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Inrush current discrimination in power transformer differential protection using wavelet packet transform based technique

In this paper wavelet packet transform based technique is implemented to discriminate the inrush current from internal fault currents of three phase 240 MVA, 345/38.5 KV star delta connected power transformer. The Daubechies (db4) mother wavelet with two number of level of resolution is found to be optimal in providing information to discriminate the inrush currents from internal fault currents of power transformer. Minimum description length (MDL) criteria is used to select the optimal mother wavelet and number of level of resolution. In this paper a model of three phase star-delta connected power transformer is built using PSCAD software to get the samples of differential currents. Wavelet packet transform with multi resolution analysis is implemented using MATLAB wavelet tool box on these samples for discrimination purpose. The results show this technique is very useful over many traditional techniques for the discrimination of inrush currents.