pre- Location of fault
Pre-location is the application of a test at the terminals of a
given cable to give an indication of the distance to the fault
from the test point
Whilst the measurement should be accurate as condition will
allow the primary purpose of pre-location is to give an
indication as quick as possible of the vicinity in which to
commence the final pin-pointing tests
69 Pre- Location of fault (cont)-
Generally there are four pre-location methods which are practised and the are as follows
1Bridge or loop Method2 Pulse Echo Time Domain Reflectometry (TDR) method3 Impulse Current Method4 Arc Reflection and secondary Impulse method
691 Bridge method
6911 Direct reading fault localizer
Conditions required
i) No break in continuity if possible one sound core available
ii) The fault resistance of cable should not be more then 20 k-ohm
iii) Applied voltage not to exceed 600 V (DC)
Case 1 Single conductor to earth with sound core available
Fault distance from front end X= n100 x L
Where n = localiser reading in
L= Cable route length
Case2 Conductor to conductor with sound core available
Fault distance from front end X= n100 x L
Where n = localiser reading in
L= Cable route length
Case 3 Three phase fault
By Open loop test and close loop test
(i) Rf2Rf1 gt 5 and (ii) Rf2 lt 20 k-ohm
Open loop test
Case 3 Three phase fault
Close loop test
Open loop and close loop test (cont)
Fault distance from front end
Lxm
mn
X 2
1001
)200200
(
Where m = open loop bridge reading
n = close loop bridge reading
L = cable route length
High Tension Bridge
Conditions required
i) No break in continuity if possible one sound core available
ii) The fault resistance of cable should be more then 20 k-ohm
iii) Applied voltage not to exceed 20kV (DC)
High Tension Bridge
Case 1Single Conductor to earth with sound core available
Fault Distance from test end X = n100 x 2L
High Tension Bridge
Case 2Conductor to conductor with sound core available
Fault Distance from test end X = n100 x 2L
High Tension Bridge
Case 3Three phase Fault with no sound core available
Rf2Rf1gt5 Rf2gt20 kohm
Lxm
)mn
(X 2
1002
1
100100
Where m = open loop bridge reading
n = close loop bridge reading
L = cable route length
6912 Universal Bridge to measure capacitance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be more then 100 ohm
iii) Earth all the conductors not under test
6912 Universal Bridge to measure capacitance of Cable
Open circuit with sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with sound core available
Fault Distance from test end x = CA x L CAB
CA = Capacitance of open circuited coreCAB = Capacitance of sound coreL = conductor length
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
Fault Distance from test endfront end = ((CA(CA+CB))x L
6913 Universal Bridge to measure Inductance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be less then 30 ohm
iii) All conductors be left free from earth
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
Fault distance X = LA x L LAB
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
Fault distance X = LA x L LA + LB
692 Pulse Echo (PE) Time Domain Reflectometry (TDR)
A simple method which works on travelling wave principles It is applicable to all series and shunt faults with Rf gt=Zo10 (5 ohm) and Rflt=10 Zo (500 ohm) respectively For power cables typical Zo=50 ohm
6921 Travelling wave principles
A pulse injected into a cable is reflected back to the source by any change in characteristics impedance (Zo) of the cable
The waveform generated can be monitored using an oscilloscope as shown in Figure below
6921 Continue-
Waveform interpretion
Shunt Fault
Series Fault
6921 Continue-
Waveform interpretion
Low Resistance Shunt Fault (Rflt=500 Ohm)
-Reflection Negative
-Amplitude depends on fault resistance
Series Fault (Rfgt=5 ohm)
- Reflection Positive
- Amplitude depends on fault resistance
Zo = Characteristics impedance = radic(LC)
L = Inductance of cable C = Capacitance of Cable
69211 Reflection Factor (r)
- Is to determine the degree of reflection
Shunt fault r (V) = -Zo1(2Rf + Zo1)
Series Fault r (V) = Rf(2Zo1 + Rf)
Example shunt fault fault resistance = 5 ohm Zo = 50 ohm
r(V) = -50 (2 x5 +50) = - 833If fault resistance = 500 ohm
r(V) = -50 (2x 500 + 50) = -475
69212 Velocity of Propagation (Vp)
- Vp depends on the cable dielectric and is defined as
Vp = Vs radicEr
- where Vs = Velocity of light in free space
Er = Relative permittivity of dielectric
Vp = 300radicEr
Dielectrics Velocity of PropagationVp (mmicroS)
Impregnated Paper 150 -171
PVC 152-175
PE Approx 200
XLPE 156-176
Typical Vp for different Dielectric
69212 Continue (Vp)
Example
Determine the velocity of propagation Vp2 for a cable length of 2500 meters t1= 3164 S and t2 = 1745 S Calculate the fault distances
6921 Continue-
The distance to the fault Lf (m) is given by
Lf = T x Vp2
6921 Continue
Solutions
Velocity of propagation = Vp2 = 25003164 = 79 mS
Distance of Fault from test end = Vp2 x t2
= 79 x 1745
= 137855
693 Impulse Current Method
Working on Travelling wave principles it is applicable to all types of fault
694 Arc Reflection or Secondary Impulse method
Basically PE and TDR associated with fault treatment It is also applicable to faults of all nature but with easily interpreted breakdown waveforms
610 PIN POINTING
Pin Pointing is the application of a test that positively confirms the exact position of the fault
Before the commencement of pin pointing the prelocated fault distance should mark on the cable route which is measured by means of a trumeter
Pin pointing is normally carried out by the shock wave discharge method The fault can be detected by the use of semisphone
611 Confirmation and Re-Test
After the pin-pointed position of the fault has been marked and exposed check for physical sign of fault If there is the fault is confirm
After confirmation the fault should be cut away IR and continuity test should be carried out on the remaining cable sections to determine the soundeness of these cable sections
The insulation resistance test are again carried out after jointing followed by pressure test before supply can be restored
Flowchart for Fault location Procedure
Example
Find a fault distance using direct reading fault localiser from end test which has a test data as below
Bridge Balance Reading =60 Length of Cable 140 meters
Solutions
Fault Distance= = 60100 x 140 = 84 meterxL
n
100
For cables route with combination of 2 or more different type of cable
Determine the resistance of cable for each section separately from Table 1 and Table 2
Example Cable 11kV Al Length 150 m size = 16mm2
From Table 1= Resistance for 1km =226 ohm
For 150 m = 1501000 x 226 = 0339 ohm
From Table 1 Calculate the equivalent resistance for all the combined section
Example for Al = 0339 ohm for copper = 0150 ohm
Equivalent Resistance = 0339+0150 =0489 ohm
Continue
Calculate equivalent resistance from obtained bridge reading
Example Bridge Reading = 60
Equivalent resistance = 06 x 0489 ohm = 02934 ohm
Determine which section of cable the fault occurs
Total resistance at Al section = 0339 ohm
As equivalent resistance at Al section is higher then 02934(Equivalent resistance from bridge test) the fault is
located at Aluminium section of cable
Table 1 Impedance data for 635011000 V Cable Aluminium
Table II Impedance data 11000 V Cable Copper
TABLE VI ndash 6001000V (ALUMINIUM)
TABLE VII ndash 38006600V (ALUMINIUM)
TABLE VIII- 635011000 (ALUMINIUM)
TABLE V
610 COMMISSIONING OF UNDERGROUND CABLE
High Voltage DC testing
High voltage testing is carried out in order to determine the electrical strength of cable insulation Site tests are performed by applying a predetermined high voltage to the insulation DC site test voltages regardless of insulation type are used to ensure that the cable cable joints and termination are correctly made and installed
High Voltage Testing (cont-)
IEC Standards specify maintenance tests after installation should be 70 of permissible factory test DC voltages (15 minutes) = 175kV
Insulation Resistance Test (cont-)
Test voltages should be slowly raised to the required value over a period of about 1 minute and the test period starts once the full voltage is reached In this way the capacitive and absorption currents will have decreased and circuit conditions stabilized such that true leakage current may be measured
68 BURNING A FAULT (continue-)
Burning a fault is achieve by passing current from a DC HT test set through the fault
Other conductors not under test should be earthed
HT is applied for about 5 to 10 minutes to burn the fault
HT test is used to determine which fault location equipment is suitable to be used
HT is the last resort often used because it sometimes produce ambiguous and unpredictable results
Therefore fault location equipment should be attempted first
69 Pre- Location of fault
Pre-location is the application of a test at the terminals of a
given cable to give an indication of the distance to the fault
from the test point
Whilst the measurement should be accurate as condition will
allow the primary purpose of pre-location is to give an
indication as quick as possible of the vicinity in which to
commence the final pin-pointing tests
69 Pre- Location of fault (cont)-
Generally there are four pre-location methods which are practised and the are as follows
1Bridge or loop Method2 Pulse Echo Time Domain Reflectometry (TDR) method3 Impulse Current Method4 Arc Reflection and secondary Impulse method
691 Bridge method
6911 Direct reading fault localizer
Conditions required
i) No break in continuity if possible one sound core available
ii) The fault resistance of cable should not be more then 20 k-ohm
iii) Applied voltage not to exceed 600 V (DC)
Case 1 Single conductor to earth with sound core available
Fault distance from front end X= n100 x L
Where n = localiser reading in
L= Cable route length
Case2 Conductor to conductor with sound core available
Fault distance from front end X= n100 x L
Where n = localiser reading in
L= Cable route length
Case 3 Three phase fault
By Open loop test and close loop test
(i) Rf2Rf1 gt 5 and (ii) Rf2 lt 20 k-ohm
Open loop test
Case 3 Three phase fault
Close loop test
Open loop and close loop test (cont)
Fault distance from front end
Lxm
mn
X 2
1001
)200200
(
Where m = open loop bridge reading
n = close loop bridge reading
L = cable route length
High Tension Bridge
Conditions required
i) No break in continuity if possible one sound core available
ii) The fault resistance of cable should be more then 20 k-ohm
iii) Applied voltage not to exceed 20kV (DC)
High Tension Bridge
Case 1Single Conductor to earth with sound core available
Fault Distance from test end X = n100 x 2L
High Tension Bridge
Case 2Conductor to conductor with sound core available
Fault Distance from test end X = n100 x 2L
High Tension Bridge
Case 3Three phase Fault with no sound core available
Rf2Rf1gt5 Rf2gt20 kohm
Lxm
)mn
(X 2
1002
1
100100
Where m = open loop bridge reading
n = close loop bridge reading
L = cable route length
6912 Universal Bridge to measure capacitance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be more then 100 ohm
iii) Earth all the conductors not under test
6912 Universal Bridge to measure capacitance of Cable
Open circuit with sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with sound core available
Fault Distance from test end x = CA x L CAB
CA = Capacitance of open circuited coreCAB = Capacitance of sound coreL = conductor length
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
Fault Distance from test endfront end = ((CA(CA+CB))x L
6913 Universal Bridge to measure Inductance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be less then 30 ohm
iii) All conductors be left free from earth
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
Fault distance X = LA x L LAB
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
Fault distance X = LA x L LA + LB
692 Pulse Echo (PE) Time Domain Reflectometry (TDR)
A simple method which works on travelling wave principles It is applicable to all series and shunt faults with Rf gt=Zo10 (5 ohm) and Rflt=10 Zo (500 ohm) respectively For power cables typical Zo=50 ohm
6921 Travelling wave principles
A pulse injected into a cable is reflected back to the source by any change in characteristics impedance (Zo) of the cable
The waveform generated can be monitored using an oscilloscope as shown in Figure below
6921 Continue-
Waveform interpretion
Shunt Fault
Series Fault
6921 Continue-
Waveform interpretion
Low Resistance Shunt Fault (Rflt=500 Ohm)
-Reflection Negative
-Amplitude depends on fault resistance
Series Fault (Rfgt=5 ohm)
- Reflection Positive
- Amplitude depends on fault resistance
Zo = Characteristics impedance = radic(LC)
L = Inductance of cable C = Capacitance of Cable
69211 Reflection Factor (r)
- Is to determine the degree of reflection
Shunt fault r (V) = -Zo1(2Rf + Zo1)
Series Fault r (V) = Rf(2Zo1 + Rf)
Example shunt fault fault resistance = 5 ohm Zo = 50 ohm
r(V) = -50 (2 x5 +50) = - 833If fault resistance = 500 ohm
r(V) = -50 (2x 500 + 50) = -475
69212 Velocity of Propagation (Vp)
- Vp depends on the cable dielectric and is defined as
Vp = Vs radicEr
- where Vs = Velocity of light in free space
Er = Relative permittivity of dielectric
Vp = 300radicEr
Dielectrics Velocity of PropagationVp (mmicroS)
Impregnated Paper 150 -171
PVC 152-175
PE Approx 200
XLPE 156-176
Typical Vp for different Dielectric
69212 Continue (Vp)
Example
Determine the velocity of propagation Vp2 for a cable length of 2500 meters t1= 3164 S and t2 = 1745 S Calculate the fault distances
6921 Continue-
The distance to the fault Lf (m) is given by
Lf = T x Vp2
6921 Continue
Solutions
Velocity of propagation = Vp2 = 25003164 = 79 mS
Distance of Fault from test end = Vp2 x t2
= 79 x 1745
= 137855
693 Impulse Current Method
Working on Travelling wave principles it is applicable to all types of fault
694 Arc Reflection or Secondary Impulse method
Basically PE and TDR associated with fault treatment It is also applicable to faults of all nature but with easily interpreted breakdown waveforms
610 PIN POINTING
Pin Pointing is the application of a test that positively confirms the exact position of the fault
Before the commencement of pin pointing the prelocated fault distance should mark on the cable route which is measured by means of a trumeter
Pin pointing is normally carried out by the shock wave discharge method The fault can be detected by the use of semisphone
611 Confirmation and Re-Test
After the pin-pointed position of the fault has been marked and exposed check for physical sign of fault If there is the fault is confirm
After confirmation the fault should be cut away IR and continuity test should be carried out on the remaining cable sections to determine the soundeness of these cable sections
The insulation resistance test are again carried out after jointing followed by pressure test before supply can be restored
Flowchart for Fault location Procedure
Example
Find a fault distance using direct reading fault localiser from end test which has a test data as below
Bridge Balance Reading =60 Length of Cable 140 meters
Solutions
Fault Distance= = 60100 x 140 = 84 meterxL
n
100
For cables route with combination of 2 or more different type of cable
Determine the resistance of cable for each section separately from Table 1 and Table 2
Example Cable 11kV Al Length 150 m size = 16mm2
From Table 1= Resistance for 1km =226 ohm
For 150 m = 1501000 x 226 = 0339 ohm
From Table 1 Calculate the equivalent resistance for all the combined section
Example for Al = 0339 ohm for copper = 0150 ohm
Equivalent Resistance = 0339+0150 =0489 ohm
Continue
Calculate equivalent resistance from obtained bridge reading
Example Bridge Reading = 60
Equivalent resistance = 06 x 0489 ohm = 02934 ohm
Determine which section of cable the fault occurs
Total resistance at Al section = 0339 ohm
As equivalent resistance at Al section is higher then 02934(Equivalent resistance from bridge test) the fault is
located at Aluminium section of cable
Table 1 Impedance data for 635011000 V Cable Aluminium
Table II Impedance data 11000 V Cable Copper
TABLE VI ndash 6001000V (ALUMINIUM)
TABLE VII ndash 38006600V (ALUMINIUM)
TABLE VIII- 635011000 (ALUMINIUM)
TABLE V
610 COMMISSIONING OF UNDERGROUND CABLE
High Voltage DC testing
High voltage testing is carried out in order to determine the electrical strength of cable insulation Site tests are performed by applying a predetermined high voltage to the insulation DC site test voltages regardless of insulation type are used to ensure that the cable cable joints and termination are correctly made and installed
High Voltage Testing (cont-)
IEC Standards specify maintenance tests after installation should be 70 of permissible factory test DC voltages (15 minutes) = 175kV
Insulation Resistance Test (cont-)
Test voltages should be slowly raised to the required value over a period of about 1 minute and the test period starts once the full voltage is reached In this way the capacitive and absorption currents will have decreased and circuit conditions stabilized such that true leakage current may be measured
69 Pre- Location of fault
Pre-location is the application of a test at the terminals of a
given cable to give an indication of the distance to the fault
from the test point
Whilst the measurement should be accurate as condition will
allow the primary purpose of pre-location is to give an
indication as quick as possible of the vicinity in which to
commence the final pin-pointing tests
69 Pre- Location of fault (cont)-
Generally there are four pre-location methods which are practised and the are as follows
1Bridge or loop Method2 Pulse Echo Time Domain Reflectometry (TDR) method3 Impulse Current Method4 Arc Reflection and secondary Impulse method
691 Bridge method
6911 Direct reading fault localizer
Conditions required
i) No break in continuity if possible one sound core available
ii) The fault resistance of cable should not be more then 20 k-ohm
iii) Applied voltage not to exceed 600 V (DC)
Case 1 Single conductor to earth with sound core available
Fault distance from front end X= n100 x L
Where n = localiser reading in
L= Cable route length
Case2 Conductor to conductor with sound core available
Fault distance from front end X= n100 x L
Where n = localiser reading in
L= Cable route length
Case 3 Three phase fault
By Open loop test and close loop test
(i) Rf2Rf1 gt 5 and (ii) Rf2 lt 20 k-ohm
Open loop test
Case 3 Three phase fault
Close loop test
Open loop and close loop test (cont)
Fault distance from front end
Lxm
mn
X 2
1001
)200200
(
Where m = open loop bridge reading
n = close loop bridge reading
L = cable route length
High Tension Bridge
Conditions required
i) No break in continuity if possible one sound core available
ii) The fault resistance of cable should be more then 20 k-ohm
iii) Applied voltage not to exceed 20kV (DC)
High Tension Bridge
Case 1Single Conductor to earth with sound core available
Fault Distance from test end X = n100 x 2L
High Tension Bridge
Case 2Conductor to conductor with sound core available
Fault Distance from test end X = n100 x 2L
High Tension Bridge
Case 3Three phase Fault with no sound core available
Rf2Rf1gt5 Rf2gt20 kohm
Lxm
)mn
(X 2
1002
1
100100
Where m = open loop bridge reading
n = close loop bridge reading
L = cable route length
6912 Universal Bridge to measure capacitance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be more then 100 ohm
iii) Earth all the conductors not under test
6912 Universal Bridge to measure capacitance of Cable
Open circuit with sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with sound core available
Fault Distance from test end x = CA x L CAB
CA = Capacitance of open circuited coreCAB = Capacitance of sound coreL = conductor length
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
Fault Distance from test endfront end = ((CA(CA+CB))x L
6913 Universal Bridge to measure Inductance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be less then 30 ohm
iii) All conductors be left free from earth
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
Fault distance X = LA x L LAB
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
Fault distance X = LA x L LA + LB
692 Pulse Echo (PE) Time Domain Reflectometry (TDR)
A simple method which works on travelling wave principles It is applicable to all series and shunt faults with Rf gt=Zo10 (5 ohm) and Rflt=10 Zo (500 ohm) respectively For power cables typical Zo=50 ohm
6921 Travelling wave principles
A pulse injected into a cable is reflected back to the source by any change in characteristics impedance (Zo) of the cable
The waveform generated can be monitored using an oscilloscope as shown in Figure below
6921 Continue-
Waveform interpretion
Shunt Fault
Series Fault
6921 Continue-
Waveform interpretion
Low Resistance Shunt Fault (Rflt=500 Ohm)
-Reflection Negative
-Amplitude depends on fault resistance
Series Fault (Rfgt=5 ohm)
- Reflection Positive
- Amplitude depends on fault resistance
Zo = Characteristics impedance = radic(LC)
L = Inductance of cable C = Capacitance of Cable
69211 Reflection Factor (r)
- Is to determine the degree of reflection
Shunt fault r (V) = -Zo1(2Rf + Zo1)
Series Fault r (V) = Rf(2Zo1 + Rf)
Example shunt fault fault resistance = 5 ohm Zo = 50 ohm
r(V) = -50 (2 x5 +50) = - 833If fault resistance = 500 ohm
r(V) = -50 (2x 500 + 50) = -475
69212 Velocity of Propagation (Vp)
- Vp depends on the cable dielectric and is defined as
Vp = Vs radicEr
- where Vs = Velocity of light in free space
Er = Relative permittivity of dielectric
Vp = 300radicEr
Dielectrics Velocity of PropagationVp (mmicroS)
Impregnated Paper 150 -171
PVC 152-175
PE Approx 200
XLPE 156-176
Typical Vp for different Dielectric
69212 Continue (Vp)
Example
Determine the velocity of propagation Vp2 for a cable length of 2500 meters t1= 3164 S and t2 = 1745 S Calculate the fault distances
6921 Continue-
The distance to the fault Lf (m) is given by
Lf = T x Vp2
6921 Continue
Solutions
Velocity of propagation = Vp2 = 25003164 = 79 mS
Distance of Fault from test end = Vp2 x t2
= 79 x 1745
= 137855
693 Impulse Current Method
Working on Travelling wave principles it is applicable to all types of fault
694 Arc Reflection or Secondary Impulse method
Basically PE and TDR associated with fault treatment It is also applicable to faults of all nature but with easily interpreted breakdown waveforms
610 PIN POINTING
Pin Pointing is the application of a test that positively confirms the exact position of the fault
Before the commencement of pin pointing the prelocated fault distance should mark on the cable route which is measured by means of a trumeter
Pin pointing is normally carried out by the shock wave discharge method The fault can be detected by the use of semisphone
611 Confirmation and Re-Test
After the pin-pointed position of the fault has been marked and exposed check for physical sign of fault If there is the fault is confirm
After confirmation the fault should be cut away IR and continuity test should be carried out on the remaining cable sections to determine the soundeness of these cable sections
The insulation resistance test are again carried out after jointing followed by pressure test before supply can be restored
Flowchart for Fault location Procedure
Example
Find a fault distance using direct reading fault localiser from end test which has a test data as below
Bridge Balance Reading =60 Length of Cable 140 meters
Solutions
Fault Distance= = 60100 x 140 = 84 meterxL
n
100
For cables route with combination of 2 or more different type of cable
Determine the resistance of cable for each section separately from Table 1 and Table 2
Example Cable 11kV Al Length 150 m size = 16mm2
From Table 1= Resistance for 1km =226 ohm
For 150 m = 1501000 x 226 = 0339 ohm
From Table 1 Calculate the equivalent resistance for all the combined section
Example for Al = 0339 ohm for copper = 0150 ohm
Equivalent Resistance = 0339+0150 =0489 ohm
Continue
Calculate equivalent resistance from obtained bridge reading
Example Bridge Reading = 60
Equivalent resistance = 06 x 0489 ohm = 02934 ohm
Determine which section of cable the fault occurs
Total resistance at Al section = 0339 ohm
As equivalent resistance at Al section is higher then 02934(Equivalent resistance from bridge test) the fault is
located at Aluminium section of cable
Table 1 Impedance data for 635011000 V Cable Aluminium
Table II Impedance data 11000 V Cable Copper
TABLE VI ndash 6001000V (ALUMINIUM)
TABLE VII ndash 38006600V (ALUMINIUM)
TABLE VIII- 635011000 (ALUMINIUM)
TABLE V
610 COMMISSIONING OF UNDERGROUND CABLE
High Voltage DC testing
High voltage testing is carried out in order to determine the electrical strength of cable insulation Site tests are performed by applying a predetermined high voltage to the insulation DC site test voltages regardless of insulation type are used to ensure that the cable cable joints and termination are correctly made and installed
High Voltage Testing (cont-)
IEC Standards specify maintenance tests after installation should be 70 of permissible factory test DC voltages (15 minutes) = 175kV
Insulation Resistance Test (cont-)
Test voltages should be slowly raised to the required value over a period of about 1 minute and the test period starts once the full voltage is reached In this way the capacitive and absorption currents will have decreased and circuit conditions stabilized such that true leakage current may be measured
69 Pre- Location of fault (cont)-
Generally there are four pre-location methods which are practised and the are as follows
1Bridge or loop Method2 Pulse Echo Time Domain Reflectometry (TDR) method3 Impulse Current Method4 Arc Reflection and secondary Impulse method
691 Bridge method
6911 Direct reading fault localizer
Conditions required
i) No break in continuity if possible one sound core available
ii) The fault resistance of cable should not be more then 20 k-ohm
iii) Applied voltage not to exceed 600 V (DC)
Case 1 Single conductor to earth with sound core available
Fault distance from front end X= n100 x L
Where n = localiser reading in
L= Cable route length
Case2 Conductor to conductor with sound core available
Fault distance from front end X= n100 x L
Where n = localiser reading in
L= Cable route length
Case 3 Three phase fault
By Open loop test and close loop test
(i) Rf2Rf1 gt 5 and (ii) Rf2 lt 20 k-ohm
Open loop test
Case 3 Three phase fault
Close loop test
Open loop and close loop test (cont)
Fault distance from front end
Lxm
mn
X 2
1001
)200200
(
Where m = open loop bridge reading
n = close loop bridge reading
L = cable route length
High Tension Bridge
Conditions required
i) No break in continuity if possible one sound core available
ii) The fault resistance of cable should be more then 20 k-ohm
iii) Applied voltage not to exceed 20kV (DC)
High Tension Bridge
Case 1Single Conductor to earth with sound core available
Fault Distance from test end X = n100 x 2L
High Tension Bridge
Case 2Conductor to conductor with sound core available
Fault Distance from test end X = n100 x 2L
High Tension Bridge
Case 3Three phase Fault with no sound core available
Rf2Rf1gt5 Rf2gt20 kohm
Lxm
)mn
(X 2
1002
1
100100
Where m = open loop bridge reading
n = close loop bridge reading
L = cable route length
6912 Universal Bridge to measure capacitance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be more then 100 ohm
iii) Earth all the conductors not under test
6912 Universal Bridge to measure capacitance of Cable
Open circuit with sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with sound core available
Fault Distance from test end x = CA x L CAB
CA = Capacitance of open circuited coreCAB = Capacitance of sound coreL = conductor length
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
Fault Distance from test endfront end = ((CA(CA+CB))x L
6913 Universal Bridge to measure Inductance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be less then 30 ohm
iii) All conductors be left free from earth
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
Fault distance X = LA x L LAB
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
Fault distance X = LA x L LA + LB
692 Pulse Echo (PE) Time Domain Reflectometry (TDR)
A simple method which works on travelling wave principles It is applicable to all series and shunt faults with Rf gt=Zo10 (5 ohm) and Rflt=10 Zo (500 ohm) respectively For power cables typical Zo=50 ohm
