模块化多电平变换器容错运行环流抑制策略

Circulating Current Suppressing Strategy of Modular Multilevel Converter in Fault-tolerant Control Mode

模块化多电平变换器(MMC)采用热备用的容错控制方案时,可实现系统的容错运行。当子模块发生故障时,系统直接旁路故障子模块,变换器运行在上、下桥臂不对称状态。为分析这种状态下MMC系统的工作性能,推导了MMC容错运行工作模式的数学模型,详细分析了桥臂电流以及子模块电容电压的波动情况。分析结果表明:MMC上、下桥臂不对称运行时,故障相间桥臂环流中的基频成分增加,该部分环流会流入直流侧;改变子模块的平均开关频率可抑制不对称运行所引起的环流。在PSCAD/EMTDC中搭建了31电平的MMC模型(其中含6个热备用模块)并进行了仿真,仿真结果验证了所推导结论的正确性。

A modular multilevel converter（MMC） using the main fault-tolerant control strategy based on redundant sub-modules can operate in fault-tolerant mode. When a sub-module fails, the system directly bypasses the fault sub-module, and the inverter operates in its asymmetrical up-bridge and down-bridge state. To obtain the performance of MMC system operating in this asymmetric state, we derived a mathematical model of MMC in fault-tolerant mode, and analyzed the fluctuation of the circulating current and the voltage of the sub-modules. The results show that, when MMC works in its asymmetrical bridge arms state, the fundamental-frequency component of the current circulation in the fault phase increases, and the current circulation flows into the DC side of MMC. Moreover, regulating the average switching frequency of sub-modules can effectively suppress this circulating current. We also simulate the operation of a 31 voltage-level MMC system which includes six reserve sub-modules in PSCAD/EMTDC, and verify the analytic results.