Research On A New Type Of Three-level Hybrid Modular Multilevel Converter

Voltage Source Converter based High Voltage Direct Current Transmission(VSC-HVDC)is the key technology for the future construction of smart grid and energy internet.The advantage is that there will be no commutation failure,active and reactive power can be controlled independently,and reactive power compensation is not required.VSC-HVDC mainly has two forms: two-level VSC-HVDC and modular multi-level VSC-HVDC,and due to the low loss of modular multilevel converter(MMC),low output voltage harmonic content,Modularity and easy expansion,making modular multilevel VSC-HVDC have great research and application value.However,the capacitance value of the sub-module of traditional MMC is very large,and the existence of large capacitance causes the volume and cost of the MMC to be high,which increases the difficulty of practical application to a certain extent.This paper proposes a new quasi three-level hybrid MMC.In this new quasi three-level hybrid MMC,the sub-module capacitor only plays a role in supporting voltage,so that the large capacitor of the sub-module can be replaced with a small capacitor,which can effectively reduce the volume of sub-modules,and fundamentally optimize the volume and cost of MMC.Firstly,the topology of the proposed quasi three-level hybrid MMC is discussed.On this basis,the operating modes of the half-bridge submodule and full-bridge submodule in the MMC topology are discussed.The operating principles of the upper arm,the lower arm and the clamping bridge arm are analyzed in detail,and a mathematical model is established based on the operating principle of the new type of three-level hybrid MMC.Secondly,the quasi three-level hybrid MMC's multilevel modulation method and capacitor voltage balance strategy are analyzed in detail.with the help of multilevel modulation method and capacitor voltage balance strategy,to achieve the effective,safe and stable operation of the quasi three-level hybrid MMC.And used Matlab/simulink software to build the quasi three-level hybrid MMC's simulation model,through simulation to verify the feasibility of the quasi three-level hybrid MMC.Thirdly,an LCL filter was designed to reduce the harmonic content of the quasi three-level hybrid MMC as a inverter to meet the standards specified by the industry.Finally,the sub-module hardware is designed,the software programming of the sub-module is completed and the basic test is conducted.