Modulation And Circulation Suppression Strategy Of Modular Multilevel Converters

Modular Multilevel Converter(MMC)is a new converter topology in the field of flexible DC power transmission.Compared with traditional converters,MMC has a higher degree of modularity,can be extended to a high voltage level,has better controllability,lower switching losses,and higher output power quality.And MMC does not have the withstand voltage problem caused by the direct series connection of IGBT devices in the voltage source converter(VSC)structure.However,due to its special topology,there are still problems such as capacitor voltage balance of submodules and circulating currents between bridge arms.Therefore,this thesis focuses on the three aspects of MMC system modulation,sub-module equalization control,and MMC circulation suppression.This thesis first analyzes the topology of MMC and the operation mode at work,and studies the topology and operation mode of a single sub-module.Based on this,the mathematical model of MMC is established,the equivalent circuit is drawn,and the method of selecting the core components in MMC was researched,and then the output characteristics and internal characteristics of MMC were further analyzed through simulation.Based on the established MMC mathematical model,the existing modulation strategies analyzed and summarized,and a modulation strategy based on two-stage model predictive control is proposed.This strategy can reduce the capacitor voltage fluctuation and switching times of the sub-module on the basis of satisfying the AC-side output current following reference value,and has a smaller calculation complexity than the traditional model predictive control.Finally,the circulation generation mechanism and circulation characteristics of MMC were analyzed and verified.A circulation-related model was established.By comparing the existing circulation suppression strategies,the advantages and disadvantages were summarized.Based on this,a new circulation suppression method was proposed.An improved repetitive controller is adopted.This method can make up for the problems of complex control structure and poor circulation suppression effect of the existing circulation suppression strategies.And through Matlab/Simulink simulation,respectively,the effectiveness and superiority of the proposed modulation strategy and circulating current suppression strategy are verified.By using the modulation strategy and circulating current suppression method proposed in this thesis,it can have better performance on the output characteristics of the AC side,and has less computational complexity,and the circulating current suppression has a better suppression effect than the existing methods.It is especially effective for high frequency harmonic suppression.