Study On Control Strategy Of Hybrid DC System Based On LCC-MMC

The energy resources are mainly distributed in Northwest China,while it is mainly consumed by the eastern coastal areas.According to the characteristics of energy and load distribution in China,the single flow direction is a significant characteristic of the large-capacity and long-distance high voltage direct current(HVDC)project in China.As to the HVDC project with a single power flow direction,the line commutated converter(LCC)will be used in the rectifier of hybrid direct current transmission system and the modular multilevel converter(MMC)will be used in the inverter.The line commutated converter-modular multilevel converter(LCC-MMC)technology combines the advantages of two kinds of DC transmission technology,high flexibility and low cost.As for its good application prospects in the areas of renewable energy convergence,asynchronous grid interconnection and long distance Island transmission,it has already become a spark project all over the world.So the following topics will be discussed in the article.Firstly,the two types of HVDC technology will be discussed.Because of the existing problems of the technology above,the hybrid DC power transmission technology is introduced.It introduces the research status of MMC,and the topology structure of Hybrid DC transmission is analyzed in this paper.Secondly,the structure of the sub module is analyzed.Based on the analysis of the working principle,the mathematical model of MMC is established.And its operating characteristics are explained.The carrier phase shift and the nearest level modulation strategy are introduced in this paper.Combined with the characteristics of the MMC level is not high,the carrier phase shift modulation strategy is selected.The MMC voltage modulation wave is given.And the simulation results are analyzed.Thirdly,the main topological structure of LCC-MMC is analyzed.And the control strategy of the system is designed.The formation mechanism of the MMC internal circulation will be researched.The circulation inhibition of dq coordinate system controller is designed at the same time.A sub module of the voltage balancing controller will be designed to stabilize the capacitor voltage within a certain range.The traditional carrier phase-shifting modulation strategy applied in the MMC method is improved,which makes the actual value track the reference value.The inhibition of the circulation size of MMC will be competed.Lastly,a direct current transmission system is built in PSCAD/EMTDC software.The LCC-MMC control strategy will be carried on the simulation research.The results show that the hybrid DC transmission system and its control strategy are correct.Circulation is also significantly suppressed.Finally,AC fault of the inverter is simulated.The system can return to stable state quickly when the fault was cancelled.It has a strong ability to clean up the fault.