Circulating current calculation for delta windings of transformer with Y/Δ connection

The equivalent instantaneous inductance algorithm is an effective way to distinguish the excitation inrush from the fault current, which could not be easily applied to the 220 kV and below three- phase transformers with Y/Δ connection due to the hardly acquired circulating current of delta winding. A method for calculating the circulating current is proposed. The proportional relationship between the circulating current of delta winding and the zero-sequence current of Y winding is derived based on the differential equations of transformer. In the unsaturated flux zone, because the circulating current is equal to the corresponding phase current at Y side, it is proportional to the zero-sequence current as well. In order to distinguish the unsaturated flux zones, the waveform correlation theory is applied to calculate the correlative coefficient of phase current with zero-sequence current for each phase at Y side, which proves that the unsaturated flux zone of at least one phase is long enough for calculating the ratio of phase current over zero-sequence current to obtain the circulating current. The accuracy of the proposed method is verified with MATLAB simulation. With the transformer parameters unknown and the closing angle uncontrolled, it calculates the circulating current accurately within one power cycle. ©, 2015, Electric Power Automation Equipment Press. All right reserved.