1. New Experience in the Transformer Diagnosis Michael Krüger, OMICRON Austria

2. Electrical Measurement Methods I

3. Electrical Measurement Methods II

4. Content Winding Resistance Measurement
Fault Finding on Transformer Windings with FRA
Moisture Measurement on Transformers
Partial Discharge Measurement on Transformers

5. Winding Resistance Measurement

6. Diverter Switch Contact of Phase V

7. Winding Resistance after Repair

8. Content Winding Resistance Measurement
Fault Finding on Transformer Windings with FRA
Moisture Measurement on Transformers
Partial Discharge Measurement on Transformers

9. Fault on a 115kV / 34.5kV Transformer
There was a breakdown in phase B of the 34.5 kV busbar in the substation
The differential protection relay of the faulty transformer tripped
Example from David Stelmach. – a person was working in the substation, one 115kV line was falling down on the 34.5kV lines, first on phase B, then also on the other phases. Transformer saw overcurrent on B phase and then tripped out on Differential.

10. DGA Phase 1: Trip out of Service, Differential Phase 2: DGA
second DGA showed 19ppm C2H2

11. Transformer Model for FRA
28/11/2006
Transformer Model for FRA

12. Swept Frequency Response Analysis (SFRA)
28/11/2006
Swept Frequency Response Analysis (SFRA)
Magnitude
Phase
This slide explains the so-called sweep frequency method. In contrast to the impulse method, here we work with a sinusoidal signal, which is swept between 10 Hz and 10/20 kHz (or more). Vin – represented in red – is the input signal, which is standardized to 1 or 100%. We sweep the signal over the whole frequency range and measure the signal Vout - represented in blue - at the other end of the winding. We see that the blue signal (output signal) compared to the red signal (input signal) shows both a damping as well as a phase shift.
The transfer function (H) in dB is the result of the calculation according the shown formula: H(dB) = ....
Advantages of the sweep FRA method:
The input amplifiers of the equipment can be realized with a very small bandwidth; by that it can be avoided that other distortion signals superimpose the measurement signal
Over the whole frequency range we have a constant amplitude of the oscillator signal and by that no limitation of / a constant energy at higher frequencies. This allows a more sensitive measurement of particularly at higher resonance frequencies.

13. FRA LV Winding – Comparison of the 3 Phases
B Phase

14. Radial Forces Buckling with inner supports Buckling without inner
Higher order or forced Buckling
The conductors can bend between the supports all along the circumference
Free mode
or free Buckling
Source: Brochure CIGRE WG : The Short Circuit Performance of Power Transformers

15. Radial Failure (Buckling)
This slides shows the standard FRA measuring method:
The yellow cable (generator output) is connected to the beginning of the winding, with the red cable the injected voltage is measured back to compensate wiring influences (reference channel). With the blue cable (output) the damped signal at the other end of the same winding is fed back to equipment to the measurement input.
This is the standard FRA method which should be applied as a minimum for all FRA measurements.
The measurement is performed for each phase: If a transformer has two voltage levels this means that six measurements are performed, if we have a three-winding transformer (upper, medium, and low voltage winding), nine such measurements are necessary.
© OMICRON Academy

16. Failure Modes due to Radial Forces
B phase with buckling
C phase without buckling
Source: Cigre Brochure 342 “ MECHANICAL-CONDITION ASSESSMENT OF TRANSFORMER WINDINGS USING FREQUENCY RESPONSE ANALYSIS (FRA)”

17. LV Winding with Cigre Case Study of Buckling
Comparison FRA
LV Winding with Cigre Case Study of Buckling
Systematic shift of several resonances
B Phase

18. B Phase
Take a closer look

19. B Phase

20. Content Winding Resistance Measurement
Fault Finding on Transformer Windings with FRA
Moisture Measurement on Transformers
Partial Discharge Measurement on Transformers

