中文摘要：

当电网电压出现不平衡时,模块化多电平换流器（modular multilevel converter,MMC）型高压直流输电（high current direct current,HVDC）系统的电能传输将受到明显影响。针对该问题,文章首先分析了不平衡故障时MMC的相关内、外特性,指出在不同不平衡控制目标下MMC内部均可能出现零序环流,零序环流流入直流侧将引起直流电压/电流的二倍频波动;然后在αβ坐标系下建立MMC不平衡控制系统,基于比例降阶谐振调节器设计电流内环和环流抑制控制器;仿真结果验证了文章所提出控制策略的有效性。

英文摘要：

Under unbalanced grid voltage conditions, the power transmission quality of MMC-HVDC （ modular multilevel converter-high current direct current） will be significantly affected. To solve this problem, the correlative inter- nal and external characteristics of MMC are analyzed at first in this paper, and it is indicated that the zero-sequence Circulating current always exists in MMC under different unbalanced control targets, and the twice fundamental har- monic frequency fluctuation of de-link voltage and current will be caused when the zero-sequence circulating current flows into de-side, and then, a control system of MMC under unbalance conditions is built in α-β coordinate system. Both the internal current-loop controller and the circulating current suppressing controller are designed based on pro- portional reduced-order resonant （P-ROR） regulator. It is verified that the proposed strategy is very effective with the simulation results