Modeling and control of the modular multilevel cascade converter based on chopper-cells

This paper presents the modular multilevel cascade converter (MMCC) based on chopper-cells suitable to medium and high voltage systems without line-frequency transformers. This converter is characterized by a modular arm structure composed by cascaded chopper-cells with floating dc capacitors. The arm structure requires average voltage control and voltage-balancing control of the dc capacitors. This paper proposes a continuous model and a control method to achieve rigid and stable voltage control of all the floating dc capacitors under common operating conditions. The validity of the theory is confirmed by computer simulation and experiment.