Differential Protection of Transformer / Merz-price method

Differential protection system or Merz-Price circulating current method

Differential protection of a 3 phase delta/delta transformer used for the protection against phase to earth and phase to phase fault. Note that CTs on the two sides of the transformer are connected in star. This compensates for the phase difference between the power transformer primary and secondary. The CTs on the two sides are connected by piolt wires and one relay is used for each pair of CTs. In a power transformer, currents in the primary and secondary are usually different; therefore, a differential current flows through the relay even under no fault conditions. This is compensated by different turn ratios of CTs. If T is the turn-ratio of power transformer, then turn ratio of CTs on the LV side is made T times that of the CTs on the HV side.

Working:

During normal operating conditions, the secondaries of CTs carry identical currents. Therefore, the currents entering and leaving the pilot wires at both ends are the same and no current flows through the relays. If a ground or phase-to-phase fault occurs, the currents in the secondaries of CTs will not be the same and the differential current flowing through the relay circuit will trip the breaker on both sides of the transformer. The-protected zone is limited to the region between CTs on the high-voltage side and  the CTs on the low-voltage side of the power transformer.