Optimal design of the modular multilevel converter for an energy storage system based on split batteries

This paper presents the optimal design of a modular multilevel converter (MMC) for use in a standalone high power energy storage system based on split batteries (sBESS). The MMC allows for the sBESS to connect directly to the medium-voltage grid without the need for a line-transformer. A free parameter variation is performed to compare designs with different numbers of modules and different power semiconductors. Many commercially available IGBTs were found to be overdimensioned for the 5MW, 20kV target system, and better results were obtained with optimized assemblies. When the converter is designed with just a few more modules than absolutely necessary, the total size of the passive components can be reduced drastically, while the power losses increase only marginally. An attractive candidate system is given a closer look, for which a peak power-conversion efficiency of 99.3% is predicted (not including power losses in the line-filter and in the dc-dc converters to interface the batteries).