Control for Efficiency of Modular Multilevel Converter with Variable-Rotation-Frequency Phase Disposition Modulation

The phase disposition (PD) pulse width modulation (PWM) method is one of the most commonly used modulation techniques for modular multilevel converter (MMC). However, the different charging and discharging times will cause unbalanced submodule-capacitor voltage when it is applied to MMC. To lighten the unbalance, the carrier wave rotation has been brought into use and proved to be effective. However, it is difficult to give attention to both voltage balancing and the power loss of the MMC, where both of them are key factors during the operation. In this paper, the tradeoff between the unbalanced capacitor voltage of submodules and the semiconductor power loss is optimized by the proposed method, which could automatically adjust the rotation frequency based on the voltage difference and the semiconductor power loss of each MMC submodule. The effectiveness of the proposed method has been confirmed by simulation and experimental results.