Discrimination Between Inrush and Short Circuit Currents in Differential Protection of Power Transformer Based on Correlation Method Using the Wavelet Transform

(Received: October 30, 2011; Accepted: December 8, 2011) Abstract: This paper presents a novel technique for transformer differential protection to prevent incorrect operation due to inrush current. The proposed method in this paper is based on time-frequency transform known as the Wavelet transform. The discrete Wavelet transform is used for analysis the differential current signals in time and frequency domains. The investigation on the energy distribution of the signal on the discrete Wavelet transform components shows the difference distribution between inrush and internal fault current signals of power transformer. The correlation factor which is a statistical parameter is used here to express the pattern of the energy distribution for different current signals. The proposed algorithm is based on the correlation factors to distinguish between internal fault and inrush currents in the transformer differential protection. The proposed algorithm is tested and simulated for several cases by simulating inrush and internal fault currents. The simulation of inrush and internal fault currents are performed using electromagnetic transient program PSCAD/EMTDC software. Simulation results show that the proposed scheme accurately identifies inrush and fault currents at the distance of the power transformer protection in a time period less than quarter of power frequency cycle. In addition the proposed method has high sensitivity and reliability. The method has low computation work and not requires determining the threshold for each new power system.