A wind turbine is designed to produce a maximum of power at
wide spectrum of wind speeds. All wind turbines are designed for a maximum wind
speed, called the survival speed, above which they do not survive. The survival
speed of commercial wind turbines is in the range of 40 m/s (144 km/h) to 72
m/s (259 km/h). The most common survival speed is 60 m/s (216 km/h). The wind
turbines have three modes of operation:
● Below rated wind speed operation
● Around rated wind speed operation
(usually at nameplate capacity)
● Above rated wind speed operation
If the rated wind speed is exceeded the power has to be
limited. There are various ways to achieve this.
Stalling works by increasing
the angle at which the relative wind strikes the blades (angle of attack), and
it reduces the induced drag (drag associated with lift). Stalling is simple
because it can be made to happen passively (it increases automatically when the
winds speed up), but it increases the cross-section of the blade face-on to the
wind, and thus the ordinary drag. A fully stalled turbine blade, when stopped,
has the flat side of the blade facing directly into the wind.
A fixed-speed HAWT inherently increases its angle of attack
at higher wind speed as the blades speed up. A natural strategy, then, is to
allow the blade to stall when the wind speed increases. This technique was
successfully used on many early HAWTs. However, on some of these blade sets, it
was observed that the degree of blade pitch tended to increase audible noise
levels.
Vortex generators may be used to control the lift
characteristics of the blade. The VGs are placed on the airfoil to enhance the
lift if they are placed on the lower (flatter) surface or limit the maximum
lift if placed on the upper (higher camber) surface.
Furling works by decreasing the angle of attack, which
reduces the induced drag from the lift of the rotor, as well as the
cross-section. One major problem in designing wind turbines is getting the
blades to stall or furl quickly enough should a gust of wind cause sudden
acceleration. A fully furled turbine blade, when stopped, has the edge of the
blade facing into the wind.
Standard modern turbines all pitch the blades in high winds.
Since pitching requires acting against the torque on the blade, it requires
some form of pitch angle control, which is achieved with a slewing drive. This
drive precisely angles the blade while withstanding high torque loads. In
addition, many turbines use hydraulic systems. These systems are usually spring
loaded, so that if hydraulic power fails, the blades automatically furl. Other
turbines use an electric servomotor for every rotor blade. They have a small
battery-reserve in case of an electric-grid breakdown. Small wind turbines
(under 50 kW) with variable-pitching generally use systems operated by
centrifugal force, either by flyweights or geometric design, and employ no
electric or hydraulic controls.
Modern large wind turbines are typically actively controlled
to face the wind direction measured by a wind vane situated on the back of the
nacelle. By minimizing the yaw angle (the misalignment between wind and turbine
pointing direction), the power output is maximized and non-symmetrical loads
minimized. However, since the wind direction varies quickly the turbine will
not strictly follow the direction and will have a small yaw angle on average.
The power output losses can simply be approximated to fall with cos3(yaw
angle).
Braking of a small wind turbine can also be done by dumping
energy from the generator into a resistor bank, converting the kinetic energy
of the turbine rotation into heat. This method is useful if the kinetic load on
the generator is suddenly reduced or is too small to keep the turbine speed
within its allowed limit.
Cyclically braking causes the blades to slow down, which
increases the stalling effect, reducing the efficiency of the blades. This way,
the turbine's rotation can be kept at a safe speed in faster winds while
maintaining (nominal) power output. This method is usually not applied on large
grid-connected wind turbines.
A mechanical drum brake or disk brake is used to hold the
turbine at rest for maintenance. Such brakes are usually applied only after
blade furling and electromagnetic braking have reduced the turbine speed, as
the mechanical brakes would wear quickly if used to stop the turbine from full
speed. There can also be a stick brake.