Overview

One of the early mechanisms used for power and speed regulation in wind turbines was based on pitch angle control. This control scheme was gradually replaced by passive stall methods, which achieve power limitation without the need for active devices. Despite their hardware simplicity, passive stall controlled WECS exhibit reduced conversion efficiency and are exposed to higher stresses potentially increasing the danger of fatigue damage. Lately, interest in pitch angle control has revived, now combined with variable speed operation. This renewed interest is due to the growing concern about mechanical loads and power quality [2, 30].

With the aim of using simple controllers, the generator torque and pitch angle of VS-VP wind turbines are often controlled separately. In low wind speeds, wind turbines are operated at variable speed and fixed pitch in order to capture as much energy as possible. In high wind speeds, the pitch angle is adjusted to limit the captured power at its rated value whereas the generator torque characteristic often remains unchanged [16]. Nevertheless, this mode of operation does not fully exploit the capabilities of VS-VP wind turbines. In fact, power regulation using only pitch angle control exhibits some limitations, which are due to constraints on the amplitude and speed of response of the pitch servos [52, 62]. For instance, high frequency load mitigation, as well as power smoothing, demands fast changes of the pitch angle that might cause fatigue damage to some mechanical devices. By combining pitch angle control with variable-speed operation, the activity...