Page 421 - Wind Energy Handbook

BLADES 395

the grid-loss case is liable to become the critical one as regards out-of-plane bending
moments, unless the tip deployment time can be reduced sufficiently.
The ‘Normal turbulence model’ specified for load case 1.1 includes random
variations of both the longitudinal wind velocity component about the 10 min mean
and the lateral component about a zero mean. Both components are taken to be
Gaussian distributed, with the standard deviation (m/s) of the longitudinal compo-
nent of turbulence given by
2
1
ó 1 ¼ 0:18( 15 þ U hub ) (7:8)
3 3
for a Class A site and the standard component of the lateral component of turbulence
taken as 0:8ó 1 . The design load case is then the set of conditions existing over at least
one rotation cycle that produces the extreme value of the loading under evaluation.
Utilizing the probability distribution of the longitudinal wind velocity component
about the 10 min mean in combination with a Weibull distribution of 10 min mean
wind speeds on the one hand, and the probability distribution of the lateral wind
velocity component on the other, it is possible to establish, for wind speed ‘bins’
corresponding to different values of the longitudinal wind speed component, the
maximum yaw angle that would occur for at least one rotation cycle over the
machine design life. For each case, the out-of-plane bending moment at 12 m radius
is calculated (allowing for shaft tilt), enabling the critical case to be identified. It is
seen from Table 7.1 that the combination of 40.4 m/s wind speed and 278 yaw angle
is the most severe, producing a moment of 130 kNm, which is 10 percent larger
than the moment produced by the worst of the deterministic load cases (load case
1.3). Note that the 40.4 m/s wind speed here is the resultant of a longitudinal
velocity component, assumed parallel to the shaft axis in plan, of 36 m/s and a
lateral component of 18.3 m/s. Thus the yaw error arises from an additional lateral
component which increases the total wind speed, in contrast to load cases 1.3 and
1.8 where the wind merely changes direction.
The derivation of the extreme 12 m radius out-of-plane bending moment for the
‘Normal turbulence model’ load case described above is conservative on three
counts, because no allowance is made for the following:

• lack of correlation of the wind over the outer 40 percent of the blade,
• limitation on maximum wind speed seen during operation by high wind cut-out,
• limitation on maximum yaw angle by yaw control.

The alleviation of extreme loadings by high wind cut-out and yaw control depend
on the averaging times applied to the wind speed and direction signals by the
control system.

Extreme loading during operation: pitch-regulated machines

The characterization of extreme operational loadings on pitch-regulated machines
is inevitably more complicated than for stall-regulated machines, although at the