6921 Travelling wave principles
A pulse injected into a cable is reflected back to the source by any change in characteristics impedance (Zo) of the cable
The waveform generated can be monitored using an oscilloscope as shown in Figure below
6921 Continue-
Waveform interpretion
Shunt Fault
Series Fault
6921 Continue-
Waveform interpretion
Low Resistance Shunt Fault (Rflt=500 Ohm)
-Reflection Negative
-Amplitude depends on fault resistance
Series Fault (Rfgt=5 ohm)
- Reflection Positive
- Amplitude depends on fault resistance
Zo = Characteristics impedance = radic(LC)
L = Inductance of cable C = Capacitance of Cable
69211 Reflection Factor (r)
- Is to determine the degree of reflection
Shunt fault r (V) = -Zo1(2Rf + Zo1)
Series Fault r (V) = Rf(2Zo1 + Rf)
Example shunt fault fault resistance = 5 ohm Zo = 50 ohm
r(V) = -50 (2 x5 +50) = - 833If fault resistance = 500 ohm
r(V) = -50 (2x 500 + 50) = -475
69212 Velocity of Propagation (Vp)
- Vp depends on the cable dielectric and is defined as
Vp = Vs radicEr
- where Vs = Velocity of light in free space
Er = Relative permittivity of dielectric
Vp = 300radicEr
Dielectrics Velocity of PropagationVp (mmicroS)
Impregnated Paper 150 -171
PVC 152-175
PE Approx 200
XLPE 156-176
Typical Vp for different Dielectric
69212 Continue (Vp)
Example
Determine the velocity of propagation Vp2 for a cable length of 2500 meters t1= 3164 S and t2 = 1745 S Calculate the fault distances
6921 Continue-
The distance to the fault Lf (m) is given by
Lf = T x Vp2
6921 Continue
Solutions
Velocity of propagation = Vp2 = 25003164 = 79 mS
Distance of Fault from test end = Vp2 x t2
= 79 x 1745
= 137855
693 Impulse Current Method
Working on Travelling wave principles it is applicable to all types of fault
694 Arc Reflection or Secondary Impulse method
Basically PE and TDR associated with fault treatment It is also applicable to faults of all nature but with easily interpreted breakdown waveforms
610 PIN POINTING
Pin Pointing is the application of a test that positively confirms the exact position of the fault
Before the commencement of pin pointing the prelocated fault distance should mark on the cable route which is measured by means of a trumeter
Pin pointing is normally carried out by the shock wave discharge method The fault can be detected by the use of semisphone
611 Confirmation and Re-Test
After the pin-pointed position of the fault has been marked and exposed check for physical sign of fault If there is the fault is confirm
After confirmation the fault should be cut away IR and continuity test should be carried out on the remaining cable sections to determine the soundeness of these cable sections
The insulation resistance test are again carried out after jointing followed by pressure test before supply can be restored
Flowchart for Fault location Procedure
Example
Find a fault distance using direct reading fault localiser from end test which has a test data as below
Bridge Balance Reading =60 Length of Cable 140 meters
Solutions
Fault Distance= = 60100 x 140 = 84 meterxL
n
100
For cables route with combination of 2 or more different type of cable
Determine the resistance of cable for each section separately from Table 1 and Table 2
Example Cable 11kV Al Length 150 m size = 16mm2
From Table 1= Resistance for 1km =226 ohm
For 150 m = 1501000 x 226 = 0339 ohm
From Table 1 Calculate the equivalent resistance for all the combined section
Example for Al = 0339 ohm for copper = 0150 ohm
Equivalent Resistance = 0339+0150 =0489 ohm
Continue
Calculate equivalent resistance from obtained bridge reading
Example Bridge Reading = 60
Equivalent resistance = 06 x 0489 ohm = 02934 ohm
Determine which section of cable the fault occurs
Total resistance at Al section = 0339 ohm
As equivalent resistance at Al section is higher then 02934(Equivalent resistance from bridge test) the fault is
located at Aluminium section of cable
Table 1 Impedance data for 635011000 V Cable Aluminium
Table II Impedance data 11000 V Cable Copper
TABLE VI ndash 6001000V (ALUMINIUM)
TABLE VII ndash 38006600V (ALUMINIUM)
TABLE VIII- 635011000 (ALUMINIUM)
TABLE V
610 COMMISSIONING OF UNDERGROUND CABLE
High Voltage DC testing
High voltage testing is carried out in order to determine the electrical strength of cable insulation Site tests are performed by applying a predetermined high voltage to the insulation DC site test voltages regardless of insulation type are used to ensure that the cable cable joints and termination are correctly made and installed
High Voltage Testing (cont-)
IEC Standards specify maintenance tests after installation should be 70 of permissible factory test DC voltages (15 minutes) = 175kV
Insulation Resistance Test (cont-)
Test voltages should be slowly raised to the required value over a period of about 1 minute and the test period starts once the full voltage is reached In this way the capacitive and absorption currents will have decreased and circuit conditions stabilized such that true leakage current may be measured
691 Bridge method
6911 Direct reading fault localizer
Conditions required
i) No break in continuity if possible one sound core available
ii) The fault resistance of cable should not be more then 20 k-ohm
iii) Applied voltage not to exceed 600 V (DC)
Case 1 Single conductor to earth with sound core available
Fault distance from front end X= n100 x L
Where n = localiser reading in
L= Cable route length
Case2 Conductor to conductor with sound core available
Fault distance from front end X= n100 x L
Where n = localiser reading in
L= Cable route length
Case 3 Three phase fault
By Open loop test and close loop test
(i) Rf2Rf1 gt 5 and (ii) Rf2 lt 20 k-ohm
Open loop test
Case 3 Three phase fault
Close loop test
Open loop and close loop test (cont)
Fault distance from front end
Lxm
mn
X 2
1001
)200200
(
Where m = open loop bridge reading
n = close loop bridge reading
L = cable route length
High Tension Bridge
Conditions required
i) No break in continuity if possible one sound core available
ii) The fault resistance of cable should be more then 20 k-ohm
iii) Applied voltage not to exceed 20kV (DC)
High Tension Bridge
Case 1Single Conductor to earth with sound core available
Fault Distance from test end X = n100 x 2L
High Tension Bridge
Case 2Conductor to conductor with sound core available
Fault Distance from test end X = n100 x 2L
High Tension Bridge
Case 3Three phase Fault with no sound core available
Rf2Rf1gt5 Rf2gt20 kohm
Lxm
)mn
(X 2
1002
1
100100
Where m = open loop bridge reading
n = close loop bridge reading
L = cable route length
6912 Universal Bridge to measure capacitance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be more then 100 ohm
iii) Earth all the conductors not under test
6912 Universal Bridge to measure capacitance of Cable
Open circuit with sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with sound core available
Fault Distance from test end x = CA x L CAB
CA = Capacitance of open circuited coreCAB = Capacitance of sound coreL = conductor length
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
Fault Distance from test endfront end = ((CA(CA+CB))x L
6913 Universal Bridge to measure Inductance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be less then 30 ohm
iii) All conductors be left free from earth
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
Fault distance X = LA x L LAB
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
Fault distance X = LA x L LA + LB
692 Pulse Echo (PE) Time Domain Reflectometry (TDR)
A simple method which works on travelling wave principles It is applicable to all series and shunt faults with Rf gt=Zo10 (5 ohm) and Rflt=10 Zo (500 ohm) respectively For power cables typical Zo=50 ohm
6921 Travelling wave principles
A pulse injected into a cable is reflected back to the source by any change in characteristics impedance (Zo) of the cable
The waveform generated can be monitored using an oscilloscope as shown in Figure below
6921 Continue-
Waveform interpretion
Shunt Fault
Series Fault
6921 Continue-
Waveform interpretion
Low Resistance Shunt Fault (Rflt=500 Ohm)
-Reflection Negative
-Amplitude depends on fault resistance
Series Fault (Rfgt=5 ohm)
- Reflection Positive
- Amplitude depends on fault resistance
Zo = Characteristics impedance = radic(LC)
L = Inductance of cable C = Capacitance of Cable
69211 Reflection Factor (r)
- Is to determine the degree of reflection
Shunt fault r (V) = -Zo1(2Rf + Zo1)
Series Fault r (V) = Rf(2Zo1 + Rf)
Example shunt fault fault resistance = 5 ohm Zo = 50 ohm
r(V) = -50 (2 x5 +50) = - 833If fault resistance = 500 ohm
r(V) = -50 (2x 500 + 50) = -475
69212 Velocity of Propagation (Vp)
- Vp depends on the cable dielectric and is defined as
Vp = Vs radicEr
- where Vs = Velocity of light in free space
Er = Relative permittivity of dielectric
Vp = 300radicEr
Dielectrics Velocity of PropagationVp (mmicroS)
Impregnated Paper 150 -171
PVC 152-175
PE Approx 200
XLPE 156-176
Typical Vp for different Dielectric
69212 Continue (Vp)
Example
Determine the velocity of propagation Vp2 for a cable length of 2500 meters t1= 3164 S and t2 = 1745 S Calculate the fault distances
6921 Continue-
The distance to the fault Lf (m) is given by
Lf = T x Vp2
6921 Continue
Solutions
Velocity of propagation = Vp2 = 25003164 = 79 mS
Distance of Fault from test end = Vp2 x t2
= 79 x 1745
= 137855
693 Impulse Current Method
Working on Travelling wave principles it is applicable to all types of fault
694 Arc Reflection or Secondary Impulse method
Basically PE and TDR associated with fault treatment It is also applicable to faults of all nature but with easily interpreted breakdown waveforms
610 PIN POINTING
Pin Pointing is the application of a test that positively confirms the exact position of the fault
Before the commencement of pin pointing the prelocated fault distance should mark on the cable route which is measured by means of a trumeter
Pin pointing is normally carried out by the shock wave discharge method The fault can be detected by the use of semisphone
611 Confirmation and Re-Test
After the pin-pointed position of the fault has been marked and exposed check for physical sign of fault If there is the fault is confirm
After confirmation the fault should be cut away IR and continuity test should be carried out on the remaining cable sections to determine the soundeness of these cable sections
The insulation resistance test are again carried out after jointing followed by pressure test before supply can be restored
Flowchart for Fault location Procedure
Example
Find a fault distance using direct reading fault localiser from end test which has a test data as below
Bridge Balance Reading =60 Length of Cable 140 meters
Solutions
Fault Distance= = 60100 x 140 = 84 meterxL
n
100
For cables route with combination of 2 or more different type of cable
Determine the resistance of cable for each section separately from Table 1 and Table 2
Example Cable 11kV Al Length 150 m size = 16mm2
From Table 1= Resistance for 1km =226 ohm
For 150 m = 1501000 x 226 = 0339 ohm
From Table 1 Calculate the equivalent resistance for all the combined section
Example for Al = 0339 ohm for copper = 0150 ohm
Equivalent Resistance = 0339+0150 =0489 ohm
Continue
Calculate equivalent resistance from obtained bridge reading
Example Bridge Reading = 60
Equivalent resistance = 06 x 0489 ohm = 02934 ohm
Determine which section of cable the fault occurs
Total resistance at Al section = 0339 ohm
As equivalent resistance at Al section is higher then 02934(Equivalent resistance from bridge test) the fault is
located at Aluminium section of cable
Table 1 Impedance data for 635011000 V Cable Aluminium
Table II Impedance data 11000 V Cable Copper
TABLE VI ndash 6001000V (ALUMINIUM)
TABLE VII ndash 38006600V (ALUMINIUM)
TABLE VIII- 635011000 (ALUMINIUM)
TABLE V
610 COMMISSIONING OF UNDERGROUND CABLE
High Voltage DC testing
High voltage testing is carried out in order to determine the electrical strength of cable insulation Site tests are performed by applying a predetermined high voltage to the insulation DC site test voltages regardless of insulation type are used to ensure that the cable cable joints and termination are correctly made and installed
High Voltage Testing (cont-)
IEC Standards specify maintenance tests after installation should be 70 of permissible factory test DC voltages (15 minutes) = 175kV
Insulation Resistance Test (cont-)
Test voltages should be slowly raised to the required value over a period of about 1 minute and the test period starts once the full voltage is reached In this way the capacitive and absorption currents will have decreased and circuit conditions stabilized such that true leakage current may be measured
Case 1 Single conductor to earth with sound core available
Fault distance from front end X= n100 x L
Where n = localiser reading in
L= Cable route length
Case2 Conductor to conductor with sound core available
Fault distance from front end X= n100 x L
Where n = localiser reading in
L= Cable route length
Case 3 Three phase fault
By Open loop test and close loop test
(i) Rf2Rf1 gt 5 and (ii) Rf2 lt 20 k-ohm
Open loop test
Case 3 Three phase fault
Close loop test
Open loop and close loop test (cont)
Fault distance from front end
Lxm
mn
X 2
1001
)200200
(
Where m = open loop bridge reading
n = close loop bridge reading
L = cable route length
High Tension Bridge
Conditions required
i) No break in continuity if possible one sound core available
ii) The fault resistance of cable should be more then 20 k-ohm
iii) Applied voltage not to exceed 20kV (DC)
High Tension Bridge
Case 1Single Conductor to earth with sound core available
Fault Distance from test end X = n100 x 2L
High Tension Bridge
Case 2Conductor to conductor with sound core available
Fault Distance from test end X = n100 x 2L
High Tension Bridge
Case 3Three phase Fault with no sound core available
Rf2Rf1gt5 Rf2gt20 kohm
Lxm
)mn
(X 2
1002
1
100100
Where m = open loop bridge reading
n = close loop bridge reading
L = cable route length
6912 Universal Bridge to measure capacitance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be more then 100 ohm
iii) Earth all the conductors not under test
6912 Universal Bridge to measure capacitance of Cable
Open circuit with sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with sound core available
Fault Distance from test end x = CA x L CAB
CA = Capacitance of open circuited coreCAB = Capacitance of sound coreL = conductor length
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
Fault Distance from test endfront end = ((CA(CA+CB))x L
6913 Universal Bridge to measure Inductance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be less then 30 ohm
iii) All conductors be left free from earth
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
Fault distance X = LA x L LAB
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
Fault distance X = LA x L LA + LB
692 Pulse Echo (PE) Time Domain Reflectometry (TDR)
A simple method which works on travelling wave principles It is applicable to all series and shunt faults with Rf gt=Zo10 (5 ohm) and Rflt=10 Zo (500 ohm) respectively For power cables typical Zo=50 ohm
6921 Travelling wave principles
A pulse injected into a cable is reflected back to the source by any change in characteristics impedance (Zo) of the cable
The waveform generated can be monitored using an oscilloscope as shown in Figure below
6921 Continue-
Waveform interpretion
Shunt Fault
Series Fault
6921 Continue-
Waveform interpretion
Low Resistance Shunt Fault (Rflt=500 Ohm)
-Reflection Negative
-Amplitude depends on fault resistance
Series Fault (Rfgt=5 ohm)
- Reflection Positive
- Amplitude depends on fault resistance
Zo = Characteristics impedance = radic(LC)
L = Inductance of cable C = Capacitance of Cable
69211 Reflection Factor (r)
- Is to determine the degree of reflection
Shunt fault r (V) = -Zo1(2Rf + Zo1)
Series Fault r (V) = Rf(2Zo1 + Rf)
Example shunt fault fault resistance = 5 ohm Zo = 50 ohm
r(V) = -50 (2 x5 +50) = - 833If fault resistance = 500 ohm
r(V) = -50 (2x 500 + 50) = -475
69212 Velocity of Propagation (Vp)
- Vp depends on the cable dielectric and is defined as
Vp = Vs radicEr
- where Vs = Velocity of light in free space
Er = Relative permittivity of dielectric
Vp = 300radicEr
Dielectrics Velocity of PropagationVp (mmicroS)
Impregnated Paper 150 -171
PVC 152-175
PE Approx 200
XLPE 156-176
Typical Vp for different Dielectric
69212 Continue (Vp)
Example
Determine the velocity of propagation Vp2 for a cable length of 2500 meters t1= 3164 S and t2 = 1745 S Calculate the fault distances
6921 Continue-
The distance to the fault Lf (m) is given by
Lf = T x Vp2
6921 Continue
Solutions
Velocity of propagation = Vp2 = 25003164 = 79 mS
Distance of Fault from test end = Vp2 x t2
= 79 x 1745
= 137855
693 Impulse Current Method
Working on Travelling wave principles it is applicable to all types of fault
694 Arc Reflection or Secondary Impulse method
Basically PE and TDR associated with fault treatment It is also applicable to faults of all nature but with easily interpreted breakdown waveforms
610 PIN POINTING
Pin Pointing is the application of a test that positively confirms the exact position of the fault
Before the commencement of pin pointing the prelocated fault distance should mark on the cable route which is measured by means of a trumeter
Pin pointing is normally carried out by the shock wave discharge method The fault can be detected by the use of semisphone
611 Confirmation and Re-Test
After the pin-pointed position of the fault has been marked and exposed check for physical sign of fault If there is the fault is confirm
After confirmation the fault should be cut away IR and continuity test should be carried out on the remaining cable sections to determine the soundeness of these cable sections
The insulation resistance test are again carried out after jointing followed by pressure test before supply can be restored
Flowchart for Fault location Procedure
Example
Find a fault distance using direct reading fault localiser from end test which has a test data as below
Bridge Balance Reading =60 Length of Cable 140 meters
Solutions
Fault Distance= = 60100 x 140 = 84 meterxL
n
100
For cables route with combination of 2 or more different type of cable
Determine the resistance of cable for each section separately from Table 1 and Table 2
Example Cable 11kV Al Length 150 m size = 16mm2
From Table 1= Resistance for 1km =226 ohm
For 150 m = 1501000 x 226 = 0339 ohm
From Table 1 Calculate the equivalent resistance for all the combined section
Example for Al = 0339 ohm for copper = 0150 ohm
Equivalent Resistance = 0339+0150 =0489 ohm
Continue
Calculate equivalent resistance from obtained bridge reading
Example Bridge Reading = 60
Equivalent resistance = 06 x 0489 ohm = 02934 ohm
Determine which section of cable the fault occurs
Total resistance at Al section = 0339 ohm
As equivalent resistance at Al section is higher then 02934(Equivalent resistance from bridge test) the fault is
located at Aluminium section of cable
Table 1 Impedance data for 635011000 V Cable Aluminium
Table II Impedance data 11000 V Cable Copper
TABLE VI ndash 6001000V (ALUMINIUM)
TABLE VII ndash 38006600V (ALUMINIUM)
TABLE VIII- 635011000 (ALUMINIUM)
TABLE V
610 COMMISSIONING OF UNDERGROUND CABLE
High Voltage DC testing
High voltage testing is carried out in order to determine the electrical strength of cable insulation Site tests are performed by applying a predetermined high voltage to the insulation DC site test voltages regardless of insulation type are used to ensure that the cable cable joints and termination are correctly made and installed
High Voltage Testing (cont-)
IEC Standards specify maintenance tests after installation should be 70 of permissible factory test DC voltages (15 minutes) = 175kV
Insulation Resistance Test (cont-)
Test voltages should be slowly raised to the required value over a period of about 1 minute and the test period starts once the full voltage is reached In this way the capacitive and absorption currents will have decreased and circuit conditions stabilized such that true leakage current may be measured
Case2 Conductor to conductor with sound core available
Fault distance from front end X= n100 x L
Where n = localiser reading in
L= Cable route length
Case 3 Three phase fault
By Open loop test and close loop test
(i) Rf2Rf1 gt 5 and (ii) Rf2 lt 20 k-ohm
Open loop test
Case 3 Three phase fault
Close loop test
Open loop and close loop test (cont)
Fault distance from front end
Lxm
mn
X 2
1001
)200200
(
Where m = open loop bridge reading
n = close loop bridge reading
L = cable route length
High Tension Bridge
Conditions required
i) No break in continuity if possible one sound core available
ii) The fault resistance of cable should be more then 20 k-ohm
iii) Applied voltage not to exceed 20kV (DC)
High Tension Bridge
Case 1Single Conductor to earth with sound core available
Fault Distance from test end X = n100 x 2L
High Tension Bridge
Case 2Conductor to conductor with sound core available
Fault Distance from test end X = n100 x 2L
High Tension Bridge
Case 3Three phase Fault with no sound core available
Rf2Rf1gt5 Rf2gt20 kohm
Lxm
)mn
(X 2
1002
1
100100
Where m = open loop bridge reading
n = close loop bridge reading
L = cable route length
6912 Universal Bridge to measure capacitance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be more then 100 ohm
iii) Earth all the conductors not under test
6912 Universal Bridge to measure capacitance of Cable
Open circuit with sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with sound core available
Fault Distance from test end x = CA x L CAB
CA = Capacitance of open circuited coreCAB = Capacitance of sound coreL = conductor length
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
Fault Distance from test endfront end = ((CA(CA+CB))x L
6913 Universal Bridge to measure Inductance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be less then 30 ohm
iii) All conductors be left free from earth
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
Fault distance X = LA x L LAB
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
Fault distance X = LA x L LA + LB
692 Pulse Echo (PE) Time Domain Reflectometry (TDR)
A simple method which works on travelling wave principles It is applicable to all series and shunt faults with Rf gt=Zo10 (5 ohm) and Rflt=10 Zo (500 ohm) respectively For power cables typical Zo=50 ohm
6921 Travelling wave principles
A pulse injected into a cable is reflected back to the source by any change in characteristics impedance (Zo) of the cable
The waveform generated can be monitored using an oscilloscope as shown in Figure below
6921 Continue-
Waveform interpretion
Shunt Fault
Series Fault
6921 Continue-
Waveform interpretion
Low Resistance Shunt Fault (Rflt=500 Ohm)
-Reflection Negative
-Amplitude depends on fault resistance
Series Fault (Rfgt=5 ohm)
- Reflection Positive
- Amplitude depends on fault resistance
Zo = Characteristics impedance = radic(LC)
L = Inductance of cable C = Capacitance of Cable
69211 Reflection Factor (r)
- Is to determine the degree of reflection
Shunt fault r (V) = -Zo1(2Rf + Zo1)
Series Fault r (V) = Rf(2Zo1 + Rf)
Example shunt fault fault resistance = 5 ohm Zo = 50 ohm
r(V) = -50 (2 x5 +50) = - 833If fault resistance = 500 ohm
r(V) = -50 (2x 500 + 50) = -475
69212 Velocity of Propagation (Vp)
- Vp depends on the cable dielectric and is defined as
Vp = Vs radicEr
- where Vs = Velocity of light in free space
Er = Relative permittivity of dielectric
Vp = 300radicEr
Dielectrics Velocity of PropagationVp (mmicroS)
Impregnated Paper 150 -171
PVC 152-175
PE Approx 200
XLPE 156-176
Typical Vp for different Dielectric
69212 Continue (Vp)
Example
Determine the velocity of propagation Vp2 for a cable length of 2500 meters t1= 3164 S and t2 = 1745 S Calculate the fault distances
6921 Continue-
The distance to the fault Lf (m) is given by
Lf = T x Vp2
6921 Continue
Solutions
Velocity of propagation = Vp2 = 25003164 = 79 mS
Distance of Fault from test end = Vp2 x t2
= 79 x 1745
= 137855
693 Impulse Current Method
Working on Travelling wave principles it is applicable to all types of fault
694 Arc Reflection or Secondary Impulse method
Basically PE and TDR associated with fault treatment It is also applicable to faults of all nature but with easily interpreted breakdown waveforms
610 PIN POINTING
Pin Pointing is the application of a test that positively confirms the exact position of the fault
Before the commencement of pin pointing the prelocated fault distance should mark on the cable route which is measured by means of a trumeter
Pin pointing is normally carried out by the shock wave discharge method The fault can be detected by the use of semisphone
611 Confirmation and Re-Test
After the pin-pointed position of the fault has been marked and exposed check for physical sign of fault If there is the fault is confirm
After confirmation the fault should be cut away IR and continuity test should be carried out on the remaining cable sections to determine the soundeness of these cable sections
The insulation resistance test are again carried out after jointing followed by pressure test before supply can be restored
Flowchart for Fault location Procedure
Example
Find a fault distance using direct reading fault localiser from end test which has a test data as below
Bridge Balance Reading =60 Length of Cable 140 meters
Solutions
Fault Distance= = 60100 x 140 = 84 meterxL
n
100
For cables route with combination of 2 or more different type of cable
Determine the resistance of cable for each section separately from Table 1 and Table 2
Example Cable 11kV Al Length 150 m size = 16mm2
From Table 1= Resistance for 1km =226 ohm
For 150 m = 1501000 x 226 = 0339 ohm
From Table 1 Calculate the equivalent resistance for all the combined section
Example for Al = 0339 ohm for copper = 0150 ohm
Equivalent Resistance = 0339+0150 =0489 ohm
Continue
Calculate equivalent resistance from obtained bridge reading
Example Bridge Reading = 60
Equivalent resistance = 06 x 0489 ohm = 02934 ohm
Determine which section of cable the fault occurs
Total resistance at Al section = 0339 ohm
As equivalent resistance at Al section is higher then 02934(Equivalent resistance from bridge test) the fault is
located at Aluminium section of cable
Table 1 Impedance data for 635011000 V Cable Aluminium
Table II Impedance data 11000 V Cable Copper
TABLE VI ndash 6001000V (ALUMINIUM)
TABLE VII ndash 38006600V (ALUMINIUM)
TABLE VIII- 635011000 (ALUMINIUM)
TABLE V
610 COMMISSIONING OF UNDERGROUND CABLE
High Voltage DC testing
High voltage testing is carried out in order to determine the electrical strength of cable insulation Site tests are performed by applying a predetermined high voltage to the insulation DC site test voltages regardless of insulation type are used to ensure that the cable cable joints and termination are correctly made and installed
High Voltage Testing (cont-)
IEC Standards specify maintenance tests after installation should be 70 of permissible factory test DC voltages (15 minutes) = 175kV
Insulation Resistance Test (cont-)
Test voltages should be slowly raised to the required value over a period of about 1 minute and the test period starts once the full voltage is reached In this way the capacitive and absorption currents will have decreased and circuit conditions stabilized such that true leakage current may be measured
Case 3 Three phase fault
By Open loop test and close loop test
(i) Rf2Rf1 gt 5 and (ii) Rf2 lt 20 k-ohm
Open loop test
Case 3 Three phase fault
Close loop test
Open loop and close loop test (cont)
Fault distance from front end
Lxm
mn
X 2
1001
)200200
(
Where m = open loop bridge reading
n = close loop bridge reading
L = cable route length
High Tension Bridge
Conditions required
i) No break in continuity if possible one sound core available
ii) The fault resistance of cable should be more then 20 k-ohm
iii) Applied voltage not to exceed 20kV (DC)
High Tension Bridge
Case 1Single Conductor to earth with sound core available
Fault Distance from test end X = n100 x 2L
High Tension Bridge
Case 2Conductor to conductor with sound core available
Fault Distance from test end X = n100 x 2L
High Tension Bridge
Case 3Three phase Fault with no sound core available
Rf2Rf1gt5 Rf2gt20 kohm
Lxm
)mn
