Simplified model and submodule capacitor voltage balancing of single‐phase AC/AC modular multilevel converter for railway traction purpose

A single‐phase AC/AC modular multilevel converter (MMC) can interact directly with 25 kV railway grid without a bulky 50 Hz step‐down transformer. This brings in great savings in size and cost. Submodule (SM) voltage balancing of this kind of MMCs remains a major technique issue. This study proposes a voltage‐balancing solution for this scenario which consists of intra‐ and inter‐arm voltage balancing methods. The former combines the advantages of carrier phase‐shifted pulse‐width modulation (PWM) and phase disposition PWM based voltage‐balancing methods. It only uses two proportional regulators, easing the control system significantly. The latter is based on a power channel between the upper and lower arms. It avoids interferences with input/output voltage and current, and gets rid of common mode current component which would be injected into the grid with conventional inter‐arm balancing methods. By assuming perfect voltage‐balancing, a simplified mathematical model is also developed, which reveals more clearly the power conversion relationship. Simulations and experiments verify the proposed voltage‐balancing methods and the mathematical model.