Current Transformers (CT) and Differential Protection

CT having the same ratio can be used in differential protection if the voltage working level is the same for both CTs, for example if used for motors and generators. If used for power transformers the ratios need to be different in the way that you have to obtain the same current value in the CT's secondary, no matter what the primary voltage is. The reason for this is that the secondaries need to be connected together, and if there are different current values in the secondary, the protection will not work; you can even destroy the equipment down the CT.

Usualy the CT can be chosen by looking at the current value needed in the secondary and at the rated voltage of the CT.

Class PS CT tobe used for differential protection.

But if we used 5P20 in busbar differential protection,then it will be OK?

The differential protection is about sensing the difference between current in and current out. For a protection to work in that way you need precision in measuring the in and out currents. Differential relays usually work with an error smaller than 5% if the relay isn't damaged of course. If you think that you can make the differential relay insensitive to the 5% measuring error than is ok to use it in busbar differential protection. The overall measuring error would be 10%, considering input and output CTs. If the busbar current is 1000 Amps you should consider that 10% is about 100 Amps. This is a lot of current and NOT to be accepted. Eventually this is the minimum current that should trigger your differential protection by using 5P20 CTs if you don't want the relay to false trigger.

Why do you want to use the 5P20 CTs anyway for differential protection when they are not meant fot this given the error ?

I know that precision equipment is expensive but could eventually save lives, or destroy lives if misused.

you mean that there is no error in PS class CTs?

There is an error. Every equipment has a built in error. If a producer states that his apparatus has zero error, surely he gives you false information. Anyway, the problem is not that it has an error but the value of that error.

I'll give an example. The differential switch at home. Say it's rated 30 Amps/30 mAmp. Which is that the rated current is 30 Amps but it will switch automatically if it detects a current leak close to 30 mA. The error needed in this case would be like 0.001, meaning 0.1 %. It saves lives and not only.

Usually the error for differential protection is below 1%. Even 1% eror is huge. Precision is what is needed for differential protection. "Swiss watch" and not "russian watch"made in China.