Matz Ohlen Director Transformer Test Systems. Megger Sweden

Transcription

1 Matz Ohlen Director Transformer Test Systems Megger Sweden

2 Frequency response analysis of power transformers Measuring and analyzing data as function of frequency, variable frequency diagnostics Impedance vs frequency FRA/SFRA (Sweep Frequency Response Analysis) Magnitude/phase vs frequency Typical frequency range 20 Hz 2 MHz Insulation characteristics vs frequency DFR/FDS (Dielectric Frequency Response/Frequency Domain Spectroscopy) Capacitance and dissipation factor vs frequency Typical frequency range a few mhz to 1 khz

3 Transformer Diagnostics Diagnostics is about collecting reliable information to make the correct decision Making the correct decisions improves reliability and saves money TTR SFRA FDS Winding Resistance

4 SFRA testing basics Off-line test The transformer is seen as a complex impedance circuit [Open] ( magnetization impedance ) and [Short] ( shortcircuit impedance ) responses are measured over a wide frequency range and the results are presented as magnitude response curves ( filter response ) Changes in the impedance can be detected and compared over time, between test objects or within test objects The method is unique in its ability to detect a variety of winding faults, core issues and other electrical faults in one test

5 SFRA measurement circuitry

6 SFRA analysis tools Visual/graphical analysis Starting db values for [Open] (excitation impedance/current) [Short] (short-circuit impedance) The expected shape of star and delta configurations Comparison of fingerprints from; The same transformer A sister transformer Symmetric phases within the same transformer New/missing resonance frequencies Correlation analysis DL/T standard Customer/transformer specific

7 Typical response from a healthy transformer LV [open] as expected for a Y tx HV [short] identical between phases HV [open] as expected for a Y tx. Double dip and one response lower Very low deviation between phases for all tests no winding defects

8 Transformer with serious issues... Large deviations between phases for LV [open] at low frequencies indicates changes in the magnetic circuit/core defects Large deviations between phases at mid and high frequencies indicates winding faults

9 SFRA standards and recommendations DL/T , Frequency Response Analysis on Winding Deformation of Power Transformers, The Electric Power Industry Standard of People s Republic of China, 2004 Cigre brochure 342 (2008), Mechanical Condition Assessment of Transformer Windings Using Frequency Response (FRA) IEEE PC /D7 (2009), Draft Trial-Use Guide for the Application and Interpretation of Frequency Response Analysis for Oil Immersed Transformers (Draft) IEC Ed1.0 (2010), Power Transformers Pert 18. Measurement of Frequency Response (Draft) Internal standards by transformer manufacturers, e.g. ABB FRA Standard

10 SFRA standards Key points Standard Dynamic range Accuracy Signal cable grounding EPIS PRC DL/T to +20 db ± db Wire, shortest length to transformer core grounding CIGRE brochure 342 IEEE PC57.149/D7 (draft) IEC (Draft) ABB FRA Technical Standard -100 to +20 db (measurement range) "Sufficient dynamic range to accommodate all transformer test objects" -100 to +10 db min 6 db S/N Better than -100 to +40 db ± db Shortest braid principle "Calibrated to an acceptable standard" ± db ± db Smoothing not allowed Grounded at both ends, documented and repeatable procedure Shortest braid principle ± db Shortest braid principle

11 SFRA measurement Range - Why you need at least -100 db... Westinghouse 40 MVA, Dyn1, 115/14 kv, HV [open]

12 Signal cable connection Shortest braid principle Source:IEC (draft)

13 SFRA Summary and conclusions SFRA is an established methodology for detecting electromechanical changes in power transformers Collecting reference curves on all mission critical transformers is an investment! Ensure accuracy by selecting a highquality instrument Ensure repeatability by following international standards and practices

14 Insulation Testing Dielectric Response Methods FDS/DFR HV Tan Delta VLF PDC Polarization Index "DC" 1 0 0, , ,0001 0,001 0,01 0, Frequency, Hz

