Research On Control Strategy Of Doubly-fed Induction Generator Grid-connected To Suppress Low Frequency Oscillation

In recent years,with the rapid development of modern society,people use more and more energy.With the over-exploitation of fossil energy,people began to focus on the development of new energy,and wind energy has become the focus of research in new energy due to its clean,easy-to-obtain,and large-volume advantages.As the grid-connected capacity of wind power increases,the impact of wind power systems on the grid is gradually increasing.In the issue of wind power grid connection,the method of using the power regulation capability of wind turbines to suppress system low-frequency oscillations has been widely studied by scholars from various countries.First,this paper discusses the influence of DFIG grid-connected capacity on system damping characteristics.Secondly,by studying the active power regulation capability of DFIG,the mechanism of DFIG active power damping system oscillation was analyzed in depth and verified on the four-machine two-area power system model with DFIG.Finally,the transient energy function method is used to analyze the mechanism of the DFIG damping system.The fuzzy sliding mode control method is applied to the rotor-side converter control of DFIG,and a fuzzy sliding mode damping controller that can suppress system oscillation is designed.This method improves the ability of DFIG to suppress system oscillations,and provides a guarantee for the safe and stable operation of the power system containing DFIG.The main work of the thesis is as follows:(1)First,the paper introduces the basic structure of DFIG and its wind turbine model.Then the mathematical model of the DFIG generator is established,and the decoupling control relationship between the rotor-side converter and the grid-side converter of DFIG is derived,which lays the foundation for the design of the DFIG damping controller.(2)This paper analyzes the causes and analysis methods of system oscillations,and establishes a four-machine two-area system with DFIG,and analyzes the influence of DFIG’s different wind power on system damping and frequency by using the eigenvalue method.(3)In order to theoretically analyze the principle of DFIG power regulation damping system oscillation,this paper establishes a simple power system model with DFIG,and linearizes the system.Through analysis,it can be known that the principle of DFIG suppressing system oscillation is that the active output delivered by DFIG to the grid will add a power increment to the electromagnetic power of the synchronous generator.That is,when the system fails,DFIG will add a damping torque to the synchronous generator.Based on the above principles,after setting three working conditions on the four-machine two-area model with DFIG,additional damping control is introduced for verification.The experimental results verify the effectiveness of the DFIG active power control loop in damping system oscillations.(4)In order to improve the robustness of DFIG in suppressing system oscillations,based on the study of the transient energy function method to explain the theory of DFIG damping system oscillation mechanism,fuzzy sliding mode control is introduced into the active power control loop of DFIG.A fuzzy sliding mode damping controller is designed and verified on a four-machine two-zone model with DFIG.Experimental results show that the fuzzy sliding mode damping controller has a better effect in suppressing system oscillations,and can more effectively improve the damping characteristics of the system.