Research On Control Strategies Of Modular Multilevel Converter

Modular multilevel converter(MMC)has the advantages of modularization,excellent voltage/power expansion capability,low output harmonic content,good redundancy and flexible front-end configuration,which make it a good application in the flexible DC transmission,electric drive,high-voltage DC power conversion,management of power quality problem,offshore wind power and other renewable resources and industrial fields.After years of development,MMC has been applied in a number of practical projects,and related technology research has made great progress,but there are still some problems like modulation,submodular voltage balance,circulating current restraining need to be further studied,the work done in this thesis is as follows:Taking MMC as the research object,based on RT-LAB,a real-time simulation model of MMC is built,which could improve the simulation speed and the reliability of the results,then the choice of the main parameters in the model was analyzed.Refer to the traditional capacitor voltage balanced control strategy,this thesis proposed a replacement method,using the integral amounts instead of voltages to sort,which could realize the method of submodules voltage sorting with no sensor;according to the maximum voltage deviation among submodules,an improvement strategy was put forward;based on the principle,the resonant circuit and controller was established to suppress the circulating current;and a double closed loop control system was designed to improve the stability of the system.The simulation results presented the output voltage waveform with low harmonic content based on the current integral substitution method,which validated the feasibility of the proposed replacement method;after introducing the maximum voltage deviation between submodules,the simulation results showed that the submodule capacitor voltages in the same bridge arm were maintained at the pre-set value,the introducing of resonant circuit effectively suppressed the circulating current;and the simulation results based on the double closed loop control system showed the good tracking and stability performance,which could have a positive effect on the capacitor voltage balanced control and the suppression of the circulating current.