(X 2
1002
1
100100
Where m = open loop bridge reading
n = close loop bridge reading
L = cable route length
6912 Universal Bridge to measure capacitance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be more then 100 ohm
iii) Earth all the conductors not under test
6912 Universal Bridge to measure capacitance of Cable
Open circuit with sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with sound core available
Fault Distance from test end x = CA x L CAB
CA = Capacitance of open circuited coreCAB = Capacitance of sound coreL = conductor length
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
Fault Distance from test endfront end = ((CA(CA+CB))x L
6913 Universal Bridge to measure Inductance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be less then 30 ohm
iii) All conductors be left free from earth
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
Fault distance X = LA x L LAB
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
Fault distance X = LA x L LA + LB
692 Pulse Echo (PE) Time Domain Reflectometry (TDR)
A simple method which works on travelling wave principles It is applicable to all series and shunt faults with Rf gt=Zo10 (5 ohm) and Rflt=10 Zo (500 ohm) respectively For power cables typical Zo=50 ohm
6921 Travelling wave principles
A pulse injected into a cable is reflected back to the source by any change in characteristics impedance (Zo) of the cable
The waveform generated can be monitored using an oscilloscope as shown in Figure below
6921 Continue-
Waveform interpretion
Shunt Fault
Series Fault
6921 Continue-
Waveform interpretion
Low Resistance Shunt Fault (Rflt=500 Ohm)
-Reflection Negative
-Amplitude depends on fault resistance
Series Fault (Rfgt=5 ohm)
- Reflection Positive
- Amplitude depends on fault resistance
Zo = Characteristics impedance = radic(LC)
L = Inductance of cable C = Capacitance of Cable
69211 Reflection Factor (r)
- Is to determine the degree of reflection
Shunt fault r (V) = -Zo1(2Rf + Zo1)
Series Fault r (V) = Rf(2Zo1 + Rf)
Example shunt fault fault resistance = 5 ohm Zo = 50 ohm
r(V) = -50 (2 x5 +50) = - 833If fault resistance = 500 ohm
r(V) = -50 (2x 500 + 50) = -475
69212 Velocity of Propagation (Vp)
- Vp depends on the cable dielectric and is defined as
Vp = Vs radicEr
- where Vs = Velocity of light in free space
Er = Relative permittivity of dielectric
Vp = 300radicEr
Dielectrics Velocity of PropagationVp (mmicroS)
Impregnated Paper 150 -171
PVC 152-175
PE Approx 200
XLPE 156-176
Typical Vp for different Dielectric
69212 Continue (Vp)
Example
Determine the velocity of propagation Vp2 for a cable length of 2500 meters t1= 3164 S and t2 = 1745 S Calculate the fault distances
6921 Continue-
The distance to the fault Lf (m) is given by
Lf = T x Vp2
6921 Continue
Solutions
Velocity of propagation = Vp2 = 25003164 = 79 mS
Distance of Fault from test end = Vp2 x t2
= 79 x 1745
= 137855
693 Impulse Current Method
Working on Travelling wave principles it is applicable to all types of fault
694 Arc Reflection or Secondary Impulse method
Basically PE and TDR associated with fault treatment It is also applicable to faults of all nature but with easily interpreted breakdown waveforms
610 PIN POINTING
Pin Pointing is the application of a test that positively confirms the exact position of the fault
Before the commencement of pin pointing the prelocated fault distance should mark on the cable route which is measured by means of a trumeter
Pin pointing is normally carried out by the shock wave discharge method The fault can be detected by the use of semisphone
611 Confirmation and Re-Test
After the pin-pointed position of the fault has been marked and exposed check for physical sign of fault If there is the fault is confirm
After confirmation the fault should be cut away IR and continuity test should be carried out on the remaining cable sections to determine the soundeness of these cable sections
The insulation resistance test are again carried out after jointing followed by pressure test before supply can be restored
Flowchart for Fault location Procedure
Example
Find a fault distance using direct reading fault localiser from end test which has a test data as below
Bridge Balance Reading =60 Length of Cable 140 meters
Solutions
Fault Distance= = 60100 x 140 = 84 meterxL
n
100
For cables route with combination of 2 or more different type of cable
Determine the resistance of cable for each section separately from Table 1 and Table 2
Example Cable 11kV Al Length 150 m size = 16mm2
From Table 1= Resistance for 1km =226 ohm
For 150 m = 1501000 x 226 = 0339 ohm
From Table 1 Calculate the equivalent resistance for all the combined section
Example for Al = 0339 ohm for copper = 0150 ohm
Equivalent Resistance = 0339+0150 =0489 ohm
Continue
Calculate equivalent resistance from obtained bridge reading
Example Bridge Reading = 60
Equivalent resistance = 06 x 0489 ohm = 02934 ohm
Determine which section of cable the fault occurs
Total resistance at Al section = 0339 ohm
As equivalent resistance at Al section is higher then 02934(Equivalent resistance from bridge test) the fault is
located at Aluminium section of cable
Table 1 Impedance data for 635011000 V Cable Aluminium
Table II Impedance data 11000 V Cable Copper
TABLE VI ndash 6001000V (ALUMINIUM)
TABLE VII ndash 38006600V (ALUMINIUM)
TABLE VIII- 635011000 (ALUMINIUM)
TABLE V
610 COMMISSIONING OF UNDERGROUND CABLE
High Voltage DC testing
High voltage testing is carried out in order to determine the electrical strength of cable insulation Site tests are performed by applying a predetermined high voltage to the insulation DC site test voltages regardless of insulation type are used to ensure that the cable cable joints and termination are correctly made and installed
High Voltage Testing (cont-)
IEC Standards specify maintenance tests after installation should be 70 of permissible factory test DC voltages (15 minutes) = 175kV
Insulation Resistance Test (cont-)
Test voltages should be slowly raised to the required value over a period of about 1 minute and the test period starts once the full voltage is reached In this way the capacitive and absorption currents will have decreased and circuit conditions stabilized such that true leakage current may be measured
Case 3 Three phase fault
Close loop test
Open loop and close loop test (cont)
Fault distance from front end
Lxm
mn
X 2
1001
)200200
(
Where m = open loop bridge reading
n = close loop bridge reading
L = cable route length
High Tension Bridge
Conditions required
i) No break in continuity if possible one sound core available
ii) The fault resistance of cable should be more then 20 k-ohm
iii) Applied voltage not to exceed 20kV (DC)
High Tension Bridge
Case 1Single Conductor to earth with sound core available
Fault Distance from test end X = n100 x 2L
High Tension Bridge
Case 2Conductor to conductor with sound core available
Fault Distance from test end X = n100 x 2L
High Tension Bridge
Case 3Three phase Fault with no sound core available
Rf2Rf1gt5 Rf2gt20 kohm
Lxm
)mn
(X 2
1002
1
100100
Where m = open loop bridge reading
n = close loop bridge reading
L = cable route length
6912 Universal Bridge to measure capacitance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be more then 100 ohm
iii) Earth all the conductors not under test
6912 Universal Bridge to measure capacitance of Cable
Open circuit with sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with sound core available
Fault Distance from test end x = CA x L CAB
CA = Capacitance of open circuited coreCAB = Capacitance of sound coreL = conductor length
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
Fault Distance from test endfront end = ((CA(CA+CB))x L
6913 Universal Bridge to measure Inductance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be less then 30 ohm
iii) All conductors be left free from earth
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
Fault distance X = LA x L LAB
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
Fault distance X = LA x L LA + LB
692 Pulse Echo (PE) Time Domain Reflectometry (TDR)
A simple method which works on travelling wave principles It is applicable to all series and shunt faults with Rf gt=Zo10 (5 ohm) and Rflt=10 Zo (500 ohm) respectively For power cables typical Zo=50 ohm
6921 Travelling wave principles
A pulse injected into a cable is reflected back to the source by any change in characteristics impedance (Zo) of the cable
The waveform generated can be monitored using an oscilloscope as shown in Figure below
6921 Continue-
Waveform interpretion
Shunt Fault
Series Fault
6921 Continue-
Waveform interpretion
Low Resistance Shunt Fault (Rflt=500 Ohm)
-Reflection Negative
-Amplitude depends on fault resistance
Series Fault (Rfgt=5 ohm)
- Reflection Positive
- Amplitude depends on fault resistance
Zo = Characteristics impedance = radic(LC)
L = Inductance of cable C = Capacitance of Cable
69211 Reflection Factor (r)
- Is to determine the degree of reflection
Shunt fault r (V) = -Zo1(2Rf + Zo1)
Series Fault r (V) = Rf(2Zo1 + Rf)
Example shunt fault fault resistance = 5 ohm Zo = 50 ohm
r(V) = -50 (2 x5 +50) = - 833If fault resistance = 500 ohm
r(V) = -50 (2x 500 + 50) = -475
69212 Velocity of Propagation (Vp)
- Vp depends on the cable dielectric and is defined as
Vp = Vs radicEr
- where Vs = Velocity of light in free space
Er = Relative permittivity of dielectric
Vp = 300radicEr
Dielectrics Velocity of PropagationVp (mmicroS)
Impregnated Paper 150 -171
PVC 152-175
PE Approx 200
XLPE 156-176
Typical Vp for different Dielectric
69212 Continue (Vp)
Example
Determine the velocity of propagation Vp2 for a cable length of 2500 meters t1= 3164 S and t2 = 1745 S Calculate the fault distances
6921 Continue-
The distance to the fault Lf (m) is given by
Lf = T x Vp2
6921 Continue
Solutions
Velocity of propagation = Vp2 = 25003164 = 79 mS
Distance of Fault from test end = Vp2 x t2
= 79 x 1745
= 137855
693 Impulse Current Method
Working on Travelling wave principles it is applicable to all types of fault
694 Arc Reflection or Secondary Impulse method
Basically PE and TDR associated with fault treatment It is also applicable to faults of all nature but with easily interpreted breakdown waveforms
610 PIN POINTING
Pin Pointing is the application of a test that positively confirms the exact position of the fault
Before the commencement of pin pointing the prelocated fault distance should mark on the cable route which is measured by means of a trumeter
Pin pointing is normally carried out by the shock wave discharge method The fault can be detected by the use of semisphone
611 Confirmation and Re-Test
After the pin-pointed position of the fault has been marked and exposed check for physical sign of fault If there is the fault is confirm
After confirmation the fault should be cut away IR and continuity test should be carried out on the remaining cable sections to determine the soundeness of these cable sections
The insulation resistance test are again carried out after jointing followed by pressure test before supply can be restored
Flowchart for Fault location Procedure
Example
Find a fault distance using direct reading fault localiser from end test which has a test data as below
Bridge Balance Reading =60 Length of Cable 140 meters
Solutions
Fault Distance= = 60100 x 140 = 84 meterxL
n
100
For cables route with combination of 2 or more different type of cable
Determine the resistance of cable for each section separately from Table 1 and Table 2
Example Cable 11kV Al Length 150 m size = 16mm2
From Table 1= Resistance for 1km =226 ohm
For 150 m = 1501000 x 226 = 0339 ohm
From Table 1 Calculate the equivalent resistance for all the combined section
Example for Al = 0339 ohm for copper = 0150 ohm
Equivalent Resistance = 0339+0150 =0489 ohm
Continue
Calculate equivalent resistance from obtained bridge reading
Example Bridge Reading = 60
Equivalent resistance = 06 x 0489 ohm = 02934 ohm
Determine which section of cable the fault occurs
Total resistance at Al section = 0339 ohm
As equivalent resistance at Al section is higher then 02934(Equivalent resistance from bridge test) the fault is
located at Aluminium section of cable
Table 1 Impedance data for 635011000 V Cable Aluminium
Table II Impedance data 11000 V Cable Copper
TABLE VI ndash 6001000V (ALUMINIUM)
TABLE VII ndash 38006600V (ALUMINIUM)
TABLE VIII- 635011000 (ALUMINIUM)
TABLE V
610 COMMISSIONING OF UNDERGROUND CABLE
High Voltage DC testing
High voltage testing is carried out in order to determine the electrical strength of cable insulation Site tests are performed by applying a predetermined high voltage to the insulation DC site test voltages regardless of insulation type are used to ensure that the cable cable joints and termination are correctly made and installed
High Voltage Testing (cont-)
IEC Standards specify maintenance tests after installation should be 70 of permissible factory test DC voltages (15 minutes) = 175kV
Insulation Resistance Test (cont-)
Test voltages should be slowly raised to the required value over a period of about 1 minute and the test period starts once the full voltage is reached In this way the capacitive and absorption currents will have decreased and circuit conditions stabilized such that true leakage current may be measured
Open loop and close loop test (cont)
Fault distance from front end
Lxm
mn
X 2
1001
)200200
(
Where m = open loop bridge reading
n = close loop bridge reading
L = cable route length
High Tension Bridge
Conditions required
i) No break in continuity if possible one sound core available
ii) The fault resistance of cable should be more then 20 k-ohm
iii) Applied voltage not to exceed 20kV (DC)
High Tension Bridge
Case 1Single Conductor to earth with sound core available
Fault Distance from test end X = n100 x 2L
High Tension Bridge
Case 2Conductor to conductor with sound core available
Fault Distance from test end X = n100 x 2L
High Tension Bridge
Case 3Three phase Fault with no sound core available
Rf2Rf1gt5 Rf2gt20 kohm
Lxm
)mn
(X 2
1002
1
100100
Where m = open loop bridge reading
n = close loop bridge reading
L = cable route length
6912 Universal Bridge to measure capacitance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be more then 100 ohm
iii) Earth all the conductors not under test
6912 Universal Bridge to measure capacitance of Cable
Open circuit with sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with sound core available
Fault Distance from test end x = CA x L CAB
CA = Capacitance of open circuited coreCAB = Capacitance of sound coreL = conductor length
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
Fault Distance from test endfront end = ((CA(CA+CB))x L
6913 Universal Bridge to measure Inductance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be less then 30 ohm
iii) All conductors be left free from earth
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
Fault distance X = LA x L LAB
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
Fault distance X = LA x L LA + LB
692 Pulse Echo (PE) Time Domain Reflectometry (TDR)
A simple method which works on travelling wave principles It is applicable to all series and shunt faults with Rf gt=Zo10 (5 ohm) and Rflt=10 Zo (500 ohm) respectively For power cables typical Zo=50 ohm
6921 Travelling wave principles
A pulse injected into a cable is reflected back to the source by any change in characteristics impedance (Zo) of the cable
The waveform generated can be monitored using an oscilloscope as shown in Figure below
6921 Continue-
Waveform interpretion
Shunt Fault
Series Fault
6921 Continue-
Waveform interpretion
Low Resistance Shunt Fault (Rflt=500 Ohm)
-Reflection Negative
-Amplitude depends on fault resistance
Series Fault (Rfgt=5 ohm)
- Reflection Positive
- Amplitude depends on fault resistance
Zo = Characteristics impedance = radic(LC)
L = Inductance of cable C = Capacitance of Cable
69211 Reflection Factor (r)
- Is to determine the degree of reflection
Shunt fault r (V) = -Zo1(2Rf + Zo1)
Series Fault r (V) = Rf(2Zo1 + Rf)
Example shunt fault fault resistance = 5 ohm Zo = 50 ohm
r(V) = -50 (2 x5 +50) = - 833If fault resistance = 500 ohm
r(V) = -50 (2x 500 + 50) = -475
69212 Velocity of Propagation (Vp)
- Vp depends on the cable dielectric and is defined as
Vp = Vs radicEr
- where Vs = Velocity of light in free space
Er = Relative permittivity of dielectric
Vp = 300radicEr
Dielectrics Velocity of PropagationVp (mmicroS)
Impregnated Paper 150 -171
PVC 152-175
PE Approx 200
XLPE 156-176
Typical Vp for different Dielectric
69212 Continue (Vp)
Example
Determine the velocity of propagation Vp2 for a cable length of 2500 meters t1= 3164 S and t2 = 1745 S Calculate the fault distances
6921 Continue-
The distance to the fault Lf (m) is given by
Lf = T x Vp2
6921 Continue
Solutions
Velocity of propagation = Vp2 = 25003164 = 79 mS
Distance of Fault from test end = Vp2 x t2
= 79 x 1745
= 137855
693 Impulse Current Method
Working on Travelling wave principles it is applicable to all types of fault
694 Arc Reflection or Secondary Impulse method
Basically PE and TDR associated with fault treatment It is also applicable to faults of all nature but with easily interpreted breakdown waveforms
610 PIN POINTING
Pin Pointing is the application of a test that positively confirms the exact position of the fault
Before the commencement of pin pointing the prelocated fault distance should mark on the cable route which is measured by means of a trumeter
Pin pointing is normally carried out by the shock wave discharge method The fault can be detected by the use of semisphone
611 Confirmation and Re-Test
After the pin-pointed position of the fault has been marked and exposed check for physical sign of fault If there is the fault is confirm
After confirmation the fault should be cut away IR and continuity test should be carried out on the remaining cable sections to determine the soundeness of these cable sections
The insulation resistance test are again carried out after jointing followed by pressure test before supply can be restored
Flowchart for Fault location Procedure
Example
Find a fault distance using direct reading fault localiser from end test which has a test data as below
Bridge Balance Reading =60 Length of Cable 140 meters
Solutions
Fault Distance= = 60100 x 140 = 84 meterxL
n
100
For cables route with combination of 2 or more different type of cable
Determine the resistance of cable for each section separately from Table 1 and Table 2
Example Cable 11kV Al Length 150 m size = 16mm2
From Table 1= Resistance for 1km =226 ohm
For 150 m = 1501000 x 226 = 0339 ohm
From Table 1 Calculate the equivalent resistance for all the combined section
Example for Al = 0339 ohm for copper = 0150 ohm
Equivalent Resistance = 0339+0150 =0489 ohm
Continue
Calculate equivalent resistance from obtained bridge reading
Example Bridge Reading = 60
Equivalent resistance = 06 x 0489 ohm = 02934 ohm
Determine which section of cable the fault occurs
Total resistance at Al section = 0339 ohm
As equivalent resistance at Al section is higher then 02934(Equivalent resistance from bridge test) the fault is
located at Aluminium section of cable
Table 1 Impedance data for 635011000 V Cable Aluminium
Table II Impedance data 11000 V Cable Copper
TABLE VI ndash 6001000V (ALUMINIUM)
TABLE VII ndash 38006600V (ALUMINIUM)
TABLE VIII- 635011000 (ALUMINIUM)
TABLE V
610 COMMISSIONING OF UNDERGROUND CABLE
High Voltage DC testing
High voltage testing is carried out in order to determine the electrical strength of cable insulation Site tests are performed by applying a predetermined high voltage to the insulation DC site test voltages regardless of insulation type are used to ensure that the cable cable joints and termination are correctly made and installed
High Voltage Testing (cont-)
IEC Standards specify maintenance tests after installation should be 70 of permissible factory test DC voltages (15 minutes) = 175kV
Insulation Resistance Test (cont-)
Test voltages should be slowly raised to the required value over a period of about 1 minute and the test period starts once the full voltage is reached In this way the capacitive and absorption currents will have decreased and circuit conditions stabilized such that true leakage current may be measured
High Tension Bridge
Conditions required
i) No break in continuity if possible one sound core available
ii) The fault resistance of cable should be more then 20 k-ohm
iii) Applied voltage not to exceed 20kV (DC)
High Tension Bridge
Case 1Single Conductor to earth with sound core available
Fault Distance from test end X = n100 x 2L
High Tension Bridge
Case 2Conductor to conductor with sound core available
Fault Distance from test end X = n100 x 2L
High Tension Bridge
Case 3Three phase Fault with no sound core available
Rf2Rf1gt5 Rf2gt20 kohm
Lxm
)mn
(X 2
1002
1
100100
Where m = open loop bridge reading
n = close loop bridge reading
L = cable route length
6912 Universal Bridge to measure capacitance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be more then 100 ohm
iii) Earth all the conductors not under test
6912 Universal Bridge to measure capacitance of Cable
Open circuit with sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with sound core available
Fault Distance from test end x = CA x L CAB
CA = Capacitance of open circuited coreCAB = Capacitance of sound coreL = conductor length
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
Fault Distance from test endfront end = ((CA(CA+CB))x L
6913 Universal Bridge to measure Inductance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be less then 30 ohm
iii) All conductors be left free from earth
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
Fault distance X = LA x L LAB
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
Fault distance X = LA x L LA + LB
692 Pulse Echo (PE) Time Domain Reflectometry (TDR)
A simple method which works on travelling wave principles It is applicable to all series and shunt faults with Rf gt=Zo10 (5 ohm) and Rflt=10 Zo (500 ohm) respectively For power cables typical Zo=50 ohm
6921 Travelling wave principles
A pulse injected into a cable is reflected back to the source by any change in characteristics impedance (Zo) of the cable
The waveform generated can be monitored using an oscilloscope as shown in Figure below
6921 Continue-
Waveform interpretion
Shunt Fault
Series Fault
6921 Continue-
Waveform interpretion
Low Resistance Shunt Fault (Rflt=500 Ohm)
-Reflection Negative
-Amplitude depends on fault resistance
Series Fault (Rfgt=5 ohm)
- Reflection Positive
- Amplitude depends on fault resistance
Zo = Characteristics impedance = radic(LC)
L = Inductance of cable C = Capacitance of Cable
69211 Reflection Factor (r)
- Is to determine the degree of reflection
Shunt fault r (V) = -Zo1(2Rf + Zo1)
Series Fault r (V) = Rf(2Zo1 + Rf)
Example shunt fault fault resistance = 5 ohm Zo = 50 ohm
r(V) = -50 (2 x5 +50) = - 833If fault resistance = 500 ohm
r(V) = -50 (2x 500 + 50) = -475
69212 Velocity of Propagation (Vp)
- Vp depends on the cable dielectric and is defined as
Vp = Vs radicEr
- where Vs = Velocity of light in free space
Er = Relative permittivity of dielectric
Vp = 300radicEr
Dielectrics Velocity of PropagationVp (mmicroS)
Impregnated Paper 150 -171
PVC 152-175
PE Approx 200
XLPE 156-176
Typical Vp for different Dielectric
69212 Continue (Vp)
Example
Determine the velocity of propagation Vp2 for a cable length of 2500 meters t1= 3164 S and t2 = 1745 S Calculate the fault distances
6921 Continue-
The distance to the fault Lf (m) is given by
Lf = T x Vp2
6921 Continue
Solutions
Velocity of propagation = Vp2 = 25003164 = 79 mS
Distance of Fault from test end = Vp2 x t2
= 79 x 1745
= 137855
693 Impulse Current Method
Working on Travelling wave principles it is applicable to all types of fault
694 Arc Reflection or Secondary Impulse method
Basically PE and TDR associated with fault treatment It is also applicable to faults of all nature but with easily interpreted breakdown waveforms
610 PIN POINTING
Pin Pointing is the application of a test that positively confirms the exact position of the fault
Before the commencement of pin pointing the prelocated fault distance should mark on the cable route which is measured by means of a trumeter
Pin pointing is normally carried out by the shock wave discharge method The fault can be detected by the use of semisphone
611 Confirmation and Re-Test
After the pin-pointed position of the fault has been marked and exposed check for physical sign of fault If there is the fault is confirm
After confirmation the fault should be cut away IR and continuity test should be carried out on the remaining cable sections to determine the soundeness of these cable sections
The insulation resistance test are again carried out after jointing followed by pressure test before supply can be restored
Flowchart for Fault location Procedure
Example
Find a fault distance using direct reading fault localiser from end test which has a test data as below
Bridge Balance Reading =60 Length of Cable 140 meters
Solutions
Fault Distance= = 60100 x 140 = 84 meterxL
n
100
For cables route with combination of 2 or more different type of cable
Determine the resistance of cable for each section separately from Table 1 and Table 2
Example Cable 11kV Al Length 150 m size = 16mm2
From Table 1= Resistance for 1km =226 ohm
For 150 m = 1501000 x 226 = 0339 ohm
From Table 1 Calculate the equivalent resistance for all the combined section
Example for Al = 0339 ohm for copper = 0150 ohm
Equivalent Resistance = 0339+0150 =0489 ohm
Continue
Calculate equivalent resistance from obtained bridge reading
Example Bridge Reading = 60
Equivalent resistance = 06 x 0489 ohm = 02934 ohm
Determine which section of cable the fault occurs
Total resistance at Al section = 0339 ohm
As equivalent resistance at Al section is higher then 02934(Equivalent resistance from bridge test) the fault is
located at Aluminium section of cable
Table 1 Impedance data for 635011000 V Cable Aluminium
Table II Impedance data 11000 V Cable Copper
TABLE VI ndash 6001000V (ALUMINIUM)
TABLE VII ndash 38006600V (ALUMINIUM)
TABLE VIII- 635011000 (ALUMINIUM)
TABLE V
610 COMMISSIONING OF UNDERGROUND CABLE
High Voltage DC testing
High voltage testing is carried out in order to determine the electrical strength of cable insulation Site tests are performed by applying a predetermined high voltage to the insulation DC site test voltages regardless of insulation type are used to ensure that the cable cable joints and termination are correctly made and installed
High Voltage Testing (cont-)
IEC Standards specify maintenance tests after installation should be 70 of permissible factory test DC voltages (15 minutes) = 175kV
Insulation Resistance Test (cont-)
Test voltages should be slowly raised to the required value over a period of about 1 minute and the test period starts once the full voltage is reached In this way the capacitive and absorption currents will have decreased and circuit conditions stabilized such that true leakage current may be measured
High Tension Bridge
Case 1Single Conductor to earth with sound core available
Fault Distance from test end X = n100 x 2L
High Tension Bridge
Case 2Conductor to conductor with sound core available
Fault Distance from test end X = n100 x 2L
High Tension Bridge
Case 3Three phase Fault with no sound core available
Rf2Rf1gt5 Rf2gt20 kohm
Lxm
)mn
(X 2
1002
1
100100
Where m = open loop bridge reading
n = close loop bridge reading
L = cable route length
6912 Universal Bridge to measure capacitance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be more then 100 ohm
iii) Earth all the conductors not under test
6912 Universal Bridge to measure capacitance of Cable
Open circuit with sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with sound core available
Fault Distance from test end x = CA x L CAB
CA = Capacitance of open circuited coreCAB = Capacitance of sound coreL = conductor length
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
Fault Distance from test endfront end = ((CA(CA+CB))x L
6913 Universal Bridge to measure Inductance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be less then 30 ohm
iii) All conductors be left free from earth
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
Fault distance X = LA x L LAB
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
Fault distance X = LA x L LA + LB
