A new main protection principle is presented for high voltage direct current（HVDC） transmission lines. The principle is based on the variation characteristics of fault component of voltage and current， together with an indepth research on the transients caused by internal faults， external faults and lightning disturbances. By a set threshold for the ratio of amplitudes of the fault components of voltage at the same side relay location on bipolar lines， faults outside HVDC lines can be excluded， and the fault line can be distinguished. The polarities of the fault component of currents at the ends of the fault line are different for internal faults， while are the same for external faults. Accordingly， internal faults and external faults can be distinguished. Simulation results based on PSCAD/EMTDC for a practical HVDC transmission system demonstrate that the fault line can be rapidly selected and internal faults and external faults can be distinguished by the protection under bipolar neutral grounding at both ends mode. Lightning disturbances can be excluded by the proposed criterion. The protection criterion can move accurately for faults with high transition resistance and lightning induced faults， and can be also applied under one pole with 70% voltage and one pole with total voltage operation mode， anti-way power delivery operation mode， monopolar metallic return operation mode and one pole with maintenance outage. The calculation requirement of the protection criterion can be satisfied with sampling frequency in the range of 10～100 kHz.