Distinguishing between Fault and Inrush Current in Presence of the CT Saturation: a New Method Based on Gravity Center in Time

Failure in distinguishing between the internal fault and inrush phenomena in transformer especially in the presence of saturation in current transformer, may lead to mal-operation of the differential relays. This paper introduces a novel index that distinguishes inrush current from fault current even in the presence of saturation. This index utilizes the concept of gravity center mass (GCM) on the current signal and it is called gravity center in time (GCT). GCT is defined as a point that the energy of signal before and after of GCT is equal. Utilizing the concept of GCM on current signals it can be mathematically proven the proposed algorithm based on the GCT categorizes normal operation, fault condition and inrush current from each other in one cycle. The application of GCT in discrimination of internal fault and inrush current is investigated and further demonstrated for some simulation and experimental inrush and internal fault signals. The results show that proposed method classifies the disturbances quickly with notable accuracy under different conditions.