Unified Algorithm Based on Rotating Phasors for Accurate Detection of Inrush Currents in Three-Phase Transformers

The detection of inrush currents is vital for transformer differential protection. The mainstream method for detecting inrush current is based on the second harmonic content of the differential current. However, the vector compensation used in the differential protection of three-phase transformers may reduce the second harmonic content in the differential current, resulting in misoperation when the transformer is energized. This study proposes an innovative unified algorithm based on rotating phasors to improve the accuracy of detecting inrush current when three-phase transformers are energized. Compared with existing methods that generally require external system parameters of the transformer and are only applicable to energizing from the wye windings of the three-phase transformers, the proposed algorithm is based solely on the measurable line currents of the transformer. Both theoretical analysis and PSCAD simulation results demonstrate that the algorithm markedly improves the accuracy of inrush current identification in three-phase transformers when the transformer is energized from the wye windings and also from the delta windings or with load. The proposed algorithm presents a new identification method for the per-phase blocking of differential protection.