Adapter Current Transformer

Current differential relays provide adjustment taps (actually internal auxiliary CTs) for just this purpose. They are used for balancing the currents from the transformer primary and secondary CTs to match them, so the differential current is as close to zero (within some tolerance) under non-fault conditions. The relay instructions should explain how to choose the taps. External auxiliary CTs should not be necessary.

That said, you should get a qualified protection engineer to develop the proper settings for your transformer protective relays. Setting them incorrectly can leave you with a very expensive piece of junk leaking out all of its magic smoke.

I think matching current transformers, also known as (Interposing Current Transformers) "ICT", are to be installed at both sides of the transformer to match the two secondary currents of the main current transformers (100/5 & 1000/5) which enter the differential relay.

Here is a rough calculation of the required ICTs based on that your differential relay is 5A relay.

At HV side of the power transformer:

The primary current of the main CT= 20 MVA/154*√3= 74.9 A

The corresponding secondary current due to the CT 100/5 A = 3.745 A

Thus, you will need ICT of ratio 3.745/5 A at the HV side.

At MV side of the power transformer:

The primary current of the main CT= 20MVA/34.5*√3= 334.7 A

The corresponding secondary current due to the CT 1000/5 A =1.673 A

Thus, you will need ICT of ratio 1.673/5A at the MV side.

It is to be noted that if your power transformer is designed for overloading (e.g. 10% overload), this % of overloading should be considered in the calculations.

Regards,

I prefer that ICT 1,673 / 3,745 A. You can use that only one side at LV side.

Is there any objection ?

Best regards...

Yes, you can; however, you will not reach the max. relay current (5A).

Best regards...