当电网电压出现不平衡时,将直接影响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。针对该问题,本文首先分析了不平衡故障时MMC的相关内、外特性,发现在不同控制目标下MMC内部均可能出现零序环流,零序环流进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；仿真结果验证了本文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 will be significantly affected. To solve this problem, the correlative internal 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 target, and the twice fundamental harmonic frequency fluctuation of dc-link voltage and current will be caused when the zero-sequence circulating current flows into dc-side, 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 proportional reduced-order resonant (P-ROR) regulator. It is verified that the proposed strategy is very effective with the simulation results.