Research On A Bipolar Sub Module Topology And Its Application In Hybrid HVDC

Due to its advantages in voltage level and capacity expansion,the HVDC transmission system based on modular multilevel converters(MMC)has many advantages in new energy access,asynchronous grid interconnection,island power supply,and urban grid capacity expansion.It has broad application prospects,so it has gradually become a hot topic of research.Since the modular multilevel converter based on the half bridge sub module topology does not have the ability to block DC short circuit fault current,which affects the application of low-cost overhead lines,in order to achieve DC fault suppression,this paper studies a new type of MMC sub module topology,its working mode and control strategy are analyzed,aiming at reducing the number of components used,reducing the operating cost,and realizing the function of DC fault suppression.Firstly,on the basis of the existing MMC topology and mathematical model,the paper analyzes the characteristics of some sub module topologies in the existing research,a new crossing three level sub module topology is studied to solve the problem of unbalanced charging of hybrid topology capacitors.At the same time,in order to cope with the possible adverse effects of blocking suppression on the converter and the power grid,a positive and negative bipolar level output sub module topology is designed,after analysis of its over modulation,lower dc voltage operation and bridge arm current direction affect to the bipolar level output requirements,the number of power devices is further reduced and their control strategies are studied.Finally,the simulation model is built in PSCAD/EMTDC,and the DC fault suppression characteristics of the new cross three-level sub-module topology are simulated and verified.Secondly,after analyzing the hybrid DC transmission syst em topology and its mathematical model,the control strategy of the hybrid DC transmission system is studied,and then combined with the advantages of both LCC-HVDC and MMC-HVDC,based on the research on the new crossing bipolar three level sub module topology in the previous paper,a hybrid DC transmission system composed of LCC and MMC is constructed.Finally,the simulation model of LCC-MMC unipolar hybrid DC transmission system is built in PSCAD/EMTDC,and the parameter changes of the system under reduced DC voltage,DC fault suppression and power flow reversal operating conditions are simulated and analyzed.