692 Pulse Echo (PE) Time Domain Reflectometry (TDR)
A simple method which works on travelling wave principles It is applicable to all series and shunt faults with Rf gt=Zo10 (5 ohm) and Rflt=10 Zo (500 ohm) respectively For power cables typical Zo=50 ohm
6921 Travelling wave principles
A pulse injected into a cable is reflected back to the source by any change in characteristics impedance (Zo) of the cable
The waveform generated can be monitored using an oscilloscope as shown in Figure below
6921 Continue-
Waveform interpretion
Shunt Fault
Series Fault
6921 Continue-
Waveform interpretion
Low Resistance Shunt Fault (Rflt=500 Ohm)
-Reflection Negative
-Amplitude depends on fault resistance
Series Fault (Rfgt=5 ohm)
- Reflection Positive
- Amplitude depends on fault resistance
Zo = Characteristics impedance = radic(LC)
L = Inductance of cable C = Capacitance of Cable
69211 Reflection Factor (r)
- Is to determine the degree of reflection
Shunt fault r (V) = -Zo1(2Rf + Zo1)
Series Fault r (V) = Rf(2Zo1 + Rf)
Example shunt fault fault resistance = 5 ohm Zo = 50 ohm
r(V) = -50 (2 x5 +50) = - 833If fault resistance = 500 ohm
r(V) = -50 (2x 500 + 50) = -475
69212 Velocity of Propagation (Vp)
- Vp depends on the cable dielectric and is defined as
Vp = Vs radicEr
- where Vs = Velocity of light in free space
Er = Relative permittivity of dielectric
Vp = 300radicEr
Dielectrics Velocity of PropagationVp (mmicroS)
Impregnated Paper 150 -171
PVC 152-175
PE Approx 200
XLPE 156-176
Typical Vp for different Dielectric
69212 Continue (Vp)
Example
Determine the velocity of propagation Vp2 for a cable length of 2500 meters t1= 3164 S and t2 = 1745 S Calculate the fault distances
6921 Continue-
The distance to the fault Lf (m) is given by
Lf = T x Vp2
6921 Continue
Solutions
Velocity of propagation = Vp2 = 25003164 = 79 mS
Distance of Fault from test end = Vp2 x t2
= 79 x 1745
= 137855
693 Impulse Current Method
Working on Travelling wave principles it is applicable to all types of fault
694 Arc Reflection or Secondary Impulse method
Basically PE and TDR associated with fault treatment It is also applicable to faults of all nature but with easily interpreted breakdown waveforms
610 PIN POINTING
Pin Pointing is the application of a test that positively confirms the exact position of the fault
Before the commencement of pin pointing the prelocated fault distance should mark on the cable route which is measured by means of a trumeter
Pin pointing is normally carried out by the shock wave discharge method The fault can be detected by the use of semisphone
611 Confirmation and Re-Test
After the pin-pointed position of the fault has been marked and exposed check for physical sign of fault If there is the fault is confirm
After confirmation the fault should be cut away IR and continuity test should be carried out on the remaining cable sections to determine the soundeness of these cable sections
The insulation resistance test are again carried out after jointing followed by pressure test before supply can be restored
Flowchart for Fault location Procedure
Example
Find a fault distance using direct reading fault localiser from end test which has a test data as below
Bridge Balance Reading =60 Length of Cable 140 meters
Solutions
Fault Distance= = 60100 x 140 = 84 meterxL
n
100
For cables route with combination of 2 or more different type of cable
Determine the resistance of cable for each section separately from Table 1 and Table 2
Example Cable 11kV Al Length 150 m size = 16mm2
From Table 1= Resistance for 1km =226 ohm
For 150 m = 1501000 x 226 = 0339 ohm
From Table 1 Calculate the equivalent resistance for all the combined section
Example for Al = 0339 ohm for copper = 0150 ohm
Equivalent Resistance = 0339+0150 =0489 ohm
Continue
Calculate equivalent resistance from obtained bridge reading
Example Bridge Reading = 60
Equivalent resistance = 06 x 0489 ohm = 02934 ohm
Determine which section of cable the fault occurs
Total resistance at Al section = 0339 ohm
As equivalent resistance at Al section is higher then 02934(Equivalent resistance from bridge test) the fault is
located at Aluminium section of cable
Table 1 Impedance data for 635011000 V Cable Aluminium
Table II Impedance data 11000 V Cable Copper
TABLE VI ndash 6001000V (ALUMINIUM)
TABLE VII ndash 38006600V (ALUMINIUM)
TABLE VIII- 635011000 (ALUMINIUM)
TABLE V
610 COMMISSIONING OF UNDERGROUND CABLE
High Voltage DC testing
High voltage testing is carried out in order to determine the electrical strength of cable insulation Site tests are performed by applying a predetermined high voltage to the insulation DC site test voltages regardless of insulation type are used to ensure that the cable cable joints and termination are correctly made and installed
High Voltage Testing (cont-)
IEC Standards specify maintenance tests after installation should be 70 of permissible factory test DC voltages (15 minutes) = 175kV
Insulation Resistance Test (cont-)
Test voltages should be slowly raised to the required value over a period of about 1 minute and the test period starts once the full voltage is reached In this way the capacitive and absorption currents will have decreased and circuit conditions stabilized such that true leakage current may be measured
High Tension Bridge
Case 2Conductor to conductor with sound core available
Fault Distance from test end X = n100 x 2L
High Tension Bridge
Case 3Three phase Fault with no sound core available
Rf2Rf1gt5 Rf2gt20 kohm
Lxm
)mn
(X 2
1002
1
100100
Where m = open loop bridge reading
n = close loop bridge reading
L = cable route length
6912 Universal Bridge to measure capacitance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be more then 100 ohm
iii) Earth all the conductors not under test
6912 Universal Bridge to measure capacitance of Cable
Open circuit with sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with sound core available
Fault Distance from test end x = CA x L CAB
CA = Capacitance of open circuited coreCAB = Capacitance of sound coreL = conductor length
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
Fault Distance from test endfront end = ((CA(CA+CB))x L
6913 Universal Bridge to measure Inductance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be less then 30 ohm
iii) All conductors be left free from earth
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
Fault distance X = LA x L LAB
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
Fault distance X = LA x L LA + LB
692 Pulse Echo (PE) Time Domain Reflectometry (TDR)
A simple method which works on travelling wave principles It is applicable to all series and shunt faults with Rf gt=Zo10 (5 ohm) and Rflt=10 Zo (500 ohm) respectively For power cables typical Zo=50 ohm
6921 Travelling wave principles
A pulse injected into a cable is reflected back to the source by any change in characteristics impedance (Zo) of the cable
The waveform generated can be monitored using an oscilloscope as shown in Figure below
6921 Continue-
Waveform interpretion
Shunt Fault
Series Fault
6921 Continue-
Waveform interpretion
Low Resistance Shunt Fault (Rflt=500 Ohm)
-Reflection Negative
-Amplitude depends on fault resistance
Series Fault (Rfgt=5 ohm)
- Reflection Positive
- Amplitude depends on fault resistance
Zo = Characteristics impedance = radic(LC)
L = Inductance of cable C = Capacitance of Cable
69211 Reflection Factor (r)
- Is to determine the degree of reflection
Shunt fault r (V) = -Zo1(2Rf + Zo1)
Series Fault r (V) = Rf(2Zo1 + Rf)
Example shunt fault fault resistance = 5 ohm Zo = 50 ohm
r(V) = -50 (2 x5 +50) = - 833If fault resistance = 500 ohm
r(V) = -50 (2x 500 + 50) = -475
69212 Velocity of Propagation (Vp)
- Vp depends on the cable dielectric and is defined as
Vp = Vs radicEr
- where Vs = Velocity of light in free space
Er = Relative permittivity of dielectric
Vp = 300radicEr
Dielectrics Velocity of PropagationVp (mmicroS)
Impregnated Paper 150 -171
PVC 152-175
PE Approx 200
XLPE 156-176
Typical Vp for different Dielectric
69212 Continue (Vp)
Example
Determine the velocity of propagation Vp2 for a cable length of 2500 meters t1= 3164 S and t2 = 1745 S Calculate the fault distances
6921 Continue-
The distance to the fault Lf (m) is given by
Lf = T x Vp2
6921 Continue
Solutions
Velocity of propagation = Vp2 = 25003164 = 79 mS
Distance of Fault from test end = Vp2 x t2
= 79 x 1745
= 137855
693 Impulse Current Method
Working on Travelling wave principles it is applicable to all types of fault
694 Arc Reflection or Secondary Impulse method
Basically PE and TDR associated with fault treatment It is also applicable to faults of all nature but with easily interpreted breakdown waveforms
610 PIN POINTING
Pin Pointing is the application of a test that positively confirms the exact position of the fault
Before the commencement of pin pointing the prelocated fault distance should mark on the cable route which is measured by means of a trumeter
Pin pointing is normally carried out by the shock wave discharge method The fault can be detected by the use of semisphone
611 Confirmation and Re-Test
After the pin-pointed position of the fault has been marked and exposed check for physical sign of fault If there is the fault is confirm
After confirmation the fault should be cut away IR and continuity test should be carried out on the remaining cable sections to determine the soundeness of these cable sections
The insulation resistance test are again carried out after jointing followed by pressure test before supply can be restored
Flowchart for Fault location Procedure
Example
Find a fault distance using direct reading fault localiser from end test which has a test data as below
Bridge Balance Reading =60 Length of Cable 140 meters
Solutions
Fault Distance= = 60100 x 140 = 84 meterxL
n
100
For cables route with combination of 2 or more different type of cable
Determine the resistance of cable for each section separately from Table 1 and Table 2
Example Cable 11kV Al Length 150 m size = 16mm2
From Table 1= Resistance for 1km =226 ohm
For 150 m = 1501000 x 226 = 0339 ohm
From Table 1 Calculate the equivalent resistance for all the combined section
Example for Al = 0339 ohm for copper = 0150 ohm
Equivalent Resistance = 0339+0150 =0489 ohm
Continue
Calculate equivalent resistance from obtained bridge reading
Example Bridge Reading = 60
Equivalent resistance = 06 x 0489 ohm = 02934 ohm
Determine which section of cable the fault occurs
Total resistance at Al section = 0339 ohm
As equivalent resistance at Al section is higher then 02934(Equivalent resistance from bridge test) the fault is
located at Aluminium section of cable
Table 1 Impedance data for 635011000 V Cable Aluminium
Table II Impedance data 11000 V Cable Copper
TABLE VI ndash 6001000V (ALUMINIUM)
TABLE VII ndash 38006600V (ALUMINIUM)
TABLE VIII- 635011000 (ALUMINIUM)
TABLE V
610 COMMISSIONING OF UNDERGROUND CABLE
High Voltage DC testing
High voltage testing is carried out in order to determine the electrical strength of cable insulation Site tests are performed by applying a predetermined high voltage to the insulation DC site test voltages regardless of insulation type are used to ensure that the cable cable joints and termination are correctly made and installed
High Voltage Testing (cont-)
IEC Standards specify maintenance tests after installation should be 70 of permissible factory test DC voltages (15 minutes) = 175kV
Insulation Resistance Test (cont-)
Test voltages should be slowly raised to the required value over a period of about 1 minute and the test period starts once the full voltage is reached In this way the capacitive and absorption currents will have decreased and circuit conditions stabilized such that true leakage current may be measured
High Tension Bridge
Case 3Three phase Fault with no sound core available
Rf2Rf1gt5 Rf2gt20 kohm
Lxm
)mn
(X 2
1002
1
100100
Where m = open loop bridge reading
n = close loop bridge reading
L = cable route length
6912 Universal Bridge to measure capacitance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be more then 100 ohm
iii) Earth all the conductors not under test
6912 Universal Bridge to measure capacitance of Cable
Open circuit with sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with sound core available
Fault Distance from test end x = CA x L CAB
CA = Capacitance of open circuited coreCAB = Capacitance of sound coreL = conductor length
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
Fault Distance from test endfront end = ((CA(CA+CB))x L
6913 Universal Bridge to measure Inductance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be less then 30 ohm
iii) All conductors be left free from earth
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
Fault distance X = LA x L LAB
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
Fault distance X = LA x L LA + LB
692 Pulse Echo (PE) Time Domain Reflectometry (TDR)
A simple method which works on travelling wave principles It is applicable to all series and shunt faults with Rf gt=Zo10 (5 ohm) and Rflt=10 Zo (500 ohm) respectively For power cables typical Zo=50 ohm
6921 Travelling wave principles
A pulse injected into a cable is reflected back to the source by any change in characteristics impedance (Zo) of the cable
The waveform generated can be monitored using an oscilloscope as shown in Figure below
6921 Continue-
Waveform interpretion
Shunt Fault
Series Fault
6921 Continue-
Waveform interpretion
Low Resistance Shunt Fault (Rflt=500 Ohm)
-Reflection Negative
-Amplitude depends on fault resistance
Series Fault (Rfgt=5 ohm)
- Reflection Positive
- Amplitude depends on fault resistance
Zo = Characteristics impedance = radic(LC)
L = Inductance of cable C = Capacitance of Cable
69211 Reflection Factor (r)
- Is to determine the degree of reflection
Shunt fault r (V) = -Zo1(2Rf + Zo1)
Series Fault r (V) = Rf(2Zo1 + Rf)
Example shunt fault fault resistance = 5 ohm Zo = 50 ohm
r(V) = -50 (2 x5 +50) = - 833If fault resistance = 500 ohm
r(V) = -50 (2x 500 + 50) = -475
69212 Velocity of Propagation (Vp)
- Vp depends on the cable dielectric and is defined as
Vp = Vs radicEr
- where Vs = Velocity of light in free space
Er = Relative permittivity of dielectric
Vp = 300radicEr
Dielectrics Velocity of PropagationVp (mmicroS)
Impregnated Paper 150 -171
PVC 152-175
PE Approx 200
XLPE 156-176
Typical Vp for different Dielectric
69212 Continue (Vp)
Example
Determine the velocity of propagation Vp2 for a cable length of 2500 meters t1= 3164 S and t2 = 1745 S Calculate the fault distances
6921 Continue-
The distance to the fault Lf (m) is given by
Lf = T x Vp2
6921 Continue
Solutions
Velocity of propagation = Vp2 = 25003164 = 79 mS
Distance of Fault from test end = Vp2 x t2
= 79 x 1745
= 137855
693 Impulse Current Method
Working on Travelling wave principles it is applicable to all types of fault
694 Arc Reflection or Secondary Impulse method
Basically PE and TDR associated with fault treatment It is also applicable to faults of all nature but with easily interpreted breakdown waveforms
610 PIN POINTING
Pin Pointing is the application of a test that positively confirms the exact position of the fault
Before the commencement of pin pointing the prelocated fault distance should mark on the cable route which is measured by means of a trumeter
Pin pointing is normally carried out by the shock wave discharge method The fault can be detected by the use of semisphone
611 Confirmation and Re-Test
After the pin-pointed position of the fault has been marked and exposed check for physical sign of fault If there is the fault is confirm
After confirmation the fault should be cut away IR and continuity test should be carried out on the remaining cable sections to determine the soundeness of these cable sections
The insulation resistance test are again carried out after jointing followed by pressure test before supply can be restored
Flowchart for Fault location Procedure
Example
Find a fault distance using direct reading fault localiser from end test which has a test data as below
Bridge Balance Reading =60 Length of Cable 140 meters
Solutions
Fault Distance= = 60100 x 140 = 84 meterxL
n
100
For cables route with combination of 2 or more different type of cable
Determine the resistance of cable for each section separately from Table 1 and Table 2
Example Cable 11kV Al Length 150 m size = 16mm2
From Table 1= Resistance for 1km =226 ohm
For 150 m = 1501000 x 226 = 0339 ohm
From Table 1 Calculate the equivalent resistance for all the combined section
Example for Al = 0339 ohm for copper = 0150 ohm
Equivalent Resistance = 0339+0150 =0489 ohm
Continue
Calculate equivalent resistance from obtained bridge reading
Example Bridge Reading = 60
Equivalent resistance = 06 x 0489 ohm = 02934 ohm
Determine which section of cable the fault occurs
Total resistance at Al section = 0339 ohm
As equivalent resistance at Al section is higher then 02934(Equivalent resistance from bridge test) the fault is
located at Aluminium section of cable
Table 1 Impedance data for 635011000 V Cable Aluminium
Table II Impedance data 11000 V Cable Copper
TABLE VI ndash 6001000V (ALUMINIUM)
TABLE VII ndash 38006600V (ALUMINIUM)
TABLE VIII- 635011000 (ALUMINIUM)
TABLE V
610 COMMISSIONING OF UNDERGROUND CABLE
High Voltage DC testing
High voltage testing is carried out in order to determine the electrical strength of cable insulation Site tests are performed by applying a predetermined high voltage to the insulation DC site test voltages regardless of insulation type are used to ensure that the cable cable joints and termination are correctly made and installed
High Voltage Testing (cont-)
IEC Standards specify maintenance tests after installation should be 70 of permissible factory test DC voltages (15 minutes) = 175kV
Insulation Resistance Test (cont-)
Test voltages should be slowly raised to the required value over a period of about 1 minute and the test period starts once the full voltage is reached In this way the capacitive and absorption currents will have decreased and circuit conditions stabilized such that true leakage current may be measured
6912 Universal Bridge to measure capacitance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be more then 100 ohm
iii) Earth all the conductors not under test
6912 Universal Bridge to measure capacitance of Cable
Open circuit with sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with sound core available
Fault Distance from test end x = CA x L CAB
CA = Capacitance of open circuited coreCAB = Capacitance of sound coreL = conductor length
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
Fault Distance from test endfront end = ((CA(CA+CB))x L
6913 Universal Bridge to measure Inductance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be less then 30 ohm
iii) All conductors be left free from earth
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
Fault distance X = LA x L LAB
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
Fault distance X = LA x L LA + LB
692 Pulse Echo (PE) Time Domain Reflectometry (TDR)
A simple method which works on travelling wave principles It is applicable to all series and shunt faults with Rf gt=Zo10 (5 ohm) and Rflt=10 Zo (500 ohm) respectively For power cables typical Zo=50 ohm
6921 Travelling wave principles
A pulse injected into a cable is reflected back to the source by any change in characteristics impedance (Zo) of the cable
The waveform generated can be monitored using an oscilloscope as shown in Figure below
6921 Continue-
Waveform interpretion
Shunt Fault
Series Fault
6921 Continue-
Waveform interpretion
Low Resistance Shunt Fault (Rflt=500 Ohm)
-Reflection Negative
-Amplitude depends on fault resistance
Series Fault (Rfgt=5 ohm)
- Reflection Positive
- Amplitude depends on fault resistance
Zo = Characteristics impedance = radic(LC)
L = Inductance of cable C = Capacitance of Cable
69211 Reflection Factor (r)
- Is to determine the degree of reflection
Shunt fault r (V) = -Zo1(2Rf + Zo1)
Series Fault r (V) = Rf(2Zo1 + Rf)
Example shunt fault fault resistance = 5 ohm Zo = 50 ohm
r(V) = -50 (2 x5 +50) = - 833If fault resistance = 500 ohm
r(V) = -50 (2x 500 + 50) = -475
69212 Velocity of Propagation (Vp)
- Vp depends on the cable dielectric and is defined as
Vp = Vs radicEr
- where Vs = Velocity of light in free space
Er = Relative permittivity of dielectric
Vp = 300radicEr
Dielectrics Velocity of PropagationVp (mmicroS)
Impregnated Paper 150 -171
PVC 152-175
PE Approx 200
XLPE 156-176
Typical Vp for different Dielectric
69212 Continue (Vp)
Example
Determine the velocity of propagation Vp2 for a cable length of 2500 meters t1= 3164 S and t2 = 1745 S Calculate the fault distances
6921 Continue-
The distance to the fault Lf (m) is given by
Lf = T x Vp2
6921 Continue
Solutions
Velocity of propagation = Vp2 = 25003164 = 79 mS
Distance of Fault from test end = Vp2 x t2
= 79 x 1745
= 137855
693 Impulse Current Method
Working on Travelling wave principles it is applicable to all types of fault
694 Arc Reflection or Secondary Impulse method
Basically PE and TDR associated with fault treatment It is also applicable to faults of all nature but with easily interpreted breakdown waveforms
610 PIN POINTING
Pin Pointing is the application of a test that positively confirms the exact position of the fault
Before the commencement of pin pointing the prelocated fault distance should mark on the cable route which is measured by means of a trumeter
Pin pointing is normally carried out by the shock wave discharge method The fault can be detected by the use of semisphone
611 Confirmation and Re-Test
After the pin-pointed position of the fault has been marked and exposed check for physical sign of fault If there is the fault is confirm
After confirmation the fault should be cut away IR and continuity test should be carried out on the remaining cable sections to determine the soundeness of these cable sections
The insulation resistance test are again carried out after jointing followed by pressure test before supply can be restored
Flowchart for Fault location Procedure
Example
Find a fault distance using direct reading fault localiser from end test which has a test data as below
Bridge Balance Reading =60 Length of Cable 140 meters
Solutions
Fault Distance= = 60100 x 140 = 84 meterxL
n
100
For cables route with combination of 2 or more different type of cable
Determine the resistance of cable for each section separately from Table 1 and Table 2
Example Cable 11kV Al Length 150 m size = 16mm2
From Table 1= Resistance for 1km =226 ohm
For 150 m = 1501000 x 226 = 0339 ohm
From Table 1 Calculate the equivalent resistance for all the combined section
Example for Al = 0339 ohm for copper = 0150 ohm
Equivalent Resistance = 0339+0150 =0489 ohm
Continue
Calculate equivalent resistance from obtained bridge reading
Example Bridge Reading = 60
Equivalent resistance = 06 x 0489 ohm = 02934 ohm
Determine which section of cable the fault occurs
Total resistance at Al section = 0339 ohm
As equivalent resistance at Al section is higher then 02934(Equivalent resistance from bridge test) the fault is
located at Aluminium section of cable
Table 1 Impedance data for 635011000 V Cable Aluminium
Table II Impedance data 11000 V Cable Copper
TABLE VI ndash 6001000V (ALUMINIUM)
TABLE VII ndash 38006600V (ALUMINIUM)
TABLE VIII- 635011000 (ALUMINIUM)
TABLE V
610 COMMISSIONING OF UNDERGROUND CABLE
High Voltage DC testing
High voltage testing is carried out in order to determine the electrical strength of cable insulation Site tests are performed by applying a predetermined high voltage to the insulation DC site test voltages regardless of insulation type are used to ensure that the cable cable joints and termination are correctly made and installed
High Voltage Testing (cont-)
IEC Standards specify maintenance tests after installation should be 70 of permissible factory test DC voltages (15 minutes) = 175kV
Insulation Resistance Test (cont-)
Test voltages should be slowly raised to the required value over a period of about 1 minute and the test period starts once the full voltage is reached In this way the capacitive and absorption currents will have decreased and circuit conditions stabilized such that true leakage current may be measured
6912 Universal Bridge to measure capacitance of Cable
Open circuit with sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with sound core available
Fault Distance from test end x = CA x L CAB
CA = Capacitance of open circuited coreCAB = Capacitance of sound coreL = conductor length
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
Fault Distance from test endfront end = ((CA(CA+CB))x L
6913 Universal Bridge to measure Inductance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be less then 30 ohm
iii) All conductors be left free from earth
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
Fault distance X = LA x L LAB
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
Fault distance X = LA x L LA + LB
692 Pulse Echo (PE) Time Domain Reflectometry (TDR)
A simple method which works on travelling wave principles It is applicable to all series and shunt faults with Rf gt=Zo10 (5 ohm) and Rflt=10 Zo (500 ohm) respectively For power cables typical Zo=50 ohm
6921 Travelling wave principles
A pulse injected into a cable is reflected back to the source by any change in characteristics impedance (Zo) of the cable
The waveform generated can be monitored using an oscilloscope as shown in Figure below
6921 Continue-
Waveform interpretion
Shunt Fault
Series Fault
6921 Continue-
Waveform interpretion
Low Resistance Shunt Fault (Rflt=500 Ohm)
-Reflection Negative
-Amplitude depends on fault resistance
Series Fault (Rfgt=5 ohm)
- Reflection Positive
- Amplitude depends on fault resistance
Zo = Characteristics impedance = radic(LC)
L = Inductance of cable C = Capacitance of Cable
69211 Reflection Factor (r)
- Is to determine the degree of reflection
Shunt fault r (V) = -Zo1(2Rf + Zo1)
Series Fault r (V) = Rf(2Zo1 + Rf)
Example shunt fault fault resistance = 5 ohm Zo = 50 ohm
r(V) = -50 (2 x5 +50) = - 833If fault resistance = 500 ohm
r(V) = -50 (2x 500 + 50) = -475
69212 Velocity of Propagation (Vp)
- Vp depends on the cable dielectric and is defined as
Vp = Vs radicEr
- where Vs = Velocity of light in free space
Er = Relative permittivity of dielectric
Vp = 300radicEr
Dielectrics Velocity of PropagationVp (mmicroS)
Impregnated Paper 150 -171
PVC 152-175
PE Approx 200
XLPE 156-176
Typical Vp for different Dielectric
69212 Continue (Vp)
Example
Determine the velocity of propagation Vp2 for a cable length of 2500 meters t1= 3164 S and t2 = 1745 S Calculate the fault distances
6921 Continue-
The distance to the fault Lf (m) is given by
Lf = T x Vp2
6921 Continue
Solutions
Velocity of propagation = Vp2 = 25003164 = 79 mS
Distance of Fault from test end = Vp2 x t2
= 79 x 1745
= 137855
693 Impulse Current Method
Working on Travelling wave principles it is applicable to all types of fault
694 Arc Reflection or Secondary Impulse method
Basically PE and TDR associated with fault treatment It is also applicable to faults of all nature but with easily interpreted breakdown waveforms
610 PIN POINTING
Pin Pointing is the application of a test that positively confirms the exact position of the fault
Before the commencement of pin pointing the prelocated fault distance should mark on the cable route which is measured by means of a trumeter
Pin pointing is normally carried out by the shock wave discharge method The fault can be detected by the use of semisphone
611 Confirmation and Re-Test
After the pin-pointed position of the fault has been marked and exposed check for physical sign of fault If there is the fault is confirm
After confirmation the fault should be cut away IR and continuity test should be carried out on the remaining cable sections to determine the soundeness of these cable sections
The insulation resistance test are again carried out after jointing followed by pressure test before supply can be restored
Flowchart for Fault location Procedure
Example
Find a fault distance using direct reading fault localiser from end test which has a test data as below
Bridge Balance Reading =60 Length of Cable 140 meters
Solutions
Fault Distance= = 60100 x 140 = 84 meterxL
n
100
For cables route with combination of 2 or more different type of cable
Determine the resistance of cable for each section separately from Table 1 and Table 2
