A new control strategy to improve the performance of a PWM AC to DC converter under unbalanced operating conditions

Abnormal harmonics of significant magnitude are generated at the output and input terminals of a pulsewidth modulated (PWM) AC-DC converter under unbalanced operating conditions. A control strategy is presented to selectively cancel the generated lower order abnormal harmonics at the output and input terminals and thereby preserve the high-performance features of a PWM AC-DC converter. The proposed technique essentially involves computing the sequence components of the unbalanced input supply and suitably counter unbalances the PWM gating signals of the converter switches to cancel the generated abnormal harmonics. The technique is essentially a feedforward approach and is suitable for higher-power GTO-type PWM AC-DC converters. A procedure for implementing this technique in real time is discussed. Selected results are verified experimentally on a prototype PWM AC-DC converter