Page 381 - Wind Energy Handbook

POWER CONTROL 355

Moreover, as regards the controller, pitch-regulation introduces the need for fast
response closed loop control, which is not required for the supervisory functions on
a stall-regulated machine. Thus the benefits of pitch control have to be weighed
carefully against all the additional costs involved, including the cost of mainte-
nance.
Another factor that needs to be considered is fatigue loading. This increases
significantly on full-span pitch-regulated machines, because the rate of change of
lift coefficient with angle of attack remains at about 2ð (see Equation (A3.9) in
Appendix A3.1) instead of reducing to zero as the blade goes into stall, with the
result that rapid changes in wind speed above rated will cause bigger thrust load
changes.
Pitch system controller design is considered in detail in Chapter 8.

6.7.3 Passive pitch control

An attractive alternative to active control of blade pitch to limit power is to design
the blade and/or its hub mounting to twist under the action of loads on the blades
in order to achieve the desired pitch changes at higher wind speeds. Unfortunately,
although the principle is easy to state, it is difficult to achieve it in practice, because
the required variation in blade twist with wind speed generally does not match the
corresponding variation in blade load. In the case of stand-alone wind turbines, the
optimization of energy yield is not the key objective, so passive pitch control is
sometimes adopted, but the concept has not been utilized as yet for many grid-
connected machines.
Corbet and Morgan (1991) give a survey of how different types of blade loads
might be utilized. Harnessing the centrifugal load is obviously promising in the
case of variable speed machines, and this has been demonstrated using a screw
cylinder and preloaded spring to passively control each tip blade, within the Dutch
FLEXHAT programme. When the centrifugal load on the tip exceeds the preload,
the tip blade is driven outwards against the spring and pitches (see Figure 6.13 for
illustration of the concept).
Joose and Kraan (1997) have proposed replacing this mechanism by a mainte-
nance-free ‘Tentortube’, which would twist under tension loading. This tube would
be carbon-fibre reinforced with all the fibres set at an angle to the axis, so that
centrifugal loading induced twist. It would be placed inside a hollow steel tip shaft,
which would carry the aerodynamic loading on the tip blade.

6.7.4 Active stall control

Active stall control achieves power limitation above rated wind speed by pitching
the blades initially into stall, i.e., in the opposite direction to that employed for
active pitch control, and is thus sometimes known as negative pitch control. At
higher wind speeds, however, it is usually necessary to pitch the blades back
towards feather in order to maintain power output at rated.
A significant advantage of active stall control is that the blade remains essentially