适用于模块化多电平换流器的环流抑制策略

Circulating Currents Suppressing Strategy for Modular Multilevel Converter

针对模块化多电平变流器(modular multilevel converter,MMC)内部环流问题,结合变流器拓扑结构特点,提出一种基于交叉耦合控制的新型环流抑制策略,该方法通过检测桥臂电流,经二倍频负序旋转坐标变换分离MMC内部环流,进而采用交叉耦合控制方式实现MMC内部环流的快速抑制,以减小子模块电容电压波动;通过建立环流控制器(circulating current suppression controller,CCSC)的传递函数,详细分析并设计了MMC内部环流交叉耦合控制器,同时分析了所设计控制器系统的稳定性。最后利用PSCAD/EMTDC搭建了系统仿真模型,验证了所提环流控制策略的有效性,使变流器的系统稳定性与整体运行性能得到显著提高。

For circulating current issue, combined with structural characteristics of modular multilevel converter （MMC）, this paper investigated a novel circulating current suppressing strategy using cross-coupling controlling method. Internal circulating currents in MMC could be separated with double frequency negative-sequence transformation of detected arm currents. As a consequence, the proposed strategy could rapidly suppress MMC circulating current with cross-coupling control scheme, effectively mitigating capacitor voltage fluctuations in sub-modules. By establishing transfer function of circulating current suppression controller （CCSC）, cross-coupling circulating current controller was fully analyzed and designed. Stability of the proposed controller was also validated. Finally, a detailed simulation model was built up in PSCAD/EMTDC simulation environment. Effectiveness of cross-coupling circulating current suppression strategy was verified. The strategy can significantly improve stability and operation performance of MMC system.