15 Dielectric Frequency Response Measurements Tan delta from mhz to khz Hi V A Lo Ground C HL Measure at several frequencies Use Ohms law: U ( ) ( ω) Z ω = Z( ω) I( ω) C, tand, PF ( ε and ε ) C L C H

16 Why perform dielectric frequency response measurements... Typical power factor 20 C "New" "Old" Warning/alert limit Power transformers, oil insulated % % > 0.5% Bushings % % > 0.5% IEEE states; The power factors recorded for routine overall tests on older apparatus provide information regarding the general condition of the ground and inter-winding insulation of transformers and reactors. While the power factors for most older transformers will also be <0.5% (20C), power factors between 0.5% and 1.0% (20C) may be acceptable; however, power factors >1.0% (20C) should be investigated.

17 Dielectric Frequency Response - Investigating high single number PF data Dry transformer with old oil (high conductivity) Wet transformer with good oil

18 What affects the response? - Moisture + - Oil Conductivity + - Temperature + - Moisture +

19 DFR Moisture estimation (1-2-3) Right click Measured DFR response Select Send to MODS

20 DFR Moisture estimation (1-2-3) Oil Capacitor model % Spacers % Barriers Measurement Master curve

21 DFR Moisture estimation (1-2-3) 2. Click Auto match 1. Confirm insulation temperature

22 DFR Moisture estimation Result Geometry Moisture Oil conductivity

23 Dielectric Frequency Response - Investigating irregular shapes CHL response CH and CL responses

24 DFR analysis irregular responses A measured irregular shape is not a mishap It is information! CH and CL has expected oil-paper response CHL looks different with higher losses at mid-frequencies Contamination/conductive layer between windings? This particular transformer had a history including an LTC replacement due to seriously burned contacts...

25 Methods for dielectric response measurements DC (Polarization-Depolarization Current measurements) Strenghts Shorter measurement time at very low frequencies Weaknesses More sensitive to AC interference More sensitive to DC interference Limited frequency range (PDC only) Data conversion necessary (combined PDC/DFR only) Discharge before measurement may be needed AC (Dielectric Frequency Response measurements) Strenghts Less sensitive to AC interference Less sensitive to DC interference Wide frequency range No discharge necessarry Weaknesses Longer measurement time for very low frequencies

26 Moisture assessment with dielectric response methods takes a while Available methods Measurement times PDC Typically hours PDC+DFR approximately minutes (2 mhz, with and without discharge) to hours (0.1 mhz with and without discharge) True AC DFR/FDS approximately 18 minutes (2 mhz) to about 5.5 hours (0.1 mhz) Availability Transformer off-line in field Typically 1 day for complete diagnostic measurements

27 Measurement time (minutes) for DR measurements mhz 1 mhz 0,1 Mhz st gen FDS 3rd gen FDS PDC+FDS, no discharge PDC+FDS, with discharge

28 Typical DFR results for transfomers with various moisture content 1.5% moisture 0.3% moisture 2.1% moisture 0.2% moisture

29 DFR results for a transfomer at various temperatures Temp Moisture, % Oil conductivity, ps 21 2,4 10,4 27 2,3 13,8 34 2,4 22,8 49 2,3 39,3

30 DFR data acqusition is pending insulation temperature 100,00 Frequency, mhz 10,00 1,00 ev=0,9 ev=0,7 ev=0,5 Corresponding data points 0, Insulation Temperature

31 Ongoing project collecting measurement results on various transformers Old distribution transformers New power transformers in factory New power transformers in the field Typical power transformers in various conditions

32 Moisture assessment of transformers with different low frequency limits 10,0 Moisture level, % 1,0 T1, 3 C T2, 7 C T3, 15 C T4, 15 C T5, 21 C T6, 23 C T7, 25 C 0,1 Low frequency limit, mhz 0,1 1 10

