Wind Turbine Aerodynamics Part A: Basic Principles - Wind Turbine Technology: Fundamental Concepts of Wind Turbine Engineering - page 316

the cube of the cosine of the yaw error, which means the effective normal wind speed is U
cos Y. It should be noted that turbulence can also result in yaw error and a corresponding
power loss.
Aerodynamic Controls
The operation of a HAWT involves starting the wind turbine from rest, stopping the
wind turbine under a wide range of normal and abnormal conditions, and modulating system
power and loads while the turbine is running. The starting of many current wind turbines
is accomplished by running the generator as a motor. For large-scale HAWTs aerodynamic
control surfaces are employed to assist in delivering the large starting torques needed. Loss-
of-load emergencies and wind speeds above the operating range require a reliable means of
stopping the turbine. Aerodynamic controls are particularly attractive for stopping the rotor,
and almost all current HAWTs employ some type of aerodynamic control to prevent rotor
overspeed . A third control function, that of regulating power output and/or system loads, has
been accomplished historically by use of aerodynamic control surfaces.
Full-Span Blade Pitch Control
First on the list of aerodynamic controls is full-span blade pitch control. Figure 5-21
shows a typical blade pitch angle schedule required to produce a ramp-plateau power curve .
The power plateau, at the turbine's rated power , has the advantage of restricting the power
and loads at high wind speeds to cost-effective levels. The ramp portion of this power curve
represents operations at below-rated wind speeds, during which blade pitch is usually held
constant. The control systems in some HAWTs change pitch angle slightly along the ramp
(either continuously or by a mid-ramp change) in order to optimize energy capture. In prac-
tice, the blade pitch changes required at the “knee” of the power curve represent a problem
area, particularly under gusty wind conditions that can cause power “spikes,” since the sharp
change in the power curve is accomplished by corresponding sharp changes in the blade pitch
rate. Control strategies are often used that “round off” these sharp knees.
Figure 5-21. Dimensionless power curve and associated full-blade pitch angles for a
typical HAWT. Blade pitch is held constant until rated power is produced.