Environmental Engineering Reference
In-Depth Information
Figure 5.35
Operating Points for a Wind Turbine with Asynchronous
Generator that is Directly Coupled to the Mains
does not use the optimal power at all wind speeds. For this example, no power
can be taken from the wind at wind speeds below 4 m/s because the rotor speed
is too high. Maximum power usage is achieved at wind speeds of about 8 m/s.
The relative share of the power that can be taken from the wind decreases with
rising wind speeds.
System concepts with two different rotational speeds achieve higher energy
gains. One concept uses two asynchronous generators that can be coupled to
the rotor one after the other. Another concept applies one asynchronous
change-pole generator
. The stator of this generator has two separate windings
with different numbers of poles. When switching from one winding to the
other the stator speed also changes because the rotor speed directly depends
on the pole pair number. Hence, systems that work over two speed ranges can
be designed. Figure 5.36 shows that the ranges for taking the optimum power
from the wind can be extended by two different generator speeds. The first
asynchronous generator of this example operates at wind speeds between
3 m/s and 7 m/s, the second at wind speeds above 7 m/s.
The main disadvantage of the asynchronous machine is the demand for
reactive current. If the asynchronous machine is connected to the mains, the
mains can provide this reactive current. However, electric utilities usually want
high compensatory payments so that it is more economical to install
reactive
power compensators
. This can be, for instance, a capacitor bank as shown in
Figure 5.33. This capacitor bank can usually be designed for some operating
points only, so that the mains must still provide the remaining reactive power.
Modern power electronics can better compensate the reactive power.
The conditions for island systems are totally different. Since the reactive
power demand rises with increasing power, it must be controlled within the
island system. Power electronic units or a synchronous machine, which
operates as a phase modifier, can provide the respective reactive power
demand.
