Research On Space Vector Modulation And Capacitor Voltage Equalization Control Strategy For Modular Multilevel Converter

With the rapid improvement of China’s economic strength,the construction of high voltage and large capacity power network system,the development of large-scale and longdistance transmission,the improvement of power transmission capacity,the enhancement of power supply reliability,and the improvement of power quality,it is necessary to improve the transformation voltage level of power electronic devices.Modular multilevel converter is widely used in the field of flexible DC transmission because of its modularity,redundancy and good output waveform quality.With the continuous improvement of voltage level,the number of converter sub modules will increase greatly,which not only makes the system add a lot of hardware,but also consumes a lot of computing resources.The modulation complexity increases with the increase of MMC sub modules,and it is difficult to realize the capacitor voltage equalization control of converter sub modules.Firstly,this paper introduces the main circuit topology,sub module topology,working principle and common modulation strategy principle of the converter.Aiming at the problem that the traditional space vector modulation strategy is difficult to extend to high-level MMC system,this paper proposes to apply the optimized space vector modulation strategy based on rotating coordinates to modular multilevel converter,which simplifies the calculation amount of traditional modulation strategy and improves the efficiency of traditional modulation strategy The output characteristics of MMC are improved,so that it can be extended to any level MMC system.Then,the causes of capacitor voltage imbalance are analyzed,and an optimized capacitor voltage balancing control strategy based on bucket sorting method is proposed,which solves the problems of large scheduling operation and high switching frequency of traditional capacitor voltage balancing control strategy based on bubble sorting method.Finally,the capacitor voltage equalization control strategy based on phase shifted space vector modulation is studied.The three-phase bridge arm three sub modules are combined to form the control unit.The two-level space vector modulation strategy is used for a single control unit,and then the modulation strategy is extended to the whole MMC system by phase-shifting.The output characteristics of different control units on the same side of the bridge arm are equivalent,and the rotation control is adopted for all control units to ensure that the operation conditions of all sub modules are consistent in a rotation cycle,and finally realize the capacitor voltage balance control.The three-phase MMC experimental platform is established,and the combined control flow of DSP and FPGA is designed,and the phaseshifting space vector modulation strategy based on rotating coordinates and the capacitor voltage equalization control strategy are verified by experiments.