Compensated-current differential relay for protection of transformers

During magnetic inrush or over-excitation, saturation of the core in a transformer draws a large excitation current, which can cause malfunction of a differential relay. The paper describes a solution, its implementation within a compensated-current differential relay and its evaluation using an EMTP-based simulator and a relay test set. The relay uses the same restraining current as a conventional relay, but the differential current is modified to compensate for the effects of the exciting current. Before the core enters saturation, the relay calculates the core-loss current and uses it to modify the measured differential current. If the core then enters saturation, the initial value of the core flux is obtained by inserting the modified differential current at the start of saturation into the magnetisation curve. The actual core flux seen during a fault or a nonfault event, such as magnetic inrush or over-excitation, is then derived and used in conjunction with the magnetisation curve to calculate the magnetising current. A modified differential current is then derived that compensates for the core-loss and magnetising currents. The operating performance of the compensated current differential relay was compared against a conventional differential relay. Results indicate that the compensated relay remained stable during severe magnetic inrush and over-excitation, because the exciting current was successfully compensated. The paper concludes by implementing the relay on a hardware platform based on a digital signal processor. The relay correctly discriminates magnetic inrush and over-excitation from an internal fault and is not affected by the level of remanent flux.