Theoretical Analysis and Control of the Modular Multilevel Cascade Converter Based on Double-Star Chopper-Cells (MMCC-DSCC)

This paper presents a modular multilevel cascade converter based on double-star chopper-cells (MMCC-DSCC), which is intended for grid connection to medium-voltage power systems without the use of line-frequency transformers. The converter is characterized by a modular arm structure consisting of cascade connection of multiple bidirectional PWM chopper-cells with floating dc capacitors. It is necessary to achieve voltage-balancing control of all the dc capacitors. However, the voltage control achieved by averaging control and individual-balancing control imposes certain limitations on the operating conditions. This paper proposes arm-balancing control to achieve voltage balancing under all the operating conditions. The validity of the arm-balancing control as well as the theory developed in this paper is shown by performing a computer simulation and experiment.