M Rasoulpoor - Faculty of Electrical and Engineering Systems,Shahrood University of Technology, Shahrood, Iran Corresponding Authro
M Banejad - Faculty of Electrical and Engineering Systems,Shahrood University of Technology, Shahrood, Iran
A Ahmadyfard - Faculty of Electrical and Engineering Systems,Shahrood University of Technology, Shahrood, Iran

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 transformknown as the Wavelet transform. The discrete Wavelet transform is used for analysis the differential currentsignals 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 toexpress the pattern of the energy distribution for different current signals. The proposed algorithm is based onthe 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 transientprogram PSCAD/EMTDC software. Simulation results show that the proposed scheme accurately identifiesinrush 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

Inrush current % Differential protection % Power transformer % Discrete wavelet transform (DWT)% Internal fault