Page 382 - Wind Energy Handbook

356 CONCEPTUAL DESIGN OF HORIZONTAL-AXIS TURBINES

stalled above the rated wind speed, so that gust slicing (see Section 6.7.2) results in
much smaller cyclic fluctuations in blade loads and power output. It is found that
only small changes of pitch angle are required to maintain the power output at
rated, so pitch rates do not need to be as large as for positive pitch control.
Moreover, full aerodynamic braking requires pitch angles of only about 208, so the
travel of the pitch mechanism is very much reduced compared with positive pitch
control.
Figure 6.11 compares schedules of pitch angle against wind speed for active stall
control and active pitch control for the same blade. The active stall control schedule
is derived from the intersection of the family of power curves for different negative
pitch angles with the 500 kW abscissa in Figure 6.12, while the active pitch control
schedule is derived from Figure 6.7.
The principal disadvantage of active stall control is the difficulty in predicting
aerodynamic behaviour accurately in stalled flow conditions. Active stall control is
considered further in Section 8.2.1.

6.7.5 Yaw control

As most horizontal-axis wind turbines employ a yaw drive mechanism to keep the
turbine headed into the wind, the use of the same mechanism to yaw the turbine
out of wind to limit power output is obviously an attractive one. However, there
are two factors which militate against the rapid response of such a system to limit
power: first, the large moment of inertia of the nacelle and rotor about the yaw axis,

30

25

Active pitch
20
control
Pitch angle ( ) 15

10

5

0
0 5 10 15 20 25 30
Active stall control
-5
Wind speed (m/s)
Figure 6.11 Specimen Pitch Angle Schedules for Active Pitch Control and Active Stall
Control