CT Selection for Differential Protection

The CT works by measuring and converting the current (based on the CT primary/secondary winding ratio), it doesn't measure the voltage like a standard transformer.

http://en.wikipedia.org/wiki/Current_transformer

You just need to choose current transformers (CT) that are insulated for the voltages in question. It is no big deal, these CTs are standard and can be brought off the shelf or cheaply made to order.

Check your differential relay (or whatever devices you are using for differential protection) for what CT secondary current and burden you need (should be either 5A or 1A with a burden of say 5VA).

Talk to a CT supplier or manufacturer for more details and advice (or check out their websites).

CTs are availble for diffrent voltage levels, the problem is where your concerntration lies. CTs on primary and secondry will indicate according to their current ratio, the most important thing to concider is that their out puts will reflect their respective inputs and will not exceed the normal CT out put range of 5 or 1 Amp. If the insullation class of ur CTs is achieved for the respective voltages and the CTs are of same burden, ur free to do any thing u want to do with it.

Dickson.

As per my knowledge, differential CTs are not required for transformers of rating 1.5 MVA. They are used for transformers above 5 MVA (= 5000KVA).

Please cross check your specifications.

Regards,

Prashanth Gaitonde

Hyderabad