Compensated current differential relay for Y-Δ transformer protection

A compensated current differential relay for Y-Delta transformer protection is described in the paper. 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. A method to estimate the circulating component of the delta winding current is proposed. To cope with the remanent flux, before saturation, the core-loss current is calculated and used to compensate the measured differential current. When the core then enters saturation, the initial value of the flux is obtained by inserting the modified differential current at saturation start into the magnetization curve. Thereafter, the core flux is then derived and used in conjunction with the magnetization curve to obtain the magnetizing current. A modified differential current is then derived that compensates for the core-loss and magnetizing currents. The performance of the proposed relay was compared against a conventional differential relay. Test results indicate that the proposed relay remained stable during severe magnetic inrush and over-excitation, because the exciting current was successfully compensated. The relay correctly discriminates magnetic inrush and over-excitation from an internal fault and is not affected by the level of remanent flux.