Abstract: A new protection principle for HVDC transmission lines only using current signal of DC-filter is proposed in order to solve the problems of traditional protection principle using one-end data of HVDC,such as poor reliability and insufficient sensitivity.By analyzing the current of DC-filter at normal operation condition and the impedance characteristic of the electric network,which is made of reactance and DC-filter,it is found that the measured current is obviously different for special frequency signals when an internal or external fault occurs.Compared with the existing whole-line quick-action protection,the proposed protection principle uses the current of DC-filter,which is very small during normal operation and has significant changes between pre and post fault.Moreover the sample frequency is lower,and is easier to be implemented.Simulation results show that the proposed protection principle can identify external fault from internal fault.It can rapidly operate with high sensitivity and selectivity in different fault conditions.