In compensating for unbalanced voltages of three-phase AC, instantaneous values of wye-phase voltages 120°
out of phase with each other are obtained from line voltages using a
centroid vector operation, symmetrical component voltages of three-phase balanced
system are obtained from the instantaneous values of wye-phase voltages, a compensation
signal to compensate unbalanced voltages of three-phase AC is generated from zero-phase-sequence
voltage of symmetrical component voltages is generated, wye-phase voltages 120°
out of phase, the unbalanced voltages of which are compensated, are obtained from the compensation
signal and the symmetrical component voltages, a
control signal of a PWM conversion is generated based on the compensated wye-phase
voltage compensated, and the unbalanced voltages of three-phase AC are compensated. The amount of time to compensate the three-phase unbalanced voltages required for detecting an unbalance of voltages and generating a
control signal can be shortened.