Lvrt Study Of DFIG-based Wind Turbines In Weak Grid

With the increasing shortage of fossil energy,the new energy industry,especially wind energy,has attracted more and more attention because of its advantages.In recent years,China's wind power industry has developed rapidly.However,wind turbines are installed in remote areas,at the end of power grid,causing the line impedance to be large and cannot be ignored.And with the increase of wind power penetration,the equivalent inertia of power system becomes insufficient.These make the grid's voltage stability and frequency stability poor,and this kind of grid is called “weak grid”.Weak grid has little support ability for wind turbines,and it contains abundant harmonics,which is easy to lose stability.Grid voltage dip is a common fault.At present,countries have formulated relevant guidelines requiring wind turbines to have low voltage ride through(LVRT)capability when connected to the grid.Scholars have also carried out a comprehensive study to improve the LVRT capability of wind turbines.But most of these are aimed at the traditional strong grid situation,LVRT studies of wind turbines in weak grids are less.Doubly fed induction generator(DFIG)is the mainstream model of wind turbines.DFIG's stator is directly connected to the grid,and needs phase-locked loop(PLL)to achieve grid-connected operation.New problems may occur in DFIG-based wind turbines when performing LVRT in weak grid.In this dissertation,the LVRT capability of DFIG-based wind turbines in weak grid is studied and improved from two aspects.1)Study of rotor overcurrent at fault moment.When a voltage dip fault occurs in weak grid,large line impedance will make the DFIG stator terminal voltage level lower,and the virtual inertia control will introduce complex rotor active compensation current while suppressing frequency fluctuations,rotor overcurrent at fault moment may become larger.System damping can be effectively increased by introducing an active damping control strategy on the DFIG rotor side to suppress rotor overcurrent.Then add virtual inductance to reduce the transition time.2)Study of small signal stability during fault.Weak grid's voltage stability is poor,even during low voltage fault,and the support for DFIG is very small.The coupling between pll and rotor-side control is strong.If system parameters such as PLL bandwidth and virtual inertia coefficient are set unreasonably,system will be unstable due to excessive damping during low voltage fault.Proportional-integral-resonant controller can reduce a certain number of harmonics in the grid and improve stability,and it could be used in the familiar wind farm.Adaptive PLL coefficient method enables DFIG to perform LVRT smoothly by setting different PLL bandwidth before and after fault moment.Active damping method can add a certain amount of damping to the system to ensure small signal stability during low voltage fault.The simulation model of 2MW DFIG-based wind turbine system was built based on MATLAB/SIMULINK platform,and the above theoretical research and control strategy are simulated and analyzed.Finally,by using 11 kW experimental platform,the effectiveness is verified.