Four-leg converter based unbalanced load sharing method in islanding microgrid

The active power-frequency, reactive power-voltage based droop control is widely used in microgrids to share the load powers among converters. The conventional droop control method has not guaranteed the unbalance current sharing accuracy. To overcome the drawback above, the three-phase four-leg topology is adopted as a microgrid converter. The principle of unbalanced load current sharing and the causes of zero sequence circulating current are investigated. A negative and zero sequence virtual impedance based load sharing method is proposed in this paper, and it is integrated with positive sequence power droop controller. The negative and zero sequence currents are sharing reasonably according to virtual impedances in each sequence, which mitigate the effect of line impedance mismatch. Finally, the proposed control strategy is verified on a four-leg paralleled converters test rig. The experimental results show that the four-leg converters with the proposed method are operating stably in parallel and not only the unbalanced currents are sharing accurately, but also the zero sequence circulating current is suppressed effectively. ©, 2015, Chinese Machine Press. All right reserved.