# Adaptive Pitch Controller of a Large-Scale Wind Turbine Using Multi-Objective Optimization
## 1 Introduction  
Wind energy has a leading role in the current great demand for renewable energy and is one of the sources that has experienced a greater growth in recent decades [1]. In order for this energy to be competitive, it is necessary to reduce its production and maintenance costs [2]. From a control point of view, this can be achieved by operating wind turbines to generate energy captured from the wind more efficiently and to reduce their different structural loads improving their reliability [3]. Furthermore, the control system design of wind turbines is even more difficult due to the fact that wind turbines are non-linear systems with multiple outputs to be controlled and are subjected to intermittent and variable disturbances such as the wind direction and wind magnitude [4, 5]. Traditionally, single controllers have been used in the different operating zones of the turbine, either to extract as much energy as possible for low wind speeds or to limit the generated power to its nominal value and thus avoid potential damages at high wind speeds [6]. Nowadays, horizontal axis variable speed and variable blade pitch (VS-VP) wind turbines with three blades are the most widespread because they allow the generation of optimal power at different wind speeds [7]. VS-VP wind turbines can work in several operational regions depending on the wind speed [8]: cut-in (I), partial load (II), transition (III), full load (IV), and cut-out (V). The ideal power curve of a wind turbine is illustrated in Figure 1, where the potential generated power according to the wind speed is shown, and the different operation regions are marked. This work is focused on region IV (full load region), where the effect of the high wind speed has to be reduced in order to avoid damages on the system. In this case, it is necessary to maintain the generator power and, consequently, the generator speed, at their nominal values. This is performed by means of controllers that act on the blade pitch angle to modify the wind turbine aerodynamics limiting the energy extracted from the wind [10]. In this region, the generated power (\(\mathrm{P_{g}}\)) depends exclusively on the generator speed (\(\omega_{\mathrm{g}}\)) according to Equation (1), since the generator torque is kept constant at its rated value. \[\mathrm{P_{g}}=\mathrm{T_{g\_rated}}\cdot\omega_{\mathrm{g}} \tag{1}\]  
On the other hand, unmitigated structural loads can cause undesirable performance or even lead to early failure of the whole wind turbine system. The various types of these loads are commonly caused by the different deflection modes in the tower and rotor blades or vibrations in the turbine drive train. The tower structure mainly shows two vibration modes: fore-aft and side-to-side. The fore-aft movement is the tower rocking from back to front. There is a wide variety of works dealing with the mitigation of this vibration mode [11; 12; 13]. Devices such as calibrated liquid column dampers (TLCDs) can be used in order to reduce these undesired oscillations. They can be fully passive [14] or semi-actively controlled [15; 16]. Another option is to use active control methods. Some commercial controllers use pitch control to actively mitigate the tower fore-aft (f-a) movement when the wind turbine is operating at region IV. This is called Active Tower Damping Control (ATDC) and is usually performed adding another control loop to the basic turbine speed controller [17]. Feedforward control is another strategy that can be used to decrease structural fatigue and improve reference tracking of the generator speed. Traditionally, wind speed was measured in wind turbines through an anemometer, which was placed on the top of the nacelle. Nowadays, a more precise wind speed measurement can be obtained by means of technology based on light detection and ranging (LIDAR) [18]. LIDAR sensors allow a better implementation of those control strategies that take into account such measured variables.