The use of wind energy has grown constantly during the last 10 years. This has led to an increase of research and development of larger and more effective wind turbine in order to offer renewable energy to the customers . This has also lead to, as the sizes of the systems increase, that controlling of wind turbines are beginning to be more flexible and also crucial.

The future promises a further increase in the use of control systems as new designs investigate controlling load and power excursions by using variable speed operation, and also aerodynamic controls and other devices in more complex schemes.

Early modern-era wind turbines used passive blade controls to regulate power, yawing and rotor braking. As the size of turbines and the knowledge of automation control systems increased, other passive devices became impractical and active control systems arose. Today a 1-2 MW wind turbine often has active pitch control.

The wind turbine is a device for conversion of kinetic energy in the wind into electricity. Although there are many different configurations of wind turbines systems they all work in the same way. The available power originates from the mass ow of the moving air, referred to as the wind speed. The transformation to mechanical torque is done by aerodynamical forces acting on the rotors blades, the actuator disc. The wind turbine shaft then transports the power to the generator which is connected to the electrical grid. Usually there is a gearbox between the slowly rotating turbine shaft and the more rapidly rotating generator shaft.

As the incoming wind contains the energy input to the system, the power output is dependent on the wind speed. A lowest and highest wind speed, WScutin respectively WScutout, determines the range in which wind turbine can operate.

The aerodynamics behavior of the wind turbines is described with mass ow rate which must be the same everywhere within the stream-tube. When kinetic energy is extracted from the wind at the actuator disc it slows down its velocity.

The mass of air which passes through a given cross section of the stream-tube in a unit length of time is AWS, where  is the air density, A the cross-sectional area and WS is the ow velocity. Because the mass ow rate must be the same everywhere along the steam-tube this gives the equation A 1 WS 1 = AdWSd = AwWSw: (2 The symbol 1 refers to conditions far upstream, d to conditions at the disc and w to conditions in the far downstream, wake. It is usual to consider that the actuator disc induces a velocity variation which must be superimposed on the free-stream velocity. The stream-wise component of this induced ow at the disc is given by -aWS 1, where a is called the axial ow induction factor, or the in ow factor. At the disc therefore the netstream-wise velocity is WSd = WS.