HVDC technology has become an efficient transmission technology and is widely used in China power grids. However, when the AC section power of DC transmission is lost, overvoltage probelm is easy to occur. Based on the back-to-back VSC-HVDC project, overvoltage problem of VSC-HVDC is analyzed in the case of total power loss and single-phase loss operation of AC section. AC section power loss discrimination system architecture is proposed, the standard information of device is designed, and the power loss discrimination logic for single phase and three-phase AC line applying in LCC-HVDC and VSC-HVDC system is also developed. Finally, an actual fault event is demonstrated for the effectiveness and correctness of the AC section power loss discrimination system, avoiding the possible problem of overvoltage and equipment damage.