Research On Rotor Excitation Control Technology Of Large-scale Grid-connected Doubly-fed Wind Turbine

With the gradual depletion of fossil energy and the increasingly prominent environmental protection issues, more and more countries begin to pay attention to the utilization of renewable energy. As a means to obtain a kind of clean and renewable energy, wind power has been developed rapidly all over the world. But the wind has characteristics of the random, the intermittent and the instability, and the wind turbine is also a nonlinear, multivariable, and strong coupling complex systems. When the wind farm is connected to the large EHV transmission system, it may affect the dynamic characteristics and stability mechanism of the original power system. Therefore, in-depth study of the technological control of grid-connected wind farms and provide theoretical basis and technical support for large-scale wind power Grid-connected smoothly has become the key point to the safe operation online of the wind power.In this paper, the no-load grid-connected control technology and the interaction with the synchronous motor of the current large-scale doubly-fed induction generator is studied in depth, and the decentralized structure model of the rotor excitation current and rotor flux for the grid-connected MW doubly-fed wind turbine is proposed respectively. Based on the decentralized structure model, this paper designs an adaptive sliding mode variable structure controller of the rotor current for the no-load grid-connection of the DFIG and a sliding mode variable structure controller of the rotor flux for the inhibition of the power angle mutation of the DFIG in order to overcome the sensitivity to the system parameter variations and external disturbances of the traditional control method and achieves the high performance adaptive control of the doubly-fed wind power generation system, and provides a theoretical support for the localization of the grid-connected control technology of the doubly-fed wind power system. The main work completed is as follows:(1) By analyzing the relationship among the rotor d-q axis voltage, the rotor d-q axis current of the excitation converter and the stator flux of the DFIG, this paper establishes a no-load decentralized structure model of the rotor excitation current of the DFIG which provides the basic model for the controller design of rotor excitation current.(2) By fully considering the parameter variations and external disturbances and other uncertainties in the process of the synchronization, this paper proposes an adaptive terminal sliding mode control strategy of the rotor excitation current of the DFIG and realizes the variable gain terminal sliding mode control of the rotor excitation current of the doubly fed induction generator, and ensure that the tracking error of the rotor excitation current can converge to zero in finite time. The simulation results show that this method can realize the flexible grid-connected control and also has strong robustness to parameter variations and external disturbances of the system.(3) The dynamic characteristics of the doubly-fed wind power generator under the disturbance of the power system is analyzed in-depth, and then taking the dynamic change of the power angle and the pitch angle as the research focus based on the single machine(DFIG) VS infinite system, this paper studies the characteristics of the DFIG under the disturbance of the system and it’s effects on system in-detail and revealed the technical problems existing in current grid-connected control of the doubly fed induction generator. The results of simulation and experiment show that the validity of the theoretical analysis.(4) Through analyzing the relationship among the rotor d-q axis voltage, the rotor d-q axis flux and the stator d-q axis voltage of the DFIG, this paper establishes the grid-connected decentralized structure model of the rotor flux of the DFIG and SG which provides the basic model for the controller design of rotor flux.(5) By fully considering the interaction and its effect on the transient stability of the power system between the DFIG and the SG in the process of the fault of the "wind-thermal" hybrid transmission system, this paper introduces a combined control strategy of the pitch angle of the DFIG group and the power angle of the DFIG in order to improve the transient stability of the power system and realized the effective control of the power angle of the DFIG and the pitch angle of the DFIG group when it is in parallel operation with the SG. The simulation results show that this strategy on the one hand can improve the characteristics of the power angle of the DFIG when it is disturbed, on the other hand it can enhance the transient stability of synchronous motor and can increase the ability to resist external disturbance of the system.