Diagnostic Techniques of Power
Transformers
ISEI 2010 –Short Course San Diego – June 6th 2010
Ali Naderian, P.Eng., PhD.Kinectrics Inc.
Toronto, [email protected]
+1-416-797-3724
1
Introduction• Myself:
ITC ( Siemens Power Transformer) 1998-2000 Designed a 3-step cascade Testing Transformer
(2000) Rebuilding High voltage Lab of the University of
Waterloo (2005) Kinectrics (Ontario Hydro Research) since 2007
working at the HV Lab, as well as Testing on Transformers/Cables
2
Objectives
• Identify the diagnostic and condition assessment methods
• Explain the selected conventional testing methods
• Identify innovative techniques of transformer diagnostics
• Interpret the test results of case studies3
Contents• Routine (conventional)
Winding/Insulation Diagnostics Tests:Polarization Index (insulation resistance)Turn ratioDC resistanceShort circuit ImpedanceExcitation CurrentPower Factor, capacitance
• Oil Tests:DGAOil quality tests (Dielectric, Power factor, …)
4
Contents• Advanced Condition Assessment
Techniques:
Partial Discharge (PD) :Electrical/Acoustic
Dielectric Frequency Response (DFR)
Frequency Response Analysis (FRA)
5
Polarization Index
• Propose: Tests : Winding Insulation + Oil Overall integrity of the winding insulation Verify that the state of dryness of insulation
• Definition:Polarization Index : PI=R10-min /R1-min
Trend Measure of dielectric deterioration
Absorption ratio : AR=R60s /R15s
• Test Method: H-LG, L-HG, HL-G Utest=5 kV-DCTime= 15 s, 60 s, 10 min
6
7
Insulation Resistance Model
iL iC
ReqRA is not a fix resistance:
8
Test Arrangement
kV 20 °C 30 °C 40 °C 50 °C 60 °C
6.6 400 200 100 50 25
6.6-19 800 400 200 100 50
22-45 1000 500 250 125 65
≥ 66 1200 600 300 100 75
Typical Insulation Resistance MΩ
When to use Guard?
9
Interpretation• R Needs temp correction
k=1.5 for oil-filled transformer
k=30 for untanked or dry-type transformers
• 1.3≤AR≤3.0: Dry Transformer
• PI (Large Power transformers)
1- Make sure transformer is grounded before and after test.
2- The energy stored must be discharged safely by short-circuiting at least x4 of the test period.
Winding
Winding
KVA
kUR 60
PI ConditionLess than 1 Dangerous
1.0 - 1.1 Poor (wet or poor dielectric)
1.1 - 1.25 Questionable1.25 - 2.0 AcceptableAbove 2.0 Very Good
Turn Ratio (TTR)
• r= Np/Ns=Ep/Es
• Deviation indicates problem in either of windings High-resistance connections in the lead circuitry or
high contact resistance in tap changers, open circuits Low resistance: shorted turn-to-turn
• Minimum accuracy : 0.1%
10
IEEE 62, IEEE C57.12.00 IEC 60076-1
+/-0.5% nameplate ratio The lesser of +/-0.5% of declared voltage ratio or 0.1*Uk%
Test Arrangement
11
• Auxiliary Transformer 230kV/13.8kV
12
Test Case
Nothing wrong with transformer!Tap changer index is off by one position. Position 1 is 2, position 2 is 3, …, position 5 is 1.
DC Resistance
13
• Measure of winding resistance • For temp-rise=55°C corrected to 75°C• For temp-rise=65°C corrected to 85°C• Before test:
(IEEE)different between top and bottom temp ≤5°C
(IEC) 3hours rest time
• Test current ≤10% of rated current• DC resistance should be ≤ 2% factory
test
DC ResistanceCan detect:• Shorted-turns• Loose connection on bushing• Loose connections or high-contact
resistance on tap changers• Broken winding strands• The above issues leads to hot-
spots, generates gases DGA
14
Short circuit impedance• Applications:
Investigate winding deformation confirm the name plate values To check shipping as a receiving
(pre- commissioning test) • If possible run three-phase if not
feasible it can be done single-phase , then average the results
15
Short circuit impedance
16
Apply voltage and measure current in HV side while shorting out the dual leg of the LV side.
