Control of Energy Storage Enabled Modular Multilevel Converters with Reduced Storage Requirements

Energy storage (ES) integration into the grid is typically achieved by using single-stage converters interconnected to long series connected strings of battery cells. Such configurations are susceptible to reliability issues as no mechanism exists to address failure of individual battery cells. DC bus fault currents are also excessively large in such systems, which degrades safety and adds complexity to protection systems. This paper examines the modularized integration of ES into the grid by exploiting the modular nature of modular multilevel converters (MMCs). The power balance analysis in this paper demonstrates the ability of a MMC to integrate ES while addressing the short-comings of existing solutions. In the proposed topology, the ES is subdivided into banks, where each bank is integrated into an individual MMC submodule. This both reduces current conduction losses within the MMC submodules and simultaneously provides a mechanism for isolating ES banks from both ac and dc side fault events. This paper focuses on determining the robustness of the proposed ES enabled MMC structure to failure, overheating or over-charge/discharge of individual ES banks. It is shown that as many as 30% of ES banks may be shut-down without affecting MMC power exchange with the grid. In cases where ES banks are operable in less than 70% of modules, it is shown that auxiliary controls may be introduced to facilitate continued MMC power exchange with the grid. This is achieved through reactive current circulation within the MMC structure.