Optimal control of modular multilevel converters for low-speed operation of motor drives

One of the main technical challenges of the Modular Multilevel Converter (MMC) for medium-voltage adjustable-speed motor drive applications is the large magnitude of the SubModule (SM) capacitor voltage ripple under low-speed operation. This paper proposes two new techniques to reduce the magnitude of SM capacitor voltage ripple of the MMC for motor drive applications. The paper presents a comprehensive analysis of the proposed techniques and provides a comparison between the proposed and the existing techniques. Furthermore, the paper formulates an optimization problem to establish a tradeoff between the performance indices and, subsequently, to find the Pareto-optimal solutions. The effectiveness and performance of the proposed techniques are verified by simulation studies in the PSCAD/EMTDC environment.