| Actively promoting renewable energy construction is an important step for countries around the world to respond to the energy crisis and environmental pollution.In the new energy field,the wind power industry has received extensive attention,which develops rapidly.In recent years,with the increase of wind power grid integration capacity,the operational stability of the power grid is facing new challenges.Wind speed characteristics such as fluctuation and randomness will affect wind power generation to a certain degree.Power electronic equipment such as converters used when the wind turbines are connected to the grid will also bring harmonic problems and new oscillation risks.Studying the stability of subsynchronous oscillation under grid-connected wind farms is conducive to better wind farm control and safe and reliable operation under grid-connected conditions.Aiming at the possible stability problems of wind farm grid connection,a dynamic model of wind farm integrated into the power grid,including a detailed wind turbine structure is established in this thesis.The impedance analysis method and stability criteria in the frequency domain are used to analyze the stability and the influencing factors of possible sub-synchronization of the system.The specific research contents are as follows:Firstly,the research background and significance of the stability of wind power grid-connected systems are described.The research status of the subsynchronous oscillation problem is introduced,including its development process in real scenarios,the theoretical analysis method of the subsynchronous stability problem,and the model required for research.These introductions give a more comprehensive understanding of the entire issue.The second is the modeling of a grid-connected wind farm.The mathematical model of a single doubly-fed induction machine is constructed.Analysis of the dynamic mathematical model of the machine itself,the grid side filter and the vector control principle,and transfer function structure of the stator side and rotor side converters are carried out.In the synchronous rotating coordinate system,the stator and rotor adopt a certain orientation method that can achieve decoupling control of active and reactive power.When the magnetic flux orientation and voltage orientation are applied,the d-q axis current component and the power type of its corresponding control will be different.The mathematical model of the wind farm will consist of several identical wind turbines.Next,based on the impedance analysis theory,a detailed derivation of the overall impedance model of the wind farm grid-connected system is explored.In the case of small disturbances and considering the possible error effects of the Phase-Locked Loop(PLL),the impedance model expressions of the doubly-fed induction wind generator in the frequency domain are built,including the machine’s impedance model,PLL model,control parts impedance model of the rotor side and grid side and grid side filter model.On this basis,the relationship between the stator voltage and the stator current,and the stator voltage and the grid side current are derived,and a complete impedance model can be obtained.Combined with the Nyquist stability criterion and Bode diagram based on the frequency domain,the theoretically derived impedance model is verified.Finally,the wind farm grid-connected system model already built is used to finish the electromagnetic transient simulation,and the system stability analysis is performed by using the frequency sweep method,impedance analysis method and frequency domain stability criterion.By injecting AC voltage harmonic disturbance to each wind turbine,the unstable situation that may occur in the grid-connected wind farm is observed.Changing the factors such as wind speed,wind turbine control parameters,the influencing factors of the subsynchronous oscillation problem are studied based on the Nyquist diagram and Bode diagram.These researches provide ideas for suppressing the occurrence of oscillation. |
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