Research The Control Strategy Of VSC-HVDC Based On SVPWM

In recent years, in order to deal with the energy crisis and environmental problemsduring the process of power development, new energy technology (such as: wind powergeneration, photovoltaic generation, etc.) achieved an unprecedented rapid development.However, because of the characteristic of inheerent dispersion, small structure, far fromthe load centers and so on, its development has been restricted. At the same time, thestability and economy of power transmission can not be guaranteed by the traditionalway—AC power transmission. Along with the development of power electronicstechnology, DC transmission attracts people’s attention again and has been playing animportant role in the stable operation of the modern large power grids. Compared to ACtransmission, DC transmission has great advantages at long-distance large-capacitytransmission with its voltage over220kV, sea-ground cable transmission and so on. Butthe tranditional HVDC transmission is also unable to supply power to weak AC system,as well as passive network, at the same time, the inverter of HVDC generates muchmore low-order harmonics and absorb more reactive power. Therefore, the traditionalHVDC transmission are subject to certain limitations at its applications and can’teffectively solve the problem of the new energy transmission.Since the1990s, VSC-HVDC projects, which is based on full-controlled powersemiconductor devices and PWM, has been appearing gradually around the world. Thenew transmission technology not only has the advantages of the traditional HVDC, butalso can be connected to decentralized power, supplying power to the passive network,independentli adjusting the active power and reactive power and improving the powerquality. Sice the1990s, VSC-HVDC, with its unique advantages, has become thedevelopment trend of the trend of the future grid and attracted more and more domesticand foreign scholars engaged in this subject. Therefore, it has a profound theoretical andpractical significance to research the probloms of operation and control strategy.In this paper, the technical characteristics, the application status as well as workingprinciple for VSC-HVDC are firstly analysed, laying the basis of the theoreticalbackground of the research. The strong coupling and nonlinear steady-statemathematical model of VSC based on dq synchronous rotating coordinate system, andthen the continuous-time state space mathematical model is discretized by forwarddifference method to get the VSC-HVDC discretization mathematics model. By using this model, the physics concepts of the VSC becomes more clear, at the same time, it iseasy to design and develop the control system. Combined with the discretizationmathematics model and the dual closed-loop control (the inner current controller andthe outer loop power controller) thinking which is applied widely in motor field， thediscretization control system for the double ended VSC-HVDC and passive networkwith are developed. In this paper, the simulation models for VSC-HVDC areconstructed for verification, throughing the electromagnetic transient simulationsoftware PSCAD/EMTDC. In addition, in order to improve the performance of thecontrol system, the functional modules of SVPWM is created in PSCAD/EMTDC byFortran programming language and achieving its application in VSC-HVDC system.The simulation results shows that the designed controller based on discretemathematical model is simple and effective. At same time they are able control the trendand stability of the system to meet the needs for various control methods, fullyembodying the advantages of VSC-HVDC. The simulation results also provide a goodreference value for the future promotion field of VSC-HVDC.