1. Alexander Dierks – Alectrix (Pty) Ltd, South Africa
Innovative Test Techniques & Diagnostic Measurements to Improve the Performance and Reliability of Power System Transformers
Dr. Michael Krüger, Alexander Kraetge – Omicron Electronics GmbH, Austria
Alexander Dierks – Alectrix (Pty) Ltd, South Africa
9/17/2018

2. The Switch Quiz Room with one light Three switches outside room
Which switch operates the light?
You are only allowed to open and enter the room ONCE!
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3. The Switch Quiz Turn Switch 1 & 2 ON and wait
Turn Switch 2 OFF, enter the room and check:
If light is ON  Switch 1
If light is OFF and bulb is warm  Switch 2
If light is OFF and bulb is cold  Switch 3
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4. Sources of Transformer Faults
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5. Classical Tests of Transformers
Ratio Measurement (Tap changer dependant)
Excitation Current Measurement (Open Circuit Impedance)
Short Circuit Impedance
Winding Resistance (static)
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6. New Test Techniques: Summary
Onload Tapchanger (OLTC) Testing
SFRA Measurement
Capacitance and Tan Delta Measurement
Di-electric Response Measurement
Partial Discharge Measurement
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7. Functioning of Tap changer
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8. OLTC Principle (1) A B Tap Selector Diverter Switch Diverter Resistors
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9. OLTC Principle (2) A B Tap Selector Diverter Switch Diverter Resistors
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10. OLTC Principle (3) A B Tap Selector Diverter Switch Diverter Resistors
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11. OLTC Principle (4) A B Tap Selector Diverter Switch Diverter Resistors
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12. OLTC Principle (5) A B Tap Selector Diverter Switch Diverter Resistors
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13. OLTC Principle (6) A B Tap Selector Diverter Switch Diverter Resistors
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14. Measurement of Winding Resistance
Measuring Lines for I
Measuring Lines for U
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15. New Voltage Selector Contact
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16. Testing of a new 500MVA Transformer
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17. 220:110 kV 100MVA TRF: A-Phase
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18. Faulty Tap Selector Contact
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19. Winding Resistance A-Phase After Repair
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20. Delta R for U-Mp „UP“-„DOWN“
Before repair
After repair
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21. Transient Current during Switching Process3 A 1
Slope 
Ripple 4 2
1 = Diverter switch switches to the first commutating resistor
2 = Both commutating resistors are in parallel
3 = Final contact of the diverter contact B is reached
4 = Current control of the CPC 100 regulates the current to the nominal test current again
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22. Ripple of a Diverter Switch in a Good Condition (1100MVA)
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23. Ripple of an Aged Diverter Switch
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24. Aged Diverter Switch
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25. New Test Techniques: Summary
Onload Tapchanger Testing
SFRA Measurement
Capacitance and Tan Delta Measurement
Di-electric Response Measurement
Partial Discharge Measurement
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26. Transformer Model for SFRA
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27. Sweep 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.
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28. SFRA Measurement: Test Set Up
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.
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29. SFRA: Healthy transformer 33kV 10MVA
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30. SFRA: Damaged Winding 400kV 700MVA
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31. What can SFRA detect?
Fingerprint (Reference Measurement) is important!!!
Geometrical changes in the core and/or windings due to transport or electrical faults:
Core deformation / Core movement (clamping)
Faulty core groundings
Fault on magnetic cores (shorted laminates)
Bulk winding movement
Coil deformation - axial & radial
Buckling of windings
Broken or open internal connections
Inter-winding / Inter-turn short circuits
(list is self explaining)
Bucklings: Certain kind of winding deformation (see picture later in this presentation)
Winding faults: Can also be localized in some cases
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32. Radial Deformation (Buckling)
Source: G. Bertagnolli, Short Circuit Duty of Power Transformers, ABB Trasformatori, Legnano
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33. Damaged Winding (Axial Movement)
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34. Grounding of the Measurement Cables Important for reproducible measurements
Braids for a low-inductive connection cable screen - flange
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.
