Three control strategies for a three-leg AC-DC converter under unbalanced AC voltage condition

Three control strategies that can be applied to a three-phase AC-DC converter under unbalanced input voltage condition are discussed. Analytic solutions are given on the a-b-c frame, and are very simple, direct and intuitive. All of the strategies guarantee sinusoidal input currents. The first strategy maintains unity input power factor per phase, the second one maintains constant instantaneous power, and the last one supplies maximum power within the current rating of the converter. One or more strategies can be chosen according to the situation. All the control functions, including decoupled current control and PWM, are implemented on the a-b-c frame. It is shown that most modern PWM schemes can be realized in the time domain, namely on the a-b-c frame, by slightly modifying SPWM. Accordingly, the controller is very simple and robust. The proposal is verified by simulation and experiments on a prototype operating at 15 kHz.