Design and evaluation of a current differential relay for transformer protection

The paper proposes a current differential protection relay for transformers. The relay uses the same restraining current as a conventional relay, but the differential current is modified to compensate for the exciting current. Before the core enters saturation, the relay calculates the coreloss current, which is used 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 magnetization curve. The actual core flux is then derived and used to calculate the magnetizing current in conjunction with the magnetization curve. A modified differential current is derived that compensates for the core-loss and magnetizing currents. The performance of the compensated-differential relay was compared against a conventional differential relay. The compensated relay remained stable during severe magnetic inrush and is not affected by the level of remanent flux.