CT saturation during unbalanced faults 1

[WHiPCPL]

Hello

I have a question regarding current transformer saturation during fault conditions as listed in the subject title.

Some background: We have a 3-phase induction motor drawing 500A during full load.
I've chosen some current transformers: 500:5, 5P5, 30VA. (Transformers will not saturate during start-up).
The short circuit currents vary between 900A(SLG)-21kA(DLG).

As you can probably tell, the CT's will saturate during a DLG fault.

My question is:
Assuming that a DLG fault happens, will the saturation caused by the DLG fault be a problem for the relay to trip, or will the saturation in the transformers prevent that?

As far as my understanding goes, the fact that 2/3 of the transformers saturate should still register as a fault, the only exception being a 3-phase fault, because then all CT's would be in saturation and the relay wouldn't register this as a problem. Although I don't know if any differential relay is capable of registering a 3-phase fault.

Can someone help?

 

[RRaghunath]

I suppose you have Instt protection set to pickup at 250% of Motor Starting current. If this protection element is configured to operate on 'Peak' and not 'RMS'. The protection element would have made a decision / issued trip command before the CT saturates. This way, CT saturation will not affect the protection of motor.
With regard to Differential protection, the protection can see all types of faults including Phase-to-ground faults in case of motor.

 

[scottf]

5P5 is an interesting rating. We normally see 5P10 or higher (like 5P20).

Do you know how much burden you will actually have connected to the CT?

For burdens up to 3.5VA, your CT should be able to pass 21kA without saturating [ 30VA/3.5VA = 8.6, 8.6 * 5 = 43, 43 x 500A = 21,500A ].