The need for control goes back to the origins of wind turbines. The main control goals were the limitation of power and speed below some specified values to prevent the turbine from unsafe operation under high wind conditions. Former wind turbines included primitive mechanical devices to attain these objectives. As wind turbines augmented in size and power, control specifications became more demanding and regulation mechanisms more sophisticated. Increasingly, control systems have been expected not merely to keep the turbine within its safe operating region but also to improve efficiency and quality of power conversion. They gradually evolved in consequence until playing today a decisive role in modern wind turbines.

The development of a wind turbine control system can be divided into several steps. The first task is to define clearly the control objectives. The second task is the selection of a suitable control strategy, which settles the operating point of the turbine for each wind speed. The third task is to decide how the control strategy will be realised. It encompasses the selection of the control schemes, the controlled variables, the reference signals, the switching procedure between different controllers, etc. This step is usually referred to as controller setup. Finally, the last task previous to the implementation is the design of the input-output map, i.e., the dynamic characteristics of the controller according to the specifications. This chapter addresses the first two steps of the control system development whereas controller setup and design are left to the following chapters.