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. 

Parents
  • HV isn't my thing, but applying general principles, it sounds to me like you've got the equivalent of two cables in parallel with an RCD in each cable. So if there's any slight differences in the impedances of each of the three lines, some of the current passing "in" along one cable could return "out" via the other cable. It need not be a very significant imbalance - 7.5A is a very small proportion of the overall current, so you'd only need a tiny percentage difference in impedances. That's my 1st guess anyway, just to get the discussion going...

        - Andy.

Reply
  • HV isn't my thing, but applying general principles, it sounds to me like you've got the equivalent of two cables in parallel with an RCD in each cable. So if there's any slight differences in the impedances of each of the three lines, some of the current passing "in" along one cable could return "out" via the other cable. It need not be a very significant imbalance - 7.5A is a very small proportion of the overall current, so you'd only need a tiny percentage difference in impedances. That's my 1st guess anyway, just to get the discussion going...

        - Andy.

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