Control.com - Nerds in Control

In system of 6.6kv there is busbar differential protection is provided.

The operating philosophy is of 2 incomer & 1 buscoulpers ( With TWO incomers ON & Buscopler OFF).

The busbar differential scheme is of 2X87 type (respective bus has its differential protection)

Case -1
TWO incomers ON & Buscopler OFF

Any fault happened say at Bus A in switchboard at the outgoing feeders of boards -The Incomer-1 Trips & also the Outgoing feeder trips

Case -2
Incomers-1 ON, Incomer -2 OFF & Buscopler -ON

1) what will happen if the fault occur at BUS -B (which is now fed from Incomer -1)?

2)what will happen if the fault occur at BUS -A?

What should be ideal Design?

Pal... does the term "2X87" refer to a specific manufacturer?

Regards, Phil Corso?

Dear Phil Corso

The term 2X87 means there are two no. numerical relay for differential protection. One for Bus A & other one Bus B.

i hope now its is clear.

regards
Umesh Pal

Phil,

I had found a post from December 21, 2009. Our situation is that we have unbalanced phase to ground voltages on a bank of single phase GSU's that are not connected to the generator so essentially unloaded. This is very similar to the situation on the above noted posting. What was the result of that situation? We are concerned that closing the GCB could cause damage to the gen although we are getting several opinions that once the GSU's are loaded the phase to ground voltages will balance. Any comments you can share?

Bob Fanslow

Bob... the GSU problem you cited had to do with HV-winding capacitance differences between two existing and the replacement xfmrs!

Your problem appears to be different. I presume your GSU HV-side winding (Grid) is Y-connected, and the LV-side (alternator) is Delta-connected!

If the phase-to-ground voltge imbalance is on the Delta-side, and it is not severe, then it is due to differences in the individual Phase-to-Ground capacitances.

Can you provide the actual measured-voltage values so an evaluation of "severe" can be made?

Regards, Phil

> Case -2
> Incomers-1 ON ,Incomer -2 OFF & Buscopler -ON

> 1) what will happen if the fault occur at BUS -B (which is now fed from Incomer -1)?

> 2 )what will happen if the fault occur at BUS -A?

> What should be ideal Design?

This is traditionally refer to as Main-Tie-Main protection. Incomer A and B are the two Mains, and the bus coupler is the tie... you could also say Incomer-Bus Coupler-Incomer, but Main-Tie-Main is the more widely recognized terminology.

1) Fault at Bus B will cause the bus coupler to trip and any outgoing feeder connected to Bus B will trip. If the bus coupler fails to trip, then A incomer and A and B outgoing feeders will trip.

2) Fault at A should cause the bus coupler to trip as well as A incomer and A/B outgoing feeders. This will cause Bus B to become de-energized. Usually the protective relays can be programmed to automatically close the B incomer when there is a loss of source on the A bus. There is a bit of logic involved in making sure this is done safely.

Google "main tie main logic" and you will get results that should help you understand the details better.

Cheers,
Nic

Pal... Thanks for the your response. Nic provided some logic comments/details as a guide. But, I'm not sure if you wanted general logic-only information or are you seeking overall strategy/planning involving:

A) Placement of the CTs?

B) Phase-Fault and Ground-Fault Relays?

C) Trip-Relay Coordination?

D) Use of Lock-out relays?

E) Effect/impact of motor contribution to bus-fault detection and isolation?

Phil

Dear Nic,

Thanks for your reply.
And I am sorry for replying late.

As you stated above i know that during fault at bus A. The relay at Buscopler will operate & trip the Bus Tie breaker to isolate the faulty bus. The faults which are covered by Main protection relay at Bus Tie are 50, 51N, 51 & 51G.

But my query was regarding the 87 (bus differential protection) as stated earlier in normal operation - any fault at bus A will actuate the differential relay located at Bus A & in turn IC-1 will be also trip along will the receptive outgoing.

Same will be also applicable for Bus B differential relay.
Note that in both BUS TIE is OFF.

However when Bus TIE is ON & BUS A is charged through BUS B, what will happen to bus differential relay of Bus A & Bus B when there is fault at Bus A.

I hope it is clear now.
kindly revert for any clarification.

regards
Pal

Dear Pal,

Bus differential scheme for a switchboard with Two Incomers and one Bus coupler shall have dedicated CTs for Incomer 1, Incomer 2,all outgoing feeders of Bus A, all outgoing feeders of Bus B and two CTs for Bus coupler. Bus coupler CTs shall be located on either side of the Bus coupler to create a overlap (let's call them CTBC1 and CTBC2). Bus differential for Bus A shall summate Incomer 1 CT, one of the Bus coupler CT CTBC1 and CTs of all outgoing feeders on Bus A.

Similarly, Bus B differential scheme shall summate Incomer 2 CT, other CT of the Bus coupler CTBC2 and CTs of all outgoing feeders on Bus B.

Please note -The polarity of Incomer CT and Bus coupler CT shall be same where as polarity of CTs of outgoing feeders shall be reversed. In short Incomer and Bus coupler CTs shall act as source CTs.

Operation of Bus A differential relay shall trip all outgoing breakers on Bus A, Incomer 1 and Bus coupler. Similarly, Bus B differential relay shall trip all outgoing breakers on Bus B , Incomer2 and Bus coupler.

Coming back to your question " When Bus Tie is ON and Bus A is charged through Bus B, what will happen to Bus differential relay of Bus A and Bus B when there is fault on Bus A?"

For a fault on Bus A, Bus A differential relay will actuate and trip all outgoing feeders of Bus A, Incomer 1 and Bus coupler. Differential relay of Bus B will not actuate as the fault is outside its zone and would be stable. Hence, power supply to Bus A will not be affected.

Thanks and regards
Veenak

Dear Veenaks

Thanks !
I understood from your reply.