Assessment of Fault Tolerance in Modular Multilevel Converters With Integrated Energy Storage

Energy storage (ES) integration into the grid is typically achieved using a two- or three-level dc/ac converter with ES interfaced directly to the inverter's dc link or through a dc/dc converter. In both cases, long-series connected strings of batteries are required to efficiently maintain the necessary dc-link voltage. Such configurations are susceptible to reliability issues, as shutdown of a battery string due to individual battery failure, overheating, or overcharging/discharging results in loss of a large fraction of ES capacity. To increase the reliability of an ES system, shorter strings of batteries are preferable. In this study, the ES is subdivided into many banks of short-series strings, which are integrated into the submodules of a modular multilevel converter (MMC). To further enhance the reliability, the MMC should also be unaffected by an ES bank shutdown. This paper investigates the robustness of the MMC to ES bank failure by assessing the power balance between submodules when a subset of ES banks is not operational. The analysis concludes that as many as 33% of ES banks may be shutdown without affecting MMC power exchange with the grid, and is supported with both simulation and experimental results.