微电网孤岛工况下基于四桥臂变流器的不平衡负载分配策略

Four-Leg Converter Based Unbalanced Load Sharing Method in Islanding Microgrid

为实现负载功率的合理分配,微电网中的变流器并联运行时通常采用基于有功功率-频率、无功功率-电压的下垂控制策略。随着系统中不平衡负载的增加,传统的下垂控制策略已不能保证不平衡负载电流的准确分配。为解决上述问题,本文采用三相四桥臂作为微电网变流器拓扑,分析了负载电流不平衡分量的分配机理以及零序环流的产生原因,提出一种基于负序、零序虚拟阻抗的电流均分策略。该策略与正序功率下垂控制结合,通过设置各序的虚拟阻抗以减小线路阻抗对变流器负载电流分配的影响,实现对负序、零序电流的合理分配。最后,通过实验对所提出的并联控制策略进行验证。实验结果表明,在不平衡负载工况下,采用该控制策略的四桥臂变流器并联运行稳定,能够实现对负序、零序电流的准确均分,并有效抑制零序环流。

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.