Direct-current power transmission power conversion device and direct-current power transmission power conversion method

A DC power transmission power conversion device comprising:

• a power converter including a plurality of leg circuits composed of a positive arm and a negative arm for each phase which are connected in series to each other and have a connection point therebetween connected to an AC line for a corresponding phase, the leg circuits being connected in parallel between positive and negative DC buses, the power converter performing power conversion between plural-phase AC and DC;
• and
• a control device for controlling the power converter, wherein
• each of the positive arm and the negative arm composing each leg circuit is composed of at least one converter cell connected in series,
• the converter cell is composed of a series unit of a plurality of semiconductor switching elements connected in series, and a DC capacitor connected in parallel to the series unit,
• the control device performs output control for each converter cell composing the positive arm and the negative arm,
• the control device includes:
• a capacitor voltage control unit for generating a current command value (ipref, ip+ref, ip−ref) for voltage control for each DC capacitor on the basis of voltage (Vcap) of the DC capacitor;
• a current control unit for, on the basis of the current command value (ipref, ip+ref, ip−ref) from the capacitor voltage control unit, generating a voltage command (Vpc+, Vpc−) for AC current control, of a positive arm voltage command (Vp+) for controlling current flowing through the positive arm and a negative arm voltage command (Vp−) for controlling current flowing through the negative arm;
• and
• a DC control unit for, on the basis of DC voltage (Vdc) between the DC buses, DC current (idc) flowing through the DC buses, and a command value (Vdcref, idcref) as a preset control target value, generating a DC voltage command (Vdc*) for DC voltage control, of the positive arm voltage command (Vp+) and the negative arm voltage command (Vp−), and
• the control device corrects the command value (Vdcref or idcref) to be given to the DC control unit, in accordance with a detected amount of voltage variation in each DC capacitor.