Theoretical Analysis of Power Flow and Stored Energy in the Capacitor of Neutral-Point-Clamped Modular Multilevel Converter

This paper describes theoretical analysis of a power flow and a total stored energy in the capacitor of a neutral-point-clamped modular multilevel converter (NPC-MMC). The NPC-MMC consists of both a three-level structure with common DC capacitors and chopper cells. It requires only half of chopper cells compared to the conventional MMC. In this paper, at first, theoretical analysis reveals the NPC-MMC can supply the AC power from not only the cells but also common DC capacitors. The direct power flow between the DC capacitors and the AC side brings 48% reduction of the total stored energy in the cell capacitors and the DC capacitors to the conventional MMC. Secondary, the 10-kVA experimental circuit using low-voltage MOSFETs validates the capacitor energy analysis. Measured efficiency reaches extremely high value of 99.3%. Therefore, it requires neither heat sink nor cooling fans. These experimental results show that the NPC-MMC can realize both downsizing and very high efficiency conversion.