Answer / yusuf

Buchholz and Pressure relays such as Over Surge Relays are
also known as Gas operated relays... Consider an internal
fault such as inturturn fault in the transformer winding,
then the oil temperature raises which in turn produces Gas
inside the Oil Tank.. This Gas passes through the pipe
connected between Oil Tank and Conservator Tank and relay is
placed in the pipe work... so the Gas passes through the
relay and gives an alarm first and then trips the
transformer when the fault is severe.... Differential
protection is based on the principle of Differential current
which compares the current at the two sides ( LV & HV )of
the Trafo... And that's the reason the Differential
protection does not operate for incipient (Slow developing)
Faults

Answer / 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. 

what is the difference between the 94 lockout relay and 86 lock out relay. I have noticed that the 94 lockout relay is activated when the distance realy is activated and 86 is activated when the 87T differential is activated. but why can't the same lockouts be used for both the protection schemes

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