CT SIZING FOR PRIMARY PROTECTION RELAY

[recs]

Lets assume that we have a 10,000 kVA 138 kV to 12.47 kV transformer with a primary FLA of 41.8 A.

The short-circuit available at the secondary side of the transformer is 5.76 kA which referred to the primary side is 520 A.

Assuming this CT feeds a typical electromechanical 50/51 relay (CO 8) about 100 feet away from the CT, would you choose a 50/5 CT for this primary relay?

Would you consider a C400 accuracy class for this application?

Are there any commercially available CT products (50/5 C400 or C800) readily available and of a reasonable size to install in an average sized equipment?

Thanks.

 

[LionelHutz]

Yes, I'd likely pick a 50:5 CT but not necessarily a C400. The class depends on how it's applied. For this application the CT burden is mostly the lead length.

 

[scottf]

It sounds like you're going to need a free-standing CT and not an internal bushing CT, due to the low ratio.

At 15 kV, C200 is the commonly available protection class for free-standing CTs. It is possible to get a 50:5A with a C400 rating, but it would be a good deal more expensive, larger, by special order. You would have to confirm for yourself, but with only 200 feet round-trip of leads, I'm guessing a C200 would work fine.

As for physical size, if you go with a free-standing CT, where would you put it? Easiest would be an outdoor CT mounted next/near to the transformer.

 

[ShooterBoter]

Does the relay protect the primary side of the transformer also?
With such a low ratio CT you need to check if the CT will saturate on a primary fault.

 

[bacon4life]

Are looking for a high side bushing CT or slipover CT? A standalone 138 kV CT is pretty expensive.

Our protection group usually specs much higher ratio CT's on the high side of all of our similarly sized substation transformers, but I am not quite sure why. Perhaps they future proof for upgrades to transformer differential protection. Perhaps it has to do with possible slow operation of overcurrent relays and high transient voltages in the case of severe saturation as described in these papers:

https://www.google.com/url?sa=t&rct...d=4&cad=rja&uact=8&ved=0CDUQFjAD&url=https://

http://www.selinc.com/WorkArea/Down...tmvE6IxLeSU8npWIJtBBetQ&bvm=bv.75558745,d.cGE

https://www.wecc.biz/library/Docume...ing Current Transformer Application Guide.pdf

 

[karls2]

If the relay is not close to the CT's it's sometimes good to use /1A on secondary, it will allow you to save some VA in the wiring (RI²) most of the protective relays accept /1 or /5A.