6 th International Conference on Large Power Transformers- Modern Trends Benefits of SFRA - Case Studies B B Ahir Gujarat Energy Transmission Corporation Limited 1
Outline Condition Monitoring in GETCO Why SFRA? Case study (1):SFRA - How it helped to find a fault in winding Case study (2): SFRA - How it helped to find a fault in core Case study (3): SFRA - How it helped to find a fault in core Conclusion 2
Condition Monitoring in GETCO Transformer Technology / Tool Online Offline Insulation Resistance and Polarisation Index Routine Low Voltage tests Capacitance & Tan δ measurements Infrared Thermograph (Bushing) Measurement of Moisture in Oil DGA SFRA Acoustic Partial Discharge Measurement moisture content in active parts of Transformer 3
Why SFRA? To assess mechanical integrity of a transformer To detect core displacement and winding deformation due to : Large electromagnetic forces from fault current Transformer transportation and relocation Winding shrinkage causing release of clamping structure To detect broken or loosened clamping structure Hoop Buckling Shorted turns and open winding 4
Case study (1): Location : 220 kv Kapadwanj S/s Rating : 220/66kV, 100 MVA Date : 22.01.2012 OSR relay operated due to LV side Isolator support insulator flash over Initially SFRA along with Low Voltage Test and DGA 5
Case study (1): APPLIED VOLTAGE LV MAGNETIC BALANCE ( Post fault Abnormal ) Tap 1 Tap 17 r - n y - n b - n r-n y-n b-n r n 255 V 255 0 0 255 0 0 y n 251 V 174 251 74 175 251 68 b n 256 V 27 225 256 24 226 256 DGA Results ( Post Tripping ) H2 O2 N2 CH4 C2H2 C2H4 C2H6 CO CO2 119 3657 14212 11 36 27 1 169 570 Nature of incipient fault Major Key Gas Minor Key Gas Arcing C 2 H 2, H 2 CH 4, C 2 H 4 Low voltage magnetic balance test found abnormal in R phase and DGA indicates arcing
Case study (1): HV N at Tap No.1 ( Post fault - Abnormal) 500Hz to 2MHz with +/- 3 db: Tap and Main winding HV R Phase HV R phase shifted -3dB with reference to other phase in main and tap winding portion 7
Case study (1): LV N at Tap No.17 ( Post fault - Abnormal) 500Hz to 2MHz with +/- 3 db: Tap and Main winding LV R Phase 20Hz 10KHz Core deformation/open ckt./ Shorted turns / residual magnetism LV R phase shifted widely with reference to other phase in main and tap winding portion. It seems like open circuit. 8
Case study (1): Analysis SFRA LV R phase found abnormal defect in tapped winding ( 500Hz to 2 MHz) and open winding ( 5Hz to 100kHz ) with +/- 3 db variation. Problem reflected in low voltage magnetic balance and DGA ( Arcing ) also. Based on above abnormal results, decided to internal inspection of this transformer. 9
Case study (1): Internal Inspection LV R phase connecting leads of preselector switch of OLTC and its fixing assembly burnt. Burned connecting leads Burnt link and fixing assembly replaced Re-insulation of leads Drying process with oil filtration Take SFRA and other supporting test 10
Case study (1): HV N at Tap No.1 ( Normal After rectification ) HV R Phase All phases are identical after rectification of problem 11
Case study (1): LV N at Tap No.17 ( Normal After rectification) LV R Phase All phases are identical after rectification of problem 12
Case study (1): LV MAGNETIC BALANCE ( Normal After rectification ) APPLIED VOLTAGE Tap 1 Tap 17 r - n y - n b - n r-n y-n b-n r n 255 V 255 224 30 255 225 25 y n 252 V 176 252 76 175 252 69 b n 255 V 28 226 255 26 226 255 Low voltage test found normal after rectification
Case study (1): Cause of failure The heavy fault current due to flash over of LV side isolator support insulator caused R phase pre-selector switch LV connection two nos. of leads between position 3 & 12 to burn. SFRA plays a role fault with other test. as a supporting test to confirm the 14
Case study (2): Location : 220kV Khanpur Substation Rating : 220/66kV, 100 MVA Date : 21.04.08 High key gases in routine DGA test Low voltage test - 15
Case study (2): DGA Results ( on 22.12.07, Normal ) H2 O2 N2 CH4 C2H2 C2H4 C2H6 CO CO2 7 3405 12150 21 Nil 48 15 34 292 DGA Results ( on 21.04.08, Abnormal ) H2 O2 N2 CH4 C2H2 C2H4 C2H6 CO CO2 355 2988 13928 472 <1 538 214 57 1085 Nature of incipient fault Major Key Gas Minor Key Gas Thermal Fault - Over heated oil CH 4, C 2 H 4 C 2 H 6, H 2 DGA indicates thermal fault as per high key gases method 16
Case study (2): Low voltage Test CURRENT MEASURED MAGNETIZING CURRENT ( Post fault - Normal) HV WINDING LV WINDING R 3.2 ma 29.3 ma Y 2.6 ma 26.3 ma B 4.1 ma 35.2 ma MAGNETIC BALANCE ( Post fault - Normal ) APPLIED VOLTAGE R - N Y - N B - N r-n y-n b-n R - N 255 V 255 193 56 87 65 18 Y - N 252 V 170 252 82 59 87 28 B - N 256 V 58 190 256 16 69 87 Low voltage test found normal after fault
Case study (2): Low voltage Test CURRENT MEASURED SHORT CIRCUIT CURRENT ( Post fault - Normal ) HV WINDING LV WINDING R 3.54 A 10.38 A Y 3.55 A 10.18 A B 3.48 A 10.41 A Low voltage test found normal after fault
Case study (2): HV - N ( Post fault - Abnormal) 20Hz 10KHz Core deformation/open ckt./ Shorted turns / residual magnetism HV B Phase B phase curve differ in core area in compare with other two phases which shows core related issue 19
Case study (2): HV - N ( LV Shorted ) ( Post fault - Abnormal ) HV B Phase LV short curve nullify effect of core, winding part seems identical and normal 20
