Statistical Discrimination Index Founded on Rate of Change of Phase Angle for Immunization of Transformer Differential Protection Against Inrush Current

Magnetizing inrush currents are known as the most critical potential threat in maloperation of the transformer differential protection. Therefore, there is a need to detect the inrush current from the internal fault current to avoid the differential protection maloperation. Due to the possibility of a current transformer (CT) saturation at a high level of inrush and fault current, the discrimination algorithm should function appropriately in CT saturation conditions. Here, a method is developed based on the rate of phase angle change (RoCoPA) to perform the discrimination process. The proposed discrimination index employs the RoCoPA of the three-phase differential currents and calculates the standard deviations of the RoCoPA of the three-phase differential currents. The discrimination procedure is designed based on the fact that during fault scenarios, the signal remains almost sinusoidal, and as a result, the RoCoPA has the minimum variations. On the contrary, owing to the non-sinusoidal wave shape of the inrush current, the RoCoPA has notable variations. Based on the various simulated and practical scenarios, the proposed index's performance is evaluated considering challenging scenarios. The results reveal that the proposed discrimination index has promising accuracy with average response delay with about half a cycle