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HIGHVOLT Prüftechnik Dresden GmbH
SUITABILITY OF DIFFERENT
TEST VOLTAGES FOR
ON-SITE TESTING OF
XLPE CABLE SYSTEMS Michael Hensel
Suitability of different test voltages for on-site testing of XLPE cable systems 2
Content
Introduction 1
Comparison of test methods
2
3
IEC Standard
Test parameters and their significance
Differences between AC/DC and MV/HV cables
Introduction Cable Life
Suitability of different test voltages for on-site testing of XLPE cable systems 3
Manufacturing D
am
ag
e
Mis
tak
e
Te
sti
ng
Transportation Installation
Testi
ng
Power-
freq.
50/60
Hz
Operation
50/60 Hz
Cable life
Re
pa
ir
Ag
ein
g
Te
sti
ng
Introduction Commissioning and Diagnostic Testing
Suitability of different test voltages for on-site testing of XLPE cable systems 4
Commissioning test
Quality control after installation of the cable system
Detecting all faults which would lead to a breakdown during
service time of the cable system Aim
Diagnostic test
Check of specific values of the cable system
Condition assessment for remaining life time estimation
of the cable system Aim
Suitability of different test voltages for on-site testing of XLPE cable systems 5
Content
Introduction 1
Comparison of test methods
2
3
IEC Standard
Test parameters and their significance
Differences between AC/DC and MV/HV cables
6 Suitability of different test voltages for on-site testing of XLPE cable systems
Comparison of test methods shall be based on
parameters describing
1) The harmfulness for healthy insulation
2) The usefulness of a test method for an application, i.e. the
capabilities to distinguish between healthy and defective
insulation
3) The applicability (for example expressed by the equipment
size, weight and its power consumption)
Numbers 2 and 3 are only worth discussing if the previous
numbers are answered satisfactorily
Test parameters and their significance Characteristics of Test Methods to be Assessed
7 Suitability of different test voltages for on-site testing of XLPE cable systems
A Cable System
Will be used with constant AC stresses (with occasional
switching and lightning overvoltages)
Quality depends only on performance under these
stresses – A good cable is one that works well a long
time, not one that passed a certain test
A Test should therefore
“Produce the same dielectric effect the insulation as
overvoltages ... in service” (IEC 60071-1)
Thereby be sensitive to those faults dangerous during
operation
Test parameters and their significance Why test with same Type of Stresses as in Service?
8 Suitability of different test voltages for on-site testing of XLPE cable systems
Purpose of test
Initiate discharges in defects so that they can be found
Withstand test: Cause sufficient growth in the defects to
lead to breakdown
Influential test parameters:
Test voltage value
Test voltage duration (both for PD inception as for growth)
Test voltage wave shape (type of stress)
Type and amount of stress is important for
significance of test
Test parameters and their significance Which parameters are interesting?
Suitability of different test voltages for on-site testing of XLPE cable systems 9
Test parameters and their significance What is a Significant Test?
Cable system without faults:
Test result cable system is healthy
Test result cable system is not healthy
Cable system with faults:
Test result cable system is not healthy
Test result cable system is healthy
Suitability of different test voltages for on-site testing of XLPE cable systems 10
Content
Introduction 1
Comparison of test methods
2
3
IEC Standard
Test parameters and their significance
Differences between AC/DC and MV/HV cables
11 Suitability of different test voltages for on-site testing of XLPE cable systems
H
V
C R
XLPE Cable System
AC Conditions:
tan d =
ic >> ir
Capacitive field control
DC conditions (without consideration of space charges):
C becomes irrelevant
Resistive field control
Defects usually have lower resistance and lower field
Differences between AC/DC and MV/HV cables Field Distribution in Layered Dielectrics
12 Suitability of different test voltages for on-site testing of XLPE cable systems
Examples for Typical Cable Geometry
MV Cable: 24 kV, insulation thickness 5.5 mm, mean
operational field strength at voltage peak 3.1 kV/mm
HV Cable: 138 kV, insulation thickness 17.8 mm, mean
operational field strength at voltage peak 6.3 kV/mm
EHV Cable: 400 kV, insulation thickness 26 mm, mean
operational field strength at voltage peak 12.6 kV/mm
Differences between AC/DC and MV/HV cables Differences between MV, HV and EHV cables
13 Suitability of different test voltages for on-site testing of XLPE cable systems
Technical Consequences
Field strength increases with cable voltage
Recommended limit for test field strength (XLPE):
27-30 kV/mm
Allowed ratio between test voltages and operational voltages
for MV 4 times and for HV 2 times higher than for EHV XLPE
cables
Test methods using higher overvoltage's can be applied for
MV cables
Using these methods for HV and EHV cables test voltages
would have to be reduced, in turn reducing sensitivity of test
Differences between AC/DC and MV/HV cables Differences between MV, HV and EHV Cables
Suitability of different test voltages for on-site testing of XLPE cable systems 14
Content
Introduction 1
Comparison of test methods
2
3
IEC Standard
Test parameters and their significance
Differences between AC/DC and MV/HV cables
DC
Suitability of different test voltages for on-site testing of XLPE cable systems 15
(1) Mainly used for testing of oil-paper cables
(2) Experience and research showed that DC voltage creates space
charges in solid, extruded insulations designed for AC applications
(3) These space charges can lead to a breakdown when AC voltage is
applied to the test object after the DC test