Applied voltage is 100 V-500 V Watch the current in LV side LTC is in neutral position
3-phase Single phase
Short circuit impedance• If deviation exists it could be due to:
Type of excitation (1-phase versus 3-phase) Different instrumentation Winding deformation
17
• Deviations of over ± 3% from the benchmark Per Phase Tests could be related to winding deformations.
• Even 3-phase test result may be different from nameplate because of the Vtest.
Excitation Current • Can possibly detect: Core problems such as:
Shorted core laminations poor joints
Winding problems such as: short circuit (turn to turn) open circuit poor connections
LTC problems such as: high hesitance connection open circuitCoking and wear of LTC and DETC contacts
18
Excitation Current Test
• Factory tested at rated voltage (no-load)
• If possible run three-phase if not ,can be done single-phase.
• Perform Excitation Test before any DC test: DC test leaves residual magnetism in the core.
• Voltage is applied to HV :5kV or 10 kV• LTC set to: 1- Neutral , 2- 1 step up, 3- 1 step down
4- Full raise , 5- Full lower
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Excitation Current Setup
20
Excitation Current Pattern
21
H1 H2 H3
ExcitationH1-H0
Similar to H3-H0
H2-H0
Higher R results higher
excitation
current
Excitation Current Results
22
Possible patterns
Characteristics of
H-L-H • 3 leg core-type (Most common)• 5 leg core or shell type with a delta secondary
L-H-L • 3 leg core-type ,Y secondary, inaccessible neutral
All equal
• 5 leg with Y secondary• 3 single phase connected as 3- phase• Shell-type with Y-connected secondary
All different
• Can be due to magnetized core• Defects, Faults
Excitation current
Criteria to detect defect
Iext<50 mA The difference between 2 higher current >10%
Iext >50 mA The difference between 2 higher current >5%
Excitation Current with LTC
23
When the Autotransformer is in the bridging position the excitation current goes up.
I
IC IR
Power Factor TestingDissipation Factor or is
the ratio of the resistive current to capacitive current
24
Dissipation factor : tan=IR/IC
I=IC+IR
Power Factor : P.F.= IR/I
To check the condition of the capacitive insulation: Between windingsBetween winding and coreBetween winding and tank
Capacitances
2-Winding Transformer
1 phase of 3 phases shown
25
• CH: HV bushing + HV winding +Oil• CL: LV bushing + LV winding + Oil• CHL: Both windings+ barriers + Oil
Test Arrangement3 test modes: Ungrounded Specimen Test (UST)
Grounded Specimen Test (GST)
Grounded Specimen with Guard (GST-g)
26
Test Arrangement
One of the most common source of measurement error: Neutral is not properly connected.
27
Test Mode Energize Ground Guard Winding
1 UST HV - - CHL
2 UST LV - - CHL
3 GST HV LV - CH+CHL
4 GST-g HV - LV CH
5 GST LV HV - CL+CHL
6 GST-g LV - HV CL
Usually 6 tests are performed to confirm the values:
P.F. Interpretation Power Factor Insulation ConditionAbove 1.0% Dangerous
wet transformer0.7 – 1.0 Investigate0.5 – 0.7 Deteriorated
Less than 0.5 Good
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3 winding transformer
Shielding between LV/HV
IEEE Std 62: Service aged transformers : P.F.<2%
Oil Test: DGA
• Rogers Ratio• Doernenburg Ratio• Duval Triangle • IEC1 - (1st Edition 1978)• IEC2 – (2nd Edition 1999)• ANSI/IEEE (C57.104-1991)•CIGRE Method• Laborelec• Japanese Method• Russian Method
29
Key Gas Method
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Key Gas
Secondary Gas Fault Pattern Possible Root cause
H2
CH4 and minor C2H6
and C2H4
Low energy Partial
Discharge
Aging of insulation, possible carbon particles in oil, poor grounding of metal objects, loosed lead, floating metal or contamination
C2H4
(ethylene)
CH4 and minor H2 and C2H6(ethane)
Oil overheatingPaper insulation destroyed. Metal discoloration. Oil heavily carbonized.