33 Distribution transformer T = 23 C, f = mHz

34 Distribution transformer T = 23 C, f = 0.1mHz-10kHz

35 Distribution transformer T = 23 C, f0 = mHz 7 6 Auto geometry X (auto) Y (auto) Moisture Oil, ps , Stop freq, mhz

36 Power transformer T = 25 C, f = 0.1mHz-1 khz

37 Power transformer T = 25 C, f = 0.1mHz-1kHz

38 Power transformer, T = 25 C, f0 = mHz 2 1,8 Auto geometry 1,6 1,4 1,2 1 0,8 0,6 X (auto) Y (auto) Moisture, % Oil cond, ps 0,4 0,2 0 Min freq, mhz 0,1 1 10

39 DR measurement frequency range Conclusions so far... Auto geometry estimation mode in MODS works good Limited value of measuring below 1-2 mhz at normal temperatures (only a few results collected so far from measurements at < 15 C) If geometry is (approximately) known, it may be possible to reduce measurement time Measurements at higher temperature can shorten the measurement time

40 Summary and conclusions Dielectric response measurement is an excellent tool for insulation diagnostics Moisture assessment using DFR measurements and transformer insulation modeling is a generally accepted standard diagnostic method Transformer outage time is expensive and it is necessary to minimize measurement time. DFR measurements down to 1-2 mhz seem to be sufficient for accurate moisture assessment at normal temperatures DFR is capable of identifying non-moisture issues like contamination/sludge and/or conductive layers

41 Questions and/or comments?

42 Additional material Sweep Frequency Response Analysis Application Examples

43 Time Based Comparison - Example 1-phase generator transformer, 400 kv SFRA measurements before and after scheduled maintenance Transformer supposed to be in good condition and ready to be put in service

44 Time Based Comparison - Example Obvious distorsion as by DL/T standard (missing core ground)

45 Time Based Comparison After repair Normal as by DL/T standard (core grounding fixed)

46 Type Based Comparisons (twin-units) Some parameters for identifying twin-units: Manufacturer Factory of production Original customer/technical specifications No refurbishments or repair Same year of production or +/-1 year for large units Re-order not later than 5 years after reference order Unit is part of a series order (follow-up of ID numbers) For multi-unit projects with new design: reference transformer should preferably not be one of the first units produced

47 Type Based Comparison - Example Three 159 MVA, 144 KV single-phase transformers manufactured 1960 Put out of service for maintenance/repair after DGA indication of high temperatures Identical units SFRA testing and comparing the two transformers came out OK indicating that there are no electromechanical changes/problems in the transformers Short tests indicated high resistance in one unit (confirmed by WRM)

48 Type Based Comparison 3x HV [open]

49 Type Based Comparison 3x HV [short]

50 3x HV [short] - details Higher resistance on A-phase

51 Type Based Comparison 3x LV [open]

52 Design Based Comparisons Power transformers are frequently designed in multi-limb assembly. This kind of design can lead to symmetric electrical circuits Mechanical defects in transformer windings usually generate non-symmetric displacements Comparing FRA results of separately tested limbs can be an appropriate method for mechanical condition assessment Pending transformer type and size, the frequency range for design-based comparisons is typically limited to about 1 MHz

53 Design Based Comparison - Example 40 MVA, 114/15 kv, manufactured 2006 Taken out of service to be used as spare No known faults No reference FRA measurements from factory SFRA testing, comparing symmetrical phases came out OK The results can be used as fingerprints for future diagnostic tests

54 Designed Based Comparison HV [open]

55 Designed Based Comparison HV [short]

56 Designed Based Comparison LV [open]

57 Design Based Comparison After Suspected Fault Power transformer, 25MVA, 55/23kV, manufactured 1985 By mistake, the transformer was energized with grounded low voltage side After this the transformer was energized again resulting in tripped CB (Transformer protection worked!) Decision was taken to do diagnostic test

58 Design Based Comparison After Suspected Fault Response (dbs) Frequency (Hz) HV-0, LV open A and C phase OK, large deviation on B-phase (shorted turn?)

59 0 Design Based Comparison After Suspected Fault Response (dbs) Frequency (Hz) HV-0 (LV shorted) A and C phase OK, deviation on B-phase (winding deformation?)

60 And how did the mid-leg look like? Insulation cylinder Core limb LV winding

61 End