Optimization Of Maximum Power Point Tracking Of Wind Turbines Considering Effects Of Turbulence Frequency

Variable-speed-constant-frequency wind turbines generally operate in the maximum power point tracking(MPPT)mode when the wind speed is below the rated value for improving the efficiency of wind energy capture as well as annual power generation.With the gradual exhaustion of ideal wind farms with high wind speed and low turbulence,wind turbines are developing towards the direction of large scale and low wind speed in order to seek a breakthrough.In this context,the rotor inertia of wind turbines increases dramatically,which leads to turbines' inability to accelerate or decelerate quickly in response to wind speed changes and the effects of turbulence frequency on MPPT efficiency has been emphasized.However,the turbulence frequency is neglected in the existing MPPT control methods,which causes the MPPT performance is not satisfactory enough.The aim of this paper is to make the consideration of turbulence characteristics more comprehensive in the process of MPPT control design and parameters optimal setting based on the mechanism exploration of effects of turbulence frequency on MPPT,and therefore further improve the wind energy capture performance of wind turbines with large inertia in the complex turbulence.To this end,this research is conducted from three aspects:single indicator for the characterization of turbulence frequency applicable to MPPT control,mechanism of the effect of turbulence frequency on MPPT and performance optimization of MPPT control with consideration of turbulence frequency.The main results obtained in this paper are summarized as the following:1.It is difficult to characterize turbulence frequency,because the turbulent wind contains many components with different frequencies.To address this problem,the integral of low frequency band of normalized power spectrum density function is employed as the indicator for characterizing turbulence frequency based on the fact that the dominant frequency will have a larger distribution proportion in the mean power than the other frequency band.The effectiveness of the indicator is verified by simulations on the simulated and measured turbulent wind.2.Aiming at the frequency characteristic of wind speed fluctuation which has not been fully considered in wind turbine control system,the regularity and mechanism of turbulence frequency affecting MPPT are analyzed.The results indicate that the variation of turbulence frequency not only has effect on MPPT of wind turbine itself,but also has effect on the sensitivity of MPPT efficiency to the variation of mean wind speed and turbulence intensity,i.e.the indirect effect.Accordingly,turbulence frequency needs to be taken into account in the optimal design of MPPT control strategies.Apart from,comprehensive consideration of the mean wind speed,the turbulence standard deviation and the turbulence frequency is necessary.3.In order to take turbulence frequency into account in MPPT control,the functional relations between the optimal tracking range and three wind speed characteristics(i.e.,mean wind speed,turbulence intensity and turbulence frequency)are constructed for the power curve modification based on the reduction of tracking range by utilizing the response surface model.And an improved optimal torque(OT)control method with comprehensive response to the variation of turbulence conditions is proposed.Since turbulent wind is more comprehensively considered,higher efficiency of wind energy capture is achieved.4.To avoid possible abnormal torque curve gain in the adaptive search process,by analyzing the effect of varying turbulence condition on the adaptive algorithm,this paper reveals a scenario that turbulence condition changes toward the conveniently extracted trend(namely,mean wind speed increases,turbulence standard deviation decreases,or turbulence frequency decreases)might result in the significant deviation of torque gain from the optimal value and the decline in wind energy production.Then,an improved adaptive torque control strategy is proposed to achieve higher MPPT efficiency under varying turbulence condition.This method introduces a dynamic power reduction indicator to identify such a scenario and amend the incorrect perturbation of torque gain.5.This paper further extends the focus from the improvement of wind energy capture efficiency to the trade-off between wind energy capture and mechanical stress.According to simulation analysis of the interaction between wind energy capture and stress,the coordination of them needs to be adjusted with the variation of turbulence conditions.The nonlinear functional relation between the optimal value of energy capture and stress balancing parameter(i.e.,bandwidth)and turbulent wind speed characteristics is constructed based on the constant-bandwidth OT method by utilizing the response surface model.Then,the multi-objective optimization of wind energy capture and stress is realized by modifying the bandwidth with variation of wind conditions simultaneously.