Research On Hybrid Flying Cascaded Multilevel DC/DC Converter

HVDC transmission technology has broad prospects in long-distance transmission and renewable energy access.By constructing DC grid,it can better realize power transmission and improve the reliability of power supply.Nowadays,a large number of HVDC transmission projects have been built around the world.As the development of DC grids has taken shape,the voltage levels of DC transmission lines are quite different.In the DC grid,there is a lack of highvoltage and large-capacity transformer devices like AC transformers in the AC grid to realize interconnection of different voltage levels.Therefore,with the development and application of DC grid technology,research on DC/DC converters will become more and more important.Aiming at the shortcomings of the existing research,this paper deeply studies the Hybrid Flying Cascaded DC/DC Converter(HFC),and the circuit structure of the converter topology.Comprehensive analysis of operating principles,parameter design,modulation strategies and control methods are studied.And compared with the existing typical non-isolated topology such as the autotransformer converter and the direct coupled converter are theoretically analyzed.The modular multi-level DC/DC converter topology with the modular idea as the core design generally adopts the traditional Half Bridge Submodule(HBSM)or Full Bridge Submodule(FBSM)structure.The traditional submodule structure was modified to propose a new three-switch submodule(TSSM).Through simulation experiments,the loss of TSSM and HBSM in HFC topology application is analyzed and compared.Aiming at the capacitor parallel problem existing in the application of TSSM,a buffer circuit for two different switching devices(IGBT,IGCT)are proposed.The correctness of the design is verified by simulation experiments.In this paper,the proposed TSSM is applied to the HFC converter,and the theoretical analysis of the parameter range,device stress,number of submodules,submodule capacitance and bridge arm inductance of the HFC topology based on TSSM is theoretically analyzed.At the same time,a small-capacity HFC converter experimental prototype with two-level control framework was built.The experimental verification on the experimental prototype further verified the correctness of the studied HFC topology and TSSM in parameter design and control strategy.