Space charges endanger an otherwise healthy cable system
DC voltage is therefore not used for commissioning and
diagnosic tests on AC cable systems
Very Low Frequency (VLF)
Suitability of different test voltages for on-site testing of XLPE cable systems 16
(1) Continous voltage with a frequency between 0.01 to 1 Hz, typically
used for testing 0.1 Hz (Europe)
(3) Due to the few voltage cycles PD measuring is not as sensitive as
with power frequency
(2) Due to the low frequency the capacitive field control is not longer dominant
reduced field strength in the faults higher voltage necessary in comparison
with AC of power frequency
Limit is here the max. field strength for XLPE of approx. 30 kV/mm
For MV cable systems well established
For EHV cable systems not standardized
(4) Experience shows that the detection/diagnostic of water trees with VLF works well
(5) Comparability between different VLF test results is well given but the comparison to
a test with AC power frequency is difficult due to the different field control effects
(capacitive/resistive)
(6) VLF testing in a well defined test procedure for MV applications
17 Suitability of different test voltages for on-site testing of XLPE cable systems
Very Low Frequency (VLF)
OWTS/DAC (Oscillating wave)
Suitability of different test voltages for on-site testing of XLPE cable systems 18
(1) Usage of the cable system as test energy storage by charging it with a DC current
(2) The DC charging ramp poses the danger for creating space charges
Might lead to a breakdown of a otherwise healthy insulation
(3) Typical test procedure of 50 „shots“ equals ~ 10 s of continuous AC voltage
(depending on the capacitance of the cable system)
Only faults with a PD inception time of ~ 10 s can be detected
(4) Does not represent stress under service conditions
(5) Test time highly depends on the parameters of the cable and the test system
very bad repeatability and thus not comparable
(6) Due to DC charging different behavior of insulation compared to power frequency AC
test and contiuous AC test not comparable with factory test or other on-site tests
Not standardized and not recommended for commissioning testing
Typical usage for diagnostic testing of MV cables where the safety margin in
the electrical field strength is bigger compared to HV and EHV cable systems
19 Suitability of different test voltages for on-site testing of XLPE cable systems
OWTS/DAC (Oscillating wave)
Continuous AC 20-300 Hz
Suitability of different test voltages for on-site testing of XLPE cable systems 20
(1) Usually resonant circuit with fixed inductance and frequency converter as supply
(2) Test voltage very similar to service voltage. Even at 20 Hz, 1 hour is equivalent with
72.000 oscillations high probability to initiate discharges in faults very good
possibilities to detect faults / e.g. PD
(3) Electric field distribution is very close to that under service voltage represents
very well the stess under service conditions
(4) Very high comparability with tests done at power frequency in the factory
(5) Test can be repeated even on other cables very good repeatability
(6) Test is very well defined in the standards and the existing experience of ~ 20.000
test conducted worldwide with continous AC between 20 - 300 Hz shows similar
performance like the tests done in the factories
Standardized and well suitable for commissioning and
diagnostic testing of MV/HV/EHV cable systems
21 Suitability of different test voltages for on-site testing of XLPE cable systems
Continuous AC 20-300 Hz
24 h rated voltage
Suitability of different test voltages for on-site testing of XLPE cable systems 22
(2) Only faults which lead to a breakdown withhin 24 h of cable system service voltage
can be found
(3) PD measuring increases the chance to find more defects, but still all the faults with
a PD inception time >24 h remain undetected
(1) Voltage form and frequency identical to service conditions
Well suitable for diagnostic testing of MV/HV/EHV cable systems
Not recommended for commissioning testing
Commissioning testing with 24 h of rated voltage is like a “1 day guarantee”
Suitability of different test voltages for on-site testing of XLPE cable systems 23
Content
Introduction 1
Comparison of test methods
2
3
IEC Standard
Test parameters and their significance
Differences between AC/DC and MV/HV cables
IEC Standard IEC 60502 old version
Suitability of different test voltages for on-site testing of XLPE cable systems 24
IEC Standard IEC 60502 new version
Suitability of different test voltages for on-site testing of XLPE cable systems 25
IEC 60502-2
Suitability of different test voltages for on-site testing of XLPE cable systems 26
IEC Standard IEC 60502 new version
IEC 60502-2
Suitability of different test voltages for on-site testing of XLPE cable systems 27
IEC Standard IEC 60840 latest version
IEC 60840
Suitability of different test voltages for on-site testing of XLPE cable systems 28
IEC Standard IEC 60840 latest version
IEC 60840
Suitability of different test voltages for on-site testing of XLPE cable systems 29
IEC Standard IEC 62067 latest version
IEC 62067
Suitability of different test voltages for on-site testing of XLPE cable systems 30
IEC Standard IEC 62067 latest version
IEC 62067
HIGHVOLT Prüftechnik Dresden GmbH
THANK YOU FOR YOUR
ATTENTION
Michael Hensel
32 Suitability of different test voltages for on-site testing of XLPE cable systems
MV cable test van 36 kV / 10 A
HV cable test system 260/80 kV – 83 A
33 Suitability of different test voltages for on-site testing of XLPE cable systems
11
Cable type: 400kV XLPE
Cable length: 16km
Cable capacitance: 2360nF
Test voltage: 260kV
Test duration: 1 hour
Parallel and Series Connections for long HV
Cable Test
L1 + L4 L2 + L5 L3 + L6
6 x WRV 83/260 T
L4 II L5 II L6
Suitability of different test voltages for on-site testing of XLPE cable systems