The transient characteristic analysis of fault on HVDC transmission lines shows that the directions of the fault components of the voltage and the current measured at the two ends of the DC lines are different for internal faults and external faults.As such，a new protection principle for HVDC transmission lines is proposed.The direction discrimination criteria of fault components of voltages and currents，the setting methods of threshold values and the corresponding protection criteria are given.Simulation results of actual HVDC system show that internal faults and external faults can be distinguished by the proposed protection principle in a variety of operating modes of the bipolar HVDC system and various fault conditions.The proposed protection acts accurately for lightning induced faults，faults with high transition resistance and faults between poles.The requirement for the communication channel，the sampling rate and the data calculation speed for this protection principle is relatively low.