Power Flow Analysis and DC-Capacitor Voltage Regulation for the MMCC-DSCC

This paper discusses the power flow analysis and dc-capacitor voltage regulation for the modular multilevel cascaded converter (MMCC) based on double-star chopper cells (DSCC). This analysis reveals the relationship between the circulating current through the dc link and the mean power flowing out of or into each chopper cell. The dc component on the circulating current supplies a half of the active output power to the upper and lower strings of chopper cells, whereas the fundamental-frequency component transfers an amount of power between the upper and lower strings. Thus, the MMCC-DSCC can balance the capacitor voltages among the cells by controlling the amplitude of the fundamental-frequency component on the circulating current. A new control method based on the power flow analysis is employed, and its effectiveness is verified by circuit simulation.