AbstractA new principle of UHVDC Line pilot protection based on internal parameters of the trigger angle of converter is proposed, in order to improve the ability of the protection to withstand transition resistance. Through the analysis of UHVDC control system, it is found that, due to the regulation of control system, in terms of trigger delay angle of rectifier side and trigger leading angle of inverter side, the change tendency in internal fault and external fault is different. Thus, the protection criterion is constructed. Compared with the traditional protection principle using voltage and current to establish protection criteria, this principle uses internal parameters as protection quantity. The simulation based on PSCAD/EMTDC verifies the effectiveness of the protection principle.