Dynamic Voltage Balancing Algorithm for Modular Multilevel Converter with Three-Level Flying Capacitor Submodules

The modular multilevel converter (MMC) has several three-level flying capacitor (3L-FC) submodules in cascade. To balance the submodule capacitors voltage, this paper describes a dynamic voltage balancing algorithm based on the carrier phase shifted pulse width modulation (CPS-PWM) scheme. The submodules are controlled based on the instantaneous value of capacitor voltage and the direction of current. For the selection of the submodules, the maximum and minimum voltage submodule selection logics are designed. These voltage logics are generating an index number for each submodule based on the relative comparison of capacitors voltage. Finally the switching state of the submodules is generated by comparing the submodule index number with the dynamic reference index number. The performance of the proposed voltage balancing algorithm at different operating conditions is evaluated on 6kV/2MVA MMC system with the MATLAB simulation and the corresponding results are presented. In addition, the performance comparison of MMC with 3L-FC and conventional two-level half bridge (2L-HB) submodules is presented.