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35. New Test Techniques: Summary
Onload Tapchanger Testing
SFRA Measurement
Capacitance and Tan Delta Measurement
Di-electric Response Measurement
Partial Discharge Measurement
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36. C / Tan Delta Progression of a Bushing
Source: RWE
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37. Bushing Fault
Source: RWE
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38. Bushing after Explosion
Source: RWE
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39. Tan Delta / Dissipation Factor
Bushings:
Aging and decomposition of insulation
Water content
Bad contacted electrodes or capacitive layers
Cracks in the insulation
Transformers:
Aging
Water content in oil and paper
Contamination by particles
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40. Capacitance Measurement
Bushings:
Partial breakdowns between layers
Oil ingress into cracks of solid insulation (RBP)
Change of Geometry between:
Transformer windings
Windings and tank
Windings and core
Core and tank
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41. CPC 100 / TD 1
Tan-delta & Capacitance Measurement
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42. 220kV RIP Bushing Stored Outside
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43. 220kV RIP Bushing Stored Outside
Minimum
Minimum
Minimum
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44. New Test Techniques: Summary
Onload Tapchanger Testing
SFRA Measurement
Capacitance and Tan Delta Measurement
Di-electric Response Measurement
Partial Discharge Measurement
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45. Design of the Winding Insulation Measurement of Water Content in Paper & Pressboard
Source: Weidmann AG
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46. Frequency Domain Spectroscopy (FDS)X
(t) N Z 1 2 C X, L
Reference
path
Measurement I CN CX
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47. PDC Analysis Polarisation-Depolarisation-Current
Principle
Recording of current
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48. Dirana: Combination of PDC and FDS
FDS  11 h Meas. Time
PDC  Meas. only up to 1 Hz
0.1 Hz – 5 kHz
0.1 mHz – 0.1 Hz
FDS+PDC  0.1 mHz – 5 kHz in less than 3 hours meas. time
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49. PDC up to 0.1Hz und FDS above 0.1Hz
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50. FDS / PDC Measurements on a 800 kVA Distribution Transformer
1969
Furan content:
12.27 mg/kg !!
 DP  130
Decision:
transformer was taken out of service
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51. FDS curves for different temperatures
FDS / PDC Measurements on a 800 kVA Distribution Transformer
FDS curves for different temperatures
50°C
25°C
12°C
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52. FDS / PDC Measurements on a 800 kVA Distribution Transformer
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53. New Test Techniques: Summary
Onload Tapchanger Testing
SFRA Measurement
Capacitance and Tan Delta Measurement
Di-electric Response Measurement
Partial Discharge Measurement
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54. Fault on a 25 MVA Furnace Transformer
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55. Overpressure valve released 200l of oil
Fault on a 25 MVA Furnace Transformer
Overpressure valve released 200l of oil
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56. DC insulation resistance
Fault on a 25 MVA Furnace Transformer
DC insulation resistance
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57. Fault on a 25 MVA Furnace Transformer
Taps
No-load current
Taps
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58. Fault on a 25 MVA Furnace Transformer
Z0 (f) = R0 (f) + j X0 (f)
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59. Fault on a 25 MVA Furnace Transformer
FRA (log view)
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60. Fault on a 25 MVA Furnace Transformer
Faulty B phase
FRA (lin view zoomed)
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61. Partial discharge measurement
Fault on a 25 MVA Furnace Transformer
Partial discharge measurement
Optical fiber
MPD 600
Measuring electrode
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62. Partial discharge measurement
Fault on a 25 MVA Furnace Transformer
Partial discharge measurement
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63. Fault on a 25 MVA Furnace Transformer
Opened transformer
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64. Fault on a 25 MVA Furnace Transformer
Melted screw
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65. Melted steel with copper marks
Fault on a 25 MVA Furnace Transformer
Melted steel with copper marks
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66. Conclusions / Summary
Defective contacts of the OLTC can be detected by measuring the static winding resistance over all taps up- and down.
Switching problems of the diverter switch of the OLTC can be found through dynamic resistance measurements.
The Sweep Frequency Response Analysis (SFRA) is a powerful tool for detecting winding displacements, winding deformations and faulty core and screen connections. Reproducible connection techniques are important to ensure reliable and reproducible measurements.
Tan Delta frequency sweeps are much more sensitive for investigating insulation problems compared to measurements at 50Hz only.
Polarization / Depolarization Current (PDC) and Frequency Domain Spectroscopy (FDS) measurements are useful tools for determining the water content in cellulose for deciding on further actions (e.g. drying).
Partial Discharge (PD) measurements are an important tool for the troubleshooting of insulation problems.
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67. Thank you for your interest !
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