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

Hmm the single core balancing CT is nice in some ways, as you are not relying on near identical 3 cores to have the same winding coupling. It probably makes the CT less sensitive, or larger, and means you cannot easily compare the individual phase currents....

(based on images from https://www.electrical-technology.com/2021/10/core-balance-ct-zero-sequence-ct.html )

There is nothing to ensure that the division of current between the two long cables is exactly the same for all 3 phases, and generally it will not be.

Mike

Hmm the single core balancing CT is nice in some ways, as you are not relying on near identical 3 cores to have the same winding coupling. It probably makes the CT less sensitive, or larger, and means you cannot easily compare the individual phase currents....

(based on images from https://www.electrical-technology.com/2021/10/core-balance-ct-zero-sequence-ct.html )

There is nothing to ensure that the division of current between the two long cables is exactly the same for all 3 phases, and generally it will not be.

Mike

Thanks  mapj1 &  AJJewsbury for your valuable reply & apologize myself for late reply!

mapj1 said:

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.

mapj1 said:

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..

Mike

Lets assume that the real imbalance is because of cores having different resistance or cross section area not same for all cores or cores copper not pure or the length is not the same than in this existing condition when B/C-1 ON, O/G-1,2,12,13 are ON &  B/C-2 IS OFF than also the difference of current in all three phases should have seen by the core balance CT & should trip the relay. ..Just guessing this condition.  

It is already proven that at this existing combinations the CB CT's are not sensing any current. 

So what we have plan is to Close the B/C-2 & same time the O/G-12 or 13 will open so in this case only for lets say mS the loop with close & then open. 

will not going to change any delay of any earth fault current. 

i think this way we can close B/C-2 & open any either O/G-12 OR13 with out facing any earth fault. 

Also would like to discuss one scenario,

lets assume will Open O/G-12 feeder. B/C-1 & 2 will be close condition & O/G-13 will also close. in this condition is there any possibility to sense any unbalance current by CB CT of O/G feeders-13 & 2...

if yes then mentioned above HW interlock will not work....

Pl see the below SLD. 

Kindly share your valuable comments.