Performance Analysis And Improvements For Maximum Power Point Tracking Control Of Low Wind Speed Wind Turbines

As the wind energy in high-wind-speed region has been sufficiently exploited,the massive wind energy potential in low-wind-speed region that is closer to central load demand is becoming a new growth point of wind power generation.However,due to the joint effects of the complex turbulent wind conditions in low-wind-speed region and the larger wind rotor inertia,highly efficient wind energy extraction for low wind speed still challenges.It is difficult for low wind speed wind turbines to obtain the satisfactory wind energy capture efficiency via the conventional maximum power point tracking(MPPT)control.Hence,key points in the design of MPPT controller for low wind speed wind turbines are summarized according to the operating wind conditions and the structural features of low wind speed wind turbines.Based on these design factors,comparative analyses are performed on the defects of two typical MPPT methods,i.e.,optimal torque control and tip speed ratio control,then the improved control strategies are proposed.The main contributions of this thesis are as follows:A.According to the turbulent wind conditions of low wind speed and the slow dynamics of large wind rotor,three factors that need to be concerned in the design of MPPT controller for low wind speed wind turbines are summarized,including the rotor speed tracking performance,wind turbine loads,and actuator restrictions.Based on these factors,this thesis reveals that the optimal torque control shows poor speed tracking performance due to the limitations on its principles;the tip speed ratio control improves the speed tracking performance while increases the loads burdened by the wind turbines;restrictions on the generator electric torque adjustment range significantly affect the performance of MPPT controller.Hence,consideration should be given to all the three design factors discussed above when improving these two types of MPPT control schemes.B.For the poor rotor speed tracking performance of the optimal torque control,an accelerated optimal torque control is proposed based on the idea of sliding mode control.The proposed method not only enhances the turbines'speed tracking performance but also effectively improves the efficiency of wind energy extraction via an additional fuzzy control that restrains the buffeting.The proposed method is free of wind speed observation,which is also an advantage of the optimal torque control,and can prevent the electric torque from exceeding its regulation range.Distinct from the existing improvement idea of adjusting the torque curve settings,the structure of the initial controller is modified in the proposed method.By introducing wind rotor acceleration,the dependence on the statistical information of wind conditions is avoided.The proposed method provides a feasible approach for the improvement of the optimal torque control.C.For the significant increase in wind turbine loads when the conventional tip speed ratio control is implemented,a variable parameter nonlinear feedback MPPT control is proposed.Considering that the radical rotor speed regulation will lead to intensive generator torque fluctuation and increase in wind turbine loads,the proposed method prevents the unnecessary generator torque fluctuation using variable parameters in order to improve the speed tracking performance and mitigate wind turbine loads simultaneously.D.The mechanism of MPPT control to achieve maximum wind energy extraction is tracking the optimum rotor speed.However,it is revealed in this thesis that for wind turbines operated under complex turbulent wind conditions of low wind speed with large moment of inertia,the rapid variation of the optimum rotor speed is beyond the wind turbines' speed tracking capability so that the turbines can hardly track the optimum rotor speed.Therefore,an optimized MPPT control strategy with the objective of rotor speed tracking is proposed.The speed tracking capability of wind turbines and the distribution of wind energy with wind speed are comprehensively considered in the proposed method.By adjusting the rotor speed reference for the wind turbine,the tracking performance under high wind speed is stressed while alleviating the change in speed tracking objectives,so that a tradeoff between the wind energy extraction efficiency and loads is achieved for low wind speed wind turbines.