A method to detect inrush currents in power transformer using current derivation characteristics

To further improve the performance of differential protection for power transformers, this paper presents a new method for distinguishing inrush currents from internal faults using the extreme value of current derivation rate. In the condition of internal faults, its derivation rate curve is close to sine waveform and the time span between the maximum value and the minimum one of the derivation rate is approximately near to half a cycle. While in case of inrush currents, due to the dead angle, the time span between the maximum value and the minimum one is less than half a cycle. Therefore, the difference of the time span determined by the extreme value within a period is used to detect inrush currents. Thus, a criterion using the current derivation rate feature is developed and the corresponding realization flow is presented. At last, the dynamic tests under different conditions verified that the developed algorithm is of favorable performance for identifying accurately inrush currents from internal faults within a period, which provides helpful theoretical basis for improving the performance of differential protection for power transformers.