Case Study

VARIOUS CASE STUDIES ON OPERATION AND CONTROL SCHEMES FOR GRID SUB-STATION Contd….

1. Introduction: For the last few months, the response of the readers to the case studies on various incidents is overwhelming. Hence this month we are again choosing the write up on similar kind of studies for developing the synchronisation of practical observation to the theoretical concepts. The analysis of each incident being supported by actual observations had been described during the situation to add awareness amongst the operation, testing and commissioning engineers to know the cause of problems and be helpful for easy rectification of the problems. This can also help to develop economic schemes for the smooth running of the operation and control system in the Grid Sub-Station.

2.1. Tripping of All Transformers: At one of the 220/132 KV Grid Sub-station, it was connected with 3 no of transformers in parallel. One day it was observed with tripping of all the transformers at the situation, when all the transformers were sharing the load within the allowable limit of setting.
Actual Observation: 
  • These transformers were of Auto Transformers of rating 160MVA, 100 MVA and 100 MVA. On the day of this incident, R phase of the 160 MVA transformer on 220 KV side due to certain reason opened, resulting the tripping of all other poles in Pole discrepancy.
  • The other transformers were also tripped on over loading, resulting black out of the grid.
Analysis of this incident: 
  • The reason for opening of the individual R phase pole on the 220 kv side of 160 MVA is not known. 
  • But due to this opening, the Pole discrepancy relay (PD) on this feeder initiated for outage of the other two poles. The PD timing was set with 2 seconds. So during this 2 seconds, current on the R phase of 160 MVA transformer became ZERO. The other two transformers were loaded with the extra R phase current. 
  • So due to unbalance current in the system 2nd transformer was tripped on Earth Fault relay (Because it was of NON_DIRECTIONAL type). Now only one transformer was left to cater the system load and resulted the tripping on over loading and total outage of the supply system.
Recommendation:
  • It was advised to reduce the PD setting to 1.2 seconds above the Over load condition tripping time of the rest of the transformers.
  • The Backup relays used in the transformers were changed to Directional type where it was with Non-directional type. 
2.2. Non-Tripping of the feeder for Ground touching of live conductor:  One of the 33 KV feeder was passing over the rocky area. One day it was resulted with the snapping of the Y phase conductor and the conductor touched the rocky area. But the relay used in the sub-station did not trip. 
Analysis of such incident: 
  1. The relay as used was of NON-directional type and setting of the E/F unit was of 40 % as the setting of PSM. 
  2. This time the snapping of jumper and touching to rocky area causes the fault as expected but the touch of ROCKY part resulted the impedance fault and the current as drained to ground was considered as the load in the system.
  3. As the E/F setting was of 40 % and the unbalance current due to this incident was of impedance in nature and the current drawal from the system was of within the limit of the setting. So the line did not trip and caused with the continuous supply from the system.
NOTE: This type of incident is always dangerous and due to falling of live conductor on ground may cause GPR (Ground potential rise) and Potential contour at the affected area. In case of any living being comes across with this affected part may be electrocuted. 
Solution: 
  1. If Back up relay is used, then the setting of the PSM to be kept minimum as per observation and practice.
  2. If DP relay of Impedance type is used then its coverage of impedance towards fault has to be raised in consideration to the load encroachment area. 
  3. However in this case setting was changed to 15% to avoid the non-tripping of the feeder due to these incidences.
2.3. Actuation of APR (Air Protective Relay): For one of the 132/33 KV grid it was planned for the charging of a new 63 MVA transformer. The air cell inflation was done as per the pressure mentioned in the manual. After inflating the cell, the oil was pushed to the conservator till oozing of the oil at the top valve. But it was found with the reduction of the oil level after some days on the conservator tank and actuation of APR. 

Actual Observation: 
  • The level was done to the point as mentioned with the required temperature of 30 deg at the monitor on the conservator tank.
  • But the filling of the oil was done at first attempt with the temperature of 50 deg centigrade.
  • When oil was cooled down, the level on the monitor was observed with less than that of the designated point.
Analysis:
  • The attempt of filling the oil was done with higher temperature than recommended during filtration of the oil in the main tank.
  • As the temperature was of 50 deg centigrade and oil was in hot condition so the level as expected should be little higher than the temperature assigned at 30 deg.
  • Moreover the attempt of inflating the air cell had been done without release/bleeding of the oil at different designated air release points. 
  • So due to cool down and stopping of circulated oil after completion of the filtration process, the air at different suitable pockets got trapped. 
  • Some air also got trapped in the conservator tank along with the inflated air cell.
  • So the level of the oil on the conservator tank got reduced and due to this discrepancy, the APR got actuated.
Rectification: 
  • The process of inflating the cell was repeated once again.
  • This time oil bleeding at different air release points (Bushing turrets, Headers, Buch holtz relays and other points...) was done.
  • The air cell was put with the pressure as mentioned in the manual and filling of the oil was done on the bottom valve till complete oozing from the top valve point.
  • The problem was rectified. 
Recommendation:
So while handling the air cell to inflate, the bleeding of air release has to be done at different points of the transformer earlier to inflate the cell.    

