Page 426 - Wind Energy Handbook

400 COMPONENT DESIGN

Behaviour of stall-regulated 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 com-
ponent 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 148) causes a
bending moment variation of 20 kNm when the blade rotates from 08 to 1808 azimuth,
compared to a variation of 17 kNm as a result of a 6m=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 inten-
sity 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 ¼ 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 life-
time 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).

Behaviour of pitch-regulated machines in fatigue

For pitch-regulated machines, the highest flapwise bending moment ranges occur
at high wind speeds and yaw angles, but the largest mean values occur around