ABB RET 670

[totoboodj]

Hi guys, we have this new relay, ABB RET 670 differential + ref. The relay has three sets of current sensing terminals for differential protection. But in our switchyard configuration, there must be four sets (1-low side of transformer, 3-high side of transformer). Since there are only 3 sets of terminals available on the relay, is it okay to parallel 2 sets of CTs in 1 terminal of the relay? Thanks!

 

[dpc]

Maybe. If you are buying and installing a new relay, you really should buy one that would allow each input to be brought in separately. The CTs can be summed if they are identical ratio and preferably matched accuracy class and secondary lead length, etc.

 

[davidbeach]

There are transformer relays available with as many as 5 sets of three-phase currents plus neutrals. Maybe more. You'd be better off with a different relay.

Paralleling CT inputs can work, but it can also leave you without sufficient restraint current if current can enter on one of the paralleled CTs and leave on the other during an external fault.

 

[totoboodj]

Dpc,

The CTs are identical. Same class, ratio, model and maker.

David,

Please elaborate further. Thanks!

 

[DTR2011]

It looks like your Relay can be ordered to accept 12 Currents, to meet this application. ABB Manual pg 10/15 . Perhaps you can replace the Analog Input module (Transformer Input Module - TRM) to accommodate your needs. I don't have experience with RET 670, but assume this would be possible (as it is with other relays). A firmware upgrade may also be required. Check with ABB and see what they have to say.