Research On Optimal Control Strategy Of Modular Multilevel Converter

At present,the new energy industry in China is developing rapidly,Modular multi-level Converter(Modular Multilevel Converter(MMC)has the advantages of very strong scalability or high modularity,so it is very common in many fields,and has become the focus of experts’ attention and research.The degree of application in the field is very common,and it has become a focus of attention and research by experts.However,in actual engineering,the two factors that determine whether the MMC-HVDC can operate well are the sub-module capacitance voltage balance of the MMC and the fault location of the sub-module.In the research of this paper,we first use MMC as the research entry point,analyze its related topological structure and operating principle,and carry out in-depth research on modulation strategy combined with related data models;derive the MMC control system based on dq rotating coordinate system of three-phase MMC Model and average model based on switching function.Enumerate four commonly used MMC modulation methods,and conduct a detailed analysis of these modulation methods to clarify the advantages and disadvantages of different methods.Secondly,although the traditional MMC capacitor voltage control strategy has certain effects in practical applications,the calculation process is more complicated and there are problems such as large switching losses.A linear time selection algorithm based on the sequence optimization of the capacitor voltage of the sub-module is proposed,which not only effectively reduces the complexity of the calculation link,but also significantly improves the calculation efficiency.With the help of Matlab/Simulink simulation platform,a 21-level MMC-HVDC simulation model was established to verify the reliability and effectiveness of the scheme.Finally,in order to ensure the reliable operation of the system,the proposed optimization control strategy is effectively implemented.Aiming at the problems of large number of system sub-modules and easy failure,a method based on fault detection and location is proposed to improve the reliability of the system.The Kalman Filter(KF)is applied to the MMC for fault detection,and the MMC fault can be detected by comparing the measured state value with the state value estimated by the KF.Analyzed the fault characteristics of the sub-module IGBT under the open circuit fault,and proposed a fault location method for the sub-module IGBT.This method can effectively and accurately locate the faulty sub-module IGBT even in the case of multiple sub-module IGBT failures.Ensure the reliable operation of the system.With the help of Matlab/Simulink simulation platform,a 21-level MMC-HVDC simulation model was established to verify the reliability and effectiveness of the scheme.