Research On Control Strategy Of Wind Power Grid Connected Through VSC-HVDC Transmission System

Wind power generation can effectively solve the problem of traditional energy shortage and reduce environmental pollution.It has a lot of room for development.With the promotion of the national energy strategy,more and more large-scale wind farms are integrated into the power grid,and the development trend of wind power at home and abroad is showing a rapid development trend.China’s wind power resources and load centers are distributed in the opposite direction,and the load centers are far away from the regions rich in wind power resources.For long-distance wind farm grid connection,the early AC grid connection mode and the traditional DC transmission mode can not meet the requirements of wind power grid connection.Flexible DC transmission system,which has outstanding advantages in transmission flexibility and system stability,has obvious technical advantages in the selection of wind farm grid connection mode,and is gradually becoming a priority solution for longdistance transmission and grid connection of large-scale wind farms.In this paper,a dynamic model based on electromagnetic transient and corresponding control strategy are proposed for the grid connected flexible DC transmission system with doubly fed wind turbines,which can track the dynamic performance of the system faster and better.First of all,the commonly used doubly-fed wind turbines in wind farms are studied.First,the basic operating principles of wind turbines are introduced.Then,the mathematical models of each part of the doubly-fed wind turbines are established in the dq synchronous rotating coordinate system,and then the wind turbines Start with the structure and function of each part,analyze its basic control method,and lay the foundation for the later study of the grid-connected model of the doubly-fed wind turbine and its control system design.Then the structure and basic principle of two terminal two-level DC grid connected system(VSC-HVDC)are introduced,and the double closed-loop control strategy based on direct current control commonly used in VSC-HVDC System is described,mainly including fixed power control,fixed AC voltage control,fixed DC voltage control,fixed frequency control,etc.At the same time,on the basis of following the design principles of converter station control strategy,a coordinated control strategy between converter stations during the failure of the flexible HVDC system is proposed,which helps to improve the system’s transient response capability.Finally,a voltage source converter(VSC)based electromagnetic transient model and control strategy of the DC transmission system is proposed.The electromagnetic transient model of the AC side in a two-phase rotating coordinate system is established,and the dynamic characteristics of the DC side are given based on the principle of instantaneous power balance.Combined with the basic model and control of wind farm and HVDC system,the effectiveness of the proposed model and control strategy is verified on the Matlab/Simulink simulation platform.