Impedance Modeling And Stability Analysis Of Doubly-fed Wind Power System Under Weak Grid Conditions

With the gradual increase of wind power penetration rate,large-scale wind power needs to be transported to the power grid over long distance through long lines,which leads to the obvious dynamic interaction between the wind turbines and the weak grid.The problem of grid-connected stability of the doubly-fed wind power system under weak grid conditions is prominent.This paper focuses on the grid-connected stability of the doubly-fed wind power system under weak grid conditions,and studies the impedance modeling,grid-connected stability analysis and control strategy to improve the stability of doubly-fed wind turbine and control system.The research content is of great significance for improving the grid-connected stable operation capability of the doubly-fed wind power system under weak grid conditions.The main research contents of this thesis include:(1)In order to analyze the influence of control parameters on the grid-connected stability of the doubly-fed wind power system,the sequence impedance model is established based on its structure.Firstly,using the method of harmonic linearization,three different types of sequence impedance models are established,including the sequence impedance model of the doubly-fed induction motor and the sequence impedance model of the doubly-fed wind power systems considering only current loop and both current loop and phase-locked loop.Secondly,the differences and adaptation ranges of three different types of sequence impedance models are compared and analyzed.Finally,the effectiveness of the built-in sequence impedance model is verified by the time domain simulation frequency scanning method.(2)Aiming at the problem that the traditional vector control strategy of the doubly-fed wind power system is difficult to adapt to the grid impedance change,the adaptive control strategy of the phase-locked loop parameters for the doubly-fed wind power system is proposed.Firstly,the influence of the phase-locked loop bandwidth and the grid impedance on the grid-connected stability of the doubly-fed wind power system is analyzed by using the sequence impedance model.Secondly,by using the grid-connected impedance stability criterion,the grid-connected stable operation area of the doubly-fed wind power system under different grid impedances is obtained.Finally,based on the grid impedance on-line detection method,the adaptive control strategy of phase-locked loop parameters is proposed,and the effectiveness of the proposed control strategy is verified by simulation.(3)Aiming at the problem that the existing impedance model is difficult to directly apply to analyze the grid-connected stability of virtual synchronous control based doubly-fed wind power system,a stability analysis method based on dq rotating coordinate system output impedance modeling is proposed.Firstly,based on the virtual synchronous control structure of the doubly-fed wind power system,the dq-axis impedance model considering the power control loop is derived by using the small disturbance analysis method.Secondly,the output impedance characteristics of the virtual synchronous control based doubly-fed wind power system are obtained and compared by the time domain simulation frequency scanning method.Then,using the generalized Nyquist criterion,the influence of different grid strengths and control parameters on the grid-connected stability of the virtual synchronous control based doubly-fed wind power system are analyzed.Finally,the output impedance frequency characteristics and the grid-connected stable operation capability of the doubly-fed wind power system under weak grid conditions are analyzed between virtual synchronous control and vector control by using impedance theory and time domain simulation.The research results of this thesis lay the theoretical foundation and provide technical support for the stability analysis method and the control strategy research for improving the stability of the grid-connected wind power system under weak grid conditions.