1A CT to 5A relay input

[stevenal]

The preferred low impedance bus differential relay can be ordered with 1A or 5A inputs, but not a combination. The existing breakers have 5A CTs, and the transformer we wish to add has 1A CTs. What problem might occur if I were to run the 1A CT into a 5A input and adjust the ratio in the relay accordingly? Should I use auxiliary CTs to drop all the 5A CTs to 1A and order the 1A version of the relay? Thanks. Merry Christmas

 

[davidbeach]

I would think that if you just put in the correct CTR all should be fine. You'll lose low-end accuracy (measurements potentially lost in the weeds) but shouldn't confuse the relay during faults. I wouldn't go the other direction though, connecting a 5A CT to a 1A input could cause trouble in that the relay A/D will "saturate" at 20% of the current it would with a 5A input.

Is the transformer a possible source or is it just a load?

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[stevenal]

David,

Thanks. We tend to treat the transformer as a source and trip the circuit switcher in case of a bus fault. In reality, it feeds radially. Normally open switches parallel substations only long enough to complete a load transfer, so the chance of a bus fault occurring during such a transfer is very low.

 

[davidbeach]

That means that the CT will perform satisfactorily for any bus fault. Then you just need to be concerned about its performance for a transformer fault. But that's a concern regardless of the secondary current.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[RRaghunath]

If I remember right, the CT secondary amps is settable for each CT individually in modern numerical relays.
I mean the relay analog input is suitable for connecting either 1A or 5A secondary CTs and the selection is part off the relay setting configuration. Just checkout.

 

[thermionic1]

Some relays (SEL) determine the Inom sec at the time of ordering. Others (GE, etc) have dual inputs.