Case study (2): LV N ( Post fault - Abnormal) 20Hz 10KHz Core deformation/open ckt./ Shorted turns / residual magnetism LV B Phase B phase curve differ in core dominated area, which shows problem in core area 21
Case study (2): Analysis SFRA showed abnormality in the core. IR between core, frame and earth was measured. Core to Frame : 2000MΩ Frame to Earth : 8000MΩ Core to Earth : 0.03MΩ As the Core to Earth insulation found quite low, internal inspection carried out but nothing seen abnormal. Further investigation was done by inserting a GI wire between core and bottom of the tank and along the core nut found under the B phase core limb which was touching core and bottom. 22
Case study (2): Schematic Diagram 23
Case study (2): Analysis After removing the nut, again IR between core, frame and earth was measured. Core to Frame : 2000MΩ Frame to Earth : 8000MΩ Core to Earth : 2000MΩ Normally core earthing provided on top of the tank. The nut was providing another earthing and it caused thermal fault due to circulating current between core and earth. 24
Case study (2): HV - N ( Normal After rectification) After attending the problem all phase curves found identical 25
Case study (2): HV - N ( LV Shorted ) ( Normal After rectification ) After attending the problem all phase curves found identical 26
Case study (2): LV N ( Normal After rectification) After attending the problem all phase curves found identical 27
Case study (2): Conclusion A metallic nut below the core at bottom of the tank was found out with the help of SFRA which created parallel path to earth and led to thermal fault. Such kind of unbelievable fault can be identified by SFRA. In this case SFRA played a vital role as a supporting tool with DGA to detect the faulty area and to reach up to the root cause of the fault. 28
Case study (3): Location : 66kV Chhala Substation Rating : 66/11.55kV, 10 MVA Date : 01.09.08 Tripped on differential relay due to failure of 66kV class Y phase bushing with reflected of 11kV feeder 29
Case study (3): Low voltage Test CURRENT MEASURED MAGNETIZING CURRENT ( Post fault - Abnormal ) HV WINDING LV WINDING R 1.8 ma 117.5 ma Y 13.5 ma 133 ma B 1.8 ma 135 ma MAGNETIC BALANCE ( Post fault - Abnormal ) APPLIED VOLTAGE R - Y Y - B B - R r-n y-n b-n R Y 433 V 433 388 176 41 37 16 Y B 431 V 230 431 296 21 40 38 B - R 437 V 309 296 437 19 38 41 Low voltage test found abnormal after fault 30
Case study (3): HV PH PH ( Post fault - Abnormal ) 20Hz 10KHz Core deformation/open ckt./ Shorted turns / residual magnetism R-Y Y-B B-R All three phases response are differ in core related area as mentioned frequency 31
Case study (3): HV PH PH ( LV Shorted ) ( Post fault Normal ) In LV short condition core area nullify and winding area curves are identical in all phases 32
Case study (3): LV PH N ( Post fault - Abnormal ) 20Hz 10KHz Core deformation/open ckt./ Shorted turns / residual magnetism R Y B All three phases response are differ in core related area as mentioned frequency 33
Case study (2): Analysis SFRA and low voltage revealed problem inside core area. Internal inspection carried out by removing top plate of transformer. 2 Nos. of bolts clamping the top channel with tank were loose and top yoke channel got lifted upwards by about 25mm which caused a little core deformation. 34
Case study (2): Internal Inspection Yoke Bolt Yoke Channel After attending the problem, SFRA was taken. 35
Case study (3): HV PH PH ( Normal After rectification ) All three phases response are identical in core dominated area after rectification of fault 36
Case study (3): HV PH PH ( LV Shorted ) ( Normal After rectification ) This response was identical from initially as problem was in core area after rectification of fault 37
Case study (3): LV PH N ( Normal After rectification ) All three phases response are identical now in core area after rectification of fault 38
Case study (3): Low voltage Test CURRENT MEASURED MAGNETIZING CURRENT ( Normal After rectification) HV WINDING LV WINDING R 2.4 ma 56.2 ma Y 1.9 ma 28.5 ma B 2.2 ma 55.6 ma MAGNETIC BALANCE ( Normal After rectification) APPLIED VOLTAGE R - Y Y - B B - R r-n y-n b-n R Y 442 V 442 365 79 41 35 6 Y B 450 V 256 450 189 23 42 17 B - R 445 V 44 396 445 4 37 41 Low voltage test found normal after rectification of fault 39
Case study (3): Cause of Failure Heavy reflected fault caused axial and radial forces exerted on transformer active part. Core structure got disturbed and it was reflected in SFRA plot. 40
Conclusion SFRA responses clearly indicate faulty conditions of transformer and it helped for root cause analysis. SFRA helps to detect fault by just compare with different phase with different task without having a initial results of same transformer or same design transformer. SFRA plays vital role as supporting test for final conclusion as its deliver diversity of information in only one test. SFRA is very useful tool to detect heavy through fault current leads to mechanical deformation of core or winding, internal connections and contacts inside transformer. GETCO has adopted such diagnosis tool while fault as well as in routine practice while shifting of transformer. 41
Thanks 42