Modified-Modular Multilevel Converter With DC Boosting Capability and Capacitor Ripple Voltage Optimization by Regulating Arm-Currents

This paper proposes a modified-MMC configuration that works in conjunction with a novel Switching Function (SF) to enable DC side voltage boosting feature. Unlike in conventional-MMC, a new variable i.e. DC Side-Voltage Boosting Factor (DCS-VBF) is incorporated in the SF, which enables independent injection of DC and AC voltage in the arm voltage. This capability allows DC side voltage boosting without underutilizing the MMC components (switches and capacitors), thereby reducing Voltage Boosting Transformer (VBT) requirements and Submodule (SM) i.e switches and capacitor count. Furthermore, the SM-Capacitor Voltage Ripple (SM CVR) analyses are carried out. A multiple objectives problem is formulated considering SM-CVR analyses to achieve maximum peak SM-CVR suppression with minimum circulating currents and zero sequence voltages (ZSV) injection. The Pareto Optimality and Particle Swarm Optimization (PSO) together are utilized to obtain the Pareto fronts for optimal circulating currents/ZVS combinations. Based on these analyses, a range of DCS-VBF is suggested. The proposed peak CVR suppression scheme is capable of reducing the CVR upto 30%. The analysis also shows that upto 16% SM reduction is possible. Representative simulation and experimental results are presented to validate the claims.