Physical models of the actuators and the electrical subsystem of the generator are seldom included in the modelling of a wind turbine. However, they are sometimes useful for the control system design and therefore, they are included here for the sake of completeness. First, generic models in the form of first-order and second-order linear models are considered, followed by physical models. Large turbines have primarily hydraulic pitch actuators but in the last time new powerful electrical drives are available, as well. For this reason, hydraulic and electrical models are considered here. In the case of the yaw systems, although there are some hydraulic mechanisms, most systems are implemented by using electrical actuation.
