All wind turbines are equipped with a certain power control feature in case the speed of the wind is too high or too low and to have a frequency coherent with the grid. The traditional and the simplest and least expensive method in this respect is a system of natural limitation known as “stall”. The blades are mounted directly on the hub at a fixed angle. When the wind speed is higher than the operational speed, the blades begin to stall.
This system regulates the power by stalling the blades when a rated power is attained. It is simple and relatively reliable, but it misses precision because it depends on the air density and of the roughness of the blades.
The second method modern wind turbines use to control power is the pitch-controlled system, which is very precise; it can control and adjust the angle of the blades several times per second if necessary. Blade pitch control is, therefore, the system which, by regulating the blade pitch angle, turns (pitches) the blades so they can take maximum advantage of the wind and, at the same time, in case of very high wind speed it functions as a brake, adjust the angle blade so that the wind goes through. Also, the increased interest in pitch system is given by the increase of the rotor size.
As mentioned above, these pitch systems continuously control the speed, when it is too high they lower the blade speed rotation and in case of low wind speed they increase the rotation speed. Therefore, pitch systems are extremely useful; they “improve the efficiency of wind energy conversion and power generation stability” . Pitch systems are usually equipped with an electrical motor, a gearbox and an electro-mechanic actuator. There are two types of pitch systems: hydraulic and electromechanical.
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