Model Predictive Control for Modular Multilevel Converter with Reduced Switching Frequency Based on Improved Insertion Priority of the Sub-modules

When modular multilevel converters are used in medium and high voltage variable frequency drive systems, the switching frequency should be limited. Capacitor voltage balancing strategy based on insertion priority of the sub-modules (SMs) can effectively reduce the switching frequency. However, this strategy does not consider the switching frequency balance of the SMs. In this paper, a capacitor balancing strategy based on improved insertion priority of SMs is proposed. It is implemented based on model predictive control. The capacitor voltage and switching frequency balancing of SMs can be achieved. The strategy first simplifies the insertion priority. When the arm current is positive, only the upper limit of the allowable voltage distribution band is applied. When the arm current is negative, only the lower limit of the allowable voltage distribution band is applied. Then, the SMs within each priority level are sorted. Re-adjust the sequence of SMs with switching frequency that are too high or too low to keep their switching frequency as consistent as possible. The MATLAB/Simulink simulation results of MMC-PMSM show that the proposed strategy significantly reduces and balances the switching frequency of the SMs while ensures good control performance.