A high voltage direct current (HVDC) transmission system, comprising: a power transmission part configured to convert converting alternating current power into direct current power; a power receiving part configured to convert converting DC power into AC power; and a DC power transmission part configured to transfer the DC power converted in the power transmission part to the power receiving part, wherein one of the power transmission part and the power receiving part includes: a measuring part which is configured to measure_an AC voltage; an AC voltage control unit configured to generate a first reactive power control signal based on the measured AC voltage and a reference AC voltage, wherein the AC voltage control unit generates a first difference value between the measured AC voltage and the reference AC voltage as the first reactive power control signal; a reactive power control unit configured to generate a second reactive power control signal based on the first reactive power control signal and a reference reactive power signal; and a power control unit configured to control reactive power of the power transmission part or the power receiving part based on the second reactive power control signal, characterized in that the reactive power control unit is further configured to generate a second difference value between a preset reactive power of the reference reactive power signal and a reactive power of the first reactive power control signal as the second reactive power control signal, the power control unit is further configured to generate a turn-on/off control signal based on the second reactive power control signal and transfer the generated turn-on/off control signal to a converter part to control the reactive power of the power transmission part or the power receiving part. a power transmission part configured to convert converting alternating current power into direct current power; a power receiving part configured to convert converting DC power into AC power; and a DC power transmission part configured to transfer the DC power converted in the power transmission part to the power receiving part, wherein one of the power transmission part and the power receiving part includes: a measuring part which is configured to measure_an AC voltage; an AC voltage control unit configured to generate a first reactive power control signal based on the measured AC voltage and a reference AC voltage, wherein the AC voltage control unit generates a first difference value between the measured AC voltage and the reference AC voltage as the first reactive power control signal; a reactive power control unit configured to generate a second reactive power control signal based on the first reactive power control signal and a reference reactive power signal; and a power control unit configured to control reactive power of the power transmission part or the power receiving part based on the second reactive power control signal, a measuring part which is configured to measure_an AC voltage; an AC voltage control unit configured to generate a first reactive power control signal based on the measured AC voltage and a reference AC voltage, wherein the AC voltage control unit generates a first difference value between the measured AC voltage and the reference AC voltage as the first reactive power control signal; a reactive power control unit configured to generate a second reactive power control signal based on the first reactive power control signal and a reference reactive power signal; and a power control unit configured to control reactive power of the power transmission part or the power receiving part based on the second reactive power control signal, characterized in that the reactive power control unit is further configured to generate a second difference value between a preset reactive power of the reference reactive power signal and a reactive power of the first reactive power control signal as the second reactive power control signal, the power control unit is further configured to generate a turn-on/off control signal based on the second reactive power control signal and transfer the generated turn-on/off control signal to a converter part to control the reactive power of the power transmission part or the power receiving part.