The synchronous generator does not have the disadvantage of requiring a certain reactive power demand since this can be controlled by the excitation current. Therefore, a synchronous generator can both generate and consume reactive power. Permanent magnets can also be used to excite the synchronous generator. In this case, reactive power cannot be controlled. Furthermore, permanent magnets are rather expensive. Hence, power electronic components such as thyristors usually convert the three-phase current of the mains to the desired DC that determines the excitation (see Figure 5.37).
In contrast to the asynchronous generator, the synchronous generator runs with a constant speed. The operation characteristics in the speed-power diagram are indicated by an absolutely vertical line that has no slight bend at higher powers as with the asynchronous generator. The slip cannot cushion power jumps so that they pass almost entirely to the mains. Besides changes in load at the mains, this can cause high mechanical strains on the wind generator itself. A slip clutch can cushion gusts but also exhibits high wear.
Therefore, synchronous generators are usually not coupled directly to the mains as shown in Figure 5.37. The concept of direct coupling is mainly used for stand-alone systems in island grids. In such grids, usage may consist of pumps driven by three-phase motors or DC loads with rectifiers and batteries. This allows the synchronous generator to work at different operating points with different speeds, which reduces the loads significantly.
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