6th International Conference on Large Power Transformers- Modern Trends Benefits of SFRA - Case Studies B B Ahir Gujarat Energy Transmission Corporation Limited 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 Condition Monitoring in GETCO Transformer Technology / Tool Online Offline Insulation Resistance and Polarisation Index √ Routine Low Voltage tests √ Capacitance & Tan δ measurements √ √ (Bushing) Infrared Thermograph √ Measurement of Moisture in Oil √ √ DGA √ √ √ √ √ SFRA Acoustic Partial Discharge Measurement moisture content in active parts of Transformer 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 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 Case study (1): LV MAGNETIC BALANCE ( Post fault – Abnormal ) APPLIED VOLTAGE Tap Tap 17 r-n y-n b-n r-n y-n b-n r–n 255 V 255 0 255 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 27 169 570 Nature of incipient fault Arcing Major Key Gas Minor Key Gas C2H2 , H2 CH4, C2H4 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 +/- dB: Tap and Main winding HV R Phase HV R phase shifted -3dB with reference to other phase in main and tap winding portion Case study (1): LV – N at Tap No.17 ( Post fault - Abnormal) 500Hz to 2MHz with +/- 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 Case study (1): Analysis  SFRA LV R phase found abnormal – defect in tapped winding ( 500Hz to MHz) and open winding ( 5Hz to 100kHz ) with +/- 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 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 (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 Rating Date : 66kV Chhala Substation : 66/11.55kV , 10 MVA : 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 MAGNETIZING CURRENT ( Post fault - Abnormal ) CURRENT MEASURED 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 32 phases 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  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 36 fault 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 37 of fault 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 MAGNETIZING CURRENT ( Normal – After rectification) CURRENT MEASURED 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 Y–B 450 V 256 450 189 23 42 17 B-R 445 V 44 396 445 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