Inherent Switching Reallocated CPS-PWM With Improved Voltage Balancing for MMC

Modular multilevel converter is an attractive solution in medium or high voltage applications, and voltage balancing is one of the most significant technical problems especially for CPS-PWM based MMCs. To solve this problem, in this paper, an inherent switching reallocated CPS-PWM is proposed for MMC. Based on arm current directions, switching states of sub-modules (SMs), voltage sorting and carrier sorting results, proposed method divides SMs and carriers into inserting and bypassing groups for each, enabling switching reallocation within these four co-optimized groups to inherently balance capacitor voltages. This effectively prevents additional switchings. Besides, most computations of the proposed CPS-PWM are logical operations implemented in FPGA, so proposed method can achieve reallocation frequency equal to sampling frequency. Unlike other CPS-PWM based methods for voltage balancing, the inherent switching reallocated CPS-PWM not only has superior converge speed in dynamic processes which accommodate a wide power range, but also has excellent balancing accuracy especially considering capacitance differences. Owing to appropriate reallocation principles, the proposed CPS-PWM has lower switching loss compared to existing carrier reallocation methods. Finally, the feasibility and superiority of the proposed method are validated by simulation and experiment results.