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The travelling wave protection and under voltage protection which are initiated by the rate of change of voltage can not detect the high impedance fault. The operation time of differential protection is so long that it can not play the role of back-up protection of the DC line fault. What's more, the HVDC control system has great influence on the performance characteristic of the DC line protection, especially in case of the low load level. The differential protection will miss tripping because of the current fluctuation caused by the control system. Based on the EMTDC simulation including the actual HVDC line parameters, control and protection models, the currently used DC line protection and the influence of the control system are evaluated in this paper, which is analyzed by combination of the fault records and the simulation results. The improvement scheme is also proposed for the actual system.