Research On Control Technology Of Modular Multilevel Converter

Modular multilevel converter(MMC),as a topology applied in the medium and high voltage field,is receiving more and more attention in industry and academia.MMC is highly modular,voltage-scalable,multi-level voltage output,fault-tolerant,and four-quadrant stable operation.It is widely used in renewable energy access,island power supply,grid interconnection and power quality management.This thesis combines simulation and experiment to study the control strategy of MMC.Based on the recent widely used modulation strategy,a hybrid modulation is proposed,and a voltage balancing strategy based on power frequency is Proposed The specific content of this thesis includes the following aspects:First of all,this thesis studies the working mechanism,control principle and dynamic model of MMC.The mathematical model of MMC is established.The relationship between the capacitor voltage fluctuation and the DC side current and the circulating current is explained from the power point of view.The influencing factors of the capacitor voltage fluctuation are analyzed.The circulating current influences the capacitor voltage fluctuation and the bridge arm current.Secondly,combining the advantages of both the nearest level modulation strategy and the carrier phase shift modulation strategy,a hybrid modulation strategy based on the nearest level modulation(HM-NLM)is proposed.The principle and implementation scheme of HM-NLM strategy are introduced.The strategy is compared based on Matlab/Simulink simulation platform,which proves the feasibility of HM-NLM.Thirdly,this thesis proposes a power frequency balancing strategy.By judging the charging and discharging capacity of the sub-module capacitors in each power frequency cycle of the driving signal,re-establishment the correspondence between the sub-module and the driving signal is rotated every power frequency cycle to achieve the purpose of balancing the voltage of the sub-module.Compared with the traditional voltage balancing strategy,this voltage balancing strategy does not need to monitor the bridge arm current.The frequency based system frequency reduces the calculation amount of the system,and the voltage balancing strategy is verified based on the Matlab/Simulink simulation platform.Finally,an experimental study on modulation strategy and voltage balancing strategy is carried out.The recent level approximation modulation strategy and HN'NLM strategy are compared in the experimental platform,and the effectiveness of the voltage balancing strategy is verified.The experimental results prove the correctness and effectiveness of the theoretical analysis and power frequency balancing strategy.