This paper studied a typical problem occurred during the commissioning period of the YunGuang UHVDC transmission system

i.e.wrong breaking sequence of the high speed neutral bus switch（HSNBS） on the faulted pole resulted in the blocking of the healthy pole.The reasons for the wrong breaking sequence of HSNBS and the course of the consequent blocking of the healthy pole were analyzed and understood according to the actual transient fault record.Four corresponding countermeasures were presented and testified via revising the control strategy for the condition without communication

improving the differential protection of bipolar neutral bus

modifying the anti-jump circuit

and avoiding the operation without communication.Therefore

the reliability of UHVDC transmission system can be enhanced.