A New Practical Approach for Discrimination between Inrush Currents and Internal Faults in Power Transformers

The power transformers are protected by differential relays. These relays use the second harmonic blocking to distinguish inrush current from internal fault current. In recent years, the performance of the second harmonic blocking has decreased and therefore new methods are needed to distinguish inrush current from internal fault current. The aim of this study is to present a new method for discrimination between magnetizing inrush currents and internal faults in differential protection of power transformers. The proposed scheme is based on calculating the dq0 transformation of current signals in the abc phases of transformer terminals for different inrush current and internal fault signals. Creating new waveforms using mathematical calculations, the signals are identifiable by their characteristics. The accurate diagnosis is based on creating a classification pattern for the discrimination algorithm. To substantiate the preciseness of proposed methodology, different states of inrush and fault currents are simulated in PSCADTM/EMTDCTM. Consequently, calculations and the distinction process are carried-out in MATLAB environment. The discrimination procedure needs only current signal data of less than a quarter of power frequency cycle and uses very simple classification rules. Eventually, both simulation and experimental results show that the accuracy of proposed method is high. The results of this study confirm that simple and accurate methods which use patterns can be developed for distinguishing the internal faulty current of transformer from inrush and healthy current.