| With the rapid growth of wind power market and large-scale development of wind turbine in most countries of the world,the safety of wind turbine has attracted great attention.Among the many accidents of wind turbine,the accidents caused by thunderstorms and strong winds account for a larger proportion.Downburst is a kind of sudden and destructive local strong wind with a wind speed of up to 75 m/s near the ground,which seriously threatens the safety of wind turbine in wind farms.Due to the harmlessness of downburst and its absence in the design standard of wind turbine,it is of great value to study the load characteristics of wind turbine under the action of downburst,which can provide reference for wind turbine wind-resistant design and risk assessment of wind turbine.In order to clarify the influence of downburst on aerodynamic characteristics and structural response of wind turbine,NREL 5 MW wind turbine was taken as the research object,and the downburst theoretical analytical model combined with IEC Kaimal spectrum was used to construct the downburst three-dimensional pulsating wind field,which was taken as the inflow condition of wind turbine.The aerodynamic load and structural dynamic response of rotor and tower were calculated and analyzed by Open FAST.The main work of this paper includes the following contents:(1)The average downburst wind field of different heights was established by theoretical analytical model.Based on IEC Kaimal fluctuating wind spectrum model,the downburst fluctuating wind field of different heights was constructed by Fourier inversion method superimposed with fluctuating wind speed.The influence of different downburst heights on aerodynamic loads of rotor and tower were studied.When the height of the downburst is located at the bottom-tip of the rotor,the incoming flow in the rotor plane is all negative wind shear,and the thrust and torque of the rotor are minimum.When the height of the downburst changes from the bottom-tip to top-tip of the rotor,the root mean square coefficients of the transverse force,longitudinal force,tilt moment and yaw moment of the rotor first increase and then decrease with the increase of the height of the downburst.When the height of downburst is the same as the height of hub,the root mean square of the coefficient of unbalanced load(transverse force,longitudinal force,tilt moment and yaw moment)is the minimum.The average aerodynamic drag coefficient in fore-aft direction of the tower is greatly affected by the change of the height of the downburst.In particular,when the tower section is 10%,the maximum aerodynamic drag coefficient in fore-aft direction is 6.57 times higher than the minimum.(2)The average downburst wind field with different horizontal maximum wind speed was established by theoretical analytical model.Based on the fluctuating wind spectrum,the inverse Fourier transform method was adopted to build the downburst fluctuating wind field with different horizontal maximum wind speed.The influence of the change of horizontal maximum wind speed on the aerodynamic load of rotor and tower under the conditions of downburst was studied.The results show that the root mean square of the rotor thrust coefficient first increases and then decreases with the increase of the horizontal maximum wind speed,and the root mean square of the torque coefficient and the unbalanced load coefficient increase with the increase of the horizontal maximum wind speed.When the horizontal maximum wind speed of the downburst is lower than the rated wind speed(11.4m/s),the root mean square changes of aerodynamic force and aerodynamic torque coefficient of the rotor are more obvious.The average aerodynamic drag coefficient of the tower at different section positions increases with the increase of the horizontal maximum wind speed,and the average aerodynamic drag coefficient of the side-side direction changes linearly with the change of the horizontal maximum wind speed.(3)The power spectrum curves of turbine thrust and torque coefficient were studied under different height and different horizontal maximum wind speed of downburst.The results show that the peak frequency of turbine thrust and torque coefficient power spectrum is related to the integral multiple of blade passing frequency.(4)Under different heights of downburst,the modal superposition method was used to study the influence of downburst height on blade root load,tower load,blade tip displacement and tower top displacement of wind turbine.The results show that the average values of blade root load,tower load,blade tip displacement and tower top displacement are minimum when the downburst height is located at the bottom-tip of the rotor.Besides,the change of the height of downburst has little effect on the blade root load and tip displacement.When the height of the downburst changes from the bottom-tip to top-tip of the rotor,it has a great influence on the load of the tower and the displacement of the tower top.(5)Under the conditions of different horizontal maximum wind speeds of downburst,the modal superposition method was used to study the influence of the horizontal maximum wind speeds of downburst on the blade root load,tower load,blade tip displacement and tower tip displacement of wind turbine.The results show that when the horizontal maximum wind speed of downburst changes from small to large,it has great influence on the load of blade root and tower,tip displacement and tower top displacement.When the horizontal maximum wind speed of the downburst is the rated wind speed,the rotor will absorb the maximum wind energy,and the average values of blade root load,blade tip fore-aft displacement,tower pitching moment and tower fore-aft displacement all reach the maximum value. |
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