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Evaluation of Required Energy Storage in Neutral-Point-Clamped Modular Multilevel Converter for Downsizing Low-Voltage Grid Converters

This article clarifies a required energy storage in a neutral-point-clamped modular multilevel converter (NPC-MMC) based on a theoretical analysis and an experimental verification to enable quantitative comparison of the size of the capacitors. The NPC-MMC consists of both a three-level structure with common dc capacitors and chopper cells. It can reduce the number of the chopper cells by half comparing to the conventional MMC. The theoretical analysis reveals that the NPC-MMC directly transfers a part of its power from the dc capacitors to the ac grid, whereas the conventional MMC transfers all of its power by way of the cells. Therefore, the NPC-MMC can reduce its required energy storage by almost half comparing to the MMC. Moreover, the NPC-MMC can further reduce its energy storage by injecting harmonic currents to its circulating current despite still smaller arm currents than the MMC. A 200-V 10-kVA experimental circuit using low-voltage MOSFETs validates the analysis and demonstrates that the NPC-MMC can achieve both downsizing and high efficiency.