Nowadays, the use of renewable energies for electricity generation is very common, being wind power one of the most efficient among them. Wind power makes it possible to obtain electricity from the wind by using wind turbines. Said wind turbines basically include a tower, a nacelle which houses a rotary electric generator and a rotor formed in turn by at least two blades, and a power train which transmits power from the rotor to the electric generator. The power train may comprise a gearbox with a low-speed shaft connected to the rotor and a high-speed shaft connected to the electric generator.
In multi-Megawatt wind turbines, there is a trend towards larger rotors, which provide energy at a lower cost. In said configurations there is an increasing importance of the control system. Said system makes it possible to maximize the energy production whilst limiting the mechanical loads produced by the wind. To do this, the control system acts on the blade pitch angle and on the torque demanded to the generator.
On the one hand, the blade pitch angle is controlled by actuators disposed in the root of each blade, which makes the blade rotate around its longitudinal axis. Said actuation manages to vary the aerodynamic behaviour of the blade. On the other hand, the control system regulates the torque demanded to the generator from the converter.
The torque control in accordance with the electric generator speed Q includes different control areas:                a first control area (at low wind speeds) wherein the torque is regulated to maintain the rotation speed constant;        as the wind speed increases, it enters into a control area wherein the electric torque demand T is made so that the ratio between the blade tip speed and the wind speed at the hub height (Tip Speed Ratio (TSR)) is maintained in an optimum value which maximizes the aerodynamic power capture of the wind;        once the maximum rotation speed is reached, the torque is regulated to maintain the rotation speed constant in said maximum value until reaching the rated power of the wind turbine. This occurs at a wind speed which will be hereinafter called rated wind speed.        
The blade pitch control also includes different control areas:                A control area below the rated wind speed, i.e. below rated power, wherein a blade pitch set-point is applied which serves to maximize the wind power capture for each incident wind speed. Typically, as incident wind signal an average wind speed is used calculated from measurements taken at the hub height by an anemometer located there. In accordance with the value of said average wind speed, a single pitch angle set-point is calculated and applied to the three blades.        A control area above the rated wind speed wherein the pitch angle is regulated in order to maintain the power constant (typically at a value equal to the rated power).The blade pitch angle for each incident wind speed is applied jointly to the three blades.        
However, in the control area below the rated wind speed, the fact that the blade pitch angle is calculated in accordance with the average wind speed (or a signal indicative thereof such as the power and/or the blade pitch angle) means that it does not consider effects such as windshear, upflow or misalignment of the nacelle with respect to the wind direction; effects which are largely independent of the average wind speed. This causes that the blade pitch angle calculated in accordance with the average wind speed media is not always optimum from the production side.
With the aim of increasing production in wind regimes below rated wind speed, document EP2556249 discloses a control method consisting of controlling the actuation of the blades (blade pitch angle or alternative control elements) to independently maximize the driving moment of each blade below the rated wind speed. The controller independently calculates the blade pitch set-points for each one of the blades so that it maximizes the driving moment of each one of them when it determines that the wind speed or the force that acts on the blades is below the rated wind speed with the aim of increasing production in said wind regime. To implement said control method it is necessary to know the driving moment of each blade, for which individual load sensors per blade are necessary, the reliability and precision of which is critical in order to maximize said production, and the blades may even be damaged if they give an incorrect measurement. Furthermore, if the sensors are damaged, it will be needed to await until repair of the same before continuing to perform the control method described.