High Impedance Differential CTs

[Thriggen]

I have a situation where it would be useful for me to put a CO relay in line with CTs that are currently only being used for BUS differential protection. That particular relay is an SEL 587Z. I know that in some places in our system, we have multiple relays on a single CT, although not with a 587Z. I have recently come across some documentation that says the 587Z should have dedicated CTs. I guess my question is will it work if I put the CO in line with the CTs if I add in the impedance that the CO will introduce to the overall CT circuit when doing my calculations. That burden is very small. Thanks in advance.

 

[davidbeach]

It's fine as long as you don't expect much of the CO. The CO does must better when CTs don't saturate; the 587Z works by forcing the CTs into saturation.

 

[living2learn]

what is a CO?

 

[Thriggen]

http://www.abb.com/product/db0003db004281/C12573E700330419C1256E7E003223E5.aspx

 

[stevenal]

So the CO adds little burden to the circuit going to the 587Z, but on the other hand the 587Z adds greatly to the burden of the circuit feeding the CO. You'll need to go to a low impedance bus differential scheme if you wish to avoid adding CTs.

 

[jghrist]

So the CO adds little burden to the circuit going to the 587Z, but on the other hand the 587Z adds greatly to the burden of the circuit feeding the CO. You'll need to go to a low impedance bus differential scheme if you wish to avoid adding CTs.

The 587Z impedance is not in the CT circuit for a through fault, so the CO current will not be restricted if it is meant for operation on a line fault, not a bus fault.

 

[stevenal]

This is true.

 

[davidbeach]

True, but only true if all of the CTs behave themselves for the through fault. One CT saturating and some of the 587Z impedance will come into play. Throw in a station services transformer hanging off the bus with no CTs and then you have standing current through the 587Z impedance.

jghrist is right that it will probably work for line faults, but not as well as might be expected. I'd try very hard to avoid the situation.

 

[cranky108]

Why a CO, when the 587Z has an over current built in?

 

[davidbeach]

But only overcurrent for the sum, not overcurrent for any individual position. The 487B on the other hand...

 

[cranky108]

No there is an independit over current that can be wired to anything you want.

Not everybody wires it for a summation.

But a 501 will do just as well, and is much more cost effective than a 487B.

Frankly, the 400 SEL relays are much over priced for what they do, for usage at lower voltages (230 kV and lower).

 

[DTR2011]

Why not use the 51/50 Elements in the 587? Good question.

 

[Thriggen]

I really appreciate the discussion. It has shed light on a lot of things I did not consider. The reasons for me trying to reuse use the CO and SC relays that are on the panel, is becuase I wanted to keep things consistent in the sub. We are adding a tie breaker in a ring bus, replacing a disconnect switch. The existing bus protection is now separated to an east and west bus. Unfortunately we are only purchasing one 587Z but I wanted to use the electromechanicals to try to stay consistent with what I was doing on the other side. I realized late yesterday that I was going to have to use the 50/51 elements on the 587Z to make things easier for myself. Thanks all. Happy Holidays.