C2H2
H2 and minor CH4 and C2H4
High Energy Arcing
Poor contacts in leads, weakened insulation from aging, carbonized oil.
CO, CO2
If the fault involves and oil-impregnated
structure CH4 and C2H4
Conductor Overheating
Overloading or cooling problem, bad connection in leads, stray magnetic flux, discoloration of paper.
Duval Triangle
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%C2H2=100x/(x+y+z);
%C2H4 = 100y/(x+y+z);
%CH4 = 100z/(x+y+z),
T1: Low-range thermal fault (below 300 C)T2 :Medium-range thermal fault (300-700 C) T3 :High-range thermal fault (above 700 C) D1: Low-energy electrical dischargeD2 :High-energy electrical dischargeDT: Indeterminate - thermal fault or electrical discharge.
x = (C2H2); y = (C2H4); z = (CH4), in ppm
IEEE Std C57.104
32
• Four-condition DGA guide to classify risks to transformers with no previous problems.
• Uses combinations of individual gases and total dissolved combustible gas concentration (TDCG).
The three-ratio version of Rogers
ratio Method uses the
following ratios: R1= C2H2/ C2H4 ,
R2= CH4/H2 , R3= C2H4/ C2H6
R2<0.1 0.1<R1<1 R3<1
1<R3<3
R1>1
R1<0.1 R3<1
1<R2<3
Gas Inputs R1,R2, R3
1<R3<3
R3>3
R3>30.1<R1<1
Case 0No fault or it cannot be detected
Case 3Low Temp Thermal Overloading
Case4Thermal <700 oC
Case5Thermal >700 oC
Case1PD low energy
Case2High energy arcing
Y
Y YY
Y Y
Y
Y
Y
Y
Y
Y
YN
N N
N
N
Rogers Fault Tree
IEEE Std C57.104
33
• Not all techniques were applicable in all cases
•CIGRE: CO2/CO <3 Excessive paper degradation
• If most of Gas levels < 1 and one or two is high, the error of regular methods might be high. Call your judgment, do not forget Key Gas Table.
Status H2 CH4 C2H2 C2H4 C2H6 CO CO2 TDCGCondition 1
100 120 35 50 65 350 2500 720
Condition 2
101-700 121-400 36-50 51-100 66-100 351-570
2500-4000
721-1920
Condition 3
701-1800
401-1000 51-80 101-200 101-150
571-1400
4001-10000
1921-4630
Condition 4
>1800 >1000 >80 >200 >150 >1400 >10000 >4630
Case 1:750 MVA,500kV
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Date H2 CH4 C2H2 C2H4 C2H6 CO2 CO TCGppm/day
April 2010
10 5 2 6 2 1398 277 47,800
May 2009
10 5 2 2 2 155 71 0.13
Key Gas IEC IEEE Roger’s ratio
IEEE Duval Method
Doernenb-urg Ratio
Suggestion
Conductor Overheating
Partial discharge or power discharge
Not conclusive
TCG levels indicate excessive decomposition
D1:Disch--arge of low energy
No diagnosis
DGA every day, On-line PD test (electric follow up with acoustic) , temporary removal from service for off-line tests
Case 2:300 MVA, 420 kV
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• The content of hydrogen detected by the on-line monitoring system increased continuously.• Concentration of hydrogen is only dependent on the oil temperature.
• Oil used was uninhibited oil considered as “moderately stray gassing”• Avoid misleading of DGA pattern (Stray Gassing) caused by catalytic effects of zinc surfaces and oil .
Oil Quality TestsRecommended oil quality tests for insulating oil
[IEEE Std C57.106-2006]
36
Test Dielectric Strength
Dissipation Factor
Interfacial tension (IFT)
Neutralization number (acidity)
PCB
Standard ASTM D1816 -97 (1 mm gap)ASTM 877[min]
ASTM D924-99 @ 25 0C[max]
ASTM D-971-91[min]
ASTM D974-92[max]
ASTM 4059-91[max]
Limit (new oil)
1816:23kV877: 26 kV
0.1% 35 mN/m 0.03 mg KOH/g
2
Limit (service aged oil)
1mm gap:23kV
0.5% 24 mN/m 0.2 mg KOH/g
50
Post Failure Test Interpretation
37
Suggested Problem Category
Test Data1st priority 2nd priority 3rd priority
Turn-to-turn fault Out of tolerance ratio
Low winding resistance
Excitation increase
Damage to major insulation
High power factor Low insulation resistance
Abnormal DGA
Lead and terminal issues
Abnormal DGA, trend inclines
DC resistance Excitation increases if load
increasesThrough fault mechanical
damage
FRA shows different pattern for faulty phase
Deviation of exciting current
Change in impulse
Core heating Abnormal DGA Low core ground resistance
Excitation increases
Moisture High insulation power factor
Low dielectric oil test
Low insulation resistance
Time for Break!
