Transformer protection based on the increment of flux linkages

The ratio of the increments of the primary and secondary winding flux linkages (RIFL) is equal to the turns ratio during normal operating conditions, magnetising inrush and over-excitation. It differs from the turns ratio during an internal winding fault. A transformer protection algorithm based on the RIFL will be described in the paper. For a single-phase and a three-phase Y-Y transformer, the increments of the flux linkages (IFL) are calculated. For a transformer with a delta winding, the differences of the IFL between the two phases are calculated to use the line currents, because the delta winding currents are practically unavailable. The ratio of the matching IFL or the differences of the IFL is compared with the turns ratio for fault detection. Compared to a conventional harmonically restrained or blocking differential relay, the algorithm improves the operating time of a relay and is independent of core characteristic and system conditions. Test results indicate that the algorithm can successfully discriminate between internal winding faults and all other operating conditions including magnetising inrush and over-excitation.