Due to the advantages of modularity, scalability and redundancy, modular multilevel converter (MMC) has been widely used in the high power applications such as HVDC transmission. However, the reliability becomes one of the most important challenges for MMC which is composed of a large number of power electronics components. To increase the reliability of MMC, the reserved sub-modules (SMs) are often utilized to realize the fault-tolerant operation. Considering MMC with the reserved SMs, the SM failures will cause the asymmetrical operation of the upper and the lower arms. Through detailed analysis in this paper, it can be concluded that there are not only even-order harmonics, but also odd-order harmonics in the circulating current under asymmetrical operation conditions. Accordingly, an improved repetitive controller of the circulating current is proposed in this paper, which guarantees not only the fault-tolerant operation of MMC, but also the suppression of all the harmonics in the circulating current. Whether MMC operates in the symmetrical or the asymmetrical condition, the repetitive controller can both work. Therefore, there is no need to switch controllers after the fault, which reduces the complexity of the fault-tolerant control. Experiments based on a downscaled single-phase MMC prototype have verified the effectiveness of the proposed control strategy.%模块化多电平变换器（MMC）以其模块化、可扩展和便于冗余容错设计等优势在高压直流输电等大功率场合得到了很多应用。然而，采用大量电力电子元器件使可靠性成为制约MMC发展的主要因素，而通过设置备用子模块进而实现容错运行成为提高其可靠性的主要手段。针对含备用子模块MMC发生子模块故障导致上、下桥臂不对称运行的情况，本文通过理论分析表明，环流中不仅包含了偶次谐波成分，还包含了不对称运行产生的奇次谐波成分。据此，本文采用改进的环流重复控制器，在实现 MMC 故障后容错运行的同时，还实现了对环流各次谐波成分的有效抑制。所提出的环流重复控制器，不仅适用于上、下桥臂不对称工况，也适用于对称工况。因此，故障前后无需进行控制器的切换，降低了容错控制的复杂度。本文通过搭建单相 MMC 硬件平台，验证了所提控制策略的有效性。
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