21. Diagnosis of Tranformer On-load Tap Changers
29 November 2006
Water in the Transformer
Most of the water is contained in the paper!
Mass of the oil:
100,000 kg = 220,000 Lbs
Water content at 60 °C:
40 ppm
Mass of the water, dissolved in the
oil:
4 kg = 8.8 Lbs
Mass of the solid insulation:
13,000 kg = 20,000 Lbs
Water content at 60 °C:
4 %
Mass of the water contained in the paper:
520 kg = 1200 Lbs
Paper 4 -

22. Diagnosis of Tranformer On-load Tap Changers
29 November 2006
Determination of the Water Content in Paper [Karl Fischer Titration and Equilibrium Curves]
Curves are only valid for new oil and new paper, for aged oil/paper different curves are necessary
Oil sampling is critical (round robin tests)
Balance between water content in the paper and in oil needs constant temperatures over a long period
Only average measurement
Paper 4 -

23. History of Dielectric Diagnostic Methods
1991 RVM for water determination
Soon questioned by users (Kachler 1996)
1960+
Dissipation factor at power frequency
Polarization index
At that time: No reliable method for onsite moisture diagnostics
1999+ Polarization Depolarization Currents
Technical University Zurich Switzerland
1999+ Frequency Domain Spectroscopy
KTH Stockholm with ABB
2007 Combination of PDC and FDS

24. Diagnosis of Tranformer On-load Tap Changers
29 November 2006
Dielektric Response in the Time Domain Polarisation Depolarisation Current (PDC)
Current = f(t)
Paper 4 -

25. Diagnosis of Tranformer On-load Tap Changers
29 November 2006
Dielektric Response in the Frequency Domain Frequency Domain Spectroscopy (FDS) or Dielectric Frequency Response (DFR)
Paper 4 -

26. Dielectric Response Measurement with Frequency Domain Spectroscopy (FDS) and Polarisation & Depolarisation Current (PDC)
Current [nA]
Time [s]
Trans-
formation
Frequency [Hz]
Power factor
0.001 1
1000
100
0.1
PDC
FDS 2 4 6 8 10 12 14
PDC
FDS
DIRANA
Duration / h
0,0001
0,001
0,01
0,1 1
100
1000
Frequency range / Hz
Combination of PDC und FDS reduces measurement time

27. Interpretation and Analysis
Transition from Capacitive to Resistive Voltage Distribution
Pressboard: Water, lmw Acids
Oil: Carbon, Sludge, hmw Acids
Öl 1pS/m 10
Pressboard
Dissipation factor 1
Pressboard
0,5
1,0
2,0
Overall Response 1%, 1pS/m, X30, Y15
0.1
0.01
0.001
0.0001
0.0001
0.001
0.01
0.1
1.0 10
100
1000
f/Hz

28. Analysis of the Water Content by Comparison to Model Results
Measurement
Laboratory Results
Water Content
Oil Conductivity
Comparison
Fitting X
Oil
XY-Model
Barriers
Spacers Y
Temperature
Moisture determination bases on a comparison of the dielectric properties of the real transformer to modelled dielectric properties. The model bases on laboratory measurements on pressboard samples (upper right photograph) and the conductivity of oil. Both are combined by the XY-model taking into account the geometry condition of the transformer and the insulation temperature. A fitting algorithm arranges the modelled dielectric properties in order to get a good agreement to the measured dielectric properties. The best fitting curve provides the moisture content and oil conductivity of the real transformer.
Here you can demonstrate how the software works using the link to C:\Program Files\OMICRON\DIRANA\Dirana.exe
Source: Weidmann

29. FDS / PDC Measurement on a 130 MVA Transformer – Situation 2006
Diagnosis of Tranformer On-load Tap Changers
29 November 2006
FDS / PDC Measurement on a 130 MVA Transformer – Situation 2006
130 MVA
230 / 115 / 48 kV
Year of manf. 1967
Rubber membrane in the expansion vessel
Paper 4 -

