Electricity Production from Renewables Energies

3.4. Power limitation by varying the power coefficient

High wind speeds enable us to obtain significant powers, but they are not very common. Wind turbines are thus not oversized for these speeds, because this would lead to additional costs. Turbines are generally designed to reach their nominal power at a wind speed of about 15 m/s and are automatically stopped at 25 m/s. To limit the power produced at high wind speeds although the aerodynamic gross power of the wind keeps on increasing in v³, there are two categories of power regulation systems:

– the “stall” or the natural aerodynamic stall system, for which blades have a profile that is designed to naturally reduce the power coefficient C_p , when the wind speed increases, i.e. when the tip speed ratio β decreases — this is the stall principle;

– the “pitch” or variable pitch angle system, with mobile blades around their longitudinal axis. This enables us to reduce the lift and therefore the C_p coefficient for significant wind speeds.

3.4.1. The “pitch” or variable pitch angle system

A blade is at variable pitch angle or at variable step, if the orientation of the blade in relation to its initial position can be modified during operation. Blades are equipped with actuators (hydraulic jacks or electric motors), which are responsible for changing their orientation. Given the inertia of a blade and the importance of the forces exerted on it, orientation is slow. For example, the temporal angle variation is about 8°/s for a wind ...