Reliability Improvement of Power Transformer s Differential Protection

Modern transformers have many superiorities such as lower power losses and reduced weight and cost in comparison with conventional grain-oriented type transformers. Despite the foregoing advantages, the inrush current in modern transformers has very small second harmonic component, which lowers the reliability of differential protection of the transformer, in which the protection fails to discriminate the inrush current from the internal faults. Hence, the transformer may be disconnected during a normal energization event. In this paper, a new algorithm based on Kalman Filter (KF) is proposed to improve the reliability of the differential protection. The three-phase inrush currents are estimated using an extended KF. Three residual signals are defined as the differences between the measured (with CTs) and estimated (with KF) currents. In normal conditions, these residual signals are nearly zero since the currents are precisely estimated by the extended KF. However, in case of the transformer internal faults, the residual signals are increased. The effectiveness of the proposed method is verified using a large number of simulation test cases. The results confirm high reliability and security of the proposed method.