A novel wavelet-based algorithm for discrimination of internal faults from magnetizing inrush currents in power transformers

In this paper, a new algorithm based on processing differential current is proposed for digital differential protection of power transformers by considering different behaviors of the differential currents under fault and inrush current conditions. In this method, a criterion function is defined in terms of difference of amplitude of wavelet coefficients over a specific frequency band. The criterion function is then used for three phases, and internal faults are precisely discriminated from inrush current less than a quarter a cycle after the disturbance; this is one advantage of the method. Another advantage of the proposed method is that the fault detection algorithm does not depend on the selection of thresholds. The merit of this method is demonstrated by simulation of different faults and switching conditions on a power transformer using PSCAD/EMTDC software. Also the proposed algorithm is tested offline using data collected from a prototype laboratory three-phase power transformer. The test results show that the new algorithm is very quick and accurate.