Example Cable 11kV Al Length 150 m size = 16mm2
From Table 1= Resistance for 1km =226 ohm
For 150 m = 1501000 x 226 = 0339 ohm
From Table 1 Calculate the equivalent resistance for all the combined section
Example for Al = 0339 ohm for copper = 0150 ohm
Equivalent Resistance = 0339+0150 =0489 ohm
Continue
Calculate equivalent resistance from obtained bridge reading
Example Bridge Reading = 60
Equivalent resistance = 06 x 0489 ohm = 02934 ohm
Determine which section of cable the fault occurs
Total resistance at Al section = 0339 ohm
As equivalent resistance at Al section is higher then 02934(Equivalent resistance from bridge test) the fault is
located at Aluminium section of cable
Table 1 Impedance data for 635011000 V Cable Aluminium
Table II Impedance data 11000 V Cable Copper
TABLE VI ndash 6001000V (ALUMINIUM)
TABLE VII ndash 38006600V (ALUMINIUM)
TABLE VIII- 635011000 (ALUMINIUM)
TABLE V
610 COMMISSIONING OF UNDERGROUND CABLE
High Voltage DC testing
High voltage testing is carried out in order to determine the electrical strength of cable insulation Site tests are performed by applying a predetermined high voltage to the insulation DC site test voltages regardless of insulation type are used to ensure that the cable cable joints and termination are correctly made and installed
High Voltage Testing (cont-)
IEC Standards specify maintenance tests after installation should be 70 of permissible factory test DC voltages (15 minutes) = 175kV
Insulation Resistance Test (cont-)
Test voltages should be slowly raised to the required value over a period of about 1 minute and the test period starts once the full voltage is reached In this way the capacitive and absorption currents will have decreased and circuit conditions stabilized such that true leakage current may be measured
6912 Universal Bridge to measure capacitance of Cable
Open circuit with sound core available
Fault Distance from test end x = CA x L CAB
CA = Capacitance of open circuited coreCAB = Capacitance of sound coreL = conductor length
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
Fault Distance from test endfront end = ((CA(CA+CB))x L
6913 Universal Bridge to measure Inductance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be less then 30 ohm
iii) All conductors be left free from earth
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
Fault distance X = LA x L LAB
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
Fault distance X = LA x L LA + LB
692 Pulse Echo (PE) Time Domain Reflectometry (TDR)
A simple method which works on travelling wave principles It is applicable to all series and shunt faults with Rf gt=Zo10 (5 ohm) and Rflt=10 Zo (500 ohm) respectively For power cables typical Zo=50 ohm
6921 Travelling wave principles
A pulse injected into a cable is reflected back to the source by any change in characteristics impedance (Zo) of the cable
The waveform generated can be monitored using an oscilloscope as shown in Figure below
6921 Continue-
Waveform interpretion
Shunt Fault
Series Fault
6921 Continue-
Waveform interpretion
Low Resistance Shunt Fault (Rflt=500 Ohm)
-Reflection Negative
-Amplitude depends on fault resistance
Series Fault (Rfgt=5 ohm)
- Reflection Positive
- Amplitude depends on fault resistance
Zo = Characteristics impedance = radic(LC)
L = Inductance of cable C = Capacitance of Cable
69211 Reflection Factor (r)
- Is to determine the degree of reflection
Shunt fault r (V) = -Zo1(2Rf + Zo1)
Series Fault r (V) = Rf(2Zo1 + Rf)
Example shunt fault fault resistance = 5 ohm Zo = 50 ohm
r(V) = -50 (2 x5 +50) = - 833If fault resistance = 500 ohm
r(V) = -50 (2x 500 + 50) = -475
69212 Velocity of Propagation (Vp)
- Vp depends on the cable dielectric and is defined as
Vp = Vs radicEr
- where Vs = Velocity of light in free space
Er = Relative permittivity of dielectric
Vp = 300radicEr
Dielectrics Velocity of PropagationVp (mmicroS)
Impregnated Paper 150 -171
PVC 152-175
PE Approx 200
XLPE 156-176
Typical Vp for different Dielectric
69212 Continue (Vp)
Example
Determine the velocity of propagation Vp2 for a cable length of 2500 meters t1= 3164 S and t2 = 1745 S Calculate the fault distances
6921 Continue-
The distance to the fault Lf (m) is given by
Lf = T x Vp2
6921 Continue
Solutions
Velocity of propagation = Vp2 = 25003164 = 79 mS
Distance of Fault from test end = Vp2 x t2
= 79 x 1745
= 137855
693 Impulse Current Method
Working on Travelling wave principles it is applicable to all types of fault
694 Arc Reflection or Secondary Impulse method
Basically PE and TDR associated with fault treatment It is also applicable to faults of all nature but with easily interpreted breakdown waveforms
610 PIN POINTING
Pin Pointing is the application of a test that positively confirms the exact position of the fault
Before the commencement of pin pointing the prelocated fault distance should mark on the cable route which is measured by means of a trumeter
Pin pointing is normally carried out by the shock wave discharge method The fault can be detected by the use of semisphone
611 Confirmation and Re-Test
After the pin-pointed position of the fault has been marked and exposed check for physical sign of fault If there is the fault is confirm
After confirmation the fault should be cut away IR and continuity test should be carried out on the remaining cable sections to determine the soundeness of these cable sections
The insulation resistance test are again carried out after jointing followed by pressure test before supply can be restored
Flowchart for Fault location Procedure
Example
Find a fault distance using direct reading fault localiser from end test which has a test data as below
Bridge Balance Reading =60 Length of Cable 140 meters
Solutions
Fault Distance= = 60100 x 140 = 84 meterxL
n
100
For cables route with combination of 2 or more different type of cable
Determine the resistance of cable for each section separately from Table 1 and Table 2
Example Cable 11kV Al Length 150 m size = 16mm2
From Table 1= Resistance for 1km =226 ohm
For 150 m = 1501000 x 226 = 0339 ohm
From Table 1 Calculate the equivalent resistance for all the combined section
Example for Al = 0339 ohm for copper = 0150 ohm
Equivalent Resistance = 0339+0150 =0489 ohm
Continue
Calculate equivalent resistance from obtained bridge reading
Example Bridge Reading = 60
Equivalent resistance = 06 x 0489 ohm = 02934 ohm
Determine which section of cable the fault occurs
Total resistance at Al section = 0339 ohm
As equivalent resistance at Al section is higher then 02934(Equivalent resistance from bridge test) the fault is
located at Aluminium section of cable
Table 1 Impedance data for 635011000 V Cable Aluminium
Table II Impedance data 11000 V Cable Copper
TABLE VI ndash 6001000V (ALUMINIUM)
TABLE VII ndash 38006600V (ALUMINIUM)
TABLE VIII- 635011000 (ALUMINIUM)
TABLE V
610 COMMISSIONING OF UNDERGROUND CABLE
High Voltage DC testing
High voltage testing is carried out in order to determine the electrical strength of cable insulation Site tests are performed by applying a predetermined high voltage to the insulation DC site test voltages regardless of insulation type are used to ensure that the cable cable joints and termination are correctly made and installed
High Voltage Testing (cont-)
IEC Standards specify maintenance tests after installation should be 70 of permissible factory test DC voltages (15 minutes) = 175kV
Insulation Resistance Test (cont-)
Test voltages should be slowly raised to the required value over a period of about 1 minute and the test period starts once the full voltage is reached In this way the capacitive and absorption currents will have decreased and circuit conditions stabilized such that true leakage current may be measured
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
Fault Distance from test endfront end = ((CA(CA+CB))x L
6913 Universal Bridge to measure Inductance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be less then 30 ohm
iii) All conductors be left free from earth
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
Fault distance X = LA x L LAB
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
Fault distance X = LA x L LA + LB
692 Pulse Echo (PE) Time Domain Reflectometry (TDR)
A simple method which works on travelling wave principles It is applicable to all series and shunt faults with Rf gt=Zo10 (5 ohm) and Rflt=10 Zo (500 ohm) respectively For power cables typical Zo=50 ohm
6921 Travelling wave principles
A pulse injected into a cable is reflected back to the source by any change in characteristics impedance (Zo) of the cable
The waveform generated can be monitored using an oscilloscope as shown in Figure below
6921 Continue-
Waveform interpretion
Shunt Fault
Series Fault
6921 Continue-
Waveform interpretion
Low Resistance Shunt Fault (Rflt=500 Ohm)
-Reflection Negative
-Amplitude depends on fault resistance
Series Fault (Rfgt=5 ohm)
- Reflection Positive
- Amplitude depends on fault resistance
Zo = Characteristics impedance = radic(LC)
L = Inductance of cable C = Capacitance of Cable
69211 Reflection Factor (r)
- Is to determine the degree of reflection
Shunt fault r (V) = -Zo1(2Rf + Zo1)
Series Fault r (V) = Rf(2Zo1 + Rf)
Example shunt fault fault resistance = 5 ohm Zo = 50 ohm
r(V) = -50 (2 x5 +50) = - 833If fault resistance = 500 ohm
r(V) = -50 (2x 500 + 50) = -475
69212 Velocity of Propagation (Vp)
- Vp depends on the cable dielectric and is defined as
Vp = Vs radicEr
- where Vs = Velocity of light in free space
Er = Relative permittivity of dielectric
Vp = 300radicEr
Dielectrics Velocity of PropagationVp (mmicroS)
Impregnated Paper 150 -171
PVC 152-175
PE Approx 200
XLPE 156-176
Typical Vp for different Dielectric
69212 Continue (Vp)
Example
Determine the velocity of propagation Vp2 for a cable length of 2500 meters t1= 3164 S and t2 = 1745 S Calculate the fault distances
6921 Continue-
The distance to the fault Lf (m) is given by
Lf = T x Vp2
6921 Continue
Solutions
Velocity of propagation = Vp2 = 25003164 = 79 mS
Distance of Fault from test end = Vp2 x t2
= 79 x 1745
= 137855
693 Impulse Current Method
Working on Travelling wave principles it is applicable to all types of fault
694 Arc Reflection or Secondary Impulse method
Basically PE and TDR associated with fault treatment It is also applicable to faults of all nature but with easily interpreted breakdown waveforms
610 PIN POINTING
Pin Pointing is the application of a test that positively confirms the exact position of the fault
Before the commencement of pin pointing the prelocated fault distance should mark on the cable route which is measured by means of a trumeter
Pin pointing is normally carried out by the shock wave discharge method The fault can be detected by the use of semisphone
611 Confirmation and Re-Test
After the pin-pointed position of the fault has been marked and exposed check for physical sign of fault If there is the fault is confirm
After confirmation the fault should be cut away IR and continuity test should be carried out on the remaining cable sections to determine the soundeness of these cable sections
The insulation resistance test are again carried out after jointing followed by pressure test before supply can be restored
Flowchart for Fault location Procedure
Example
Find a fault distance using direct reading fault localiser from end test which has a test data as below
Bridge Balance Reading =60 Length of Cable 140 meters
Solutions
Fault Distance= = 60100 x 140 = 84 meterxL
n
100
For cables route with combination of 2 or more different type of cable
Determine the resistance of cable for each section separately from Table 1 and Table 2
Example Cable 11kV Al Length 150 m size = 16mm2
From Table 1= Resistance for 1km =226 ohm
For 150 m = 1501000 x 226 = 0339 ohm
From Table 1 Calculate the equivalent resistance for all the combined section
Example for Al = 0339 ohm for copper = 0150 ohm
Equivalent Resistance = 0339+0150 =0489 ohm
Continue
Calculate equivalent resistance from obtained bridge reading
Example Bridge Reading = 60
Equivalent resistance = 06 x 0489 ohm = 02934 ohm
Determine which section of cable the fault occurs
Total resistance at Al section = 0339 ohm
As equivalent resistance at Al section is higher then 02934(Equivalent resistance from bridge test) the fault is
located at Aluminium section of cable
Table 1 Impedance data for 635011000 V Cable Aluminium
Table II Impedance data 11000 V Cable Copper
TABLE VI ndash 6001000V (ALUMINIUM)
TABLE VII ndash 38006600V (ALUMINIUM)
TABLE VIII- 635011000 (ALUMINIUM)
TABLE V
610 COMMISSIONING OF UNDERGROUND CABLE
High Voltage DC testing
High voltage testing is carried out in order to determine the electrical strength of cable insulation Site tests are performed by applying a predetermined high voltage to the insulation DC site test voltages regardless of insulation type are used to ensure that the cable cable joints and termination are correctly made and installed
High Voltage Testing (cont-)
IEC Standards specify maintenance tests after installation should be 70 of permissible factory test DC voltages (15 minutes) = 175kV
Insulation Resistance Test (cont-)
Test voltages should be slowly raised to the required value over a period of about 1 minute and the test period starts once the full voltage is reached In this way the capacitive and absorption currents will have decreased and circuit conditions stabilized such that true leakage current may be measured
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
Fault Distance from test endfront end = ((CA(CA+CB))x L
6913 Universal Bridge to measure Inductance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be less then 30 ohm
iii) All conductors be left free from earth
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
Fault distance X = LA x L LAB
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
Fault distance X = LA x L LA + LB
692 Pulse Echo (PE) Time Domain Reflectometry (TDR)
A simple method which works on travelling wave principles It is applicable to all series and shunt faults with Rf gt=Zo10 (5 ohm) and Rflt=10 Zo (500 ohm) respectively For power cables typical Zo=50 ohm
6921 Travelling wave principles
A pulse injected into a cable is reflected back to the source by any change in characteristics impedance (Zo) of the cable
The waveform generated can be monitored using an oscilloscope as shown in Figure below
6921 Continue-
Waveform interpretion
Shunt Fault
Series Fault
6921 Continue-
Waveform interpretion
Low Resistance Shunt Fault (Rflt=500 Ohm)
-Reflection Negative
-Amplitude depends on fault resistance
Series Fault (Rfgt=5 ohm)
- Reflection Positive
- Amplitude depends on fault resistance
Zo = Characteristics impedance = radic(LC)
L = Inductance of cable C = Capacitance of Cable
69211 Reflection Factor (r)
- Is to determine the degree of reflection
Shunt fault r (V) = -Zo1(2Rf + Zo1)
Series Fault r (V) = Rf(2Zo1 + Rf)
Example shunt fault fault resistance = 5 ohm Zo = 50 ohm
r(V) = -50 (2 x5 +50) = - 833If fault resistance = 500 ohm
r(V) = -50 (2x 500 + 50) = -475
69212 Velocity of Propagation (Vp)
- Vp depends on the cable dielectric and is defined as
Vp = Vs radicEr
- where Vs = Velocity of light in free space
Er = Relative permittivity of dielectric
Vp = 300radicEr
Dielectrics Velocity of PropagationVp (mmicroS)
Impregnated Paper 150 -171
PVC 152-175
PE Approx 200
XLPE 156-176
Typical Vp for different Dielectric
69212 Continue (Vp)
Example
Determine the velocity of propagation Vp2 for a cable length of 2500 meters t1= 3164 S and t2 = 1745 S Calculate the fault distances
6921 Continue-
The distance to the fault Lf (m) is given by
Lf = T x Vp2
6921 Continue
Solutions
Velocity of propagation = Vp2 = 25003164 = 79 mS
Distance of Fault from test end = Vp2 x t2
= 79 x 1745
= 137855
693 Impulse Current Method
Working on Travelling wave principles it is applicable to all types of fault
694 Arc Reflection or Secondary Impulse method
Basically PE and TDR associated with fault treatment It is also applicable to faults of all nature but with easily interpreted breakdown waveforms
610 PIN POINTING
Pin Pointing is the application of a test that positively confirms the exact position of the fault
Before the commencement of pin pointing the prelocated fault distance should mark on the cable route which is measured by means of a trumeter
Pin pointing is normally carried out by the shock wave discharge method The fault can be detected by the use of semisphone
611 Confirmation and Re-Test
After the pin-pointed position of the fault has been marked and exposed check for physical sign of fault If there is the fault is confirm
After confirmation the fault should be cut away IR and continuity test should be carried out on the remaining cable sections to determine the soundeness of these cable sections
The insulation resistance test are again carried out after jointing followed by pressure test before supply can be restored
Flowchart for Fault location Procedure
Example
Find a fault distance using direct reading fault localiser from end test which has a test data as below
Bridge Balance Reading =60 Length of Cable 140 meters
Solutions
Fault Distance= = 60100 x 140 = 84 meterxL
n
100
For cables route with combination of 2 or more different type of cable
Determine the resistance of cable for each section separately from Table 1 and Table 2
Example Cable 11kV Al Length 150 m size = 16mm2
From Table 1= Resistance for 1km =226 ohm
For 150 m = 1501000 x 226 = 0339 ohm
From Table 1 Calculate the equivalent resistance for all the combined section
Example for Al = 0339 ohm for copper = 0150 ohm
Equivalent Resistance = 0339+0150 =0489 ohm
Continue
Calculate equivalent resistance from obtained bridge reading
Example Bridge Reading = 60
Equivalent resistance = 06 x 0489 ohm = 02934 ohm
Determine which section of cable the fault occurs
Total resistance at Al section = 0339 ohm
As equivalent resistance at Al section is higher then 02934(Equivalent resistance from bridge test) the fault is
located at Aluminium section of cable
Table 1 Impedance data for 635011000 V Cable Aluminium
Table II Impedance data 11000 V Cable Copper
TABLE VI ndash 6001000V (ALUMINIUM)
TABLE VII ndash 38006600V (ALUMINIUM)
TABLE VIII- 635011000 (ALUMINIUM)
TABLE V
610 COMMISSIONING OF UNDERGROUND CABLE
High Voltage DC testing
High voltage testing is carried out in order to determine the electrical strength of cable insulation Site tests are performed by applying a predetermined high voltage to the insulation DC site test voltages regardless of insulation type are used to ensure that the cable cable joints and termination are correctly made and installed
High Voltage Testing (cont-)
IEC Standards specify maintenance tests after installation should be 70 of permissible factory test DC voltages (15 minutes) = 175kV
Insulation Resistance Test (cont-)
Test voltages should be slowly raised to the required value over a period of about 1 minute and the test period starts once the full voltage is reached In this way the capacitive and absorption currents will have decreased and circuit conditions stabilized such that true leakage current may be measured
6912 Universal Bridge to measure capacitance of Cable
Open circuit with no sound core available
Fault Distance from test endfront end = ((CA(CA+CB))x L
6913 Universal Bridge to measure Inductance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be less then 30 ohm
iii) All conductors be left free from earth
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
Fault distance X = LA x L LAB
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
Fault distance X = LA x L LA + LB
692 Pulse Echo (PE) Time Domain Reflectometry (TDR)
A simple method which works on travelling wave principles It is applicable to all series and shunt faults with Rf gt=Zo10 (5 ohm) and Rflt=10 Zo (500 ohm) respectively For power cables typical Zo=50 ohm
6921 Travelling wave principles
A pulse injected into a cable is reflected back to the source by any change in characteristics impedance (Zo) of the cable
The waveform generated can be monitored using an oscilloscope as shown in Figure below
6921 Continue-
Waveform interpretion
Shunt Fault
Series Fault
6921 Continue-
Waveform interpretion
Low Resistance Shunt Fault (Rflt=500 Ohm)
-Reflection Negative
-Amplitude depends on fault resistance
Series Fault (Rfgt=5 ohm)
- Reflection Positive
- Amplitude depends on fault resistance
Zo = Characteristics impedance = radic(LC)
L = Inductance of cable C = Capacitance of Cable
69211 Reflection Factor (r)
- Is to determine the degree of reflection
Shunt fault r (V) = -Zo1(2Rf + Zo1)
Series Fault r (V) = Rf(2Zo1 + Rf)
Example shunt fault fault resistance = 5 ohm Zo = 50 ohm
r(V) = -50 (2 x5 +50) = - 833If fault resistance = 500 ohm
r(V) = -50 (2x 500 + 50) = -475
69212 Velocity of Propagation (Vp)
- Vp depends on the cable dielectric and is defined as
Vp = Vs radicEr
- where Vs = Velocity of light in free space
Er = Relative permittivity of dielectric
Vp = 300radicEr
Dielectrics Velocity of PropagationVp (mmicroS)
Impregnated Paper 150 -171
PVC 152-175
PE Approx 200
XLPE 156-176
Typical Vp for different Dielectric
69212 Continue (Vp)
Example
Determine the velocity of propagation Vp2 for a cable length of 2500 meters t1= 3164 S and t2 = 1745 S Calculate the fault distances
6921 Continue-
The distance to the fault Lf (m) is given by
Lf = T x Vp2
6921 Continue
Solutions
Velocity of propagation = Vp2 = 25003164 = 79 mS
Distance of Fault from test end = Vp2 x t2
= 79 x 1745
= 137855
693 Impulse Current Method
Working on Travelling wave principles it is applicable to all types of fault
694 Arc Reflection or Secondary Impulse method
Basically PE and TDR associated with fault treatment It is also applicable to faults of all nature but with easily interpreted breakdown waveforms
610 PIN POINTING
Pin Pointing is the application of a test that positively confirms the exact position of the fault
Before the commencement of pin pointing the prelocated fault distance should mark on the cable route which is measured by means of a trumeter
Pin pointing is normally carried out by the shock wave discharge method The fault can be detected by the use of semisphone
611 Confirmation and Re-Test
After the pin-pointed position of the fault has been marked and exposed check for physical sign of fault If there is the fault is confirm
After confirmation the fault should be cut away IR and continuity test should be carried out on the remaining cable sections to determine the soundeness of these cable sections
The insulation resistance test are again carried out after jointing followed by pressure test before supply can be restored
Flowchart for Fault location Procedure
Example
Find a fault distance using direct reading fault localiser from end test which has a test data as below
Bridge Balance Reading =60 Length of Cable 140 meters
Solutions
Fault Distance= = 60100 x 140 = 84 meterxL
n
100
For cables route with combination of 2 or more different type of cable
Determine the resistance of cable for each section separately from Table 1 and Table 2
Example Cable 11kV Al Length 150 m size = 16mm2
From Table 1= Resistance for 1km =226 ohm
For 150 m = 1501000 x 226 = 0339 ohm
From Table 1 Calculate the equivalent resistance for all the combined section
Example for Al = 0339 ohm for copper = 0150 ohm
Equivalent Resistance = 0339+0150 =0489 ohm
Continue
Calculate equivalent resistance from obtained bridge reading
Example Bridge Reading = 60
Equivalent resistance = 06 x 0489 ohm = 02934 ohm
Determine which section of cable the fault occurs
Total resistance at Al section = 0339 ohm
As equivalent resistance at Al section is higher then 02934(Equivalent resistance from bridge test) the fault is
located at Aluminium section of cable
Table 1 Impedance data for 635011000 V Cable Aluminium
Table II Impedance data 11000 V Cable Copper
TABLE VI ndash 6001000V (ALUMINIUM)
TABLE VII ndash 38006600V (ALUMINIUM)
TABLE VIII- 635011000 (ALUMINIUM)
TABLE V
610 COMMISSIONING OF UNDERGROUND CABLE
High Voltage DC testing
High voltage testing is carried out in order to determine the electrical strength of cable insulation Site tests are performed by applying a predetermined high voltage to the insulation DC site test voltages regardless of insulation type are used to ensure that the cable cable joints and termination are correctly made and installed
High Voltage Testing (cont-)
IEC Standards specify maintenance tests after installation should be 70 of permissible factory test DC voltages (15 minutes) = 175kV
Insulation Resistance Test (cont-)
Test voltages should be slowly raised to the required value over a period of about 1 minute and the test period starts once the full voltage is reached In this way the capacitive and absorption currents will have decreased and circuit conditions stabilized such that true leakage current may be measured
6913 Universal Bridge to measure Inductance of Cable
Requirement
i) Open Circuit Fault
ii) Fault resistance of cable should be less then 30 ohm
iii) All conductors be left free from earth
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
Fault distance X = LA x L LAB
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
Fault distance X = LA x L LA + LB
692 Pulse Echo (PE) Time Domain Reflectometry (TDR)
A simple method which works on travelling wave principles It is applicable to all series and shunt faults with Rf gt=Zo10 (5 ohm) and Rflt=10 Zo (500 ohm) respectively For power cables typical Zo=50 ohm
6921 Travelling wave principles
A pulse injected into a cable is reflected back to the source by any change in characteristics impedance (Zo) of the cable
The waveform generated can be monitored using an oscilloscope as shown in Figure below
6921 Continue-
Waveform interpretion
Shunt Fault
Series Fault
6921 Continue-
Waveform interpretion
Low Resistance Shunt Fault (Rflt=500 Ohm)
-Reflection Negative
-Amplitude depends on fault resistance
Series Fault (Rfgt=5 ohm)
- Reflection Positive
- Amplitude depends on fault resistance
Zo = Characteristics impedance = radic(LC)
L = Inductance of cable C = Capacitance of Cable
69211 Reflection Factor (r)
- Is to determine the degree of reflection
Shunt fault r (V) = -Zo1(2Rf + Zo1)
Series Fault r (V) = Rf(2Zo1 + Rf)
Example shunt fault fault resistance = 5 ohm Zo = 50 ohm
r(V) = -50 (2 x5 +50) = - 833If fault resistance = 500 ohm
r(V) = -50 (2x 500 + 50) = -475
69212 Velocity of Propagation (Vp)
- Vp depends on the cable dielectric and is defined as
Vp = Vs radicEr
- where Vs = Velocity of light in free space
Er = Relative permittivity of dielectric
Vp = 300radicEr
Dielectrics Velocity of PropagationVp (mmicroS)
Impregnated Paper 150 -171
PVC 152-175
PE Approx 200
XLPE 156-176
Typical Vp for different Dielectric
69212 Continue (Vp)
Example
Determine the velocity of propagation Vp2 for a cable length of 2500 meters t1= 3164 S and t2 = 1745 S Calculate the fault distances
6921 Continue-
The distance to the fault Lf (m) is given by
Lf = T x Vp2
6921 Continue
Solutions
Velocity of propagation = Vp2 = 25003164 = 79 mS
Distance of Fault from test end = Vp2 x t2
= 79 x 1745
= 137855
693 Impulse Current Method
Working on Travelling wave principles it is applicable to all types of fault
694 Arc Reflection or Secondary Impulse method
Basically PE and TDR associated with fault treatment It is also applicable to faults of all nature but with easily interpreted breakdown waveforms
610 PIN POINTING
Pin Pointing is the application of a test that positively confirms the exact position of the fault
Before the commencement of pin pointing the prelocated fault distance should mark on the cable route which is measured by means of a trumeter
Pin pointing is normally carried out by the shock wave discharge method The fault can be detected by the use of semisphone
611 Confirmation and Re-Test
After the pin-pointed position of the fault has been marked and exposed check for physical sign of fault If there is the fault is confirm
After confirmation the fault should be cut away IR and continuity test should be carried out on the remaining cable sections to determine the soundeness of these cable sections
The insulation resistance test are again carried out after jointing followed by pressure test before supply can be restored
Flowchart for Fault location Procedure
Example
Find a fault distance using direct reading fault localiser from end test which has a test data as below
