The Research On VSC-HVDC Based On NPC Five-Level Converter

VSC-HVDC greatly extends the range of application of HVDC, opens up vast areas of application for HVDC, and has been a highlight of transmission and distribution field for the excellent characteristics itself.However, there are many application questions needing to be further studied for no practical VSC-HVDC project operating in our country at present. Moreover, Most studies are VSC-HVDC based on the three-level or two levels converter, the research of VSC-HVDC based on the five-level converter is few.Therefore, this paper selects VSC-HVDC based on NPC(neutral point clamped) five-level to do study and simulation, and particular presents its basic principle, control strategies, the design of control system etc. The main research production is shown as follows.Firstly, further research on the topology structure and the modulation principle of multi-level voltage source converter is done in this paper. Aiming at the characteristics of multi-level voltage source converter with different topology structure, this paper adopts appropriate modulation methods, and separately does simulation for five-level voltage source converter with different topology structure by using matlab/simulink soft. The simulation results show that the modulation methods are correct for five-level voltage source converter with different topology structure.Secondly, the mathematical models of VSC-HVDC based on NPC five-level converter are derived in abc still reference frame and d-q reference frame, and based on d-q reference frame, a dynamic equivalent circuit of VSC-HVDC is founded. The dynamic performances of VSC-HVDC are easily gained through the dynamic equivalent circuit, and also provide quite convenience for the design of controllers.Finally, the structure framework of VSC-HVDC control system is built in this paper, analyzes control principle and modes of all the hierarchies,and designs the inner current decoupling controller and the out voltage controller for five-level VSC-HVDC in d-q reference frame. what's more, the simulation model of supplying to active network is built for VSC-HVDC by using matlab/simulink soft. The simulation results indicate that PI controllers are accorded with anticipant target.