Motor differential CT dimensioning 1

[AMBMI]

To verify the dimensioning of the CTs of a motor differential relay are usually checked both the stability for external faults and the relay trip for internal faults or only the stability?
If the bus bar where the motor is has a very high short circuit level (i.e. 35 kA at 11 KV close to the generators without any intermediate transformer) I think that the trip of the differential relay should be more critical.
Indeed we should be sure that the relay could trip before the CT saturation. The stability in this case is less critical because only the motor starting current (obviously multiplied by a K factor taking care of the motor time constant) is passing through the CTs. In any case I would check both conditions.
Am I totally crazy or not? The point is that we get a quite great value for the CT VA.
Thanks in advance for any answer

 

[DiscoP]

You are not crazy ! I would say that both are critical.

The kick in the backside is just as hard for protection not operating as it is for protection incorrectly operating !

Depending on the relay used and the scheme being applied the CTs can sometimes be designed to remain stable for a certain level of satuation.

The manual for the relay you are using may give you some suggestions. I would look for some guidance in the manual, or talk to the technical staff of the relay manufacturer.

 

[slavag]

I 100000000% agree with DiscoP.
I think once we told about it. Always check CT performence according to relay manual and add some margin ( if you want sleep good).
Regards.
Slava