Additional Check CTs for double bus bar proection

[EPEngineer]

Hi everyone,
I have been encountered with a situation where my client wants me to put "SEPARATE CHECK CTs FOR EACH BUS SECTION" across the Bus Coupler, balanced by other check CTs in that particular bus section. The the usual bus wires stuff put into operation by isolator aux switches. Its a 132kv double bus configuration and we are in U.K (if that may help). I want to know is it required to put separate check CTs for each bus zone beacuse this is not a genaral practice. What could be the pros and cons (except price related matters)?
Your expert opinion is requested.
Regards
SMA

 

[KJvR]

I am not sure if I fully understand what they want but it seems as if they want a separate check zone for each main zone. I can see no big advantage in such a scheme except that your check zone will consist of less CTs and therefore slightly more reliable but not by much. Maybe they are confusing back-up protection with check zone?

 

[EPEngineer]

Well I am exactly of the same opinion. Not only it involves more CTs and more wiring in an existing switchgear but more tripping relays make it more vulnerable to failure. The main question is, do wee need to put additional check CTs around bus coupler and wire in the same manner as main (or discriminating CTs)? I am sorry I am not familiar to draw or upload a cct diagram here.
Your expert opinion is requested.
Regards
SMA

 

[KJvR]

Sorry, I think I must have misunderstood the scheme. I understood that they proposed two independant check zones that is identical to the two separate main zones (one check zone for each main zone). The only advantage for such a scheme will be, if the bus coupler is closed, with a fault in Zone 1 and a false trip in Main Zone 2 due to the high through fault currents (and a wire fault) in Zone 2, only Zone 1 will trip. For a standard scheme with one overall check zone, both Zone 1 and 2 will trip.

What will be the switching arrangement of the CTs be?

 

[SMB1]

Is it a numerical BB protection?

If yes, most or all of BB numerical relays do not have check zone feature.
But for old relays (High impedance) it is required to have a seperate check zone relay supplied form different CTs in order to insure correct operation of BB protection.

 

[EPEngineer]

It is an old scheme with high impedance relays.