Abstract: For the Y/△ wiring transformer, if the circulating current of delta winding could be obtained to compensate the primary current, the accuracy of the inrush current identification method will be improved. The circulating current of delta winding is essentially an unbalanced current generated spontaneously to "offset" the transient unbalanced amounts of the primary winding. So the circulating current could prevent unbalanced amounts from being passed to the other side of the system. At present, the calculation method for the circulating current is still insufficient. The mapping relationship between circulating current and unbalanced amounts（zero-mode current, zero-mode voltage, and neutral point voltage） is obtained by deriving the transformer differential equation. A method for calculating the circulating current based on unbalanced amounts is proposed. To solve the problem that the leakage inductance of each side is difficult to measure, a parameter acquisition method based on the interval identification of unsaturated phase current is proposed according to the characteristic that the primary current is approximately equal to the circulating current when unsaturated. In the above calculation method for the circulating current, only the unbalanced amounts of the primary winding needs to be measured, and there is no requirement for the grounding mode and the system voltage. Simulation and dynamic tests verify the correctness and effectiveness of the method.