The paper makes analysis of bipolar protection misoperation occurred in the process of unipolar fault pole isolation in a Ultra-High Voltage Direct Current(UHVDC) transmission system. It is pointed out that when the fault pole is isolated from the line

due to the coupling of the line

the induced voltage is generated by the influence of the operating pole

which makes the voltage criterion of the neutral bus high-speed grounding switch cannot meet the requirements

and the disconnection command cannot be executed. The overcurrent protection action of the grounding pole in the station is locked at the operating pole. In this paper

a UHVDC transmission line model is established

the waveform characteristics of DC voltage and current during the faults are analyzed. It is derived that the magnitude of the induced voltage at the fault pole is positively correlated with the magnitude of the operating pole voltage

and the direction is the same as that of the operating pole voltage. The problem of bipolar interlocking caused by the coupling voltage of the fault pole line in the sequential control operation of neutral busbar switch faults can be avoided by modifying the control criteria.