38
Selected Advanced Techniques
• Partial Discharge
• Dielectric Frequency Response (or Frequency Domain Spectroscopy)
• Frequency Response Analysis
39
Partial Discharge
40
Draft IEEE PC57.113™
IEEE Std C57.127
Partial Discharge-IEC 60270
41
• PD should be done along with induced voltage test• Background noise < 100 pC
max (pC)=100 at 110%Urated max (pC)=300 at 130%Urated max (pC)=500 at 150%Urated
Wide-Band PD measurement Narrow-band PD measurement
Partial Discharge Measurement
42
• Signal from coupling capacitor or bushing tap
coupling device in series with the coupling capacitor frequently used circuit in test laboratories
Via Bushing Tap often applied in on-site offline /online PD investigations
Bushing Tap Sensors
43
To get down the voltage to U, it is necessary to use capacitance (Cz)
Possible power that can be taken from the test tap:
To avoid bushing damage:
Partial Discharge Patterns
44
Corona at HV electrode Corona at Ground
Floated metal object Creeping discharges
1 Cycle screenshot 1 Cycle screenshot
1 Cycle screenshot PD Resolved Pattern
UHF Measurement Via Oil Valve
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• PD-signals :UHF frequency range (300 MHz – 1 GHz)
• Sensor application at oil valves, which are available e.g. for oil filling or draining.
Case1 Case2
Courtesy of LDS GmbH (Lemke)
Drawback: High attenuation (high frequency); cannot determine which phase?; Hard to use for ball valves.
How to distinguish PD?
46
• External noises appear often independent from the applied AC test voltage level.
• Pulse-shaped noises may appear unsynchronized with the applied AC test voltage, whereas PD pulses occur always phase-correlated.
• Rise-time of PD is different from noise signals are different from pulse.
• Pattern recognition techniques help a lot!
T-F Classification Map
47
Acoustic PD :Off-line/On-line
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1. All-Acoustic (minimum 3 sensors)2. Acoustic with Electrical PD trigger
Velocity of sound in oil :1413 m/s at 20 °C
Acoustic PD
49
• Vibration noises (core,fan,pump) <50kHZ• Band Pass filter: f1=50 kHz, f2=350 kHz• Propagation path: direct , indirect
Sensor locations when phase of PD source is not known
Acoustic PD
50
• Before placing the AE transducer, wipe the area (dirt, oil, bugs,…).
• Acoustic couplant needed for enhancing the mechanical and acoustical coupling between the transducer and the tank surface.
• A sound transmitting epoxy to be used if the mounting location is non-magnetic.
• Magnetic tank shielding causes extra signal attenuation.
• LTC operation contains a high electromechanical energy that usually propagates through the entire transformer. To be distinguished in post-analysis.
• Initial place to start: one sensor in the bottom connection of each bushing.
Application of Acoustic PD
51
• When electrical PD is detected, for confirmation and source location
• When DGA indicates the possible presence of PD
• For PD detection during factory impulse testing
• When static electrification is suspected
Acoustic PD
52
• Electrical PD has a threshold 300 (500)pC. There is no similar threshold for acoustic systems.
• A strong signal buried deep within a winding may be very weak by the time it reaches the acoustic sensor.
• Sometimes longer monitoring period is necessary: weeks
• Only high PD levels can be detected.• The correlation is weak between measured and real
PD level due to attenuation.
Wall propagationDirect signal
Test Case:
53
Single phase autotransformer: 500/230/13.8 kV,146/194/243 MVA
• After two years in operation started gassing.
