This could be accomplished by relay coordination between incomer OCR and feeder OCR’s. For this stability study, feeder OCR shall operate with no time delay by an instantaneous element (50), whereas upstream differential relay for the transformer shall not operate because it is external fault. Figure 10(a) shows that the feeder side of CT
is partially saturated on the high fault current. However, it does not affect the relay operation. The amount of the distorted current is high enough to operate instantaneous OCR.