In differential protection, the methods to correct amplitude and phase deviation of currents in primary and secondary winding of the Y/Delta-connected transformer will weaken the harmonic characteristics. Researchers have proposed the inrush restraint criterion of the primary inrush current directly. However, the helping effect of circulating current in the delta winding may cause symmetrical inrush current or waveform distortion, and the reliability of the restraint criterion based on the typical magnetizing inrush current characteristics will be affected. This paper reveals the generating mechanism, the helping effect and the influencing factors of the circulating current.

Delta winding,circulating current,phasor analysis,helping effect