Research On Control Strategy Of VSC-HVDC System For Wind Farm-grid Connection

In recent years,with the promotion of national wind energy policies,the wind power generation technology develop rapidly,and application market continues to enlarge,all of these make wind farm interconnection become a mainstream way.Wind power's strong fluctuation and intcrmittcncy,and most large wind farms arc far away from the load center,which make AC grid connection technology for wind farms run into the bottleneck.The voltage source converter based high voltage direct current(VSC-HVDC)for wind farm-grid connection have many unique advantages,it can not only control the active and reactive power independently,but also do not need to add reactive power compensation device,which is an effective way for interconnection between the wind farms and grid.But because of the coupled nonlinear characteristics,conventional VSC-HVDC control strategy is hard to get ideal control effect.This paper focuses on the innovation and explore control strategy of VSC-HVDC system for wind farm-grid connection.Firstly,severol methods of wind farm interconnection are analyzed in the paper.And the research status of VSC-HVDC technology are also stated.In order to analyze the character of wind farms,the wind turbine model are introduced,and paper focus on analyzing the mathematical model of doubly-fed induction generator(DFIG)and the vector control system of machine side and grid side converter.The results of simulation model in Matlab/Simlink show that the control strategies is effective.Lastly,based on the research requirement of this paper,the methods of establishing the wind farms'equivalent generator model are presented.In chapter 3,the mathematic model in abc three-phase coordinates and dq synchronous rotating coordinates of VSC-HVDC is established.On this basis,direct current control strategy for VSC-HVDC is given,And the simulation results in Matlab/Simlink show the correctness of the model.Paper directs at nonlinear control characteristics and the difficulty of PI controller parameter setting of the VSC-HVDC system for wind farm interconnection,two kinds of passive nonlinearity control methods are presented according to passivity-based control theory in chapter 4:One is a decoupling control based on Eider-Lagrange(EL)model.It is based on the energy dissipation of VSC.an energy storage function,which has the characters of Lyapunov function,is set up according to the state error of the system.By damping assigning,the VSC-HVDC system cans astringe to the desired stable balance point rapidly.The EL passivity control rule is designed according to the energy storage function's astringency condition.The other robust control strategy is based on Port-controlled Hamiltonian with Dissipation(PCHD)model.the Hamiltonian function of system is builded and its Lyapunov stability is proved,the IDA-PB controller is designed according the interconnection and damping assignment passivity-based control theory.In the two kinds of control systems above,in order to guarantee the stabling of active power transmission and AC bus bar voltage on the side of the wind farm,it use the given active power control and given reactive power control,without of outer loop control.It can achieve the DC voltage constantly control by using the constant AC voltage and DC voltage control on the side of the grid rectifier.Lastly,the simulating model of VSC-HVDC system for wind farm-grid connection in Matlab/Simlink is built,the results in different operation conditions show that the both of the proposed control methods have good strong robustness and performance either in dynamic and static properties.