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Development of wavelet transform based numeric relay for differential protection of power transformer

This paper proposes the wavelet transform (WT) based algorithm useful in digital differential protection of transformer. It is shown that, the wavelet transform has more distinct feature extraction property, due to its ability to extract information from the transient signals, simultaneously both in time and frequency. A decision logic has been devised using extracted feature from differential current, so as to distinguish an internal fault from an inrush/over-excitation. The proposed algorithm is evaluated using simulated inrush, over-excitation and internal fault current signals. For this purpose a transient behavioral model of power transformer has been developed using Matlab software. Results of evaluation study show that the proposed WT based differential protection scheme can overcome the problem of false tripping due to inrush/over-excitation.