Abstract: The capacitor voltage balance of modular multilevel converter is the key to its stable operation, aiming at problem of large calculation amount and high switching frequency existing in traditional capacitor voltage balancing strategy. This paper proposes an optimized balancing control strategy of the capacitor voltage based on the comparison of variable reference values. The number of current input modules is calculated by the nearest level modulation, then the reference value is adjusted according to the number of input modules, and based on this reference value, the sub-module capacitor voltage sequences are divided into two groups. At the same time, the random scramble algorithm is used to sort the elements in the group. According to the direction of the bridge arm current, the sequencing order of the groups is determined, and the information of the input module is finally determined. Meanwhile, the permissible deviation threshold of capacitor voltage and the regulatory factor are selected by the optimization function of the capacitor voltage to achieve optimal control of the device's average switching frequency and capacitor voltage consistency. A simulation model of three-phase inverter based on MMC is set up in MATLAB/Simulink to verify the proposed strategy can significantly reduce the amount of calculations and the average switching frequency of the device under the premise of achieving a good effect of capacitor voltage balancing.

Key words: modular multi-level converter, voltage balance of capacitance, recent level approach modulation, comparison of variable reference value, switching frequency