Dual current control scheme for PWM converter under unbalanced input voltage conditions

Voltage unbalance in a three-phase system causes performance deterioration of a pulsewidth modulation (PWM) converter by producing the 120-Hz voltage ripples in the de link and by increasing the reactive power. To eliminate the de-link voltage ripple and the de component of the reactive power, both positive- and negative-sequence currents should be controlled simultaneously, according to the paper by Rioual et al. We used two synchronous reference frames: a positive-sequence current regulated by a proportional integral (PI) controller in a positive synchronous reference frame (SRF), and a negative-sequence current regulated by a PI controller in a negative SRF, In the positive SRF, which rotates counterclockwise, the positive sequence appears as de, while the negative sequence appears as 120 Hz, In contrast, in the negative SRF, which rotates clockwise, the negative sequence appears as de, while the positive sequence appears as 120 Hz. By deleting 120-Hz components using a notch filter in each SRF, one can measure positive- and negative-sequence currents separately, and use them for constructing two feedback controllers. Since the negative-sequence current is also controlled in its own SRF by a de command, this approach yields better performance without increasing the control gain. Note that, since the controller is implemented by a software routine in the digital signal processor chip, using two SRF's does not require additional hardware. We demonstrated the effectiveness of the proposed control scheme by using computer simulation and experiments.