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CASE STUDIES - PROTECTION OF POWER DISTRIBUTION SYSTEM - II by R. N. Kumar GM , NDPL

CASE STUDIES - PROTECTION OF POWER DISTRIBUTION SYSTEM - II by R. N. Kumar GM , NDPL. CASE STUDY 1 Tripping of 33/66 kV Transformers on OLTC OSR Relay Indication. Occurrence.

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CASE STUDIES - PROTECTION OF POWER DISTRIBUTION SYSTEM - II by R. N. Kumar GM , NDPL

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  1. CASE STUDIES - PROTECTION OF POWER DISTRIBUTION SYSTEM - II by R. N. KumarGM , NDPL

  2. CASE STUDY 1 Tripping of 33/66 kV Transformers on OLTC OSR Relay Indication

  3. Occurrence HV breaker of Power Transformers of following mentioned NDPL Grid stations were tripped on OLTC OSR & 11KV breaker tripped on inter-trip: GRIDS 1. Jahangirpuri DATE: 14/09/07 2. Rewari Line 03/11/07 3. Rohtak Rd 04/12/07 4. RG4 05/12/07 3

  4. Analysis & Observation • Following are observations for all tripping events mentioned above: • All transformers tripped in early hours of the day • All transformers tripped due to low oil level in OLTC conservator tank. • It has been observed in all above cases that during early hours of the day, oil in the tank/conservator contracts and hence oil level reduces • 4. It is also observed that this phenomenon is more predominant in OLTC conservator as the capacity of the same is very low and is of the tune of 20 liters. • 5. All above cases clearly indicate that these tripping's are due to: • a. Low oil level in OLTC conservator • b. Change in atmospheric temperature due to change in weather conditions. 4

  5. Recommendations • Tripping of power transformer on OLTC OSR due to weather change can be prevented by keeping oil level of OLTC conservator up to 70% of its capacity. • Oil level of 70% of OLTC conservator will sustain any sudden change in temperature due to change in weather condition. • Check up of OLTC conservator oil level should be monitored regularly and especially during change in weather condition. 5

  6. CASE STUDY 2 Tripping of 11kV O/G Feeder at Azadpur Grid Station

  7. Occurrence Tripping of 11KV GTK G feeder on IDMT & HS indication at 33kV Azadpur Grid station. SLD WITH CTR, FAULT CURRENT RECORDED & RELAY SETTINGS: 11KV O/G GTK G feeder: CTR 400/5 Relay Settings P/F IDMT 300A/0.1 HS 3.0KA/200ms SLD Azadpur 33KV Bus 25 MVA TX#2 CTR 1200/5 11KV Bus CTR 400/5 GTK G Tripped 7

  8. Event List 8

  9. Analysis & Observation On investigation it was observed that S-20 relay of O/G feeder has issued tripping command on its IDMT P/F & HS elements. The date and timing of the relay is also differing from the actual time. EVENTS, INVESTIGATIONS As it is clear from event list that feeder is tripping at the current which is less than its setting. Hence relay is tested by injecting current upto 300A and found ok. On further investigation it was found that CT secondary metering and protection control wiring was got interchanged due to wrong ferruling. ( metering core ferule i.e D and protection core ferule i.e C were got interchanged. This led to the wrong connection at the relay input resulting the tripping of the feeder CAPA CT secondary control wiring was rectified as per standard drawing. 9

  10. CASE STUDY 3 Tripping from Two Different Sources at Ashok Vihar

  11. Occurrence • GRID: Ashok Vihar DATE: 11/09/08 • TRIPPING / OCCURANCE : R &Y Phase CT of 33 KV Azad Pur – Tri Nagar Ckt T-Off, Flashed at ASHOK VIHAR Grid, caused following trippings: • 33 KV O/G Tri Nagar circuit from AZP Grid Grid • 33 KV WP2-Ashok Vihar circuit from both ends. • 33 KV SMB220kV-WP2 circuit-2 from both ends. 11

  12. General Observations • Flash Marks at the Upper Limb Of CB • Flash had occurred in R & Y Phase CTs of AZP-Tri Nagar feeder at Ashok Vihar Grid station. • All the isolators i.e. line as well as front bus isolator were in closed condition due to unmanning of Grid. • Flash marks observed at R,Y & B phases of the upper limb of AZP-Tri Nagar Breaker. • This shows that the flash of CT fault had reached up to breaker also and caused the tripping of I/C WP 2 Ckt at Ashok Vihar. 12

