Research On Second-order Sliding Mode Control Strategies Of Variable Speed Constant Frequency DFIG-based Wind Turbine

With the rapid development of the new energies,variable speed constant frequency doubly-fed induction generator(DFIG)-based wind turbine is one of mainstream model.Therefore,the control scheme of DFIG,which can improve the efficiency of wind and/or ride-through performance,has been one of the key research directions.Owing to the intermittency of wind and the nonlinearity,many uncertain factors and disturbances of the unit,it is more complex to study control strategy.Second-order sliding mode(SOSM)control is used to design controllers of the DFIG-based wind turbine due to its strong robustness to external disturbance and parameters change.Firstly,dynamic models of DFIG-based wind turbine,which include wind turbine,drive train and DFIG,are built.And analyze the sliding mode control theory and its applicability for control of the DFIG-based wind turbine,which lay a foundation for controller designs and simulations.Next,the maximum power point tracking(MPPT)control strategy is analyzed and the MPPT SOSM controller is designed.Based on stator flux-oriented vector control,the DFIG state space model is reduced.And the simplified is used to design a SOSM controller aiming at maximizing the power extraction of wind turbine.The control law is designed based on the Super-Twisting algorithm which only needs the information of sliding surface.The numerical analysis of the designed controller's stability has also been done to prove the controller is stable.Then,the low voltage ride-through(LVRT)control strategy is investigated and the LVRT SOSM controller is designed.By analyzing electromagnetic transient process,the SOSM controller is designed based on Super-Twisting algorithm that takes into account grid demand on LVRT.Through theoretical derivation,the reachability of the desired states and the stability of the system are verified.Finally,the performances of the MPPT and LVRT controllers have been simulated.And in case of the wind model close to the accurate,simulation results verified the effectiveness of the MPPT controller and the LVRT controller in case of deep sags.The MPPT and LVRT control achieved based on the theory of SOSM control provide a new opportunity for the design of the mastercontrol system for doubly-fed wind turbine.