Power conversion device and control method for power conversion device

A power conversion device comprising:

• a power converter which performs power conversion between an AC grid with a plurality of phases and a DC grid;
• and
• control circuitry which controls the power converter, wherein
• the power converter includes leg circuits respectively corresponding to the plurality of phases of AC, the leg circuits each having a pair of a positive arm and a negative arm connected in series,
• each of the positive arm and the negative arm includes one converter cell or a plurality of converter cells connected in series, the one or each converter cell including a series unit of a plurality of semiconductor switching elements connected in series and a capacitor connected in parallel to the series unit,
• a connection point between the positive arm and the negative arm is connected to the AC grid, and the plurality of leg circuits are connected in parallel between positive and negative DC buses of the DC grid, and
• the control circuitry includes
• a first voltage control circuitry which performs control so that a first representative value which is an average-value corresponding value of capacitor voltages of all the converter cells follows a predetermined overall voltage command value, to generate a first voltage command value,
• a phase balance control circuitry which performs control so that a second representative value which is an average-value corresponding value of the capacitor voltages of the converter cells in the leg circuit for each phase becomes equal between the leg circuits, to generate a second voltage command value,
• a positive-negative balance control circuitry which performs control so that third representative values which are average-value corresponding values of the capacitor voltages of the converter cells in the positive arm and the negative arm of the leg circuit for each phase become equal between the positive arm and the negative arm of the leg circuit for each phase, to generate a third voltage command value,
• an arm modulation command value calculation circuitry which generates an arm voltage command value for each arm on the basis of the first voltage command value, the second voltage command value, and the third voltage command value, and generates an arm modulation command value for each arm on the basis of a result of comparison between each arm voltage command value and carrier waves corresponding to the respective converter cells in each arm,
• a capacitor voltage balance control circuitry which determines whether to insert or bypass each converter cell, on the basis of the arm modulation command value for each arm and arm current in each arm, and
• a gate signal generation circuitry which generates driving signals for the semiconductor switching elements of the converter cells on the basis of commands for inserting and bypassing of the converter cells determined by the capacitor voltage balance control circuitry.