Control of parallel three-phase PWM converters under generalized unbalanced operating conditions

This paper proposes a new control scheme for parallel three-phase pulse width modulation converters under generalized unbalanced operating conditions. An average model of the parallel system in positive-sequence synchronous reference frame is derived to analyze the influence of generalized unbalanced operating conditions in ac side. It is seen that the unbalance factors in filter inductance will not only give rise to negative-sequence circulating current, but also contribute to generating zero-sequence circulating current (ZSCC). The negative-sequence circulating current can be inhibited by suppressing the negative-sequence components in ac output currents of parallel modules with a proportional integral resonant (PIR) controller. An improved feed-forward strategy and a PIR controller for ZSCC control are proposed for unbalanced operating conditions. The disturbances in ZSCC caused by unbalance factors in filter inductance can be rejected with feed-forward strategy. Because the disturbance in ZSCC is the fluctuation in grid frequency which can be suppressed by a resonance controller, therefore, a PIR controller is adopted in ZSCC controller. The proposed scheme can effectively suppress the circulating currents between the parallel modules and as a result, the distortions in output currents can be greatly reduced. Experimental results confirm the performance and effectiveness of the proposed method.