1)REF relay should be non-operative for an external fault. Knowing the maximum sc current you should calculate
the set point voltage of the REF relay. Once it is known say Vr=50V, then verify whether the CT knee point voltage
is at least TWO times Vr. If not you may have to select a different CT for relay stability.
2)Knowing the Imag of the CT and calculate the minimum operating current of the relay. In my opinion
it should be not more than 5% of the maximum allowable earth fault current.
3)It is better if you could please transfer this post to "ELECTRICAL POWER & TRANS & DISTRIBUTION" forum.
