Is it recommeded that the CT for Differential Relay is also connected to breaker fail and metering?

[apbanal]

Dear All,

Just want to ask for your technical recommendations on this area.

Just a background, we plan to purchase a new 15kV metalclad switchgear for our substation and our protection group proposed that we should install two bus differential protections. One differential relay is for main protection with one set of CTs (dedicated) and another differential relay is for back-up with another set of CTs but on this back-up differential relay, one CT (located on the Tie CB) is intended for metering and breaker fail but they proposed to just connect the back-up differential relay in series with the metering and breaker fail CT.
My questions are:
1. Is it ok to use the same CT for bus differential, breaker fail and metering?
2. Is it necessary or common to have two bus differential protections in the 13.8kV switchgear?
3. Are their any standards available for protection design especially with bus protection concern?
Note: The switchgear is for distribution feeders, fault level is just around 25kA, and they are planning to use high-impedance bus differential relay (SEL-587Z)

Thanks in advance.

 

[davidbeach]

1. Yes, particularly for a low impedance bus differential, but you can get good data most of the time even with a high impedance bus differential.
2. Not common, but I'd certainly defer to the local protection group.
3. Local practice counts for a lot.

Why argue with your local Subject Matter Experts?

 

[apbanal]

Thanks Davidbeach for the inputs. Were just seeking for some inputs/ideas from other experts based on their experience and also because our consultant is also suggesting that high impedance relaying should have a dedicated ct and not to be connected with other protection devices or metering for a certain reasons.