Page 377 - Wind Energy Handbook

POWER CONTROL 351

machines suffer from the disadvantage of uncertainties in aerodynamic behaviour
post-stall which can result in inaccurate prediction of power levels and blade
loadings at rated wind speed and above. These aspects are considered in greater
detail in Section 4.2.2.

6.7.2 Active pitch control

Active pitch control achieves power limitation above rated wind speed by rotating
all or part of each blade about its axis in the direction which reduces the angle of
attack and hence the lift coefficient – a process known as blade feathering. The
main benefits of active pitch control are increased energy capture, the aerodynamic
braking facility it provides and the reduced extreme loads on the turbine when
shut-down (see also Sections 4.2.5, 4.2.7 and 8.2.1).
The pitch change system has to act rapidly, i.e., to give pitch change rates of 58=s
or better in order to limit power excursions due to gusts enveloping the whole rotor
to an acceptable value. However, it is not normally found practicable to smooth the
cyclic power fluctuations at blade passing frequency due to blades successively
slicing through a localized gust (Section 5.7.5) with the result that the large power
swings of up to about 100 percent can sometimes occur.
The extra energy obtainable with pitch control is not all that large. A pitch-
regulated machine with the same power rating as a stall-regulated machine,
utilizing the same blades and rotating at the same speed will operate at a larger
pitch angle below rated wind speed than the stall-regulated machine, in order to
reduce the angle of attack and hence increase the power output at wind speeds
approaching rated. If the 500 kW, 40 m diameter, 30 r.p.m. stall-regulated machine
described in Section 6.5.3 is taken as baseline, a 500 kW, 30 r.p.m. pitch-regulated
machine utilizing the same blades at optimum pitch would produce about 2 percent
more energy. The optimum rotational speed is found to be about 33 r.p.m., which
increases the energy gain to about 4 percent. The power curve of the 500 kW, 40 m
diameter pitch controlled machine rotating at 33 r.p.m. is compared with the power
curve of the corresponding stall-regulated machine utilizing the same blades, but
rotating at 30 r.p.m. in Figure 6.4. Note that the knee in the power curve at rated
speed will be more rounded in practice because the pitch control will not keep pace
with the higher frequency components of turbulence.
Figure 6.7 shows a family of power curves for a range of positive pitch angles for
the 500 kW, 40 m diameter pitch-controlled machine rotating at 33 r.p.m.. The
intersections of these curves with the 500 kW abscissa define the relationship
between steady wind speed and pitch angle required for power control. It is readily
apparent from the power curve gradients at the intersection points that rapid
changes of wind speed will result in large power swings when the mean wind
speed is high.
The range of blade pitch angles required for power control is typically from 08
(often referred to as ‘fine pitch’), at which the tip chord is in the plane of rotation or
very close to it, and about 358. However, for effective aerodynamic braking, the
blades have to be pitched to 908 or full feather, when the tip chord is parallel to the
rotor shaft with the leading edge into the wind.