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.

Review of MVDC Applications, Technologies, and Future Prospects

“…Multilevel modular DC-DC converters can be implemented with an AC link between the DC-DC input and output, which is also the case with the DAB MMC or with a direct DC-DC architecture, as illustrated in Figures 16 and 17, respectively, where the symbol SM signifies the submodule. A multilevel architecture allows for higher voltage gains for connections to distribution systems [27,149]. DAB MMCs can handle high power and DC voltage and have DC fault-blocking capability, with isolation benefits [150].…”