Research On Control Strategy Based On C-MMC Modular Multilevel Converter

In modular multilevel converters(MMC),C-MMC has attracted more and more attention in the field of high voltage and direct current transmission(HVDC)with unique structural characteristics.By analyzing the mathematical model of C-MMC,the working principle of C-MMC is analyzed in detail,and the model is verified by Matlab/Simulink.Finally,an experimental prototype is designed to verify the C-MMC’s control method and modulation strategy.The main work of this thesis is as follows:First,the principle of general MMC is analyzed by using the mathematical model.At the same time,several commonly used sub-module structures are compared,and the working principle of the clamping double sub-module(CDSM)is analyzed in detail.Meanwhile,the C-MMC with system which is constructed with CDSM structure is modeled,and the mathematical relationship between the output voltage of C-MMC and the inserted sub-module is derived.Next,the detailed analysis of the modulation and control strategy of C-MMC is presented in this thesis.The advantages and disadvantages of various modulation methods,such as carrier-disposition pulse-width modulation are compared and analyzed,and the specific implementation method is given.Then two voltage sharing strategies in the capacitor voltage control of sub-modules are studied in detail.At the same time,through mathematical deduction,the formation and circulation characteristics of the circulating current in C-MMC are analyzed in detail.In view of the characteristics of the circulating current,two kinds of suppression methods are introduced: the three-phase coordinate transformation method and the quasi-proportional resonance control method.Finally,an adaptive improved quasi-proportional resonant controller is proposed,so that the suppression effect of the circulating current can be kept unchanged after the frequency change of the power grid.Then the control strategy of C-MMC-HVDC is analyzed in detail.The inner loop structure and outer loop structure of C-MMC-HVDC system are given in detail,and the effectiveness of inner loop controller and outer loop controller in C-MMC-HVDC system is verified by simulation.At the same time,according to the characteristics of C-MMC,a detailed analysis of DC fault blocking characteristics is given,therefore,the process of DC fault ride through can be realized by using C-MMC it can be concluded that the fault current cannot be recovered in C-MMC after the DC fault current blocking.At last,a prototype of C-MMC was built.The design process of the distributed controller and the design principle of the main hardware circuit are introduced,and the functions and tasks of the control part in the main controller DSP and the auxiliary controller FPGA are given in detail.Finally,the control method and modulation strategy which are mentioned in this thesis,as well as the proposed adaptive quasiproportional resonant controller,are verified on the experimental platform.