A Comparison of DC/AC and DC/DC Modular Multilevel Energy Conversion Processes

Abstract

Using strings of cascaded submodules for multilevel dc/ac conversion has attracted considerable attention owing to the widespread popularity of the modular multilevel converter (MMC). Recently, it has been shown that such conventional submodule strings can be adapted to achieve single-stage dc/dc conversion. Although the modular string configurations for dc/ac and dc/dc conversion are similar, the underlying power transfer mechanisms employed by each differ significantly. This paper compares the dc/ac and dc/dc energy conversion processes for series-cascaded submodules and highlights their key similarities and differences. In addition, converter operation and control requirements for dc/ac and dc/dc conversion are discussed and compared. A capacitor voltage regulation scheme for dc/dc conversion is proposed, whereby vars generation can be arbitrarily allocated between the converter arms in order to maximize conversion efficiency. It is shown that single-stage dc/dc conversion can reduce converter cost and operating losses by up to 50% as compared to conventional cascaded dc/ac stages, i.e. dc/ac-ac/dc conversion.