Environmental Engineering Reference
In-Depth Information
Figure 5.39 Operating Points for a Variable-Speed Wind Generator with
Power Limited by Constant Speed (1) or by a Converter (2)
plants have other advantages such as lower noise levels. If pulse width
modulated inverters are used, the power electronics can also control the
reactive power demand.
Speed-controlled asynchronous generators
For directly connected asynchronous generators it has already been shown that
a variable slip could change the speed of the generator. Since high slips cause
high losses in the rotor circuit, the slip was limited to 10 per cent in the
examples above. However, it is also possible to use the rotor power and feed it
into the mains (see Figure 5.40). If there is only the possibility to vary the
generator speed above the mains frequency, the circuit is called an
oversynchronous converter cascade . A disadvantage of this circuit is the high
reactive power demand.
Whereas an oversynchronous converter cascade can only transfer power
from the rotor to the mains, a double-fed asynchronous generator can also
transfer power from the mains to the rotor. Therefore, a DC link converter or
a direct converter can be used (see Figure 5.41).
Converter
cascade
Rotor
Transformer
Gearbox
ASG
Generator
Grid
Figure 5.40 Variable Speed Asynchronous Generator with
Converter Cascade
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