CT Requirment for Line Differential Protection

You should ask the manufacturer of the line differential relays you intend to use whether they can accommodate it. Most manufacturers have models for either 5A or 1A CT secondaries. I imagine you could use a 1A relay at one end, and a 5A at the other, but the mfr should verify. I have never seen it done.

By definition, for line relaying you will want the same final CT ratio (n:1) at both ends, since there is no transformation in the line. Therefore if you had 3000:5 CTs at one end and 600:1 at the other, they both have the same ratio.

As for CT accuracy class, again, ask the relay manufacturer, but as long as the CTs meet the minimum required by the relays, I don't believe having a more accurate CT at one of the ends will hurt you. There is always some inaccuracy in CTs, and the relays take that into account.

Ideally, . . the CTs in a differential protection system will be of the same ratio same rating and have nearly the same burden connected. If the CT ratios are not the same . . . they should be made the same by using auxiliary CTs to match the ratios. This automatically compromises the protection to some extent. Some differential protection relays will have input matching transformers that will allow you to use different ratio CTs. Oleh

No you can not. C.T,s are designed based on the knee point voltage in accordance with the network protection. For line, bus, cable differential protection you need to have a class X protection C.t which has a much higher saturation level. In using a Class P C.T will cause the C.T to saturate.

Regards

Ronald Naidoo

Electrical Engineer

Different ratio CT may be used if the protection relay has the possibility of selecting transformer ratio for current input, separately for each side. The digital relays have this option

As the class of the transformer, it had better be the same or else you'll have to counteract the error difference by raising the value of the differential current - not a good idea.