A transformer protective relaying algorithm using the ratio of the flux linkages

The ratio of the increments of the primary and secondary winding flux linkages (RIFL) is the same as the turns ratio during normal operating conditions such as magnetic inrush, overexcitation and external faults. For an internal fault, the RIFL is not equal to the turns ratio. This paper proposes a transformer protective relaying algorithm using the RIFL. For a single-phase or a three-phase Y-Y transformer, the increments of the flux linkages are calculated. For a three-phase Y-Δ transformer, the differences between the two phases of the increments of the flux linkages are calculated to use the line currents, because the delta winding current is unavailable. The RIFL is compared against the turns ratio; if they are different, it is an internal fault. Test results indicate that the proposed algorithm successfully discriminates between internal winding faults and magnetic inrush. This work also describes the results of implementation of the algorithm into a digital signal processor based prototype relay.