Modular Multilevel Converters with Integrated Energy Storage

Abstract

Battery energy storage (BES) integration into the grid is typically achieved using a 2- or 3- level dc/ac converter with BES 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 failure or shutdown of an individual battery cells results in loss of a large fraction of BES capacity. This thesis addresses the aforementioned issue by subdividing the BES into units and integrating each unit into the submodules of a modular multilevel converter (MMC). This reduces conduction losses within the MMC submodules and provides a mechanism for electronically protecting BES units from both ac and dc side fault events. However, integrating BES units into the submodules disrupts the power flow of the MMC. Therefore, this thesis analyzes the power flow of the MMC to identify the different power transfer mechanisms, which enables energy balance between BES units. To easily understand the analysis, a graphical user interface tool was developed to assess the impact of integrating BES units into a select number of submodules, and identify alternate BES distributions within the MMC. From the power flow analysis, a control method is developed to maintain energy balance across all submodule capacitors and BES units in the MMC. Validation of the analysis is performed through simulation and experimental work. As part of this work, a 600V/100kVA MMC was developed with 4MJ of supercapacitor energy storage, which is used to represent the distributed BES. The results from both simulation and experiment verified the analytic conclusions, which found that the integration of BES into the submodules of a MMC increases reliability by using short series strings of batteries and reduces conduction losses. This is achieved without impacting the terminal characteristics of the MMC as state of charge balance between BES units can be achieved through internally circulating currents. Finally, the MMC is found to be capable of operating with BES integrated into a select number of submodules, which can reduce BES installation costs and increase reliability.