DIFFERENTIAL PROTECTION OF POWER TRANSFORMER

Differential protection is based on the fact that any fault within electrical equipment would cause the current entering it, to be different, from that leaving it. Thus, we can compare the two currents either in magnitude or in phase or both and issue a trip output if the difference exceeds a predetermined set value. This method of detecting faults is very attractive when both ends of the apparatus are physically located near each other. A typical situation, where this is true, is in the case of a transformer, a generator or a bus bar. In the case of transmission lines, the ends are too far apart for conventional differential relaying to be directly applied.

Differential protection is a unit-type protection for a specified zone or piece of equipment. It is based on the fact that it is only in the case of faults internal to the zone that the differential current (difference between input and output currents) will be high. However, the differential current can sometimes be substantial even without an internal fault. This is due to certain characteristics of current transformers (different saturation levels, nonlinearities) measuring the input and output currents, and of the power transformer being protected.

with the exception of the inrush and over excitation currents, most of the other problems, can be solved by means of the percent differential relay, which adds to the normal differential relay two restraining coils fed by the zone-through current, by proper choice of the resulting percent differential characteristic, and by proper connection of the current transformers on each side of the power transformer.