Unbalanced PWM converter analysis and corrective measures

The analysis of an unbalanced PWM AC-to-DC converter by the transfer function approach is presented. The proposed analysis procedure provides for closed-form assessment of all harmonics generated under unbalanced operating conditions by suitably decomposing the converter transfer function into positive and negative sequence components. A three-phase converter operating with programmed PWM patterns is fully analyzed. The distortion at the output of the PWM converter is evaluated under unbalance. The increase in distortion under unbalance is attributed primarily to the appearance of a second-harmonic component. An algorithm for selectively cancelling the generated second-harmonic component at the converter output under unbalance is presented. Suitable implementation techniques for the proposed algorithm are also discussed.<>