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Topic Title: Proteciton CT Overloading Permitted?
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Created On: 15 November 2012 10:39 AM
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 15 November 2012 10:39 AM
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clarky280

Posts: 84
Joined: 10 June 2008

Hi guys can I get your thoughts.

On an 11kV switchboard, there are existing outgoing circuit breakers with 200:1 protection CTs fitted. It is intended to use these circuit breakers to feed some 4MVA 11kV/415V transformers.

Ideally these CTs could be replaced with 300:1 CTs to accomodate the 210A rated primary current of the transformers, however due to the restrictions of a live site this is problematic, therefore - if permissible - it would be preferred that the existing 200:1 CTs are retained.

I understand that CTs have a thermal current rating as well as a primary current rating and most CTs are able to handle more than their full rated primary current without thermal damage. Is this acceptable with regard to protection of the CT - the pickup setting for the HV relay would be set at 250A to provide grading and some nuistance tripping buffer.

Regards,

Craig
 17 November 2012 07:36 PM
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ArthurHall

Posts: 736
Joined: 25 July 2008

What is the primary of the CT? If they are wound primarys they are often only good for their rating if operating continuously.
Also what is the load on the transformer likely to be?
Protection CT,s are designed too still opperate on overcurrent but only for a limited time.
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