Rotating phasor-based algorithm for the identification of inrush currents of three-phase transformers

Identification of the inrush current is a prominent research issue pertaining to transformer protection. This study first analyzes the possible low-second-harmonic content of the differential current when a three-phase trans-former with delta-connected windings is energized by wye windings. Further, we propose a rotating phasor-based algorithm for the second harmonic content. The algorithm first rotates the corresponding phasors of the fundamental wave and the second harmonic of one phase current, then recombines them with the fundamental wave and the second harmonic of another phase current to form a new second harmonic content calculation formula. Finally, several tests were conducted using PSCAD/EMTDC and recorded data of field transformer protection. The simulation and test results show that the algorithm can lead to considerable improvements in the accuracy of inrush current identification of the three-phase transformer. The proposed algorithm provides a new identification method for the per-phase blocking of differential protection.