There is a serious hidden trouble in overvoltage protection (open electrode protection (59EL)) of Tian-Guan and Gui-Guang I HVDC transmission project which is used to detect excessive voltages that may occur if the electrode opens (both electrode line conductors): the running pole maybe blocked because of the incorrect action of open electrode protection of other pole. In this paper

the overvoltage protection is discussed first

then analyzed the defect based on the configuration of HVDC transmission system

which is confirmed by simulation on RTDS and actual case

at last

the improved suggestion is introduced to optimize the function of Electrode Line Protection and improve the reliability of HVDC transmission system.