Research On Nonlinear Control Of Variable Speed Constant Frequency Doubly-fed Wind Turbine Converter

With the growth in global energy demand and use,wind energy is playing a vital role in meeting the challenge of energy demand.Due to the high wind energy utilisation efficiency,as well as the smaller size,cost and losses,the double-fed wind turbine has become a mainstream model in the wind power market and has been widely used in wind energy industry.The control of the double-fed wind turbine converter becomes the key to the proper running of the double-fed wind turbine.However,the characteristics of wind turbines are large inertia,multi-coupling and non-linearity,and there are various disturbances in the actual system,the traditional converter control strategy cannot meet the requirements of wind turbines for high dynamics and strong robustness.In this paper,the non-linear composite control strategy is used to improve the doubly-fed wind turbine converter on the basis of traditional vector control,which is mainly worked as follows.Firstly,a brief introduction is given to the background significance of the research on variable speed and constant frequency doubly-fed wind power generation,as well as the current status of research on wind power generation at home and abroad.Then,an extensive discussion of the converter non-linear control strategy developed on the basis of the traditional doubly-fed wind turbine converter control strategy is given,with a focus on the objectives of this paper,which include the maximum wind energy capture and the low voltage ride-through problem of doubly-fed wind turbines.Secondly,the mathematical model of the wind turbine,the doubly-fed wind turbine and the converter system is established by analysing the system structure and principles of the doubly-fed wind turbine.Based on coordinate transformation and feed-forward decoupling,a double closed loop vector control model for doubly-fed wind turbine converters under vector orientation conditions is developed.And the vector control model is then non-linearly decoupled according to state feedback linearisation theory.Then,based on a vector control model with feedback linearisation decoupling,this paper designs a rotor-side non-linear composite controller for the maximum wind energy capture problem of doubly-fed wind turbines.The wind speed is predicted by a Markov grey residual prediction model to obtain the outer loop reference speed based on the optimum leaf tip speed ratio.An expansive state observer is used to observe the outer-loop rotor speed and comprehensive system disturbances to improve the anti-interference capability of the system,which is combined with sliding-mode control.Thereby,a double closed loop control strategy combining an speed loop self-antidisturbance sliding mode controller and an current loop feedback linearised sliding mode controller is proposed to improve the robustness and dynamic response speed of the system.Finally,this paper designs a composite non-linear controller for the grid-side converter of wind turbines in order to maintain its stability during low-voltage ride-through and to improve the anti-interference and dynamic response of the grid-side converter.On the basis of a feedback linearisation model that takes into account internal and external disturbances in the system,high gain dilated state observers for load current and grid voltage are introduced in the voltage and current loops respectively as feed-forward compensation controllers.In the feedback control double closed loop,the outer loop uses a fast super-twisting sliding mode with improved reaching law and the inner loop uses a fast terminal sliding mode controller,which improves the response time and reduces high frequency chatter in the system.Its response time is reduced by90% compared to conventional vector control,and its total current harmonic distortion is reduced by 90% compared to conventional sliding mode control.