Transformer Differential Protection Using Wavelet Packet Transform

When a transformer is energized the magnetizing inrush current occurs in the primary side. The magnitude of this inrush current may be very high upto ten times of transformer rated current which causes malfunction of differential protection system. Therefore, it is necessary to differentiate inrush current from faults. In this paper, a new approach of discrimination between fault currents and inrush current is proposed based on wavelet packet transform (WPT). This method efficiently helps in the development of differential protective relay to automatically detect and differentiate internal incipient faults from magnetizing inrush currents. The proposed approach is tested for signal tripping using MATLAB for data collected from Electromagnetic Transient Program/ Alternative Transient Program (EMPT/ATP) model of three phase power transformer for different cases that included normal operation, inrush current and internal fault currents. Results obtained from MATLAB clearly show that the proposed approach facilitates the accurate discrimination between inrush and fault currents in transformer differential protection. It prevents false tripping of differential relay due to inrush current.