25kV bus prot and CT cables 2

[Sn00ze]

hello,

I have a difference of opinion that i am hoping people on here can help me weigh in on. we have a switchgear with 5 cells. only 2 of the cells have breakers and are being used (incomer and line 1). The client wants to run CT cables from the switchgear building (tappered off and shorted at the switchgear building's MRs) all the way to the bus protection in the main building. The problem arises when shortening and grounding these cables.

The Relay (MFAC34)'s sumation will ideally have all of the CT cables connected but if we ground and short, would this not cause the relay to not function properly as any fault on the other relays would simply flow back through the ground on these spare cables?

After talking with an acceptor, one proposed idea would be fully wiring out and connecting the spare cells bus prot CT’s to the summation module.


Otherwise the summation module would have to be isolated from wiring in order to no add in the extra short or grounds into it.

suggestions?

 

[DTR2011]

You are correct that of the wires are shorted, the protection is disabled. If I understand correctly, wiring to the spare cells will not hurt, it is like having an open breaker. One other option is to leave the summation point for the spare units open at the summation box.

 

[stevenal]

Short where ever and as often as you wish. Grounding, however, should be done in one location only. Since you likely have your CT secondary neutral grounded already, I suggest you short at this location. Any other shorts on the circuit should be left un-grounded. Multiple ground paths will provide a path for stray ground currents through the relay inputs, and you will see non-zero current through the no breaker positions.

 

[Sn00ze]

Thank you for your responses guys, i really appreciate it. You are both correct in my opinion. I like the idea of just leaving the Summation circuit isolated from the wiring all together.