Bridge Balance Reading =60 Length of Cable 140 meters
Solutions
Fault Distance= = 60100 x 140 = 84 meterxL
n
100
For cables route with combination of 2 or more different type of cable
Determine the resistance of cable for each section separately from Table 1 and Table 2
Example Cable 11kV Al Length 150 m size = 16mm2
From Table 1= Resistance for 1km =226 ohm
For 150 m = 1501000 x 226 = 0339 ohm
From Table 1 Calculate the equivalent resistance for all the combined section
Example for Al = 0339 ohm for copper = 0150 ohm
Equivalent Resistance = 0339+0150 =0489 ohm
Continue
Calculate equivalent resistance from obtained bridge reading
Example Bridge Reading = 60
Equivalent resistance = 06 x 0489 ohm = 02934 ohm
Determine which section of cable the fault occurs
Total resistance at Al section = 0339 ohm
As equivalent resistance at Al section is higher then 02934(Equivalent resistance from bridge test) the fault is
located at Aluminium section of cable
Table 1 Impedance data for 635011000 V Cable Aluminium
Table II Impedance data 11000 V Cable Copper
TABLE VI ndash 6001000V (ALUMINIUM)
TABLE VII ndash 38006600V (ALUMINIUM)
TABLE VIII- 635011000 (ALUMINIUM)
TABLE V
610 COMMISSIONING OF UNDERGROUND CABLE
High Voltage DC testing
High voltage testing is carried out in order to determine the electrical strength of cable insulation Site tests are performed by applying a predetermined high voltage to the insulation DC site test voltages regardless of insulation type are used to ensure that the cable cable joints and termination are correctly made and installed
High Voltage Testing (cont-)
IEC Standards specify maintenance tests after installation should be 70 of permissible factory test DC voltages (15 minutes) = 175kV
Insulation Resistance Test (cont-)
Test voltages should be slowly raised to the required value over a period of about 1 minute and the test period starts once the full voltage is reached In this way the capacitive and absorption currents will have decreased and circuit conditions stabilized such that true leakage current may be measured
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
Fault distance X = LA x L LAB
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
Fault distance X = LA x L LA + LB
692 Pulse Echo (PE) Time Domain Reflectometry (TDR)
A simple method which works on travelling wave principles It is applicable to all series and shunt faults with Rf gt=Zo10 (5 ohm) and Rflt=10 Zo (500 ohm) respectively For power cables typical Zo=50 ohm
6921 Travelling wave principles
A pulse injected into a cable is reflected back to the source by any change in characteristics impedance (Zo) of the cable
The waveform generated can be monitored using an oscilloscope as shown in Figure below
6921 Continue-
Waveform interpretion
Shunt Fault
Series Fault
6921 Continue-
Waveform interpretion
Low Resistance Shunt Fault (Rflt=500 Ohm)
-Reflection Negative
-Amplitude depends on fault resistance
Series Fault (Rfgt=5 ohm)
- Reflection Positive
- Amplitude depends on fault resistance
Zo = Characteristics impedance = radic(LC)
L = Inductance of cable C = Capacitance of Cable
69211 Reflection Factor (r)
- Is to determine the degree of reflection
Shunt fault r (V) = -Zo1(2Rf + Zo1)
Series Fault r (V) = Rf(2Zo1 + Rf)
Example shunt fault fault resistance = 5 ohm Zo = 50 ohm
r(V) = -50 (2 x5 +50) = - 833If fault resistance = 500 ohm
r(V) = -50 (2x 500 + 50) = -475
69212 Velocity of Propagation (Vp)
- Vp depends on the cable dielectric and is defined as
Vp = Vs radicEr
- where Vs = Velocity of light in free space
Er = Relative permittivity of dielectric
Vp = 300radicEr
Dielectrics Velocity of PropagationVp (mmicroS)
Impregnated Paper 150 -171
PVC 152-175
PE Approx 200
XLPE 156-176
Typical Vp for different Dielectric
69212 Continue (Vp)
Example
Determine the velocity of propagation Vp2 for a cable length of 2500 meters t1= 3164 S and t2 = 1745 S Calculate the fault distances
6921 Continue-
The distance to the fault Lf (m) is given by
Lf = T x Vp2
6921 Continue
Solutions
Velocity of propagation = Vp2 = 25003164 = 79 mS
Distance of Fault from test end = Vp2 x t2
= 79 x 1745
= 137855
693 Impulse Current Method
Working on Travelling wave principles it is applicable to all types of fault
694 Arc Reflection or Secondary Impulse method
Basically PE and TDR associated with fault treatment It is also applicable to faults of all nature but with easily interpreted breakdown waveforms
610 PIN POINTING
Pin Pointing is the application of a test that positively confirms the exact position of the fault
Before the commencement of pin pointing the prelocated fault distance should mark on the cable route which is measured by means of a trumeter
Pin pointing is normally carried out by the shock wave discharge method The fault can be detected by the use of semisphone
611 Confirmation and Re-Test
After the pin-pointed position of the fault has been marked and exposed check for physical sign of fault If there is the fault is confirm
After confirmation the fault should be cut away IR and continuity test should be carried out on the remaining cable sections to determine the soundeness of these cable sections
The insulation resistance test are again carried out after jointing followed by pressure test before supply can be restored
Flowchart for Fault location Procedure
Example
Find a fault distance using direct reading fault localiser from end test which has a test data as below
Bridge Balance Reading =60 Length of Cable 140 meters
Solutions
Fault Distance= = 60100 x 140 = 84 meterxL
n
100
For cables route with combination of 2 or more different type of cable
Determine the resistance of cable for each section separately from Table 1 and Table 2
Example Cable 11kV Al Length 150 m size = 16mm2
From Table 1= Resistance for 1km =226 ohm
For 150 m = 1501000 x 226 = 0339 ohm
From Table 1 Calculate the equivalent resistance for all the combined section
Example for Al = 0339 ohm for copper = 0150 ohm
Equivalent Resistance = 0339+0150 =0489 ohm
Continue
Calculate equivalent resistance from obtained bridge reading
Example Bridge Reading = 60
Equivalent resistance = 06 x 0489 ohm = 02934 ohm
Determine which section of cable the fault occurs
Total resistance at Al section = 0339 ohm
As equivalent resistance at Al section is higher then 02934(Equivalent resistance from bridge test) the fault is
located at Aluminium section of cable
Table 1 Impedance data for 635011000 V Cable Aluminium
Table II Impedance data 11000 V Cable Copper
TABLE VI ndash 6001000V (ALUMINIUM)
TABLE VII ndash 38006600V (ALUMINIUM)
TABLE VIII- 635011000 (ALUMINIUM)
TABLE V
610 COMMISSIONING OF UNDERGROUND CABLE
High Voltage DC testing
High voltage testing is carried out in order to determine the electrical strength of cable insulation Site tests are performed by applying a predetermined high voltage to the insulation DC site test voltages regardless of insulation type are used to ensure that the cable cable joints and termination are correctly made and installed
High Voltage Testing (cont-)
IEC Standards specify maintenance tests after installation should be 70 of permissible factory test DC voltages (15 minutes) = 175kV
Insulation Resistance Test (cont-)
Test voltages should be slowly raised to the required value over a period of about 1 minute and the test period starts once the full voltage is reached In this way the capacitive and absorption currents will have decreased and circuit conditions stabilized such that true leakage current may be measured
6913 Universal Bridge to measure Inductance of Cable
Open circuited with sound core available
Fault distance X = LA x L LAB
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
Fault distance X = LA x L LA + LB
692 Pulse Echo (PE) Time Domain Reflectometry (TDR)
A simple method which works on travelling wave principles It is applicable to all series and shunt faults with Rf gt=Zo10 (5 ohm) and Rflt=10 Zo (500 ohm) respectively For power cables typical Zo=50 ohm
6921 Travelling wave principles
A pulse injected into a cable is reflected back to the source by any change in characteristics impedance (Zo) of the cable
The waveform generated can be monitored using an oscilloscope as shown in Figure below
6921 Continue-
Waveform interpretion
Shunt Fault
Series Fault
6921 Continue-
Waveform interpretion
Low Resistance Shunt Fault (Rflt=500 Ohm)
-Reflection Negative
-Amplitude depends on fault resistance
Series Fault (Rfgt=5 ohm)
- Reflection Positive
- Amplitude depends on fault resistance
Zo = Characteristics impedance = radic(LC)
L = Inductance of cable C = Capacitance of Cable
69211 Reflection Factor (r)
- Is to determine the degree of reflection
Shunt fault r (V) = -Zo1(2Rf + Zo1)
Series Fault r (V) = Rf(2Zo1 + Rf)
Example shunt fault fault resistance = 5 ohm Zo = 50 ohm
r(V) = -50 (2 x5 +50) = - 833If fault resistance = 500 ohm
r(V) = -50 (2x 500 + 50) = -475
69212 Velocity of Propagation (Vp)
- Vp depends on the cable dielectric and is defined as
Vp = Vs radicEr
- where Vs = Velocity of light in free space
Er = Relative permittivity of dielectric
Vp = 300radicEr
Dielectrics Velocity of PropagationVp (mmicroS)
Impregnated Paper 150 -171
PVC 152-175
PE Approx 200
XLPE 156-176
Typical Vp for different Dielectric
69212 Continue (Vp)
Example
Determine the velocity of propagation Vp2 for a cable length of 2500 meters t1= 3164 S and t2 = 1745 S Calculate the fault distances
6921 Continue-
The distance to the fault Lf (m) is given by
Lf = T x Vp2
6921 Continue
Solutions
Velocity of propagation = Vp2 = 25003164 = 79 mS
Distance of Fault from test end = Vp2 x t2
= 79 x 1745
= 137855
693 Impulse Current Method
Working on Travelling wave principles it is applicable to all types of fault
694 Arc Reflection or Secondary Impulse method
Basically PE and TDR associated with fault treatment It is also applicable to faults of all nature but with easily interpreted breakdown waveforms
610 PIN POINTING
Pin Pointing is the application of a test that positively confirms the exact position of the fault
Before the commencement of pin pointing the prelocated fault distance should mark on the cable route which is measured by means of a trumeter
Pin pointing is normally carried out by the shock wave discharge method The fault can be detected by the use of semisphone
611 Confirmation and Re-Test
After the pin-pointed position of the fault has been marked and exposed check for physical sign of fault If there is the fault is confirm
After confirmation the fault should be cut away IR and continuity test should be carried out on the remaining cable sections to determine the soundeness of these cable sections
The insulation resistance test are again carried out after jointing followed by pressure test before supply can be restored
Flowchart for Fault location Procedure
Example
Find a fault distance using direct reading fault localiser from end test which has a test data as below
Bridge Balance Reading =60 Length of Cable 140 meters
Solutions
Fault Distance= = 60100 x 140 = 84 meterxL
n
100
For cables route with combination of 2 or more different type of cable
Determine the resistance of cable for each section separately from Table 1 and Table 2
Example Cable 11kV Al Length 150 m size = 16mm2
From Table 1= Resistance for 1km =226 ohm
For 150 m = 1501000 x 226 = 0339 ohm
From Table 1 Calculate the equivalent resistance for all the combined section
Example for Al = 0339 ohm for copper = 0150 ohm
Equivalent Resistance = 0339+0150 =0489 ohm
Continue
Calculate equivalent resistance from obtained bridge reading
Example Bridge Reading = 60
Equivalent resistance = 06 x 0489 ohm = 02934 ohm
Determine which section of cable the fault occurs
Total resistance at Al section = 0339 ohm
As equivalent resistance at Al section is higher then 02934(Equivalent resistance from bridge test) the fault is
located at Aluminium section of cable
Table 1 Impedance data for 635011000 V Cable Aluminium
Table II Impedance data 11000 V Cable Copper
TABLE VI ndash 6001000V (ALUMINIUM)
TABLE VII ndash 38006600V (ALUMINIUM)
TABLE VIII- 635011000 (ALUMINIUM)
TABLE V
610 COMMISSIONING OF UNDERGROUND CABLE
High Voltage DC testing
High voltage testing is carried out in order to determine the electrical strength of cable insulation Site tests are performed by applying a predetermined high voltage to the insulation DC site test voltages regardless of insulation type are used to ensure that the cable cable joints and termination are correctly made and installed
High Voltage Testing (cont-)
IEC Standards specify maintenance tests after installation should be 70 of permissible factory test DC voltages (15 minutes) = 175kV
Insulation Resistance Test (cont-)
Test voltages should be slowly raised to the required value over a period of about 1 minute and the test period starts once the full voltage is reached In this way the capacitive and absorption currents will have decreased and circuit conditions stabilized such that true leakage current may be measured
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
Fault distance X = LA x L LA + LB
692 Pulse Echo (PE) Time Domain Reflectometry (TDR)
A simple method which works on travelling wave principles It is applicable to all series and shunt faults with Rf gt=Zo10 (5 ohm) and Rflt=10 Zo (500 ohm) respectively For power cables typical Zo=50 ohm
6921 Travelling wave principles
A pulse injected into a cable is reflected back to the source by any change in characteristics impedance (Zo) of the cable
The waveform generated can be monitored using an oscilloscope as shown in Figure below
6921 Continue-
Waveform interpretion
Shunt Fault
Series Fault
6921 Continue-
Waveform interpretion
Low Resistance Shunt Fault (Rflt=500 Ohm)
-Reflection Negative
-Amplitude depends on fault resistance
Series Fault (Rfgt=5 ohm)
- Reflection Positive
- Amplitude depends on fault resistance
Zo = Characteristics impedance = radic(LC)
L = Inductance of cable C = Capacitance of Cable
69211 Reflection Factor (r)
- Is to determine the degree of reflection
Shunt fault r (V) = -Zo1(2Rf + Zo1)
Series Fault r (V) = Rf(2Zo1 + Rf)
Example shunt fault fault resistance = 5 ohm Zo = 50 ohm
r(V) = -50 (2 x5 +50) = - 833If fault resistance = 500 ohm
r(V) = -50 (2x 500 + 50) = -475
69212 Velocity of Propagation (Vp)
- Vp depends on the cable dielectric and is defined as
Vp = Vs radicEr
- where Vs = Velocity of light in free space
Er = Relative permittivity of dielectric
Vp = 300radicEr
Dielectrics Velocity of PropagationVp (mmicroS)
Impregnated Paper 150 -171
PVC 152-175
PE Approx 200
XLPE 156-176
Typical Vp for different Dielectric
69212 Continue (Vp)
Example
Determine the velocity of propagation Vp2 for a cable length of 2500 meters t1= 3164 S and t2 = 1745 S Calculate the fault distances
6921 Continue-
The distance to the fault Lf (m) is given by
Lf = T x Vp2
6921 Continue
Solutions
Velocity of propagation = Vp2 = 25003164 = 79 mS
Distance of Fault from test end = Vp2 x t2
= 79 x 1745
= 137855
693 Impulse Current Method
Working on Travelling wave principles it is applicable to all types of fault
694 Arc Reflection or Secondary Impulse method
Basically PE and TDR associated with fault treatment It is also applicable to faults of all nature but with easily interpreted breakdown waveforms
610 PIN POINTING
Pin Pointing is the application of a test that positively confirms the exact position of the fault
Before the commencement of pin pointing the prelocated fault distance should mark on the cable route which is measured by means of a trumeter
Pin pointing is normally carried out by the shock wave discharge method The fault can be detected by the use of semisphone
611 Confirmation and Re-Test
After the pin-pointed position of the fault has been marked and exposed check for physical sign of fault If there is the fault is confirm
After confirmation the fault should be cut away IR and continuity test should be carried out on the remaining cable sections to determine the soundeness of these cable sections
The insulation resistance test are again carried out after jointing followed by pressure test before supply can be restored
Flowchart for Fault location Procedure
Example
Find a fault distance using direct reading fault localiser from end test which has a test data as below
Bridge Balance Reading =60 Length of Cable 140 meters
Solutions
Fault Distance= = 60100 x 140 = 84 meterxL
n
100
For cables route with combination of 2 or more different type of cable
Determine the resistance of cable for each section separately from Table 1 and Table 2
Example Cable 11kV Al Length 150 m size = 16mm2
From Table 1= Resistance for 1km =226 ohm
For 150 m = 1501000 x 226 = 0339 ohm
From Table 1 Calculate the equivalent resistance for all the combined section
Example for Al = 0339 ohm for copper = 0150 ohm
Equivalent Resistance = 0339+0150 =0489 ohm
Continue
Calculate equivalent resistance from obtained bridge reading
Example Bridge Reading = 60
Equivalent resistance = 06 x 0489 ohm = 02934 ohm
Determine which section of cable the fault occurs
Total resistance at Al section = 0339 ohm
As equivalent resistance at Al section is higher then 02934(Equivalent resistance from bridge test) the fault is
located at Aluminium section of cable
Table 1 Impedance data for 635011000 V Cable Aluminium
Table II Impedance data 11000 V Cable Copper
TABLE VI ndash 6001000V (ALUMINIUM)
TABLE VII ndash 38006600V (ALUMINIUM)
TABLE VIII- 635011000 (ALUMINIUM)
TABLE V
610 COMMISSIONING OF UNDERGROUND CABLE
High Voltage DC testing
High voltage testing is carried out in order to determine the electrical strength of cable insulation Site tests are performed by applying a predetermined high voltage to the insulation DC site test voltages regardless of insulation type are used to ensure that the cable cable joints and termination are correctly made and installed
High Voltage Testing (cont-)
IEC Standards specify maintenance tests after installation should be 70 of permissible factory test DC voltages (15 minutes) = 175kV
Insulation Resistance Test (cont-)
Test voltages should be slowly raised to the required value over a period of about 1 minute and the test period starts once the full voltage is reached In this way the capacitive and absorption currents will have decreased and circuit conditions stabilized such that true leakage current may be measured
6913 Universal Bridge to measure Inductance of Cable
Open circuited with no sound core available
Fault distance X = LA x L LA + LB
692 Pulse Echo (PE) Time Domain Reflectometry (TDR)
A simple method which works on travelling wave principles It is applicable to all series and shunt faults with Rf gt=Zo10 (5 ohm) and Rflt=10 Zo (500 ohm) respectively For power cables typical Zo=50 ohm
6921 Travelling wave principles
A pulse injected into a cable is reflected back to the source by any change in characteristics impedance (Zo) of the cable
The waveform generated can be monitored using an oscilloscope as shown in Figure below
6921 Continue-
Waveform interpretion
Shunt Fault
Series Fault
6921 Continue-
Waveform interpretion
Low Resistance Shunt Fault (Rflt=500 Ohm)
-Reflection Negative
-Amplitude depends on fault resistance
Series Fault (Rfgt=5 ohm)
- Reflection Positive
- Amplitude depends on fault resistance
Zo = Characteristics impedance = radic(LC)
L = Inductance of cable C = Capacitance of Cable
69211 Reflection Factor (r)
- Is to determine the degree of reflection
Shunt fault r (V) = -Zo1(2Rf + Zo1)
Series Fault r (V) = Rf(2Zo1 + Rf)
Example shunt fault fault resistance = 5 ohm Zo = 50 ohm
r(V) = -50 (2 x5 +50) = - 833If fault resistance = 500 ohm
r(V) = -50 (2x 500 + 50) = -475
69212 Velocity of Propagation (Vp)
- Vp depends on the cable dielectric and is defined as
Vp = Vs radicEr
- where Vs = Velocity of light in free space
Er = Relative permittivity of dielectric
Vp = 300radicEr
Dielectrics Velocity of PropagationVp (mmicroS)
Impregnated Paper 150 -171
PVC 152-175
PE Approx 200
XLPE 156-176
Typical Vp for different Dielectric
69212 Continue (Vp)
Example
Determine the velocity of propagation Vp2 for a cable length of 2500 meters t1= 3164 S and t2 = 1745 S Calculate the fault distances
6921 Continue-
The distance to the fault Lf (m) is given by
Lf = T x Vp2
6921 Continue
Solutions
Velocity of propagation = Vp2 = 25003164 = 79 mS
Distance of Fault from test end = Vp2 x t2
= 79 x 1745
= 137855
693 Impulse Current Method
Working on Travelling wave principles it is applicable to all types of fault
694 Arc Reflection or Secondary Impulse method
Basically PE and TDR associated with fault treatment It is also applicable to faults of all nature but with easily interpreted breakdown waveforms
610 PIN POINTING
Pin Pointing is the application of a test that positively confirms the exact position of the fault
Before the commencement of pin pointing the prelocated fault distance should mark on the cable route which is measured by means of a trumeter
Pin pointing is normally carried out by the shock wave discharge method The fault can be detected by the use of semisphone
611 Confirmation and Re-Test
After the pin-pointed position of the fault has been marked and exposed check for physical sign of fault If there is the fault is confirm
After confirmation the fault should be cut away IR and continuity test should be carried out on the remaining cable sections to determine the soundeness of these cable sections
The insulation resistance test are again carried out after jointing followed by pressure test before supply can be restored
Flowchart for Fault location Procedure
Example
Find a fault distance using direct reading fault localiser from end test which has a test data as below
Bridge Balance Reading =60 Length of Cable 140 meters
Solutions
Fault Distance= = 60100 x 140 = 84 meterxL
n
100
For cables route with combination of 2 or more different type of cable
Determine the resistance of cable for each section separately from Table 1 and Table 2
Example Cable 11kV Al Length 150 m size = 16mm2
From Table 1= Resistance for 1km =226 ohm
For 150 m = 1501000 x 226 = 0339 ohm
From Table 1 Calculate the equivalent resistance for all the combined section
Example for Al = 0339 ohm for copper = 0150 ohm
Equivalent Resistance = 0339+0150 =0489 ohm
Continue
Calculate equivalent resistance from obtained bridge reading
Example Bridge Reading = 60
Equivalent resistance = 06 x 0489 ohm = 02934 ohm
Determine which section of cable the fault occurs
Total resistance at Al section = 0339 ohm
As equivalent resistance at Al section is higher then 02934(Equivalent resistance from bridge test) the fault is
located at Aluminium section of cable
Table 1 Impedance data for 635011000 V Cable Aluminium
Table II Impedance data 11000 V Cable Copper
TABLE VI ndash 6001000V (ALUMINIUM)
TABLE VII ndash 38006600V (ALUMINIUM)
TABLE VIII- 635011000 (ALUMINIUM)
TABLE V
610 COMMISSIONING OF UNDERGROUND CABLE
High Voltage DC testing
High voltage testing is carried out in order to determine the electrical strength of cable insulation Site tests are performed by applying a predetermined high voltage to the insulation DC site test voltages regardless of insulation type are used to ensure that the cable cable joints and termination are correctly made and installed
High Voltage Testing (cont-)
IEC Standards specify maintenance tests after installation should be 70 of permissible factory test DC voltages (15 minutes) = 175kV
Insulation Resistance Test (cont-)
Test voltages should be slowly raised to the required value over a period of about 1 minute and the test period starts once the full voltage is reached In this way the capacitive and absorption currents will have decreased and circuit conditions stabilized such that true leakage current may be measured
692 Pulse Echo (PE) Time Domain Reflectometry (TDR)
A simple method which works on travelling wave principles It is applicable to all series and shunt faults with Rf gt=Zo10 (5 ohm) and Rflt=10 Zo (500 ohm) respectively For power cables typical Zo=50 ohm
6921 Travelling wave principles
A pulse injected into a cable is reflected back to the source by any change in characteristics impedance (Zo) of the cable
The waveform generated can be monitored using an oscilloscope as shown in Figure below
6921 Continue-
Waveform interpretion
Shunt Fault
Series Fault
6921 Continue-
Waveform interpretion
Low Resistance Shunt Fault (Rflt=500 Ohm)
-Reflection Negative
-Amplitude depends on fault resistance
Series Fault (Rfgt=5 ohm)
- Reflection Positive
- Amplitude depends on fault resistance
Zo = Characteristics impedance = radic(LC)
L = Inductance of cable C = Capacitance of Cable
69211 Reflection Factor (r)
- Is to determine the degree of reflection
Shunt fault r (V) = -Zo1(2Rf + Zo1)
Series Fault r (V) = Rf(2Zo1 + Rf)
Example shunt fault fault resistance = 5 ohm Zo = 50 ohm
r(V) = -50 (2 x5 +50) = - 833If fault resistance = 500 ohm
r(V) = -50 (2x 500 + 50) = -475
69212 Velocity of Propagation (Vp)
- Vp depends on the cable dielectric and is defined as
Vp = Vs radicEr
- where Vs = Velocity of light in free space
Er = Relative permittivity of dielectric
Vp = 300radicEr
Dielectrics Velocity of PropagationVp (mmicroS)
Impregnated Paper 150 -171
PVC 152-175
PE Approx 200
XLPE 156-176
Typical Vp for different Dielectric
69212 Continue (Vp)
Example
Determine the velocity of propagation Vp2 for a cable length of 2500 meters t1= 3164 S and t2 = 1745 S Calculate the fault distances
6921 Continue-
The distance to the fault Lf (m) is given by
Lf = T x Vp2
6921 Continue
Solutions
Velocity of propagation = Vp2 = 25003164 = 79 mS
Distance of Fault from test end = Vp2 x t2
= 79 x 1745
= 137855
693 Impulse Current Method
Working on Travelling wave principles it is applicable to all types of fault
694 Arc Reflection or Secondary Impulse method
Basically PE and TDR associated with fault treatment It is also applicable to faults of all nature but with easily interpreted breakdown waveforms
610 PIN POINTING
Pin Pointing is the application of a test that positively confirms the exact position of the fault
Before the commencement of pin pointing the prelocated fault distance should mark on the cable route which is measured by means of a trumeter
Pin pointing is normally carried out by the shock wave discharge method The fault can be detected by the use of semisphone
611 Confirmation and Re-Test
After the pin-pointed position of the fault has been marked and exposed check for physical sign of fault If there is the fault is confirm
After confirmation the fault should be cut away IR and continuity test should be carried out on the remaining cable sections to determine the soundeness of these cable sections
The insulation resistance test are again carried out after jointing followed by pressure test before supply can be restored
Flowchart for Fault location Procedure
Example
Find a fault distance using direct reading fault localiser from end test which has a test data as below
Bridge Balance Reading =60 Length of Cable 140 meters
Solutions
Fault Distance= = 60100 x 140 = 84 meterxL
n
100
For cables route with combination of 2 or more different type of cable
Determine the resistance of cable for each section separately from Table 1 and Table 2
Example Cable 11kV Al Length 150 m size = 16mm2
From Table 1= Resistance for 1km =226 ohm
For 150 m = 1501000 x 226 = 0339 ohm
From Table 1 Calculate the equivalent resistance for all the combined section
Example for Al = 0339 ohm for copper = 0150 ohm
Equivalent Resistance = 0339+0150 =0489 ohm
Continue
Calculate equivalent resistance from obtained bridge reading
Example Bridge Reading = 60
Equivalent resistance = 06 x 0489 ohm = 02934 ohm
Determine which section of cable the fault occurs
Total resistance at Al section = 0339 ohm
As equivalent resistance at Al section is higher then 02934(Equivalent resistance from bridge test) the fault is
located at Aluminium section of cable
Table 1 Impedance data for 635011000 V Cable Aluminium
Table II Impedance data 11000 V Cable Copper
TABLE VI ndash 6001000V (ALUMINIUM)
TABLE VII ndash 38006600V (ALUMINIUM)
TABLE VIII- 635011000 (ALUMINIUM)
TABLE V
610 COMMISSIONING OF UNDERGROUND CABLE
High Voltage DC testing
High voltage testing is carried out in order to determine the electrical strength of cable insulation Site tests are performed by applying a predetermined high voltage to the insulation DC site test voltages regardless of insulation type are used to ensure that the cable cable joints and termination are correctly made and installed
