Earth Fault operated in 11KV Solid BUS when made close loop.

Dear All,

We have two 11KV BUS's BUS 1 & 2, In BUS-1 there are two sections BUS-A & B. Same two sections BUS C & D are there in BUS-2. 

In BUS 1 the BUS coupler is always closed position(for closing using synchroscope) & BUS-2, BUS coupler keeping always Open condition. BUS-2 getting power through BUS C incomer BKR 1 & BUS-D from Incomer BKR2.

Since the BUS coupler-1 is closed position & as per i understand the voltage at BUS-C & BUS-D should be same. Also we verified the voltage across BUS coupler -2 with HV sticks, there were no any voltage difference found. 

In this scenario the BUS coupler -2 can closed without synchronization, i think that's why we don't have synchroscope installed on this BUS coupler-2.  

when we tried to close BUS Coupler-2 with following conditions,

1. BUS Coupler -1 Close position. 

2. O/G BKR-1 & Incomer BKR-1 Close position.

3. O/G BKR-2 & Incomer BKR-2 Close position.

when we were closed BUS Coupler -2 same time following BKR's tripped on Earth fault,

1. O/G BKR-1 & Incomer BKR-1 Close position.

2.O/G BKR-2 & Incomer BKR-2 Close position.

For earth fault protection for O/G & I/C BKR 1 & 2 cables are equipped with CB C.T(Core balancing).  CB CT ratio is 50/1 & settings is 7.50 Amp with 500 ms. 

Running load on BUS-C is 4.18 MW & BUS-D is 4.79 MW. I don't think this difference of active power can cause of this issue. 

Please find the attached SLD of our switchgear. Kindly please go through the above case & share your valuable comments. 

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  • Thanks  &  for your valuable reply & apologize myself for late reply!

    I think we may be slightly at cross purposes in terms of understanding. Once you close the bus couplers, if you like it or not, the two one km lines are resistors in parallel, and will share the load current approximate equally. But not exactly , and that is the root of our  problem, equally on all 3 phases.

    Reply - Yes I do agreed that difference of current flowing after closing of Bus coupler -2 in all three phases could be caused of this issue. 

    Just thinking that if we can increase the delay time & Io>> setting may this issue can resolve because the system will get some more duration to settle down the unbalance three phase current in all phases.

    What problem requires you  to close bus coupler 2 when bus coupler 1 is already closed, if the 4 breakers are in the on position ?

    What faul

    In the current scenario if Bus coupler -2 is OFF then if there is any pre planned maintenance of Breaker or renew the protection relays on breakers is not possible unless interrupt the load. If Bus coupler -2 can close without any issue then online load can be shift. this is the main cause behind closing the BC-2

    We are now working on the hardware interlock or through PLC automation that as soon as BC-2 will close then it will open the pre selected BKR.I think this way we can achieve our site requirement to avoid the interruption of load whenever any BKR need any maintenance.  

    Thanks to all for sharing their comments. 

  • A slower break may help ride over your switching transients, but if the imbalance is because the phase cores are not all the same resistance, one being a larger or smaller cross-section by a percent, or made from a batch of a slightly more or less pure copper core, or indeed just not quite the same length,  those things will not really be helped by a delay..