Large-scale and uncoordinated integration of distributed photovoltaics and electric vehicle chargers in sustainable distribution networks significantly aggravates three-phase imbalance, resulting in double-frequency power oscillations on the AC side. These oscillations are coupled to the DC bus through the AC/DC interface, producing pronounced DC-side second-harmonic (double-frequency) voltage ripple. Although the three-phase four-leg (3P4L) converter provides strong capability for imbalance compensation, DC ripple mitigation challenging. To address this issue, this paper proposes a multiplexed-leg three-phase four-leg converter (M-3P4L) to achieve coordinated compensation of AC-side three-phase imbalance and DC-side second-harmonic ripple. The key idea is to multiplex the fourth zero-sequence leg as a ripple-decoupling leg, so that the DC-side ripple can be suppressed with only one additional capacitor. A mathematical model is further established to reveal the formation mechanism of the DC-side double-frequency ripple under unbalanced conditions, based on which a phase-separated power decoupling control strategy is developed. The proposed controller effectively suppresses the DC-side second-harmonic component while maintaining the imbalance-compensation performance. PLECS simulations, hardware-in-the-loop and scale-down prototype experiments verify the effectiveness of the proposed scheme, showing reductions of voltage unbalance by 0.87%, current unbalance by 20.72%, and DC voltage ripple by 89.5%.
Wang et al. (Tue,) studied this question.