Power Conversion Device

A power conversion device connected to an AC power system, the power conversion device comprising:

• a power converter comprising a plurality of arms;
• and
• a control device to control the power converter, wherein
• each of the arms comprises a plurality of cascaded converter cells,
• each of the converter cells comprises:
• a pair of input and output terminals;
• a plurality of semiconductor switching elements;
• and
• a power storage element connected to the pair of input and output terminals with the plurality of semiconductor switching elements being interposed,
• the control device comprises:
• an AC voltage control unit to compute a positive-sequence reactive current command value based on a detection value of an AC voltage of the AC power system;
• a DC voltage control unit to compute a positive-sequence active current command value based on a representative value of voltages of power storage elements in the converter cells which implement the power converter;
• a phase balance control unit to generate respective arm current command values for the plurality of arms for suppression of imbalance among the voltages of the power storage elements among the plurality of arms;
• a first negative-sequence current command value computing unit to compute a first negative-sequence reactive current command value and a first negative-sequence active current command value based on the arm current command values;
• and
• an output current control unit to control an AC current of the AC power system based on the positive-sequence reactive current command value, the positive-sequence active current command value, the first negative-sequence reactive current command value, and the first negative-sequence active current command value, and
• when magnitude of arm currents that flow through the plurality of arms, respectively, is equal to or larger than an allowable current maximum value, the output current control unit restricts a positive-sequence reactive current, as compared with the positive-sequence reactive current when magnitude of the arm currents is smaller than the allowable current maximum value.