30. FDS / PDC Measurement on a 130 MVA Transformer – Water Content Distribution5
Oilsample
Karl Fischer
mg/kg
(Oommen
equilibrium) 4
Oilsample
RH %
Average
Moisture in paper [%] 3
FDS HV-LV
Tertiary not in use 2 1
FDS LV- Tertiary
FDS Tertiary- Tank
FDS/PDC
1mHz-1kHz

31. 130 MVA Transformer On-Line Drying
Diagnosis of Tranformer On-load Tap Changers
29 November 2006
130 MVA Transformer On-Line Drying
Paper 4 -

32. History 2006 –

33. Content Winding Resistance Measurement
Fault Finding on Transformer Windings with FRA
Moisture Measurement on Transformers
Partial Discharge Measurement on Transformers

34. Diagnostic Measurement on a Cast Resin Type Transformer

35. Cracks in the Epoxy

36. PD Measurement Circuit

37. Test Arangement
CPC100
CP CU1
Compensating Capacitors

38. Test Arangement Coupling Capacitor and MPD600

39. Separation of PD Sources in 3CFRD
500kHz
2 MHz
8 MHz
500kHz
8MHz
2MHz
timeframe 1 µs
500 kHz
2 MHz
8 MHz
3CFRD = 3 Center Frequency Relation Diagram

40. PD Measurement without 3CFRD Filtering

41. 3CFRD Filtering

42. PD Measurement with 3CFRD Filtering

43. PD Measurement without 3CFRD Filtering

44. On-Line PD Measurement on a Transformer

45. On-Line PD Measurement on a Transformer GIS - Cable Connection (GIS Side)2 1 3

46. Online PD Measurement at the GIS Side
red phase GIS
yellow phase GIS
blue phase GIS

47. Sensors 1, 2 and 3 at the GIS Side and Sensor 4 at the Transformer Side

48. PD Measurement at the GIS and the Transformer Side
blue phase GIS
blue phase Tr

49. PD Measurement at GIS and TR Side Trigger = TR Side - Delay on GIS Side
trigger = blue phase Tr
delayed signal blue phase GIS

50. Jumper between Oil/Oil Bushing and Cable Termination

51. Jumper between Oil/Oil Bushing and Cable Termination

52.
Questions and Remarks?

53. A last Example: Fault Investigation on a 220kV Transformer

54. DGA

55. Ratio Measurement HV to LV

56. Ratio HV to LV Phase A=R
2kV on HV -> 23mV on LV on all taps !

57. Ratio HV to LV Phase B=S
All taps have the same ratio 7.2:1 of position 11 !

58. Ratio HV to LV Phase C=T
All taps have the same ratio 7.2:1 of position 11 !

59. FRA HV
Interruption in phase R, HV side

60. FRA LV with HV open
Short circuit in phase R, but on LV or HV side ???

61. FRA LV with HV Open and Shorted
A LV with open loop HV (red line)
B LV with open loop HV (blue solid line)
B LV with shorted HV (blue dashed line)

62. Winding Resistance of LV
Phase R LV doesn’t look damaged

63. Conclusions Phase R=A is interrupted on HV side
The tap changer is broken – in position 11
The limb R has a shorted winding
The short circuit in limb R is not on the LV side but on the HV side

64. Opening of the OLTC Compartment

65. Opened OLTC Compartment
Interruption in phase R, HV side

66. Interruption due to Open OLTC Contacts

67. Removed OLTC – Cables to the Tap Winding

68. FRA on the Single Tap Winding Segments

69. FRA on the Single Tap Winding Segments
9 - 11
8 - 9
7 - 8
6 - 7
5 - 6
4 - 5
3 - 4
2 - 3

70. FRA on the Single Tap Winding Segments

71. Winding Resistance of the Tap Winding Segments4
The fault was between tap conductor 4 and 6 !
25mΩ
7mΩ
14mΩ
6mΩ 5 6

72.
Questions and Remarks?