Theoretical analysis and control of the modular multilevel cascade converter based on double-star chopper-cells (MMCC-DSCC)

This paper presents the modular multilevel cascade converter based on double-star chopper-cells (MMCC-DSCC), which is intended for installation on the 6.6-kV Japanese industrial and utility distribution systems without using line-frequency transformers. The converter is characterized by an arm structure based on the module consisting of cascade connection of multiple bidirectional PWM chopper-cells and floating dc capacitors per arm. This arm structure requires voltage-balancing control for all the chopper-cells. However, the voltage control combining averaging- with individual-balancing controls imposes certain limitations on operating conditions. This paper proposes an arm-balancing control to achieve voltage balancing in all the operating conditions. The validity of the arm-balancing control as well as the theory developed in this paper is confirmed by computer simulation.