Mechanism analysis of unbalanced three-phase inrush current of Y0D transformer

Abstract

During the energization of transformer with high-impedance (HT), the zero-sequence overcurrent protection (ZSOCP) often malfunction with the inrush current of primary winding (iP-inrush) shows a large unbalance. In order to clarify the cause of the misoperation and prepare for a solution, the imbalance characteristic is studied in this paper. The imbalance is visually displayed by dynamic phasor diagram. Through the method of numerical simulation, it can be obtained that the peak value of the zero-sequence current always appears near characteristic opening and switching angles (COSAs). At this time, the two phases are deep saturated and one phase is not saturated. The iP-inrushof the saturated phase is large, and that of the unsaturated phase is numerically the same as the circulating current (iD). According to the parameters characteristics of HT, the iDis small, so iPpresents a large imbalance. The research conclusions provide a theoretical basis for zero-mode inrush current suppression.

Keywords

• High-impedance transformer
• Imbalance
• Circulating current
• Dynamic phasor diagram