Behaviour of stallregulated machines in fatigue

For stall-regulated machines, the highest out-of-plane bending moment ranges and means normally occur at high wind speeds and yaw angles. This is illustrated in Figure 7.9, which shows the variation in this moment with wind speed and yaw angle at 60 percent radius for a 40 metre diameter machine, based on the three-dimensional data referred to at the start of Section 7.1.8 above. Note that above rated wind speed, the bending moment plots level off, so that a given departure of the lateral wind component from the zero mean, sustained over half a revolution, results in a larger bending moment fluctuation than a change in the longitudinal component of twice this magnitude. For example, if the mean wind speed is 24 m/s, a lateral component of 6 m/s (corresponding to a yaw angle of 14°) causes a bending moment variation of 20 kNm when the blade rotates from 0° to 180° azimuth, compared to a variation of 17 kNm as a result of a ±6 m/s fluctuation in longitudinal wind speed (which, in any case, could only occur after many blade rotations).

Similar comments apply to vertical wind speed fluctuations, but here there is a built-in initial tilt angle between the air flow and the shaft axis because of shaft angle tilt and updraft. Thus bending moment plots derived from three-dimensional wind simulations above rated are dominated by fluctuations at blade-passing frequency which bloom and decay as the angle between the air flow and the shaft axis rises and falls. Superimposed on these are lower frequency fluctuations caused by changes in the longitudinal wind speed.

Clearly high wind/high yaw cycles will be a major source of fatigue damage, although the contribution of cycles at wind speeds below stall may also be important, because of the more rapid variation of moment with wind speed there, and the much increased number of cycles.

Thomsen (1998) has investigated for blade root out-of-plane bending on a 1.5 MW, 64 m diameter three-bladed machine, taking a constant turbulence intensity of 15 percent and a S- N curve index of 12. The results, including allowance for mean stress, are plotted in Figure 7.13 (dotted), and indicate that the damage is concentrated at wind speeds of 20 m/s and above. The figure also shows the effect of adopting a steeper S - N curve (with m = 10) and the IEC Class A turbulence distribution (with increasing intensities as mean wind speed decreases). In each case, the relative damage contribution at high wind speeds is reduced, but the switch to the IEC turbulence distribution causes the more significant change.

It should be noted that the relative contributions of different wind speeds to lifetime fatigue damage are also dependent on the shape of the bending moment/wind speed characteristics. Thus for the machine with the bending moment/wind speed characteristics at 60 percent radius presented in Figure 7.9, the peak damage occurs at 10 m/s, if the IEC Class A turbulence intensity distribution is assumed (see Figure 7.15).

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