  13. Events, FDR & Investigations GRID: Ashok Vihar Feeder: AZP-Tri NgrFAULT 11/09/08 05:02:37.900 G1, PHASE A LS IDMTL HS1 IA= 5.56xIn, PHASE B LS IDMTL <HS1> IB= 5.35xIn, PHASE C IC= 13

  14. Events, FDR & Investigations GRID: Ashok Vihar Feeder: I/C WP2FAULT 1 11/09/08 05:02:38.300 G1, PHASE A LS IDMTL IA= 9.81xIn, PHASE B LS IDMTL <HS1> IB= 10.69xIn, PHASE C LS IDMTLIC=9.76xIn, EARTH FAULT IDMTL IE= 1.86xIn, CB ON 14

  15. Events, FDR & Investigations GRID: WP2 Feeder: O/G Asok Vihar FDR FAULT 1 11/09/08 05:02:40.5300 G1 PHASE A LS IDMTL HS1 IA=10.63xIn PHASE B LS <IDMTL> HS1 IB=11.53xIn PHASE C LS IDMTL HS1 IC=11.45xIn EARTH FAULT IE= 0.01xIn 15

  16. Events, FDR & Investigations GRID: WP2 Feeder: I/C SMB ckt2 FAULT 1 11/09/08 05:02:40.5850 G1 PHASE A [REV] LS IDMTL IA=10.97xIn PHASE B [REV] LS <IDMTL> IB=11.45xIn PHASE C [REV] LS IDMTL IC=10.93xIn EARTH FAULT IDMTL IE= 0.26xIn 16

  17. Conclusion • Following is the conclusion of all the trippings due to R & Y Phase CT Flash at Ashok Vihar Grid: • 33 KV O/G Tri Nagar Ckt tripped at Azad Pur Grid because it was the first upstream breaker to trip for isolating the CT fault at Ashok Vihar Grid • 33 KV I/C WP 2 CKT at Ashok Vihar end tripped as it has come into the flash range of faulty CT. All the isolators were in closed condition, only the breaker was off. The flash Spark had engulfed the both sides of the Azp-Tri Nagar breaker. This has been confirmed as breaker is having flash marks on upper limbs of all phases. • 33 kV O/G Ashok Viharckt and I/C SMB ckt2 at WP2 tripped due to very high intensity of the fault . The fault current was of the order of 9.5 kA. • 33 KV O/G WP2 Ckt2 at 220 KV SMB end had tripped under racing condition. • CAPA • This is a rarest kind of fault which was fed from two different sources (AZP-Tringr CKT and WP2). Leading to the cascade tripping. The only way to avoid these types of tripping is by not keeping a CB to be charged from two different sources. 17

  18. CASE STUDY 4 Transformer Tripping at BADLI

  19. Occurrence Reporting: Power Transformer tripping on Differential. Observations Basic testing of the transformer was carried out and following are the observations 1. No fault was observed on HV Side of the transformer 2. CT Circuit was found healthy 3. Stability of the transformer was OK 4. Load at the time of tripping was 510 Amps on LV Side i.e 40% of rated capacity of Tansformer. 19

  20. Investigations • It was observed from the waveforms of the tripping event that stray DC Current component was present in R Phase current at the time of event. (Please refer to snapshot of disturbance in badli-1doc attached along with). • It was also confirmed from last tripping event dated 8.3.2006 that DC Ingress has caused the tripping earlier also. • Complete circuit was checked to find out the source of DC Ingress in the Relay, It was ensured that no dc ingress was there through external cabling.Even after isolating external circuit, Relay was generating trip command. Waveform of this particular event indicated that tripping was purely due to DC Component of the current in CT Circuit. It also shows that after decay of the DC Current Output relay got reset. • On further investigations, 4 ma DC leakage current was detected on 220 V Battery charger, rendering Positive to earth voltage to be 12 volts and negative to earth voltage to be 220 V DC. • This leakage was found in adjacent Line feeder panel. 20

  21. Conclusions 1. It is confirmed that Tripping was due to intermittent DC Ingress in R Phase secondary of HV CT Circuit . 2. In modular type Duo-bias relay, two separate modules are there for power supply as well as CT Circuit. It seems DC was getting mixed up in CT circuit within the relay. CAPA Corrective Action: As It could not be concluded that where from DC is getting ingresses in CT Circuit, It was strongly suspected that it was an internal phenomenon within the relay. Based on this relay was replaced with new one. Future course of action: Expert comments from Relay manufacturer (ER) were solicited. Enclosed: Snapshots of disturbance records 21