High Voltage Testing (cont-)
IEC Standards specify maintenance tests after installation should be 70 of permissible factory test DC voltages (15 minutes) = 175kV
Insulation Resistance Test (cont-)
Test voltages should be slowly raised to the required value over a period of about 1 minute and the test period starts once the full voltage is reached In this way the capacitive and absorption currents will have decreased and circuit conditions stabilized such that true leakage current may be measured
6921 Continue-
Waveform interpretion
Shunt Fault
Series Fault
6921 Continue-
Waveform interpretion
Low Resistance Shunt Fault (Rflt=500 Ohm)
-Reflection Negative
-Amplitude depends on fault resistance
Series Fault (Rfgt=5 ohm)
- Reflection Positive
- Amplitude depends on fault resistance
Zo = Characteristics impedance = radic(LC)
L = Inductance of cable C = Capacitance of Cable
69211 Reflection Factor (r)
- Is to determine the degree of reflection
Shunt fault r (V) = -Zo1(2Rf + Zo1)
Series Fault r (V) = Rf(2Zo1 + Rf)
Example shunt fault fault resistance = 5 ohm Zo = 50 ohm
r(V) = -50 (2 x5 +50) = - 833If fault resistance = 500 ohm
r(V) = -50 (2x 500 + 50) = -475
69212 Velocity of Propagation (Vp)
- Vp depends on the cable dielectric and is defined as
Vp = Vs radicEr
- where Vs = Velocity of light in free space
Er = Relative permittivity of dielectric
Vp = 300radicEr
Dielectrics Velocity of PropagationVp (mmicroS)
Impregnated Paper 150 -171
PVC 152-175
PE Approx 200
XLPE 156-176
Typical Vp for different Dielectric
69212 Continue (Vp)
Example
Determine the velocity of propagation Vp2 for a cable length of 2500 meters t1= 3164 S and t2 = 1745 S Calculate the fault distances
6921 Continue-
The distance to the fault Lf (m) is given by
Lf = T x Vp2
6921 Continue
Solutions
Velocity of propagation = Vp2 = 25003164 = 79 mS
Distance of Fault from test end = Vp2 x t2
= 79 x 1745
= 137855
693 Impulse Current Method
Working on Travelling wave principles it is applicable to all types of fault
694 Arc Reflection or Secondary Impulse method
Basically PE and TDR associated with fault treatment It is also applicable to faults of all nature but with easily interpreted breakdown waveforms
610 PIN POINTING
Pin Pointing is the application of a test that positively confirms the exact position of the fault
Before the commencement of pin pointing the prelocated fault distance should mark on the cable route which is measured by means of a trumeter
Pin pointing is normally carried out by the shock wave discharge method The fault can be detected by the use of semisphone
611 Confirmation and Re-Test
After the pin-pointed position of the fault has been marked and exposed check for physical sign of fault If there is the fault is confirm
After confirmation the fault should be cut away IR and continuity test should be carried out on the remaining cable sections to determine the soundeness of these cable sections
The insulation resistance test are again carried out after jointing followed by pressure test before supply can be restored
Flowchart for Fault location Procedure
Example
Find a fault distance using direct reading fault localiser from end test which has a test data as below
Bridge Balance Reading =60 Length of Cable 140 meters
Solutions
Fault Distance= = 60100 x 140 = 84 meterxL
n
100
For cables route with combination of 2 or more different type of cable
Determine the resistance of cable for each section separately from Table 1 and Table 2
Example Cable 11kV Al Length 150 m size = 16mm2
From Table 1= Resistance for 1km =226 ohm
For 150 m = 1501000 x 226 = 0339 ohm
From Table 1 Calculate the equivalent resistance for all the combined section
Example for Al = 0339 ohm for copper = 0150 ohm
Equivalent Resistance = 0339+0150 =0489 ohm
Continue
Calculate equivalent resistance from obtained bridge reading
Example Bridge Reading = 60
Equivalent resistance = 06 x 0489 ohm = 02934 ohm
Determine which section of cable the fault occurs
Total resistance at Al section = 0339 ohm
As equivalent resistance at Al section is higher then 02934(Equivalent resistance from bridge test) the fault is
located at Aluminium section of cable
Table 1 Impedance data for 635011000 V Cable Aluminium
Table II Impedance data 11000 V Cable Copper
TABLE VI ndash 6001000V (ALUMINIUM)
TABLE VII ndash 38006600V (ALUMINIUM)
TABLE VIII- 635011000 (ALUMINIUM)
TABLE V
610 COMMISSIONING OF UNDERGROUND CABLE
High Voltage DC testing
High voltage testing is carried out in order to determine the electrical strength of cable insulation Site tests are performed by applying a predetermined high voltage to the insulation DC site test voltages regardless of insulation type are used to ensure that the cable cable joints and termination are correctly made and installed
High Voltage Testing (cont-)
IEC Standards specify maintenance tests after installation should be 70 of permissible factory test DC voltages (15 minutes) = 175kV
Insulation Resistance Test (cont-)
Test voltages should be slowly raised to the required value over a period of about 1 minute and the test period starts once the full voltage is reached In this way the capacitive and absorption currents will have decreased and circuit conditions stabilized such that true leakage current may be measured
6921 Continue-
Waveform interpretion
Low Resistance Shunt Fault (Rflt=500 Ohm)
-Reflection Negative
-Amplitude depends on fault resistance
Series Fault (Rfgt=5 ohm)
- Reflection Positive
- Amplitude depends on fault resistance
Zo = Characteristics impedance = radic(LC)
L = Inductance of cable C = Capacitance of Cable
69211 Reflection Factor (r)
- Is to determine the degree of reflection
Shunt fault r (V) = -Zo1(2Rf + Zo1)
Series Fault r (V) = Rf(2Zo1 + Rf)
Example shunt fault fault resistance = 5 ohm Zo = 50 ohm
r(V) = -50 (2 x5 +50) = - 833If fault resistance = 500 ohm
r(V) = -50 (2x 500 + 50) = -475
69212 Velocity of Propagation (Vp)
- Vp depends on the cable dielectric and is defined as
Vp = Vs radicEr
- where Vs = Velocity of light in free space
Er = Relative permittivity of dielectric
Vp = 300radicEr
Dielectrics Velocity of PropagationVp (mmicroS)
Impregnated Paper 150 -171
PVC 152-175
PE Approx 200
XLPE 156-176
Typical Vp for different Dielectric
69212 Continue (Vp)
Example
Determine the velocity of propagation Vp2 for a cable length of 2500 meters t1= 3164 S and t2 = 1745 S Calculate the fault distances
6921 Continue-
The distance to the fault Lf (m) is given by
Lf = T x Vp2
6921 Continue
Solutions
Velocity of propagation = Vp2 = 25003164 = 79 mS
Distance of Fault from test end = Vp2 x t2
= 79 x 1745
= 137855
693 Impulse Current Method
Working on Travelling wave principles it is applicable to all types of fault
694 Arc Reflection or Secondary Impulse method
Basically PE and TDR associated with fault treatment It is also applicable to faults of all nature but with easily interpreted breakdown waveforms
610 PIN POINTING
Pin Pointing is the application of a test that positively confirms the exact position of the fault
Before the commencement of pin pointing the prelocated fault distance should mark on the cable route which is measured by means of a trumeter
Pin pointing is normally carried out by the shock wave discharge method The fault can be detected by the use of semisphone
611 Confirmation and Re-Test
After the pin-pointed position of the fault has been marked and exposed check for physical sign of fault If there is the fault is confirm
After confirmation the fault should be cut away IR and continuity test should be carried out on the remaining cable sections to determine the soundeness of these cable sections
The insulation resistance test are again carried out after jointing followed by pressure test before supply can be restored
Flowchart for Fault location Procedure
Example
Find a fault distance using direct reading fault localiser from end test which has a test data as below
Bridge Balance Reading =60 Length of Cable 140 meters
Solutions
Fault Distance= = 60100 x 140 = 84 meterxL
n
100
For cables route with combination of 2 or more different type of cable
Determine the resistance of cable for each section separately from Table 1 and Table 2
Example Cable 11kV Al Length 150 m size = 16mm2
From Table 1= Resistance for 1km =226 ohm
For 150 m = 1501000 x 226 = 0339 ohm
From Table 1 Calculate the equivalent resistance for all the combined section
Example for Al = 0339 ohm for copper = 0150 ohm
Equivalent Resistance = 0339+0150 =0489 ohm
Continue
Calculate equivalent resistance from obtained bridge reading
Example Bridge Reading = 60
Equivalent resistance = 06 x 0489 ohm = 02934 ohm
Determine which section of cable the fault occurs
Total resistance at Al section = 0339 ohm
As equivalent resistance at Al section is higher then 02934(Equivalent resistance from bridge test) the fault is
located at Aluminium section of cable
Table 1 Impedance data for 635011000 V Cable Aluminium
Table II Impedance data 11000 V Cable Copper
TABLE VI ndash 6001000V (ALUMINIUM)
TABLE VII ndash 38006600V (ALUMINIUM)
TABLE VIII- 635011000 (ALUMINIUM)
TABLE V
610 COMMISSIONING OF UNDERGROUND CABLE
High Voltage DC testing
High voltage testing is carried out in order to determine the electrical strength of cable insulation Site tests are performed by applying a predetermined high voltage to the insulation DC site test voltages regardless of insulation type are used to ensure that the cable cable joints and termination are correctly made and installed
High Voltage Testing (cont-)
IEC Standards specify maintenance tests after installation should be 70 of permissible factory test DC voltages (15 minutes) = 175kV
Insulation Resistance Test (cont-)
Test voltages should be slowly raised to the required value over a period of about 1 minute and the test period starts once the full voltage is reached In this way the capacitive and absorption currents will have decreased and circuit conditions stabilized such that true leakage current may be measured
69211 Reflection Factor (r)
- Is to determine the degree of reflection
Shunt fault r (V) = -Zo1(2Rf + Zo1)
Series Fault r (V) = Rf(2Zo1 + Rf)
Example shunt fault fault resistance = 5 ohm Zo = 50 ohm
r(V) = -50 (2 x5 +50) = - 833If fault resistance = 500 ohm
r(V) = -50 (2x 500 + 50) = -475
69212 Velocity of Propagation (Vp)
- Vp depends on the cable dielectric and is defined as
Vp = Vs radicEr
- where Vs = Velocity of light in free space
Er = Relative permittivity of dielectric
Vp = 300radicEr
Dielectrics Velocity of PropagationVp (mmicroS)
Impregnated Paper 150 -171
PVC 152-175
PE Approx 200
XLPE 156-176
Typical Vp for different Dielectric
69212 Continue (Vp)
Example
Determine the velocity of propagation Vp2 for a cable length of 2500 meters t1= 3164 S and t2 = 1745 S Calculate the fault distances
6921 Continue-
The distance to the fault Lf (m) is given by
Lf = T x Vp2
6921 Continue
Solutions
Velocity of propagation = Vp2 = 25003164 = 79 mS
Distance of Fault from test end = Vp2 x t2
= 79 x 1745
= 137855
693 Impulse Current Method
Working on Travelling wave principles it is applicable to all types of fault
694 Arc Reflection or Secondary Impulse method
Basically PE and TDR associated with fault treatment It is also applicable to faults of all nature but with easily interpreted breakdown waveforms
610 PIN POINTING
Pin Pointing is the application of a test that positively confirms the exact position of the fault
Before the commencement of pin pointing the prelocated fault distance should mark on the cable route which is measured by means of a trumeter
Pin pointing is normally carried out by the shock wave discharge method The fault can be detected by the use of semisphone
611 Confirmation and Re-Test
After the pin-pointed position of the fault has been marked and exposed check for physical sign of fault If there is the fault is confirm
After confirmation the fault should be cut away IR and continuity test should be carried out on the remaining cable sections to determine the soundeness of these cable sections
The insulation resistance test are again carried out after jointing followed by pressure test before supply can be restored
Flowchart for Fault location Procedure
Example
Find a fault distance using direct reading fault localiser from end test which has a test data as below
Bridge Balance Reading =60 Length of Cable 140 meters
Solutions
Fault Distance= = 60100 x 140 = 84 meterxL
n
100
For cables route with combination of 2 or more different type of cable
Determine the resistance of cable for each section separately from Table 1 and Table 2
Example Cable 11kV Al Length 150 m size = 16mm2
From Table 1= Resistance for 1km =226 ohm
For 150 m = 1501000 x 226 = 0339 ohm
From Table 1 Calculate the equivalent resistance for all the combined section
Example for Al = 0339 ohm for copper = 0150 ohm
Equivalent Resistance = 0339+0150 =0489 ohm
Continue
Calculate equivalent resistance from obtained bridge reading
Example Bridge Reading = 60
Equivalent resistance = 06 x 0489 ohm = 02934 ohm
Determine which section of cable the fault occurs
Total resistance at Al section = 0339 ohm
As equivalent resistance at Al section is higher then 02934(Equivalent resistance from bridge test) the fault is
located at Aluminium section of cable
Table 1 Impedance data for 635011000 V Cable Aluminium
Table II Impedance data 11000 V Cable Copper
TABLE VI ndash 6001000V (ALUMINIUM)
TABLE VII ndash 38006600V (ALUMINIUM)
TABLE VIII- 635011000 (ALUMINIUM)
TABLE V
610 COMMISSIONING OF UNDERGROUND CABLE
High Voltage DC testing
High voltage testing is carried out in order to determine the electrical strength of cable insulation Site tests are performed by applying a predetermined high voltage to the insulation DC site test voltages regardless of insulation type are used to ensure that the cable cable joints and termination are correctly made and installed
High Voltage Testing (cont-)
IEC Standards specify maintenance tests after installation should be 70 of permissible factory test DC voltages (15 minutes) = 175kV
Insulation Resistance Test (cont-)
Test voltages should be slowly raised to the required value over a period of about 1 minute and the test period starts once the full voltage is reached In this way the capacitive and absorption currents will have decreased and circuit conditions stabilized such that true leakage current may be measured
69212 Velocity of Propagation (Vp)
- Vp depends on the cable dielectric and is defined as
Vp = Vs radicEr
- where Vs = Velocity of light in free space
Er = Relative permittivity of dielectric
Vp = 300radicEr
Dielectrics Velocity of PropagationVp (mmicroS)
Impregnated Paper 150 -171
PVC 152-175
PE Approx 200
XLPE 156-176
Typical Vp for different Dielectric
69212 Continue (Vp)
Example
Determine the velocity of propagation Vp2 for a cable length of 2500 meters t1= 3164 S and t2 = 1745 S Calculate the fault distances
6921 Continue-
The distance to the fault Lf (m) is given by
Lf = T x Vp2
6921 Continue
Solutions
Velocity of propagation = Vp2 = 25003164 = 79 mS
Distance of Fault from test end = Vp2 x t2
= 79 x 1745
= 137855
693 Impulse Current Method
Working on Travelling wave principles it is applicable to all types of fault
694 Arc Reflection or Secondary Impulse method
Basically PE and TDR associated with fault treatment It is also applicable to faults of all nature but with easily interpreted breakdown waveforms
610 PIN POINTING
Pin Pointing is the application of a test that positively confirms the exact position of the fault
Before the commencement of pin pointing the prelocated fault distance should mark on the cable route which is measured by means of a trumeter
Pin pointing is normally carried out by the shock wave discharge method The fault can be detected by the use of semisphone
611 Confirmation and Re-Test
After the pin-pointed position of the fault has been marked and exposed check for physical sign of fault If there is the fault is confirm
After confirmation the fault should be cut away IR and continuity test should be carried out on the remaining cable sections to determine the soundeness of these cable sections
The insulation resistance test are again carried out after jointing followed by pressure test before supply can be restored
Flowchart for Fault location Procedure
Example
Find a fault distance using direct reading fault localiser from end test which has a test data as below
Bridge Balance Reading =60 Length of Cable 140 meters
Solutions
Fault Distance= = 60100 x 140 = 84 meterxL
n
100
For cables route with combination of 2 or more different type of cable
Determine the resistance of cable for each section separately from Table 1 and Table 2
Example Cable 11kV Al Length 150 m size = 16mm2
From Table 1= Resistance for 1km =226 ohm
For 150 m = 1501000 x 226 = 0339 ohm
From Table 1 Calculate the equivalent resistance for all the combined section
Example for Al = 0339 ohm for copper = 0150 ohm
Equivalent Resistance = 0339+0150 =0489 ohm
Continue
Calculate equivalent resistance from obtained bridge reading
Example Bridge Reading = 60
Equivalent resistance = 06 x 0489 ohm = 02934 ohm
Determine which section of cable the fault occurs
Total resistance at Al section = 0339 ohm
As equivalent resistance at Al section is higher then 02934(Equivalent resistance from bridge test) the fault is
located at Aluminium section of cable
Table 1 Impedance data for 635011000 V Cable Aluminium
Table II Impedance data 11000 V Cable Copper
TABLE VI ndash 6001000V (ALUMINIUM)
TABLE VII ndash 38006600V (ALUMINIUM)
TABLE VIII- 635011000 (ALUMINIUM)
TABLE V
610 COMMISSIONING OF UNDERGROUND CABLE
High Voltage DC testing
High voltage testing is carried out in order to determine the electrical strength of cable insulation Site tests are performed by applying a predetermined high voltage to the insulation DC site test voltages regardless of insulation type are used to ensure that the cable cable joints and termination are correctly made and installed
High Voltage Testing (cont-)
IEC Standards specify maintenance tests after installation should be 70 of permissible factory test DC voltages (15 minutes) = 175kV
Insulation Resistance Test (cont-)
Test voltages should be slowly raised to the required value over a period of about 1 minute and the test period starts once the full voltage is reached In this way the capacitive and absorption currents will have decreased and circuit conditions stabilized such that true leakage current may be measured
Dielectrics Velocity of PropagationVp (mmicroS)
Impregnated Paper 150 -171
PVC 152-175
PE Approx 200
XLPE 156-176
Typical Vp for different Dielectric
69212 Continue (Vp)
Example
Determine the velocity of propagation Vp2 for a cable length of 2500 meters t1= 3164 S and t2 = 1745 S Calculate the fault distances
6921 Continue-
The distance to the fault Lf (m) is given by
Lf = T x Vp2
6921 Continue
Solutions
Velocity of propagation = Vp2 = 25003164 = 79 mS
Distance of Fault from test end = Vp2 x t2
= 79 x 1745
= 137855
693 Impulse Current Method
Working on Travelling wave principles it is applicable to all types of fault
694 Arc Reflection or Secondary Impulse method
Basically PE and TDR associated with fault treatment It is also applicable to faults of all nature but with easily interpreted breakdown waveforms
610 PIN POINTING
Pin Pointing is the application of a test that positively confirms the exact position of the fault
Before the commencement of pin pointing the prelocated fault distance should mark on the cable route which is measured by means of a trumeter
Pin pointing is normally carried out by the shock wave discharge method The fault can be detected by the use of semisphone
611 Confirmation and Re-Test
After the pin-pointed position of the fault has been marked and exposed check for physical sign of fault If there is the fault is confirm
After confirmation the fault should be cut away IR and continuity test should be carried out on the remaining cable sections to determine the soundeness of these cable sections
The insulation resistance test are again carried out after jointing followed by pressure test before supply can be restored
Flowchart for Fault location Procedure
Example
Find a fault distance using direct reading fault localiser from end test which has a test data as below
Bridge Balance Reading =60 Length of Cable 140 meters
Solutions
Fault Distance= = 60100 x 140 = 84 meterxL
n
100
For cables route with combination of 2 or more different type of cable
Determine the resistance of cable for each section separately from Table 1 and Table 2
Example Cable 11kV Al Length 150 m size = 16mm2
From Table 1= Resistance for 1km =226 ohm
For 150 m = 1501000 x 226 = 0339 ohm
From Table 1 Calculate the equivalent resistance for all the combined section
Example for Al = 0339 ohm for copper = 0150 ohm
Equivalent Resistance = 0339+0150 =0489 ohm
Continue
Calculate equivalent resistance from obtained bridge reading
Example Bridge Reading = 60
Equivalent resistance = 06 x 0489 ohm = 02934 ohm
Determine which section of cable the fault occurs
Total resistance at Al section = 0339 ohm
As equivalent resistance at Al section is higher then 02934(Equivalent resistance from bridge test) the fault is
located at Aluminium section of cable
Table 1 Impedance data for 635011000 V Cable Aluminium
Table II Impedance data 11000 V Cable Copper
TABLE VI ndash 6001000V (ALUMINIUM)
TABLE VII ndash 38006600V (ALUMINIUM)
TABLE VIII- 635011000 (ALUMINIUM)
TABLE V
610 COMMISSIONING OF UNDERGROUND CABLE
High Voltage DC testing
High voltage testing is carried out in order to determine the electrical strength of cable insulation Site tests are performed by applying a predetermined high voltage to the insulation DC site test voltages regardless of insulation type are used to ensure that the cable cable joints and termination are correctly made and installed
High Voltage Testing (cont-)
IEC Standards specify maintenance tests after installation should be 70 of permissible factory test DC voltages (15 minutes) = 175kV
Insulation Resistance Test (cont-)
Test voltages should be slowly raised to the required value over a period of about 1 minute and the test period starts once the full voltage is reached In this way the capacitive and absorption currents will have decreased and circuit conditions stabilized such that true leakage current may be measured
69212 Continue (Vp)
Example
Determine the velocity of propagation Vp2 for a cable length of 2500 meters t1= 3164 S and t2 = 1745 S Calculate the fault distances
6921 Continue-
The distance to the fault Lf (m) is given by
Lf = T x Vp2
6921 Continue
Solutions
Velocity of propagation = Vp2 = 25003164 = 79 mS
Distance of Fault from test end = Vp2 x t2
= 79 x 1745
= 137855
693 Impulse Current Method
Working on Travelling wave principles it is applicable to all types of fault
694 Arc Reflection or Secondary Impulse method
Basically PE and TDR associated with fault treatment It is also applicable to faults of all nature but with easily interpreted breakdown waveforms
610 PIN POINTING
Pin Pointing is the application of a test that positively confirms the exact position of the fault
Before the commencement of pin pointing the prelocated fault distance should mark on the cable route which is measured by means of a trumeter
Pin pointing is normally carried out by the shock wave discharge method The fault can be detected by the use of semisphone
611 Confirmation and Re-Test
After the pin-pointed position of the fault has been marked and exposed check for physical sign of fault If there is the fault is confirm
After confirmation the fault should be cut away IR and continuity test should be carried out on the remaining cable sections to determine the soundeness of these cable sections
The insulation resistance test are again carried out after jointing followed by pressure test before supply can be restored
Flowchart for Fault location Procedure
Example
Find a fault distance using direct reading fault localiser from end test which has a test data as below
Bridge Balance Reading =60 Length of Cable 140 meters
Solutions
Fault Distance= = 60100 x 140 = 84 meterxL
n
100
For cables route with combination of 2 or more different type of cable
Determine the resistance of cable for each section separately from Table 1 and Table 2
Example Cable 11kV Al Length 150 m size = 16mm2
From Table 1= Resistance for 1km =226 ohm
For 150 m = 1501000 x 226 = 0339 ohm
From Table 1 Calculate the equivalent resistance for all the combined section
Example for Al = 0339 ohm for copper = 0150 ohm
Equivalent Resistance = 0339+0150 =0489 ohm
Continue
Calculate equivalent resistance from obtained bridge reading
Example Bridge Reading = 60
Equivalent resistance = 06 x 0489 ohm = 02934 ohm
Determine which section of cable the fault occurs
Total resistance at Al section = 0339 ohm
As equivalent resistance at Al section is higher then 02934(Equivalent resistance from bridge test) the fault is
located at Aluminium section of cable
Table 1 Impedance data for 635011000 V Cable Aluminium
Table II Impedance data 11000 V Cable Copper
TABLE VI ndash 6001000V (ALUMINIUM)
TABLE VII ndash 38006600V (ALUMINIUM)
TABLE VIII- 635011000 (ALUMINIUM)
TABLE V
610 COMMISSIONING OF UNDERGROUND CABLE
High Voltage DC testing
High voltage testing is carried out in order to determine the electrical strength of cable insulation Site tests are performed by applying a predetermined high voltage to the insulation DC site test voltages regardless of insulation type are used to ensure that the cable cable joints and termination are correctly made and installed
High Voltage Testing (cont-)
IEC Standards specify maintenance tests after installation should be 70 of permissible factory test DC voltages (15 minutes) = 175kV
Insulation Resistance Test (cont-)
Test voltages should be slowly raised to the required value over a period of about 1 minute and the test period starts once the full voltage is reached In this way the capacitive and absorption currents will have decreased and circuit conditions stabilized such that true leakage current may be measured
6921 Continue-
The distance to the fault Lf (m) is given by
Lf = T x Vp2
6921 Continue
Solutions
Velocity of propagation = Vp2 = 25003164 = 79 mS
Distance of Fault from test end = Vp2 x t2
= 79 x 1745
= 137855
693 Impulse Current Method
Working on Travelling wave principles it is applicable to all types of fault
694 Arc Reflection or Secondary Impulse method
Basically PE and TDR associated with fault treatment It is also applicable to faults of all nature but with easily interpreted breakdown waveforms
610 PIN POINTING
Pin Pointing is the application of a test that positively confirms the exact position of the fault
Before the commencement of pin pointing the prelocated fault distance should mark on the cable route which is measured by means of a trumeter
Pin pointing is normally carried out by the shock wave discharge method The fault can be detected by the use of semisphone
611 Confirmation and Re-Test
After the pin-pointed position of the fault has been marked and exposed check for physical sign of fault If there is the fault is confirm
After confirmation the fault should be cut away IR and continuity test should be carried out on the remaining cable sections to determine the soundeness of these cable sections
The insulation resistance test are again carried out after jointing followed by pressure test before supply can be restored
Flowchart for Fault location Procedure
Example
Find a fault distance using direct reading fault localiser from end test which has a test data as below
Bridge Balance Reading =60 Length of Cable 140 meters
Solutions
Fault Distance= = 60100 x 140 = 84 meterxL
n
100
For cables route with combination of 2 or more different type of cable
Determine the resistance of cable for each section separately from Table 1 and Table 2
Example Cable 11kV Al Length 150 m size = 16mm2
From Table 1= Resistance for 1km =226 ohm
For 150 m = 1501000 x 226 = 0339 ohm
From Table 1 Calculate the equivalent resistance for all the combined section
Example for Al = 0339 ohm for copper = 0150 ohm
Equivalent Resistance = 0339+0150 =0489 ohm
Continue
Calculate equivalent resistance from obtained bridge reading
Example Bridge Reading = 60
Equivalent resistance = 06 x 0489 ohm = 02934 ohm
Determine which section of cable the fault occurs
Total resistance at Al section = 0339 ohm
