Evaluation for overall volume of capacitor and heat-sink in step-down rectifier using modular multilevel converter

This paper discusses a volume evaluation of a step-down rectifier using a Modular Multilevel Converter (MMC) for a power system connected to a utility grid of 6.6 kV in order to obtain the high power density. Specifically, this paper evaluates a relationship among an output voltage of the MMC, a volume of capacitors and a volume of a heat-sink. At first, formulas of semiconductor losses are clarified. Second, the relationship between the ripple current and the volume of electrolytic capacitors is considered. In addition, the relationship between the number of cells per leg and an overall volume is also shown. According to these considerations, the evaluation result of the overall volume shows that it is necessary to reduce a step-down ratio of the MMC in order to obtain the high power density. Moreover, it is also necessary to approach a withstand voltage ratio to 1.0 when the withstand voltage ratio is defined as the ratio of the required withstand voltage to an actual withstand voltage with series connection capacitors. In conclusion, the volume of the proposed system becomes approximately 20% of the volume of a conventional power system of 200 kVA connected to the utility grid of 6.6 kV.