## 1 Introduction  
The current interest in integrating wind energy systems into green energy grids has elevated the requirements for the operation of wind turbines. An important aspect of operating a wind turbine inside a power network is that it has to satisfy the same conditions as are normally required for traditional power plants. Since this is not possible, wind turbines are grouped in wind farms, which in turn have to provide the behaviour of a power plant. In other words, wind farms must offer grid services such as, e.g., frequency stabilisation, frequency response, reactive power management, scheduling and dispatching, management of reserves, etc. [1, 2]. They are also called ancillary services [3]. Hence, it can be profitable to reduce active power delivery in order to sell reserve power. In addition, there are other reasons to reduce active power generation, such as, for example, when the load dispatching requires only a power fraction of the full power plant capacity, or when the wind turbines have to reduce the rotational speed because of noise generation during the night [4]. On the other hand, sometimes it is very profitable if the power plant is able to deliver a power boot for a short time [5]. In other words, the power plant is required to reduce or increase its power output compared to its nominal value [6]. In the case of wind farms, these features could be achieved by connecting and disconnecting wind turbines from the grid. However, in the case of large three-bladed horizontal-axis wind turbines with variable speed and variable pitch, it is more convenient to reduce or increase the power output of all the machines individually but as a whole according to an individual power profile. Thus, the traditional operation in which a wind turbine should be operated in such a way that it is able to deliver as much power as possible is no longer valid in general, and a new paradigm for wind turbine control is required. The idea here is to use reference variables provided by the wind farm control for each machine and to configure the local control loops in such a way that the control system works to solve a tracking control problem [7]. Thus, local controllers will be able to follow a power profile, which is precisely the reference variable provided by the wind farm control. If the power profile goes under the power rated value, the operation is called down-regulation [8; 9], de-rating [10], or de-loaded operation [11; 12]. When the power profile is overrated, the operation can be named up-regulation or up-rating [6]. There are also other reasons for the down-regulation such as, for instance, wake [13] and fatigue reduction [14; 15], as well as optimisation of wind farm efficiency [16]. In the traditional concept, wind turbines are operated for power production in two possible operational states. The first one is characterised by wind speeds under the rated value. It is called control in Region II and is mainly implemented by generator torque control. The second one is entered when the wind speed goes over this rated value, i.e., control in Region III. The main control loop is the pitch control, which is used to regulate the rotational speed at its rated value [17]. Since the wind speed is actually a 3D wind field, it is useful to use the effective wind speed to this end. Here, the operating point changes with the wind speed. In the new concept, the operating point changes with the power profile, and the wind speed acts as a constraint. This change in the control paradigm leads to additional complexity in the control system of the individual wind turbines, where not only the configuration of the control loops but also the control strategies have to be adjusted to accommodate the up/down-regulating control features. In addition, a complicated discrete logic has to be included in order to reconfigure the control system and change strategies. In the present work, the idea is to design a general control system that can inherently track power references without special changes or reconfigurations.