A high voltage direct current (1, HVDC) transmission system comprising: a rectifier is configured to convert alternating current power into DC power; a cooler is configured to cool heat emitting from the rectifier; an inverter is configured to convert the DC power into the AC power; a cooler is configured to cool heat emitting from the inverter; DC transmission lines W1 and W2 are configured to transmit the DC power obtained from the rectifier; a first active power measurement unit is configured to measure a first active power input to the rectifier; a second active power measurement unit is configured to measure a second active power output from the inverter; a first control unit is configured to control the operations of the rectifier based on the first active power measured; and a second control unit is configured to control the operations of the inverter based on the second active power measured, characterized in that at least one of the first control unit and the second control unit comprises an oscillation sensing unit is configured to sense oscillation generated in the HVDC transmission system, a damping control unit is configured to generate a control signal for damping the sensed oscillation based on the sensed oscillation, and a signal output unit is configured to transmit the generated control signal to one or more of the rectifier, the inverter and the cooler, wherein the first control unit is configured to control the operation of the cooler by transmitting the generated control signal to the cooler and the second control unit is configured to control the operation of the cooler by transmitting the generated control signal to the cooler. a rectifier is configured to convert alternating current power into DC power; a cooler is configured to cool heat emitting from the rectifier; an inverter is configured to convert the DC power into the AC power; a cooler is configured to cool heat emitting from the inverter; DC transmission lines W1 and W2 are configured to transmit the DC power obtained from the rectifier; a first active power measurement unit is configured to measure a first active power input to the rectifier; a second active power measurement unit is configured to measure a second active power output from the inverter; a first control unit is configured to control the operations of the rectifier based on the first active power measured; and a second control unit is configured to control the operations of the inverter based on the second active power measured, characterized in that at least one of the first control unit and the second control unit comprises an oscillation sensing unit is configured to sense oscillation generated in the HVDC transmission system, a damping control unit is configured to generate a control signal for damping the sensed oscillation based on the sensed oscillation, and a signal output unit is configured to transmit the generated control signal to one or more of the rectifier, the inverter and the cooler, wherein the first control unit is configured to control the operation of the cooler by transmitting the generated control signal to the cooler and the second control unit is configured to control the operation of the cooler by transmitting the generated control signal to the cooler.