why are the likely reasons for bochholz & pressure relays
oprate due to a internal fault but not differential relay ?

Answer Posted / s.panneer selvam

I agree with the answer but i found that the differential relay is high impedance relay, in which we cannot used differential relay for transformer internal fault. Because due to mismatch in CT's and lead connection ur circuit will trip spuriously even if there is no abnormality when CT are requested for 100% protection. so we compelled to use some stabilizing resistor which creates non operative zone for some part of winding.where a incipient faults are not continous fault which exists in circuit , it is discontinuous fault after some it may not be in circuit. 

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