Application Of Soft Switch Technology In MMC Converter

With the development of semiconductor and power electronics,modular multilevel converter?MMC?has become a new generation of power electronic converter suitable for medium high voltage and high power applications.The converter based on MMC converter structure is more suitable for DC transmission.However,the number of neutron modules of each arm of MMC converter is increasing with the simultaneous interpreting of power and voltage levels.In view of this phenomenon,this paper proposes a method of applying soft switch technology in MMC converter.By using soft switch technology,the switching loss of the switch tube is reduced,so as to effectively reduce the power loss of the converter,reduce electromagnetic interference and save economic cost.The main contents of this paper are as follows:Firstly,this paper analyzes the topology and working principle of MMC converter,establishes the sub module?SM?model of MMC converter for simulation,designs the basic parameters of MMC converter circuit as the simulation basis,and puts forward the problem of switching loss in the switch tube by using the simulation results.Secondly,aiming at the problem of switching loss in the process of switching,this paper analyzes the related principles and requirements of soft switch technology in detail,and deeply studies the LLC?Logical Link Control,LLC?resonant half bridge circuit and simulation analysis are carried out.Based on this,soft switch technology can be applied to MMC converter,and a new topology and operation scheme of applying soft switch technology to MMC is proposed.By connecting the bridge arm inductance L₀ to each sub module respectively,the inductance in the sub module is recorded as Lm,and two capacitors CP1 and CP2 are introduced to each sub module respectively In the sub module,two power switches T₁ and T₂ are connected in parallel to the inductor Lm,which leads to the resonance process,and realizes the soft switching process of MMC's switches.Thirdly,the simulation software is used to compare the switching process waveforms of the sub modules of MMC converter with and without the new soft switch topology,and the reliability of the proposed soft switch operation scheme is verified by simulation.The simulation results are consistent with the theoretical analysis,and the soft switch technology is used to reduce the number of sub modules in the new MMC converter topology The switch loss of the switch tube can improve the working efficiency of the circuit and realize the optimal system efficiency.Fourthly,the simulation analysis and parameter design of 21-level MMC are carried out by using MATLAB/Simulink simulation software.The modulation strategy and capacitor voltage control strategy of MMC converter with soft switching process and the selection of parameters of parallel capacitors C_P11 and C_P22 are analyzed.The simulation results verify the feasibility of the proposed topology,reduce the switching loss of MMC converter,and improve the system efficiency of MMC converter.