AC transmission lines often cross DC transmission lines in the AC/DC hybrid power system

which will easily cause a touching fault between the AC transmission line and the DC transmission line. In regard to two different clearing actions for AC/DC transmission lines touching fault used in HVDC transmission projects of State Grid Corporation of China(SGCC)and China Southern Grid(CSG)

this paper analyzes the hidden problems of two different clearing actions. Under a forever touching fault

a DC overvoltage appears after blocking the converter by using clearing actions of the CSG project. The initial restart logic is in fact disabled by using the clearing actions in SGCC projects. According to the analysis of the characteristics of AC/DC transmission lines touching fault

this paper proposes a suggestion on the clearing action: AC protections trip and isolate the fault while DC protections only set an alarm for a touching fault. The EMTDC simulation experiments show that it is a proper clearing action suggestion. Simulation experiments using differential blocking methods or setting multi-line touching fault are also included.