Design and Implementation of a Modular Multilevel Converter With Hierarchical Redundancy Ability for Electric Ship MVDC System

Modular multilevel converter (MMC) can be considered as a promising candidate for modern electric ship mediumvoltage direct current (MVDC) power system due to its highvoltage quality, low switching loss, good expansibility, and good redundancy. As the voltage level in MVDC is much lower than that in high-voltage direct current case, the number of submodules (SMs) of MMC in MVDC system is also much less. Therefore, the SM failure will have more significant effect on the safety operation. Since uninterrupted operation is one of the most important requirements in electric ship application, the special redundancy design for MMC is required. This paper thus proposes an MMC system with hierarchical redundancy ability, aiming to guarantee the ride-through operation under SM failure. The '' hot-reserved '' and '' cold-reserved '' SMs are prepared in the MMC by compromising the faulty SM number, voltage and current stress of SMs, SM power loss, and so on, and the hierarchical redundancy strategy is divided into several progressive layers to ride through the SM failure in different cases. The pretreatment units are designed to realize the proposed hierarchical redundant strategy smoothly, and without changing the existent modulation and sort-and-selection strategies designed for normal condition. The proposed design is explained in this paper, and verified by a three-phase MMC simulation and a single-phase MMC prototype.