The percentage differential protection or Merz-Price protection based on the circulating current principle can also be used for the transformer. This system gives protection against pass to phase faults to the power transformers.
The principle of such a protection scheme is the comparison of the currents entering and leaving the ends of transformer, two currents at the two ends of ends of the transformer are equal and balance is maintenance . So no current flows through the operating coil of the relay is inoperative .But when there is phase to phase fault , this balance gets disturbed. The difference current flows through the operating coil due to which relay operates, tripping the circuit breaker.
Compared to the differential protection used in generators, there are certain important points which must be taken care of while using such protection for the power transformer. These points are.
1. In a power transformer, the voltage rating of the two winding is different. The high voltage winding is low current winding while low voltage winding is high current winding. Thus there always exists difference in current on the primary and secondary sides of then relay may get operated through there is no fault existing.
To compensate for this difficulty, the current ratios of CT s. on each side are different. These ratios depend on the line current of the power transformer and the connection of CT s. Due to different turns ratio, the currents fed into the pilot wire from each end are same under normal conditions so that the relay remains inoperative
