SEL 487E

[cuban1024]

hi guys, if I want to use the element 50/51 in SEL487E to protect a feeder out of the differential zone, should I use a different CT or I can use the same CT used for the 87 element?

Thank you.

 

[davidbeach]

No reason not to use the same CT.

When one this sentence into the German to translate wanted, would one the fact exploit, that the word order and the punctuation already with the German conventions agree.

-- Douglas Hofstadter, Jan 1982

 

[wcaseyharman]

I know I’ve used the same CTs for overcurrent and differential.

 

[cuban1024]

@wcaseyharma even if the overcurrent is looking outside of the differential zone as feeder backup?

 

[dpc]

A 50/51 element is non-directional. It will respond to current regardless of direction or polarity. It will see current into the differential zone as well as out of the diff zone.

 

[cranky108]

I like the 487E, because I can use the CT's for a 67 function.

 

[bacon4life]

I think there would be very little dependability gain by using 2 different CTs on a single bushing that both feed a single relay.

Using a CT in a different location could slightly improve dependability if you were trying to wrap protection zones using CTs in different locations. An example could be using the transformer bushing CT for feeder backup protection and circuit breaker bushing CT for the bank differential protection. The overall reliability might not improve much since both CTs feed the same relay.

Back in the days of separate electromechanical relays for each function, dedicated CTs were often used for differential to reduce the likelihood of saturation. There is obviously no extra CT burden by turning on the 51 element in the SEL-487E relay. Even if the CT feeds additional microprocessor based relays, the burden of microprocessor relays is trivially small.

 

[wcaseyharman]

Typically I’ve set the phase overcurrent in a 487 for transformer backup overcurrent, which can look out into the system and into the plant. I believe I did use the CT on the station service feeder as both differential and overcurrent protection for the downstream feeder and transformer.

 

[cranky108]

Use the second CT for the secondary relay.