High Power Density Design for a Modular Multilevel Converter With an H-Bridge Cell Based on a Volume Evaluation of Each Component

This paper presents a high power density design for a step-down rectifier incorporating a modular multilevel converter (MMC) in a power system connected to a 6.6 kV ac grid. In particular, the relationship among the number of cells, the output voltage of the MMC, and the overall volume is clarified. A proposed design flowchart focuses on minimizing volumes of cell capacitors, heat sinks, and arm inductors by using optimal number of cells. Moreover, the commercial electrolytic capacitors are used as the cell capacitor. Besides, the formulae used to determine the ripple currents in the electrolytic capacitor and the arm inductor are presented, along with that for the semiconductor loss. Each of these formulae were verified experimentally using a miniature model and theoretical values from all formulae agree with the measured values within minimal deviations that are discussed with regard to the design of a practical converter for a 6.6 kV system. Finally, the conditions necessary to achieve high power density in an MMC are provided, based on volume evaluations of the electrolytic capacitor, the arm inductor, and the heat sink. Using this optimization process, an MMC design achieved a volume reduction of approximately 90% compared to a conventional system.