Page 208 - Wind Energy Handbook

182 WIND-TURBINE PERFORMANCE

100
+12
+6 +24
80 +18
Electrical power (kW) 60

40

20
0 Power curve
0
5 10 15 20 25
Wind speed (m/s)
Figure 4.10 Pitching to Feather Power Regulation Requires Large Changes of Pitch Angle

flow around the blade remains attached. Figure 4.10 is for the same turbine as
Figure 4.9 but only the zero degree power curve is shown below the rated level.
Above the rated level fragments of power curves for higher pitch angles are shown
as they cross the rated power line; the crossing points give the necessary pitch
angles to maintain rated power at the corresponding wind speeds. As can be seen
in Figure 4.10, the required pitch angles increase progressively with wind speed
and are generally much larger than is needed for the pitching to stall method. In
gusty conditions large pitch excursions are needed to maintain constant power and
the inertia of the blades will limit the speed of the control system’s response.
Because the blades remain unstalled if large gusts occur at wind speeds above the
rated level large changes of angle of attack will take place with associated large
changes in lift. Gust loads on the blades can therefore be more severe than for
stalled blades. The advantages of the pitching to feather method are that the flow
around the blade remains attached, and so well-understood, and provides good,
positive damping. Feathered blade parking and assisted starting are also available.
Pitching to feather has been the preferred pitch control option mainly because the
blade loads can be predicted with more confidence than for stalled blades.

4.3 Comparison of Measured with Theoretical
Performance

The turbine considered in this section is run at constant rotational speed, the most
common mode of operation, because this allows electricity to be generated at
constant frequency. More detail about this method of operation will be discussed in
the next section but the main feature is that there is, theoretically, a unique power
output for a given wind speed.