Circulating current control for a modular multilevel converter

A power conversion device connected to an AC power grid, comprising:

• a power conversion unit including at least one arm having a plurality of converter cells connected to each other in cascade;
• and
• a converter control unit to control the power conversion unit,
• each of the converter cells including
• a pair of input/output terminals,
• a plurality of switching elements, and
• a power storage element electrically connected to the input/output terminals through the switching elements,
• the converter control unit including
• a voltage command value computing unit to compute an output voltage command value for each of the at least one arm, and
• a gate signal generating unit to generate an on/off control signal of each of the switching elements of each of the converter cells of each the arm in accordance with the output voltage command value,
• wherein the voltage command value computing unit generates the output voltage command value with compensation for an AC oscillation component, extracted for each arm, of a predetermined frequency of stored energy of the power storage elements of the converter cells included in the arm, wherein
• the AC oscillation component of stored energy of the power storage elements is extracted from temporal change for each arm in average value of voltage detection values of the power storage elements of the converter cells included in the arm, or it is extracted from temporal change in arm power estimate value obtained by multiplying an estimate value of the arm voltage and a command value of the arm current,
• wherein
• the power conversion device is connected between DC lines on a positive electrode side and a negative electrode side and the AC power grid, and
• for each of three phases of the AC power grid, the power conversion unit includes the arms connected between an AC end connected to the AC power grid through a reactor component and the DC line on the positive electrode side and between the AC end and the DC line on the negative electrode side,
• wherein the converter control unit further includes
• an output current control unit to generate a first voltage command value for each phase for controlling an output current of the power conversion device in accordance with a command value,
• a circulating current control unit to generate a second voltage command value for each phase for controlling circulating current of each phase that circulates in the power conversion device to pass through the arms on the positive electrode side and the negative electrode side in each of the phases, such that voltages of the power storage elements of the converter cells in the phase are balanced in each of the phases,
• a voltage balance control unit to generate a third voltage command value for each phase for balancing voltages of the power storage elements between the converter cells in the arm on the positive electrode side and the converter cells in the arm on the negative electrode side in each of the phases, and
• a modulation index correcting unit to extract the AC oscillation component for each arm and generate a modulation index correction signal having a frequency component in accordance with the AC oscillation component,
• the voltage command value computing unit generates the output voltage command value for each arm to compensate for the AC oscillation component of stored energy, based on the first voltage command value, the second voltage command value, the third voltage command value, a fourth voltage command value that is a command value of DC voltage applied to each of the arms, and the modulation index correction signal, and
• the first voltage command value and the third voltage command value are reflected in the output voltage command value, with polarities opposite to each other between the arm on the positive electrode side and the arm on the negative electrode side in each of the phases,
• wherein
• in each of the phases, in order to balance voltages of the power storage elements of the converter cells in the phase, the circulating current control unit calculates a command value of the circulating current for each phase and computes the second voltage command value for each phase to control the circulating current in accordance with the command value, and
• the command value of the circulating current for each phase is increased in a first state in which a command value of the output current of the power conversion device is smaller than a first preset value and an imbalance amount in voltage of the power storage elements of the converter cells in the phase is greater than a second preset value, compared to a second state that is not the first state.