Add Current Transformer to Relay

Ask your electrician. He should know what the codes, if you worry about such, require.

Hire someone who is competent. It appears you have no one knowledgeable on staff, at the moment.

Get a copy of these AND USE THEM!!

International electrical standards & regulations - Legrand

THE INDIAN ELECTRICITY RULES, 1956 - Coal India Limited

World Electricity Standards

Is there a fourth place on the relay for adding another CT input? Do you have a manual and/or circuit diagram for this relay?

Well, the best people to ask would be the equipment manufacturers. The tool needed to address this issue, is the telephone.

The information should be in the relay manual or in the application notes. It's that simple - read the manual, we don't know what make and model relay you have or how your substation is wired up.

You have said before you work in a 132kV substation, are you actually qualified to perform this work?

I am really concerned by your questions in such a dangerous environment. Read the manual or better yet get someone qualified to perform the work before you kill yourself or someone else.

If this truly is a high-impedance diff relay, then the three sets of current transformers are all paralleled (phase by phase) and their currents sum to (approximately) zero, thus giving a zero voltage input to the relay. Unless the relay's instructions say you can only parallel a specific number of CTs, and I suspect that it not the case, you can add another set of CTs to the scheme. However, you must be sure that they are identical to the existing CTs in ratio, accuracy class, burden, etc. You also want to try to assure that the lengths of the wires from all 4 sets of CTs to the relay are approximately the same gauge and length, so that the burden (and voltage drop) of the wires is similar. Otherwise you could end up having false trips or no trips when it should.

Dear sir

thank you very much