Research On Control Strategy And Stability Of Wind Turbine Generator System Connected To Weak AC Power Grid

With the rapid growth of social economy,energy has gradually become the focus of attention.The shortage of energy supply is a realistic problem that all mankind has to face,but it has also greatly promoted the development of renewable energy.As a clean and environmentally friendly renewable energy,wind energy is gradually favored by people with its own superior characteristics.The installed capacity of wind power is increasing,and its proportion in the power system is also increasing.Due to the characteristics of wind power clustering,the access point of wind power generation system is gradually in the weak AC power grid and the security and stability of the system is facing great challenges.How to improve the adaptability of the wind power system to the weak AC power grid and improve the stability of the wind power generation system is the significance of this research.In this paper,the grid connected doubly fed wind power generation system is taken as the research object,the mathematical model of three phase coordinates and DQ coordinates of DFIG are derived respectively.The stability analysis of grid connected doubly fed wind power generation system is carried out,and an optimization control method to improve the stability of the system is put forward.The specific contents include the following aspects:Firstly,the control principle of doubly fed wind power generation system is introduced.In a single machine infinite system,starting from the dynamic process of the electromagnetic time scale and ignoring the dynamic mechanical time scale,the speed of the wind generator and the power are regarded as fixed value.The control effect of active power ring,DC voltage loop,side voltage loop and phase locked loop on the system is considered,and a linearized small signal model is established.As the connection element of the power grid and the wind power generation system,the working principle of the PLL is introduced in detail,and a detailed model is set up.The rationality of the proposed linearized small signal model is verified by comparison with the simulation analysis of the detailed model.Follow by,based on the proposed linearized small signal model,the operation stability of the system is analyzed by eigenvalue analysis from four control loops of active power loop,phase locked loop,DC voltage loop and machine side voltage loop.According to the trend analysis of system root trajectories under different network strength,the leading eigenvalues of the system are given,and the influence of these four control loops on the dominant mode under weak AC power grid is analyzed.According to the change trend of the damping ratio with the short circuit ratio of the grid connected doubly fed wind power generation system,the boundary between the strong and weak power grid of the grid connected doubly fed wind power generation system is given.Finally,taking the control loop parameters and the virtual bus voltage as the research object,the selection of the control loop parameters and the range of the correlation coefficient of the virtual bus voltage optimization control method are discussed.The design of control parameters is discussed from three aspects: optimization of phase-locked loop control parameters,optimization of DC voltage loop control parameters and optimization of terminal voltage loop control parameters.An optimal control method for virtual bus voltage is proposed from the consideration of the access point of the doubly fed wind power generation system and the effectiveness of it is verified by small signal model.