Study On Control Strategy And Fault Diagnosis Of Modular Multilevel Converter

The shortage of energy supply is a severe test for the current social development,and people’s demands on the power industry are increasing day by day.Modular multilevel converter(MMC)has been widely used in flexible HVDC,power transmission,high-voltage DC/DC power conversion and other medium and high-voltage high-power applications by virtue of its outstanding advantages of high modularity,easy realization and good harmonic characteristicsIn this paper,the application of MMC in inverter is the main research direction,and the modulation strategy,capacitor voltage balance strategy and fault diagnosis method of modular multilevel converter are studied.Based on the analysis of its topology,working mechanism and mathematical modeling,different control strategies and methods of open fault diagnosis of sub-modules are discussed emphatically,and the simulation verification and result analysis are carried out.The main work of this paper is as follows:Firstly,the structure of MMC and the working mode of sub-modules are introduced,and the working principle of MMC is further expounded by taking the five-level topology as an example.The circuit topology is simplified and equivalent appropriately,the capacitor-voltage equation and the AC-DC side equation based on the switch function are established.The single equivalent circuit diagram and the AC-DC side equivalent diagram are drawn,and the corresponding relations among the electrical parameters in the equivalent diagram are analyzed in detail.This paper studies the important parameter selection in MMC main circuit,and gives the basis for determining the value of each parameter.On this basis,the main circuit simulation model based on MATLAB/Simulink is established.Secondly,the common modulation method of MMC and the method of capacitor voltage balance control are described.This paper focuses on the analysis and design of two control strategies:the layered control strategy based on carrier phase shift and the unified control strategy based on capacitor voltage sort,including the control process,the principle block diagram and the functions of each part.The simulation model of layered control strategy based on carrier phase shift is built on MATLAB/Simulink platform.The simulation results show that the capacitor voltage can be kept near the rated value by adopting this control strategy,and the output harmonic content and overall fluctuation coefficient are significantly reduced compared to before this method.In addition,the N+1 level mode and the 2N+1 level mode are compared and analyzed.The experimental results show that the equivalent switching frequency is proportional to the number of sub-modules in each bridge arm.Compared with the N+1 level mode,the 2N+1 level mode can significantly increase the number of output levels and reduce the harmonic content of output voltage.However,the circulation and the voltage drop of bridge inductance will increase obviously in this mode.Finally,a sliding mode observer is designed for the open-circuit fault of sub-module IGBT with the bridge arm current as the observation variable.The residual difference between the observed value and the measured value of the bridge arm current is compared to detect the occurrence of the fault.The fault sub-module is located according to the fault characteristic value calculated by capacitor voltage and bridge arm current in each control period.Based on MATLAB/Simulink,the fault model of sub-module is built,which verifies that the proposed fault diagnosis method can quickly and accurately detect the occurrence of sub-module open-fault and identify the location of fault.