  22. Transformer Tripping at Badli 22

  23. CASE STUDY 5 Cascade Tripping in Zone 1303 (Motinagar)

  24. Sudarshan Park grid Gadaria S/Stn Achpal S/Stn Rattan Park S/Stn Ramgarh S/Stn Single Line Diagram Trip X X Trip Trip BasaiDarapur X F 24

  25. S/Stn Gadaria S/Stn Achpal / / / / / / Schneider RN2C VIP300 ABB ,WIC1 X X X X I/C Sudarshan Park Grid I/C Bali Nagar O/G Achpal I/C Gadaria O/G Rattan Park 630 KVA 1000 KVA 2X250 KVA DT S/Stn Rattan Park Schneider RM6 VIP300 / / / / X X X X I/C Achpal Tie Spare O/G Ram Garh I/C Bali Nagar 630 KVA 630 KVA S/Stn Ram Garh CG Lucy WIP1 / / / / X X I/C Rattan Park O/G Old Bsai Dara Pur O/G Achpal 630 KVA Single Line Diagram 25

  26. Occurrence • For any fault beyond Ramgarh S/Stn, O/G CB of all the S/Stns in this circuit were getting tripped in cascading. • Investigations • As per protection philosophy circuit breaker is required to be installed only at first switching S/station for O/G feeder. • Purpose of RMU is defeated. • 3. Cascade tripping of circuit breakers installed in series can not be avoided as there is a little time margin to discriminate between these breakers. • CAPA • Zonal staff has been advised to connect the O/G feeder of the S/Station other than that of First Switching S/Station (Rattan Park S/Station) on isolator to avoid cascade tripping of breakers. 26

  27. Tripping Analysis, SLDs, FDRs & CAPA Tripping of 33/11 kV Transformer on CBFP protection at DIFR GRID: DIFR DATE: 23/04/07 & 24/04/07 OCCURRENCE : Tripping of 11KV I/C-2 on O/C Hi set indication along with Tx-2 on CBFP on both the occurrences. SLD showing exact location of the fault: 27

  28. EVENTS , FDR & INVESTIGATIONS Date 23/04/07 On investigation, it was observed that 11KV I/C tripped due rat electrocution on the circuit breaker limb on cable side resulting in the flashover at the poles of the CB and its chamber. 33KC Tx-2 was tripped on CBFP due to continuous fault feeding from source end. Fault was cleared when 33KV CB was tripped on CBFP. Date 24/04/07 On 24/04/07 also exactly same occurrence was repeated. Again 11KV I/C tripped due rat electrocution inside the circuit breaker resulting in the flashover in the CB and CB chamber. 33KC Tx-2 was tripped on CBFP due to continuous fault feeding from source end. Fault was cleared when 33KV CB was tripped on CBFP. 28

  29. FDR Date :23/04/07 Time: 2.54 11KV I/C-2 tripped on O/C Hi-Set element. Fault currents are as follows: Ia= 4.36 KA Ib= 4.36 KA Ic= 4.34 KA 6

  30. FDR 33KV Tx-2 tripped on circuit Breaker failure protection 6

  31. FDR Date: 24/04/07 Time: 00.2911KV I/C-2 tripped on O/C Hi-Set element. Fault currents are as follows:Ia= 4.55 KAIb= 4.34 KAIc= 4.24 KA33KV Tx-2 tripped on circuit Breaker failure protection 6

  32. FDR 33KV Tx-2 tripped on circuit Breaker failure protection 6

  33. CONCLUSIONS On both the occurrences 11KV I/C-2 was tripped on O/C Hi-Set element within 100ms which is meant for bus fault. 33KV CB of Transformer tripped on circuit breaker failure protection because even after the tripping of11KV CB fault was not cleared as fault occurred on the cable side limb of 11KV CB. Hence to clear the fault 33KV CB tripped on CBFP. Hence this is well coordinated tripping. CAPA The 11KV I/C-2 CB was heavily carbonized due to fault on 23/04/07. M/S Schneider have replaced the CB by new one. Same has been tested and taken into service. The necessary vermin proofing /sealing of panels was undertaken by Schneider engineer in the presence of project engineer. 4

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