1.Hubei Provincial Key Laboratory for Operation and Control of Cascaded Hydropower Station (China Three Gorges University), Yichang 443002, China;2.State Grid Gansu Maintenance Company, Lanzhou 730000, China;3.China Yangtze Power Co., Ltd., Yichang 443002, China

Aiming at the case of mal-operation of zero-sequence differential protection for the converter transformer in the high voltage direct current (HVDC) transmission project, this paper analyzes the phenomenon that the neutral-line zero-sequence current has a large amplitude and decays slowly under the recovery inrush current condition caused by removal of external faults and the inrush current condition caused by the energization of unloaded converter transformers, based on the HVDC simulation model using actual engineering parameters. By identifying differences in similarity characteristics between the waveforms of neutral-line zero-sequence current and the self-produced zero-sequence current under inrush current and internal fault conditions, this paper proposes a new criterion of the zero-sequence differential protection for converter transformers, based on the dynamic time warping (DTW) distance of zero-sequence current. The simulation and recorded cases show that the criterion has the same operation performance as the conventional zero-sequence differential protection in aspects of protecting range and transition resistance endurance ability during various internal faults. The false differential current caused by abnormal transmission of current transformers under the inrush current condition can be correctly identified, and the risk of mal-operation of zero-sequence differential protection can be reduced.