In this chapter the main wind turbine modes of operation will be discussed. In general, the following modes of operation can be distinguished :
• Power generation
In the startup mode the rotor of the wind turbine is accelerated, and the generator is connected to the utility grid.
In the power generation mode, power is extracted from the wind and converted to electricity. In this mode the wind velocity ranges from the cut-in wind velocity Vci, via the rated velocity Vr to the cut-out wind velocity Vco as illustrated in Fig. E.1 for a fictive variable rotational speed wind turbine. For the purpose of illustration, the values for Vci, Vr and Vco are 3.0, 12.5 and 25.0 m/s respectively for the Lagerwey LW-50/750. In the power generating mode, two different operating areas can be distinguished, viz. partial load (Vci < Vw < Vr) and full load (Vr < Vw < Vco) . In partial load, the aerodynamic (or rotor) power is proportional to the cubic wind velocity. In this operating area maximum energy capture can be achieved by tracking the optimum tip-speed ratio curve (i.e. the Aopt-curve). Above the rated wind velocity the aerodynamic power P (and by that the rated generator power) is kept at a constant level: Pr. This is a given fact from generator design, in what, among other things, the wind regime, heat rise and loads acting on the construction are considered. This can be achieved either by stall or, in case of adjustable blades, by pitching the rotor blades towards a smaller (i.e. feathering control) or larger angle of attack (active stall control). In Molenaar  it has been concluded that i) adjustable blades are preferred to blades with a fixed pitch angle (despite the higher initial costs), and ii) pitching the blades towards a smaller angle of attack is preferred to active stall control.
Figure E.1: Rotor power (P) as a function of the undisturbed wind velocity (Vw) of a fictive variable rotational speed wind turbine, with Vci: cut-in wind velocity, Vr: rated wind velocity, Vco: cut-out wind velocity, Pci: start power, Pr: rated aerodynamic power, and tip-speed ratio.
In the shutdown mode the generator is disconnected and the turbine is decelerated. The system is in an emergency mode if a serious malfunction has been detected (e.g. failed pitch mechanism). If happens, the backup systems are invoked by the supervisory control system to provide for a safe shutdown.
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Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.