Over-high neutral bus voltage will affect the normal operation of a high voltage direct current(HVDC)transmission system

especially when the system is in monopolar operation condition. This paper describes an event of ground return transfer switch protection action caused by induction voltage when one of the double-circuit DC transmission systems is in monopolar operation condition and an event of converter differential protection action caused by commutation failure. To avoid recurrence of similar accidents

an overall analysis is made on the protection action process of two events and improving measures are put forward. Validity of the improving measures is proved by the test and actual operations.