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CT rating

CT rating

CT rating

(OP)
I am try to specify current transformers for the protection of a 25MVA hydro generator.  Can somebody help me understand what the CT ratings mean?  C400?  C800?  T400?

RE: CT rating

C means calculated, T means tested. And the number following is roughly the voltage at the knee point, at the maximum ratio.

It's a quick answer, and you probally need to read more about it.

RE: CT rating

Those number refer to the accuracy of the CT at relay currents.  The higher the better.  

For high currents, the key to accurate current into the relay is how much secondary voltage the CT can produce before it saturates.  

Keep in mind that for multi-ratio CTs, this accuracy rating applies ONLY to the maximum ratio.  As you tap down, the accuracy goes down.  So a 1200/5 C400 on the 600/5 tap is essentially a C200 accuracy, just as an example.  

 

RE: CT rating

(OP)
Thank You, this helps.

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