This paper introduces the the action principle of the bridge differential protection of HVDC transmission system, and studies the transient response characteristics of the converter transformer from bipolar operation to unipolar operation. The reason of the accident is analyzed. The analysis results show that in the zero sequence equivalent circuit of the system, the change of the zero sequence voltage or the change of the zero sequence impedance is the reason for the transient bias of the neutral zero sequence current. In the transient process of the zero sequence current bias, the grounding transmission line is subjected to lightning strikes in the near area, resulting in the earthing current which flows into the converter station earthing network. The converter transformer has DC magnetic bias saturation. The variable transient transmission characteristics of converter transformers, result in the differenct currents of converter Y bridge and converter D bridge, which cause the converter bridge differential protection action after 200ms. Finally, the corrective measures have been given, and have a good engineering effect.
