Abstract: In the context of large-scale new energy flexible HVDC transmission systems, the high harmonic characteristics of differential currents during transformer faults at the sending end, as well as the inrush current characteristics during single-phase ground faults on the valve side of the receiving end transformer, present new challenges for transformer protection. To address the failure of traditional protection methods under certain fault conditions in this scenario, a transformer time-domain equivalent inductance protection method based on cumulative fitting error weighting is proposed. This method, incorporating parameter identification, calculates the equivalent inductance to directly reflect the transformer's state and uses the cumulative weighting of fitting errors to compensate for the high volatility typically seen in traditional time-domain calculations. The results demonstrate that this method effectively avoids the problem of incorrect blocking in second harmonic restraint criteria and exhibits superior performance in simulation tests, unaffected by power source characteristics and control strategies. This method has significant potentials for engineering applications.