The conventional inrush current identification method by the variability of equivalent excitation inductance is based on the equivalent T-circuit of single phase transformer， which relies on the measurements of all winding currents. However， the delta winding side current of a Y-d connected transformer is not measurable by typically configured current transformers; therefore the method is not applicable. The paper proposes an algorithm by filtering the zero sequence current to calculate the excitation inductance of a Y-delta connection transformer. The algorithm can calculate the excitation inductance representing the combined effects of both positive and negative components by using the delta side line currents. The calculation results are significantly different in various operating conditions， therefore can identify internal faults or excitation inrushes effectively. PSCAD simulation results validate the method.