Reliability Modelling of Series Modular Multilevel Converters for MVDC Applications

In medium-voltage DC (MVDC) applications, the lower DC voltage leads to a reduced number of sub-modules (SMs) in modular multilevel converters (MMCs). This reduction changes the reliability modelling, analysis, and lifetime calculations for these converters. With the series connection of SMs and phases, series modular multilevel converters (SMMCs) are well-suited for MVDC applications. Taking into account the correlation between component lifetime within a single converter, a methodology is proposed for evaluating reliability in SMMCs. It is demonstrated that, despite other operational and component-related drawbacks, SMMCs can offer enhanced reliability, primarily due to the H-bridge-like structure within each series-connected phase, which provides alternate pathways in case of component failure. The proposed approach can further assist with identifying high-reliability options for SMMC and corresponding SM structures, and be employed for redundant design optimization of SMMCs in MVDC applications.