Continuous advances in power electronics applied to wind turbines have now drawn the attention of almost all wind turbine manufacturers to turbines which possess the ability to continuously vary rotational speed with the wind velocity, although world wide they are still outnumbered by constant rotational speed wind turbines. Constant speed operation has not been the choice of wind turbine designers, but rather a necessity brought about by the fixed relationship between the speed of the AC generators and the fixed utility grid frequency. The two main advantages of variable speed operation over constant speed operation are additional energy capture at partial load and potential reduction of fatigue loads on rotor and drive-train. At present, however, the convincing argument is the ability to meet the (stringent) power quality requirements.
This can be easily seen by recognizing that the power available in the wind varies with the cube of the wind velocity. Therefore it is desirable to let the wind turbine speed vary over a wide range to an optimum value depending on the operating conditions. This would not be possible if the (three-phase) generator were directly connected to the utility grid. To allow the generator (i.e. wind turbine) rotational speed to vary, a power electronic interface is needed. In such an interface the three-phase generator output is, in general, rectified into DC and subsequently interfaced with the three-phase utility source by means of a power electronic converter as illustrated in Fig. 3.22. In general, a rectifier at the generator side as well as an inverter at the utility grid site is required to provide both control and power quality requirements (e.g. power factor). Both the rectifier and the inverter act as a voltage and frequency changer.
In addition, variable speed operation enables reduction of periodic torque pulsations, caused by e.g. tower shadow or wind shear, by short-term kinetic energy
storage in the rotor. Fatigue load reduction, however, is the most important and still underestimated advantage of variable rotational speed operation. After all, in 1994 it has already been demonstrated by Bongers  on an experimental test-rig (consisting of a drive-train of a variable speed wind turbine with a torque generator replacing the rotor system) that fatigue loads acting on the wind turbine structure can be significantly reduced by means of advanced controller design and implementation. Whether these advantages outweigh the disadvantages (higher initial costs, increased complexity and potential lower reliability respectively) is dependent on the design of the complete wind turbine.
However, in general, the bandwidth of the active blade pitch system is too small to achieve fatigue load reduction. As a result, pitch control has to be used to follow minute-to-minute fluctuations in aerodynamic power, while the electromechanical torque control will focus on fatigue load reduction. The design of a robust frequency converter controller for high dynamic performance of a variable speed generator requires an accurate dynamic model of the electromagnetic part. In essence, there are two aspects of a generator that need to be modeled, viz. the mechanical and the electromagnetic part. The mechanical part can be modeled using the techniques outlined in the previous section. In this section we will restrict ourselves to the dynamic modeling of the electromagnetic part.
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