Fluctuation voltage control and fault-tolerant operation of modular multilevel converters with zero-sequence injection.

SUMMARY This paper focuses on voltage fluctuation mitigation and fault-tolerant control methods for modular multilevel converters (MMC). The reason for MMC output fluctuation caused by the floating capacitor voltage fluctuation at low frequency is carefully analyzed. To reduce voltage fluctuation, two control freedom degrees of high-frequency zero-sequence voltage and phase-to-phase circulating current are proposed. In fully implementing MMC functionalities, we comprehensively investigated its fault-tolerant operation without redundant submodules for fail-safe energy transfer. A fundamental-frequency zero-sequence-based neutral-shift method is adopted to enable continuous operation and avoid complete shutdown of the MMC system. These methods enhance both the reliability and availability of MMC while avoiding the undesired unbalanced stresses among the healthy submodules. Rigorous simulation studies are performed to demonstrate the validity and effectiveness of the proposed control methods. Copyright © 2013 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]