Power curves for wind turbines - Wind Power Generation and Wind Turbine Design - page 599

Figure 3 : LEFT: Static (steady-state) power curve P ( u ) of an active stall controlled
wind turbine showing the different power operation states: stop, partial
and full load. RIGHT: Corresponding power coeffi cient c p ( u ).
increase of the power output with the wind speed. The functional behavior of the
power coeffi cient c p ( u ) is the result of certain control strategies as well as of Betz
physical limit.
In the mechanical power extraction the usual way to control power production
is achieved by stall effects on the rotor blades. Stall effects happen when the criti-
cal angle of attack for an airfoil is exceeded, resulting in a sudden reduction in the
lift forces generated by the airfoil. In modern wind turbines this is achieved by
so-called active stall control or pitch control [2]. This consists of a rotation of the
blades into the plane of rotation and the blade cross-section. The blade rotation
angle is known as blade pitch angle q . The power coeffi cient c p is in this case a
function of the blade pitch angle q and the tip-speed ratio l = w R / u (where w is the
angular velocity of the rotor, R the rotor radius, i.e. blade length and u the wind
speed), i.e. c p = c p [ l ( u ), q ]. Thus, the power extraction of wind turbines is opti-
mized via c p [ l ( u ), q ] to a desired power production. In particular for high wind
conditions c p is lowered to protect the turbine machinery and avoid overshoots in
power production.
To achieve an effi cient pitch control during wind energy conversion the
wind turbine is equipped by a power controller system. This is generally com-
posed of several composite mechanical-electrical components that, depending
on the type of design, operate actively for the optimum power performance.
As a consequence the power output operation for active stall wind turbine
systems can be separated into two states: partial load, with maximum c p value,
and full load, with reduced c p values. A complete detail of the overall structure
of the power operation system for different wind turbine types is described
in [2]. Numerical wind turbine simulation can be found, e.g. in [9]. The theo-
retical power curve P ( u ) together with the corresponding c p ( u ) is represented
in Fig. 3.