A comparison of modular multilevel energy conversion processes: DC/AC versus DC/DC

Utilizing strings of cascaded submodules for multilevel dc/ac conversion has become well known due to the widespread popularity of the modular multilevel converter (MMC). Recently, it has been shown that these submodule strings can be adapted to achieve single-stage dc/dc conversion. Although the modular string configurations for dc/ac versus dc/dc conversion are similar, the underlying power transfer mechanisms employed by each are significantly different. This paper compares the dc/ac and dc/dc energy conversion processes for series-cascaded submodules and highlights their key similarities and differences. Converter operation and control requirements for dc/ac versus dc/dc conversion are discussed and contrasted. Case study simulations performed in PLECS illustrate both energy conversion processes for a single submodule string.