2.3. Observance of low PI during testing of Transformer: While taking the PI value of a transformer for the points (HT-Ground), it was observed with lower PI at 5 KV insulation tester.
Actual Observation: 
  • The attempt of PI value was done with HT to Ground, the IR value came in Mohm. But PI came of 1.2 for the new transformer under commissioning stage. 
  • The attempt was taken on repetition and on second attempt, the value came as of 1.3.
  • But this value of 1.3 was not suitable for the new transformer. 
  • So detail connection checking was done.
  • It was observed with the connection of HT lead being wrap over the HT neutral busing. 
  • The operator has done this to have better grip of the contact on the HT neutral bushing.
  • Now this wrapping HT test lead was put direct connection to the HTN bushing conductor and then the testing was done for HTN- Ground.
  • This time the PI value came of 1.8 and allowed of better value.
  • The transformer was charged and loaded successfully.  
Analysis:
  • The test lead as connected being wrapped over the HT bushing was resulting the radial leakage current causing lesser IR value during the considerable time of 1 minutes to 10 minutes. 
  • When the test lead was taken directly from the HTN bushing, this effect was reduced and required value was obtained of 1.8.  
Recommendation: So it was recommended to use test lead connection direct to the insulation tester instead of wrapping on the bushing or any external support.

2.4. Failure of BUS connector ring: At one of the 132/33 KV GIS sub-station for the 33 system, it was observed spark on one of the bus connector ring, during the condition of high voltage on system.
Observation: 
  1. This system was connected with 5 numbers of out-going feeders with load catering of 80 MW.
  2. One of the feeder was feeding the load of 25 MW by twin connected cables system of 20KM line. 
  3. Especially when this feeder was tripping at remote end, the occurrence of spark at the connector ring was becoming prominent.
  4. Some cases, situation to cause for the outage of main incomers and interruption of the supply.
  5. But again for the availability of the feeder with required load on it was causing the reduction of sparking on it.
Rectification:
  1. The affected ring on the bus joint was checked thoroughly.
  2. It was observed with the missing of the earth/ body connecting link to ground. 
  3. This ring with its earth link was strongly connected by the flexible bond. 
  4. The other earth bonds were also checked and tightened.
  5. Then the system was charged and found with no such problems further. 
Analysis: 
  1. In practice this ring is provided above the joint insulation of the bus connector for developing the voltage equalization around it. 
  2. But this ring needs to be connected to earth potential to avoid the development of phase voltage with respect to ground. 
  3. During heavy loading condition (approximately of 80 MWatt), it was observed with lowering of Bus voltage and accordingly the appearance of the voltage on this ring was resulting of less and the available insulation to the body earth was managing this voltage without resulting the spark. 
  4. But during the outage of the remote end breaker with connected load, the voltage at the sending end was causing higher as available.
  5. This appearance of voltage was resulting the spark on the ring due to break down of the surrounding medium between ring and metal body.
  6. But after availing shutdown, when the rectification was done, the ring remained at equipotential voltage of ZERO potential and no sparking thereafter.
2.5. No Oil passing to OLTC tank: During the filtration of a 63 MVA 132/33 KV transformer, it was observed with no flow of oil to OLTC tank from the main tank, equalization pipe being in open condition between them. 
Observation: 
  1. Before to the filtration, the equalization pipe being in open condition, the vacuumisation had been done. Then oil was pushed to the main tank and also to the OLTC tank through equalization pipe.
  2. The oil was filled to the level of pipe line connected between main tank and conservator. 
  3. This level was above to the OLTC tank and its conservator.
  4. But on observation it was found with no oil on the OLTC conservator tank.
Rectification: 
  1. The valve used in the equalization pipe was checked and found in open condition.
  2. The valve used between the OLTC tank and its conservator was checked and found in open condition. 
  3. Now the air release point on top of the OLTC tank was vented and found with release of trap air and then oozing of oil out of it.
  4. Similarly top valve of the OLTC conservator tank was also opened to release of the air out of it. 
  5. Of doing this activities, oil started passing to the OLTC conservator tank and quickly filled with the oil. 
Analysis:
  1. On inquiry of the process of vacuumisation it was confirmed that the equalization pipe was in open condition and valve between the OLTC tank and its conservator was in closed condition. 
  2. After vacumisation and before filling the oil to the main tank, the valve between the OLTC tank and its conservator was opened. 
  3. So the air in the OLTC conservator now rushed to the conservator tank and trapped to certain level. 
  4. So during filling of the oil, required level in the OLTC tank got maintained due to trap of air and its pressure, but not to the level of OLTC tank. 
  5. Then when the air was released from the required valves, the oil started rushing to the space to maintain level of oil and resulted the filling of OLTC conservator tank.
  6. Then the equalization pipe was closed and filtration was done for the oil in the MAIN tank only. 
Recommendation:
  1. During vacuumisation of the main and OLTC tank, the equalization pipe used in between must be kept in OPEN condition. 
  2. After vacuumisation and before filling of oil to the main tank, none of the valve should be opened to cause the trap of air, otherwise due to air trap, the oil circulation shall be hampered.
  3. During filtration, the OLTC tank being filled with oil should be separated from the main by closing the valve of the equalization pipe.  


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