Overview on reliability of modular multilevel cascade converters

Multi-level converters have been used extensively in modern industry which calls for energy conversion with high-power and high-or medium-voltage. Because of its modularity and scalability, the multi-level converter with modular structure can be extended to different voltage levels and has a variety of forms in practical applications. It has attracted much attention from academia in the past decade, however, as a result of the numerous vulnerable power electronics sub-modules, significant challenges remain with regards to reliability. After summarizing the current research status of modular multilevel cascade converters, the main issues of reliability are reviewed in the paper. Firstly, the failure cases are thoroughly surveyed and classified, and the main failure causes are analyzed. Secondly, the reliability evaluation methods are reviewed and applied to the modular multilevel cascade converters. Thirdly, some promising measures to improve the reliability are presented and discussed, including parameter selection, redundancy design, fault-tolerant control and so on. Then, a complete reliability-oriented design procedure for the modular multilevel cascade converters is proposed. Finally, the challenges and opportunities to improve the reliability are concluded.