Page 207 - Wind Energy Handbook

CONSTANT ROTATIONAL SPEED OPERATION 181

120

100
+4
+2
80
Electrical power (kW) 60 -2 0

40

20 -4

0
5 10 15 20 25
Wind speed (m/s)
Figure 4.9 Effect on Extracted Power of Blade Pitch Set Angle

starting torque can be generated as a rotor begins to turn. A 908 pitch angle is
usually used when shutting down because this minimises the rotor idling speed at
which the parking brake is applied. At 908 of positive pitch the blade is said to be
‘feathered’. The principal disadvantages of pitch control are reliability and cost.
Power regulation can be achieved either by pitching to promote stalling or pitching
to feather which reduces the lift force on the blades by reducing the angle of attack.

4.2.6 Pitching to stall

Figure 4.9 shows the power curves for a turbine rated at 60 kW, which is achieved
at 12 m/s. At wind speeds below the rated level the blade pitch angle is kept at 08.
As rated power is reached only a small negative pitch angle, initially of about 28,is
necessary to promote stalling and so to limit the power to the rated level. As the
wind speed increases small adjustments in both the positive and negative directions
are all that are needed to maintain constant power. The small sizes of the pitch
angle adjustments make pitching to stall very attractive to designers but the blades
have the same damping and fatigue problems as fixed pitch turbines.

4.2.7 Pitching to feather

By increasing the pitch angle as rated power is reached the angle of attack can be
reduced. A reduced angle of attack will reduce the lift force and the torque. The