Wind Shear and Turbulence Effects on Rotor Fatigue and Loads Control

Abstract

The effects of wind shear and turbulence on rotor fatigue and loads control are explored for a large horizontal axis wind turbine in variable speed operation from 4 to 20 m/s. Two and three blade rigid rotors are considered over a range of wind shear exponents up to 1.25 and a range of turbulence intensities up to 17%. RMS blade root flatwise moments are predicted to be very substantially increased at higher wind shear, and resultant fatigue damage is increased by many orders of magnitude. Smaller but similar trends occur with increasing turbulence levels. In-plane fatigue damage is driven by 1P gravity loads and exacerbated by turbulence level at higher wind speeds. This damage is higher by one to two orders of magnitude at the roots of the three blade rotor. Individual blade pitch control of fluctuating flatwise moments markedly reduces flatwise fatigue damage due to this source, and to a lesser degree the in-plane damage due to turbulence. The same is true of fluctuating rotor torque moments driven by turbulence and transmitted to the drive train. Blade root moments out of the plane of rotation aggregate to create rotor pitching and yawing moments transmitted to the turbine structure through the drive train to the yaw drive system and the tower. These moments are predicted to be relatively insensitive to turbulence level and essentially proportional to the wind shear exponent for the two blade rotor. Fluctuating moments are substantially reduced with individual blade pitch control, and addition of a teeter degree of freedom should further contribute to this end. Fluctuating pitching and yawing moments of the three blade rotor are substantially less sensitive to wind shear, more sensitive to turbulence level, and substantially lower than those for the two blade rotor. Mean rotor torque and hence power are essentially the same for both rotors, independent of wind shear, and somewhat reduced with individual blade pitch control of fluctuating flatwise moments. The same is true of mean rotor thrust, however fluctuations in rotor thrust are substantially reduced with individual blade pitch control. It appears, on balance, that higher wind shear coupled with turbulence effects should be accounted for in the fatigue design of large, long life turbines. Much more work is required on this problem.

Effects of wind shear on wind turbine rotor loads and planetary bearing reliability

“…To answer this question, we use published results based on standardized simulation methods. Eggers et al 10 (9) and (10)).…”

“…The problem becomes one of determining the magnitudes of the reactions. From momentum theory, the thrust on a differential ring of a general rotor at radius, r, is dF Z = 4πρ Á r Á V(y) 2 Á (a À a 2 ) Á dr. 10,11 The differential thrust is only a function of air density, radius, wind speed and the axial induction factor, a, which is governed by blade aerodynamics. The differential thrust must be partitioned among n blades; we chose n = 3 given the prevalence of this design in practice.…”

Spectral behaviour of the turbulence-driven power fluctuations of wind turbines

“…Intermittent gusts, rapid changes in the wind direction, and passage of energetic flow structures are the features of the ABL that can induce dynamic loads on individual wind turbines and blades. [42][43][44] Atmospheric stability is another important mechanism that modulates the dynamics of turbulence and, therefore the response and fatigue damage of wind turbines and farms (e.g., [45,46]). In fact, Kelley [47] found a (critical) stability that maximises blade fatigue of wind turbines within wind farms; however, statistical analysis performed by Nelson et al [48] found a small influence of atmospheric stability on fatigue and extreme loads in turbine blades.…”

On the scale-to-scale coupling between a full-scale wind turbine and turbulence

“…[8] The ABL exhibits highly variable flow patterns including gusts, rapid wind direction changes, mean shear and passage of energetic and coherent flow structures that impose severe loads on individual wind turbines and blades. [9][10][11] Mücke et al [12] demonstrated the correlation of extreme events in atmospheric wind fields with the load alternation on the turbine blade and main shaft. The follow-up research by Wachter et al [13] showed the strong intermittent statistics of wind fluctuation and its impact on intermittent characteristics in the wind energy conversion process.…”