This chapter describes the main components and characteristics of a WECS. Our primary objective is to derive a control-orientated model of the entire system. With this aim, the WECS is organised into four main functional blocks, namely the aerodynamic, mechanical, electrical, and pitch servo subsystems.

The aerodynamic subsystem is devoted to the conversion of the harnessed wind energy into useful mechanical energy. The mechanical subsystem fulfils two main functions. The first one, carried out by the drive-train, is to transfer the torque from the rotor to the electric generator. The second one is to support the rotor and other devices in height while withstanding the thrust force. The electrical subsystem performs the conversion of mechanical power at the generator shaft into electricity. Finally, the pitch servo subsystem consists of a hydraulic or electromechanical device that rotates (part of) the blades around their longitudinal axes, thus modifying the pitch angle.

Models for the individual subsystems are developed through the chapter and are subsequently interconnected to obtain a model for the entire WECS. Also, a model of the wind is presented. This model takes into account the mean wind speed variations, the turbulence and the cyclic disturbances inherent to rotational sampling. The realistic wind signals so obtained will be used in future chapters to assess the power quality and load attenuation features of the control systems analysed therein.