CT secondary winding 5A. vs 1A.

[raptor6458]

Hi guys,

our power transformers, 6.6kV/433V needs to be protected against restricted earth fault (87N) and standby earth fault (51G) and is connected in a 6.6kV HV switchgear.
at the onset of the project all the secondary winding of the protection ct is 5A. There was a change in the design of the HV switchgear and all the secondary winding of the protection CT's are now 1A (except for the 87N & 51G). The transfo manufacturer have already purchase the 5A CTs. My question is, can we still use the 5A CT's? the protection relay in the HV switchgear is a SIPROTEC 7UT613.
thanks for any input.

 

[rbulsara]

May be, but I would do the right thing and replace the CTs with 1A secondary. (or replace the relays with 5A secondary)

Rafiq Bulsara

http://www.srengineersct.com

 

[electricpete]

One other possible solution would be an interposing CT, but performance would need to be evaluated for effect on phase angle (affects differential), saturation, etc. I suspect Rafiq's solutions are best though.

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[raptor6458]

Yup, already talked to our system engineer and we do have to replace the 5A Cts.

Thanks for the input guys!!!