• Acoustic monitoring for 5 days :significant activity
Load is minimumVoltage is maximum
Test Case:3-D Plot
54
Dielectric Frequency Response
55
• “Dielectric Frequency Response” also called “Frequency Domain Spectroscopy”
• DFR is Power factor measured in a wide range of frequency (mHZ-kHZ) unlike conventional 60Hz measurement.
Power Factor @60Hz DFR
How to Measure DFR
• Measurement modes: UST, GST, GTS-g
56
Application
• Originally to Estimate the Moisture Content
• Now extend to find out other problems : Insulation Contamination Degradation of overall insulation system High resistance contacts Bushings issues ….
57
DFR to Estimate Moisture
58Using DFR curve, the Wt% can be estimated.
How DFR Estimates Moisture?• X-Y Model of Insulation
59)(
1
)()(
1
)(
1
boardoilspacercomb CCCC
How DFR Estimates Moisture? Oil
– Non-polar liquid; r = 2.2 Pressboard, oil impregnated cellulose
– More polar; r = 4.5
– Sensitive to moisture
60
Why moisture is important?
61
Conventional Moisture Estimation
• Low accuracy at low temperatures because the water has migrated to the paper
62
MOISTURE EQUILIBRIUM BETWEEN OIL AND CELLULOSE
Uncertain area
Conventional Method Cons
• Transformer needs to reach Thermal Equilibrium
• Oil Moisture changes with temperature
• Not accurate in lower temperatures
• Aged oil resolved higher Water% than New oil
63
DFR found to be the most accurate method to estimate the insulation moisture.
DFR & Moisture %
64
Test Case :500 MVA 230kV GSU,1976
• CH and CL increased 100%!
• DGA had gone up in the past but was constant lately.
• The unit was taken out.
• One reason for increasing P.F. is moisture. Should the unit be dry-out?
• Just shipping to the closest maintenance cost $500K.
• Kinectrics offered DFR to diagnose.65
Measurement Year 2000 Year 2009CHL 0.16 0.2CH 0.29 0.59CL 0.48 0.99
Conventional Power Factor :
Test Case
• CHL is normal showing Water content 0.5-1%
• CL does not match with any moisture model curve, So it is not moisture.
• The whole curves shifted up. Insulation conductivity has changed.
• Conclusion: Contamination, no dry-out needed. Shipped out to replace the windings.
66
After opening tank: carbon deposit found on LV and HV winding due to
arcing of tank shipping bolt to the core.
DFR Result
CHCL
CHL
DFR Is Able to:Estimate the moisture content of power
transformers accurately. Identify unsatisfactory conditions during routine
testing. Detect contamination on insulating system. Characterize transformers to avoid potential
catastrophic failures.
67
Drawbacks:
Needs basic transformer design data Needs expertise to interpret results
FRA (SFRA)
Two methods:- Applying LV Impulse (Obsolete) :Time domain- AC , Sweep Frequency (SFRA)
68
Test techniques
Freq response analysis (FRA)• Impedance/admittance/transfer
measurements• Typically 1kHz – 1MHz• Network analyzer or equivalent• Detects deformations/displacements• Compare against other phases , previous
measurement, or sister unit• The lead lengths need to be as short as
possible, and the test configuration must remain constant for repeated tests.
69
Application
70
• Detecting Faults which involves:Winding deformations Core movementsFaulty core groundsPartial winding collapseHoop bucklingBroken clamping structuresShorted turns and open windings
• Generate a base-line data for future comparison
• To Confirm Smooth Transportation
FRA
71
FRA
72
FRA
Repeatability is Key in FRA Measurement
73
Core NOT grounded
Core grounded
FRA-Typical Curves
74
FRA-Interpretation
• Low frequencies
– Core problems
– shorted/open windings
• Medium frequencies
– Winding deformations
• High frequencies
– Tap connections
– Other winding connection problems
75
FRA-Applied voltage
It is usually 10V.
76
2.8 VOmicron
10 VFRAX, Doble and others
FRA-Proper Grounding
77
Good grounding practice
Poor grounding practice
FRA-Test Case -180 MVA off-shore• Acetylene up
• Change in winding resistance
• LV distorted at 5 kHz – 500 kHz 78
Shipped for repair
79
Time for More Questions!
80