Research On Sliding Mode Control For DFIG Based VSCF Wind Turbine System

Recent years wind energy conversion systems have advanced rapidly, due to the depletion of fossil fuel resources and the increasing concern for the environment pollution caused by the traditional energy sources. However, the nature wind conditions are complex. Sometimes the wind turbines will operate under gust or wind shear. Thus, it is important to improve the operation efficiency of wind turbine system and obtain more power output under the limited wind resources. Meanwhile, in order to achieve both the economic efficiency and security, it is also important to prevent the equipment damage in high wind speed.Within the wind power generation systems, the variable speed constant frequency (VSCF) wind turbine with doubly fed induction generator (DFIG) is continuously increasing its marketing share, since it can track the changing wind speed by adapting shaft speed and thus maintaining optimal power generation and lower mechanical stress. In this paper, the DFIG based VSCF wind turbine is selected as the research model. A complete dynamic model of the wind turbine system is established by integrating both aerodynamic and electrical parts, together with parameter uncertainties and perturbations. Since DFIG based VSCF wind turbine system is a complex and highly nonlinear system with strong coupling features and uncertainty in both the aerodynamics and the electrical parts, the second order sliding mode (SOSM) control method is used to solve its control problem in this paper. Besides, based on the dynamic characteristics of the system, the range of control parameters is obtained by adopting quadratic Lyapunov function, to guarantee the finite time convergence of the SOSM control system.In order to improve the energy efficiency under the rated wind speed, a SOSM control scheme is proposed to maximize the wind energy capture and regulate the stator reactive power to follow the grid requirements, by regulating the generator rotor voltage to control the wind turbine rotor speed and the generator rotor current, respectively. Above rated wind speed, a SOSM pitch angle controller is designed to maintain the output power at its rated value, to ensure the safe and stable operation of the wind turbine system.Finally, the effectiveness of the proposed SOSM control strategies is verified by the simulations based on a DFIG based VSCF wind turbine system.