Hybrid HVDC Transmission Scheme Design Based On Cell-hybrid Modular Multilevel Converter

Hybrid DC transmission technology based on line commutated converter(LCC)and voltage source converter(VSC)combines the economic advantages of LCC with the technical advantages of VSC,and has gradually become a Research hotspots in the field of direct current transmission have broad prospects for development.From the perspective of upgrading and reforming the conventional DC engineering system,considering the flexible transformation of the existing conventional DC system,it is more economical and feasible to transform the receiving end converter station into a flexible DC converter station to form a hybrid DC transmission system.Ideas.Based on the analysis of conventional DC engineering,the paper summarizes the hybrid DC system-level topology and various converter topology schemes,summarizes the comprehensive characteristics of several types of VSC topologies,and applies the conventional DC transmission receiving end.Flexible transformation of VSC topology selection work.The half-bridge-full-bridge hybrid modular multilevel converter(MMC)topology and the number of submodules are described.A scheme for upgrading the original conventional DC system to a flexible DC converter station is proposed.The conventional soft-reverse converter valve is replaced by a half-bridge full-bridge hybrid MMC converter valve,which retains and utilizes the original Converter transformers reduce the cost of retrofitting.Firstly,for the case of conventional DC engineering transformation,the two main loop topologies of the LCC and the bipolar and symmetric bipolar MMC are analyzed and studied.The research work on various MMC topologies and their variants is carried out.and various types of MMC are comprehensively analyzed.The technical characteristics of the topology and the advantages and disadvantages of the economy,the targeted selection of the end station topology suitable for the conventional DC transmission flexibility transformation.Secondly,on the basis of determining the system main loop topology and the end station topology scheme,this paper introduces a topological structure and working principle of the end station topology scheme,half bridge full bridge hybrid MMC,and introduces the configuration method of the number of submodules.Thirdly,the research on the HVDC transmission system of ±500kV/3000MW class is carried out,and the main parameter design method of HVDC transmission system is introduced.Including main loop current calculation,sub-module quantity calculation,sub-module capacitance design,bridge arm reactance design,etc.Finally,using the overmodulation operation capability of the half-bridge-full-bridge hybrid MMC,a flexible transformation scheme for the conventional DC engineering receiving end station is proposed,and the main circuit parameters are designed for the proposed scheme.The results show that the proposed retrofit scheme can realize the flexible transformation of the original converter transformer of the retention system and significantly reduce the cost of system transformation.The simulation model is established in the PSCAD/EMTDC environment.The simulation results verify the feasibility of the scheme and the correctness of the main loop parameter design.