Abstract 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.
Li Kai,Zhao Zhengming,Yuan Liqiang. Repetitive Control of Circulating Current in MMC with Asymmetrical Operation of Upper and Lower Arms[J]. Transactions of China Electrotechnical Society, 2016, 31(20): 122-129.
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