As equivalent resistance at Al section is higher then 02934(Equivalent resistance from bridge test) the fault is
located at Aluminium section of cable
Table 1 Impedance data for 635011000 V Cable Aluminium
Table II Impedance data 11000 V Cable Copper
TABLE VI ndash 6001000V (ALUMINIUM)
TABLE VII ndash 38006600V (ALUMINIUM)
TABLE VIII- 635011000 (ALUMINIUM)
TABLE V
610 COMMISSIONING OF UNDERGROUND CABLE
High Voltage DC testing
High voltage testing is carried out in order to determine the electrical strength of cable insulation Site tests are performed by applying a predetermined high voltage to the insulation DC site test voltages regardless of insulation type are used to ensure that the cable cable joints and termination are correctly made and installed
High Voltage Testing (cont-)
IEC Standards specify maintenance tests after installation should be 70 of permissible factory test DC voltages (15 minutes) = 175kV
Insulation Resistance Test (cont-)
Test voltages should be slowly raised to the required value over a period of about 1 minute and the test period starts once the full voltage is reached In this way the capacitive and absorption currents will have decreased and circuit conditions stabilized such that true leakage current may be measured
6921 Continue
Solutions
Velocity of propagation = Vp2 = 25003164 = 79 mS
Distance of Fault from test end = Vp2 x t2
= 79 x 1745
= 137855
693 Impulse Current Method
Working on Travelling wave principles it is applicable to all types of fault
694 Arc Reflection or Secondary Impulse method
Basically PE and TDR associated with fault treatment It is also applicable to faults of all nature but with easily interpreted breakdown waveforms
610 PIN POINTING
Pin Pointing is the application of a test that positively confirms the exact position of the fault
Before the commencement of pin pointing the prelocated fault distance should mark on the cable route which is measured by means of a trumeter
Pin pointing is normally carried out by the shock wave discharge method The fault can be detected by the use of semisphone
611 Confirmation and Re-Test
After the pin-pointed position of the fault has been marked and exposed check for physical sign of fault If there is the fault is confirm
After confirmation the fault should be cut away IR and continuity test should be carried out on the remaining cable sections to determine the soundeness of these cable sections
The insulation resistance test are again carried out after jointing followed by pressure test before supply can be restored
Flowchart for Fault location Procedure
Example
Find a fault distance using direct reading fault localiser from end test which has a test data as below
Bridge Balance Reading =60 Length of Cable 140 meters
Solutions
Fault Distance= = 60100 x 140 = 84 meterxL
n
100
For cables route with combination of 2 or more different type of cable
Determine the resistance of cable for each section separately from Table 1 and Table 2
Example Cable 11kV Al Length 150 m size = 16mm2
From Table 1= Resistance for 1km =226 ohm
For 150 m = 1501000 x 226 = 0339 ohm
From Table 1 Calculate the equivalent resistance for all the combined section
Example for Al = 0339 ohm for copper = 0150 ohm
Equivalent Resistance = 0339+0150 =0489 ohm
Continue
Calculate equivalent resistance from obtained bridge reading
Example Bridge Reading = 60
Equivalent resistance = 06 x 0489 ohm = 02934 ohm
Determine which section of cable the fault occurs
Total resistance at Al section = 0339 ohm
As equivalent resistance at Al section is higher then 02934(Equivalent resistance from bridge test) the fault is
located at Aluminium section of cable
Table 1 Impedance data for 635011000 V Cable Aluminium
Table II Impedance data 11000 V Cable Copper
TABLE VI ndash 6001000V (ALUMINIUM)
TABLE VII ndash 38006600V (ALUMINIUM)
TABLE VIII- 635011000 (ALUMINIUM)
TABLE V
610 COMMISSIONING OF UNDERGROUND CABLE
High Voltage DC testing
High voltage testing is carried out in order to determine the electrical strength of cable insulation Site tests are performed by applying a predetermined high voltage to the insulation DC site test voltages regardless of insulation type are used to ensure that the cable cable joints and termination are correctly made and installed
High Voltage Testing (cont-)
IEC Standards specify maintenance tests after installation should be 70 of permissible factory test DC voltages (15 minutes) = 175kV
Insulation Resistance Test (cont-)
Test voltages should be slowly raised to the required value over a period of about 1 minute and the test period starts once the full voltage is reached In this way the capacitive and absorption currents will have decreased and circuit conditions stabilized such that true leakage current may be measured
693 Impulse Current Method
Working on Travelling wave principles it is applicable to all types of fault
694 Arc Reflection or Secondary Impulse method
Basically PE and TDR associated with fault treatment It is also applicable to faults of all nature but with easily interpreted breakdown waveforms
610 PIN POINTING
Pin Pointing is the application of a test that positively confirms the exact position of the fault
Before the commencement of pin pointing the prelocated fault distance should mark on the cable route which is measured by means of a trumeter
Pin pointing is normally carried out by the shock wave discharge method The fault can be detected by the use of semisphone
611 Confirmation and Re-Test
After the pin-pointed position of the fault has been marked and exposed check for physical sign of fault If there is the fault is confirm
After confirmation the fault should be cut away IR and continuity test should be carried out on the remaining cable sections to determine the soundeness of these cable sections
The insulation resistance test are again carried out after jointing followed by pressure test before supply can be restored
Flowchart for Fault location Procedure
Example
Find a fault distance using direct reading fault localiser from end test which has a test data as below
Bridge Balance Reading =60 Length of Cable 140 meters
Solutions
Fault Distance= = 60100 x 140 = 84 meterxL
n
100
For cables route with combination of 2 or more different type of cable
Determine the resistance of cable for each section separately from Table 1 and Table 2
Example Cable 11kV Al Length 150 m size = 16mm2
From Table 1= Resistance for 1km =226 ohm
For 150 m = 1501000 x 226 = 0339 ohm
From Table 1 Calculate the equivalent resistance for all the combined section
Example for Al = 0339 ohm for copper = 0150 ohm
Equivalent Resistance = 0339+0150 =0489 ohm
Continue
Calculate equivalent resistance from obtained bridge reading
Example Bridge Reading = 60
Equivalent resistance = 06 x 0489 ohm = 02934 ohm
Determine which section of cable the fault occurs
Total resistance at Al section = 0339 ohm
As equivalent resistance at Al section is higher then 02934(Equivalent resistance from bridge test) the fault is
located at Aluminium section of cable
Table 1 Impedance data for 635011000 V Cable Aluminium
Table II Impedance data 11000 V Cable Copper
TABLE VI ndash 6001000V (ALUMINIUM)
TABLE VII ndash 38006600V (ALUMINIUM)
TABLE VIII- 635011000 (ALUMINIUM)
TABLE V
610 COMMISSIONING OF UNDERGROUND CABLE
High Voltage DC testing
High voltage testing is carried out in order to determine the electrical strength of cable insulation Site tests are performed by applying a predetermined high voltage to the insulation DC site test voltages regardless of insulation type are used to ensure that the cable cable joints and termination are correctly made and installed
High Voltage Testing (cont-)
IEC Standards specify maintenance tests after installation should be 70 of permissible factory test DC voltages (15 minutes) = 175kV
Insulation Resistance Test (cont-)
Test voltages should be slowly raised to the required value over a period of about 1 minute and the test period starts once the full voltage is reached In this way the capacitive and absorption currents will have decreased and circuit conditions stabilized such that true leakage current may be measured
610 PIN POINTING
Pin Pointing is the application of a test that positively confirms the exact position of the fault
Before the commencement of pin pointing the prelocated fault distance should mark on the cable route which is measured by means of a trumeter
Pin pointing is normally carried out by the shock wave discharge method The fault can be detected by the use of semisphone
611 Confirmation and Re-Test
After the pin-pointed position of the fault has been marked and exposed check for physical sign of fault If there is the fault is confirm
After confirmation the fault should be cut away IR and continuity test should be carried out on the remaining cable sections to determine the soundeness of these cable sections
The insulation resistance test are again carried out after jointing followed by pressure test before supply can be restored
Flowchart for Fault location Procedure
Example
Find a fault distance using direct reading fault localiser from end test which has a test data as below
Bridge Balance Reading =60 Length of Cable 140 meters
Solutions
Fault Distance= = 60100 x 140 = 84 meterxL
n
100
For cables route with combination of 2 or more different type of cable
Determine the resistance of cable for each section separately from Table 1 and Table 2
Example Cable 11kV Al Length 150 m size = 16mm2
From Table 1= Resistance for 1km =226 ohm
For 150 m = 1501000 x 226 = 0339 ohm
From Table 1 Calculate the equivalent resistance for all the combined section
Example for Al = 0339 ohm for copper = 0150 ohm
Equivalent Resistance = 0339+0150 =0489 ohm
Continue
Calculate equivalent resistance from obtained bridge reading
Example Bridge Reading = 60
Equivalent resistance = 06 x 0489 ohm = 02934 ohm
Determine which section of cable the fault occurs
Total resistance at Al section = 0339 ohm
As equivalent resistance at Al section is higher then 02934(Equivalent resistance from bridge test) the fault is
located at Aluminium section of cable
Table 1 Impedance data for 635011000 V Cable Aluminium
Table II Impedance data 11000 V Cable Copper
TABLE VI ndash 6001000V (ALUMINIUM)
TABLE VII ndash 38006600V (ALUMINIUM)
TABLE VIII- 635011000 (ALUMINIUM)
TABLE V
610 COMMISSIONING OF UNDERGROUND CABLE
High Voltage DC testing
High voltage testing is carried out in order to determine the electrical strength of cable insulation Site tests are performed by applying a predetermined high voltage to the insulation DC site test voltages regardless of insulation type are used to ensure that the cable cable joints and termination are correctly made and installed
High Voltage Testing (cont-)
IEC Standards specify maintenance tests after installation should be 70 of permissible factory test DC voltages (15 minutes) = 175kV
Insulation Resistance Test (cont-)
Test voltages should be slowly raised to the required value over a period of about 1 minute and the test period starts once the full voltage is reached In this way the capacitive and absorption currents will have decreased and circuit conditions stabilized such that true leakage current may be measured
611 Confirmation and Re-Test
After the pin-pointed position of the fault has been marked and exposed check for physical sign of fault If there is the fault is confirm
After confirmation the fault should be cut away IR and continuity test should be carried out on the remaining cable sections to determine the soundeness of these cable sections
The insulation resistance test are again carried out after jointing followed by pressure test before supply can be restored
Flowchart for Fault location Procedure
Example
Find a fault distance using direct reading fault localiser from end test which has a test data as below
Bridge Balance Reading =60 Length of Cable 140 meters
Solutions
Fault Distance= = 60100 x 140 = 84 meterxL
n
100
For cables route with combination of 2 or more different type of cable
Determine the resistance of cable for each section separately from Table 1 and Table 2
Example Cable 11kV Al Length 150 m size = 16mm2
From Table 1= Resistance for 1km =226 ohm
For 150 m = 1501000 x 226 = 0339 ohm
From Table 1 Calculate the equivalent resistance for all the combined section
Example for Al = 0339 ohm for copper = 0150 ohm
Equivalent Resistance = 0339+0150 =0489 ohm
Continue
Calculate equivalent resistance from obtained bridge reading
Example Bridge Reading = 60
Equivalent resistance = 06 x 0489 ohm = 02934 ohm
Determine which section of cable the fault occurs
Total resistance at Al section = 0339 ohm
As equivalent resistance at Al section is higher then 02934(Equivalent resistance from bridge test) the fault is
located at Aluminium section of cable
Table 1 Impedance data for 635011000 V Cable Aluminium
Table II Impedance data 11000 V Cable Copper
TABLE VI ndash 6001000V (ALUMINIUM)
TABLE VII ndash 38006600V (ALUMINIUM)
TABLE VIII- 635011000 (ALUMINIUM)
TABLE V
610 COMMISSIONING OF UNDERGROUND CABLE
High Voltage DC testing
High voltage testing is carried out in order to determine the electrical strength of cable insulation Site tests are performed by applying a predetermined high voltage to the insulation DC site test voltages regardless of insulation type are used to ensure that the cable cable joints and termination are correctly made and installed
High Voltage Testing (cont-)
IEC Standards specify maintenance tests after installation should be 70 of permissible factory test DC voltages (15 minutes) = 175kV
Insulation Resistance Test (cont-)
Test voltages should be slowly raised to the required value over a period of about 1 minute and the test period starts once the full voltage is reached In this way the capacitive and absorption currents will have decreased and circuit conditions stabilized such that true leakage current may be measured
Flowchart for Fault location Procedure
Example
Find a fault distance using direct reading fault localiser from end test which has a test data as below
Bridge Balance Reading =60 Length of Cable 140 meters
Solutions
Fault Distance= = 60100 x 140 = 84 meterxL
n
100
For cables route with combination of 2 or more different type of cable
Determine the resistance of cable for each section separately from Table 1 and Table 2
Example Cable 11kV Al Length 150 m size = 16mm2
From Table 1= Resistance for 1km =226 ohm
For 150 m = 1501000 x 226 = 0339 ohm
From Table 1 Calculate the equivalent resistance for all the combined section
Example for Al = 0339 ohm for copper = 0150 ohm
Equivalent Resistance = 0339+0150 =0489 ohm
Continue
Calculate equivalent resistance from obtained bridge reading
Example Bridge Reading = 60
Equivalent resistance = 06 x 0489 ohm = 02934 ohm
Determine which section of cable the fault occurs
Total resistance at Al section = 0339 ohm
As equivalent resistance at Al section is higher then 02934(Equivalent resistance from bridge test) the fault is
located at Aluminium section of cable
Table 1 Impedance data for 635011000 V Cable Aluminium
Table II Impedance data 11000 V Cable Copper
TABLE VI ndash 6001000V (ALUMINIUM)
TABLE VII ndash 38006600V (ALUMINIUM)
TABLE VIII- 635011000 (ALUMINIUM)
TABLE V
610 COMMISSIONING OF UNDERGROUND CABLE
High Voltage DC testing
High voltage testing is carried out in order to determine the electrical strength of cable insulation Site tests are performed by applying a predetermined high voltage to the insulation DC site test voltages regardless of insulation type are used to ensure that the cable cable joints and termination are correctly made and installed
High Voltage Testing (cont-)
IEC Standards specify maintenance tests after installation should be 70 of permissible factory test DC voltages (15 minutes) = 175kV
Insulation Resistance Test (cont-)
Test voltages should be slowly raised to the required value over a period of about 1 minute and the test period starts once the full voltage is reached In this way the capacitive and absorption currents will have decreased and circuit conditions stabilized such that true leakage current may be measured
Example
Find a fault distance using direct reading fault localiser from end test which has a test data as below
Bridge Balance Reading =60 Length of Cable 140 meters
Solutions
Fault Distance= = 60100 x 140 = 84 meterxL
n
100
For cables route with combination of 2 or more different type of cable
Determine the resistance of cable for each section separately from Table 1 and Table 2
Example Cable 11kV Al Length 150 m size = 16mm2
From Table 1= Resistance for 1km =226 ohm
For 150 m = 1501000 x 226 = 0339 ohm
From Table 1 Calculate the equivalent resistance for all the combined section
Example for Al = 0339 ohm for copper = 0150 ohm
Equivalent Resistance = 0339+0150 =0489 ohm
Continue
Calculate equivalent resistance from obtained bridge reading
Example Bridge Reading = 60
Equivalent resistance = 06 x 0489 ohm = 02934 ohm
Determine which section of cable the fault occurs
Total resistance at Al section = 0339 ohm
As equivalent resistance at Al section is higher then 02934(Equivalent resistance from bridge test) the fault is
located at Aluminium section of cable
Table 1 Impedance data for 635011000 V Cable Aluminium
Table II Impedance data 11000 V Cable Copper
TABLE VI ndash 6001000V (ALUMINIUM)
TABLE VII ndash 38006600V (ALUMINIUM)
TABLE VIII- 635011000 (ALUMINIUM)
TABLE V
610 COMMISSIONING OF UNDERGROUND CABLE
High Voltage DC testing
High voltage testing is carried out in order to determine the electrical strength of cable insulation Site tests are performed by applying a predetermined high voltage to the insulation DC site test voltages regardless of insulation type are used to ensure that the cable cable joints and termination are correctly made and installed
High Voltage Testing (cont-)
IEC Standards specify maintenance tests after installation should be 70 of permissible factory test DC voltages (15 minutes) = 175kV
Insulation Resistance Test (cont-)
Test voltages should be slowly raised to the required value over a period of about 1 minute and the test period starts once the full voltage is reached In this way the capacitive and absorption currents will have decreased and circuit conditions stabilized such that true leakage current may be measured
For cables route with combination of 2 or more different type of cable
Determine the resistance of cable for each section separately from Table 1 and Table 2
Example Cable 11kV Al Length 150 m size = 16mm2
From Table 1= Resistance for 1km =226 ohm
For 150 m = 1501000 x 226 = 0339 ohm
From Table 1 Calculate the equivalent resistance for all the combined section
Example for Al = 0339 ohm for copper = 0150 ohm
Equivalent Resistance = 0339+0150 =0489 ohm
Continue
Calculate equivalent resistance from obtained bridge reading
Example Bridge Reading = 60
Equivalent resistance = 06 x 0489 ohm = 02934 ohm
Determine which section of cable the fault occurs
Total resistance at Al section = 0339 ohm
As equivalent resistance at Al section is higher then 02934(Equivalent resistance from bridge test) the fault is
located at Aluminium section of cable
Table 1 Impedance data for 635011000 V Cable Aluminium
Table II Impedance data 11000 V Cable Copper
TABLE VI ndash 6001000V (ALUMINIUM)
TABLE VII ndash 38006600V (ALUMINIUM)
TABLE VIII- 635011000 (ALUMINIUM)
TABLE V
610 COMMISSIONING OF UNDERGROUND CABLE
High Voltage DC testing
High voltage testing is carried out in order to determine the electrical strength of cable insulation Site tests are performed by applying a predetermined high voltage to the insulation DC site test voltages regardless of insulation type are used to ensure that the cable cable joints and termination are correctly made and installed
High Voltage Testing (cont-)
IEC Standards specify maintenance tests after installation should be 70 of permissible factory test DC voltages (15 minutes) = 175kV
Insulation Resistance Test (cont-)
Test voltages should be slowly raised to the required value over a period of about 1 minute and the test period starts once the full voltage is reached In this way the capacitive and absorption currents will have decreased and circuit conditions stabilized such that true leakage current may be measured
Continue
Calculate equivalent resistance from obtained bridge reading
Example Bridge Reading = 60
Equivalent resistance = 06 x 0489 ohm = 02934 ohm
Determine which section of cable the fault occurs
Total resistance at Al section = 0339 ohm
As equivalent resistance at Al section is higher then 02934(Equivalent resistance from bridge test) the fault is
located at Aluminium section of cable
Table 1 Impedance data for 635011000 V Cable Aluminium
Table II Impedance data 11000 V Cable Copper
TABLE VI ndash 6001000V (ALUMINIUM)
TABLE VII ndash 38006600V (ALUMINIUM)
TABLE VIII- 635011000 (ALUMINIUM)
TABLE V
610 COMMISSIONING OF UNDERGROUND CABLE
High Voltage DC testing
High voltage testing is carried out in order to determine the electrical strength of cable insulation Site tests are performed by applying a predetermined high voltage to the insulation DC site test voltages regardless of insulation type are used to ensure that the cable cable joints and termination are correctly made and installed
High Voltage Testing (cont-)
IEC Standards specify maintenance tests after installation should be 70 of permissible factory test DC voltages (15 minutes) = 175kV
Insulation Resistance Test (cont-)
Test voltages should be slowly raised to the required value over a period of about 1 minute and the test period starts once the full voltage is reached In this way the capacitive and absorption currents will have decreased and circuit conditions stabilized such that true leakage current may be measured
Table 1 Impedance data for 635011000 V Cable Aluminium
Table II Impedance data 11000 V Cable Copper
TABLE VI ndash 6001000V (ALUMINIUM)
TABLE VII ndash 38006600V (ALUMINIUM)
TABLE VIII- 635011000 (ALUMINIUM)
TABLE V
610 COMMISSIONING OF UNDERGROUND CABLE
High Voltage DC testing
High voltage testing is carried out in order to determine the electrical strength of cable insulation Site tests are performed by applying a predetermined high voltage to the insulation DC site test voltages regardless of insulation type are used to ensure that the cable cable joints and termination are correctly made and installed
High Voltage Testing (cont-)
IEC Standards specify maintenance tests after installation should be 70 of permissible factory test DC voltages (15 minutes) = 175kV
Insulation Resistance Test (cont-)
Test voltages should be slowly raised to the required value over a period of about 1 minute and the test period starts once the full voltage is reached In this way the capacitive and absorption currents will have decreased and circuit conditions stabilized such that true leakage current may be measured
TABLE VI ndash 6001000V (ALUMINIUM)
TABLE VII ndash 38006600V (ALUMINIUM)
TABLE VIII- 635011000 (ALUMINIUM)
TABLE V
610 COMMISSIONING OF UNDERGROUND CABLE
High Voltage DC testing
High voltage testing is carried out in order to determine the electrical strength of cable insulation Site tests are performed by applying a predetermined high voltage to the insulation DC site test voltages regardless of insulation type are used to ensure that the cable cable joints and termination are correctly made and installed
High Voltage Testing (cont-)
IEC Standards specify maintenance tests after installation should be 70 of permissible factory test DC voltages (15 minutes) = 175kV
Insulation Resistance Test (cont-)
Test voltages should be slowly raised to the required value over a period of about 1 minute and the test period starts once the full voltage is reached In this way the capacitive and absorption currents will have decreased and circuit conditions stabilized such that true leakage current may be measured
TABLE VII ndash 38006600V (ALUMINIUM)
TABLE VIII- 635011000 (ALUMINIUM)
TABLE V
610 COMMISSIONING OF UNDERGROUND CABLE
High Voltage DC testing
High voltage testing is carried out in order to determine the electrical strength of cable insulation Site tests are performed by applying a predetermined high voltage to the insulation DC site test voltages regardless of insulation type are used to ensure that the cable cable joints and termination are correctly made and installed
High Voltage Testing (cont-)
IEC Standards specify maintenance tests after installation should be 70 of permissible factory test DC voltages (15 minutes) = 175kV
Insulation Resistance Test (cont-)
Test voltages should be slowly raised to the required value over a period of about 1 minute and the test period starts once the full voltage is reached In this way the capacitive and absorption currents will have decreased and circuit conditions stabilized such that true leakage current may be measured
TABLE VIII- 635011000 (ALUMINIUM)
TABLE V
610 COMMISSIONING OF UNDERGROUND CABLE
High Voltage DC testing
High voltage testing is carried out in order to determine the electrical strength of cable insulation Site tests are performed by applying a predetermined high voltage to the insulation DC site test voltages regardless of insulation type are used to ensure that the cable cable joints and termination are correctly made and installed
High Voltage Testing (cont-)
IEC Standards specify maintenance tests after installation should be 70 of permissible factory test DC voltages (15 minutes) = 175kV
Insulation Resistance Test (cont-)
Test voltages should be slowly raised to the required value over a period of about 1 minute and the test period starts once the full voltage is reached In this way the capacitive and absorption currents will have decreased and circuit conditions stabilized such that true leakage current may be measured
TABLE V
610 COMMISSIONING OF UNDERGROUND CABLE
High Voltage DC testing
High voltage testing is carried out in order to determine the electrical strength of cable insulation Site tests are performed by applying a predetermined high voltage to the insulation DC site test voltages regardless of insulation type are used to ensure that the cable cable joints and termination are correctly made and installed
High Voltage Testing (cont-)
IEC Standards specify maintenance tests after installation should be 70 of permissible factory test DC voltages (15 minutes) = 175kV
Insulation Resistance Test (cont-)
Test voltages should be slowly raised to the required value over a period of about 1 minute and the test period starts once the full voltage is reached In this way the capacitive and absorption currents will have decreased and circuit conditions stabilized such that true leakage current may be measured
610 COMMISSIONING OF UNDERGROUND CABLE
High Voltage DC testing
High voltage testing is carried out in order to determine the electrical strength of cable insulation Site tests are performed by applying a predetermined high voltage to the insulation DC site test voltages regardless of insulation type are used to ensure that the cable cable joints and termination are correctly made and installed
High Voltage Testing (cont-)
IEC Standards specify maintenance tests after installation should be 70 of permissible factory test DC voltages (15 minutes) = 175kV
Insulation Resistance Test (cont-)
Test voltages should be slowly raised to the required value over a period of about 1 minute and the test period starts once the full voltage is reached In this way the capacitive and absorption currents will have decreased and circuit conditions stabilized such that true leakage current may be measured
High Voltage Testing (cont-)
IEC Standards specify maintenance tests after installation should be 70 of permissible factory test DC voltages (15 minutes) = 175kV
Insulation Resistance Test (cont-)
Test voltages should be slowly raised to the required value over a period of about 1 minute and the test period starts once the full voltage is reached In this way the capacitive and absorption currents will have decreased and circuit conditions stabilized such that true leakage current may be measured
Insulation Resistance Test (cont-)
Test voltages should be slowly raised to the required value over a period of about 1 minute and the test period starts once the full voltage is reached In this way the capacitive and absorption currents will have decreased and circuit conditions stabilized such that true leakage current may be measured