High Fault Currents, Low Ratio CTs

[Pad82]

I have read from several different sources the severe implications that can result from the selection of protection CTs based on full load current rather than prospective fault currents.

my question is as follows:

A feeder circuit breaker with associated relay and protection CT (75/1A, 10P20), the minimum prospective fault level on the downstream cable is approximately 14kA. Instantaneous OC protection has been set to 10In (750A). Will the saturation of the CT inhibit the instantaneous operation of this protection relay?

I would appreciat any help you can give me on this!

 

[slavag]

Hi Pad82.
Isn't so correct Q.
Isn't full information about CT and protection relay.
What is VA of CT? What is a secondary load on the CT?

I suggest, you asked about MV application, with protective relays are installed in the MV switchgear, CT about 3-5VA.
Distance between CT and relays about 2-3m, 2.5mm^2.
In this case, is OK, no problem with saturation, CT will gennerate enough current for correct operation of 10In setting of relay. Saturation will start in the region 70-100In (6-7.5kA).
Regards.
Slava

 

[Pad82]

The CT would be rated for 5VA burden, the distance between the CT and relay would be 3M 2.5mm^2, the relay would have max burden of 0.1VA.

The minimum expected fault current is 14kA >750A. For this fault will the relay operate when the CT saturates, considering the Instantaneous protection is set so low relative to the prospective fault current?

 

[slavag]

Hi.
Without any problem.
It's standard application with standard CT in MV switchgears. Are load 1000-2500kVA trafos?
Don't worry.
Good Luck.
Slava

 

[jghrist]

See The Impact of High Fault Current and CT Rating
Limits on Overcurrent Protection, Gabriel Benmouyal & Stanley E. Zocholl, Schweitzer Engineering Laboratories, Inc. at

http://www.selinc.com/techpprs/6142.pdf

 

[Pad82]

Loads are sub distribution boards.
Thanks for your help Slava!