Research On Multi-carrier Modulation Strategies And Capacitor Voltage Balancing Technology Of Modular Multilevel Converter

Modular multilevel converters(MMC)is one of the most promising topologies in medium/high power/voltage applications and have emerged in flexible HVDC transmissions.In this paper,the topology and mathematical model of modular multilevel converter are introduced in detail.The modulation strategy and capacitor voltage balance technology are analyzed and comprehensively studied.MMC's modulation strategy is one of its core technologies.Studying the modulation strategy has important implications for the normal operation of MMC.In this paper,the multi-carrier modulation strategy--carrier phase-shifting-based modulation strategy and carrier-cascade-based modulation strategy are studied,and the existing sub-module capacitor voltage balance control method and its two modulation methods are compared and analyzed.Inadequacies.Capacitor voltage balance is an effective guarantee for the normal operation of the MMC.In this regard,the carrier rotation technology is applied to the multi-carrier modulation strategy of MMC to solve the problem of balance of capacitor voltage of each sub-module.Aiming at the problem of additional switching action caused by carrier rotation,the corresponding optimal rotation time algorithm is proposed,which effectively reduces the number of additional switching operations and avoids additional switching losses.The effectiveness of the proposed method is verified by simulation.In order to verify the effectiveness of the proposed method,this paper builds an MMC physical platform with three sub-modules per phase.The design details related to the platform are introduced,and the specific software design details are introduced for different modulation strategies and their phase shifting methods.The effectiveness of the rotation strategy is verified by comparison between simulation and experiment.