Environmental Engineering Reference
In-Depth Information
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.
Synchronous generator with DC-link converter
Modern power electronics can avoid the disadvantages of a synchronous
generator that is directly coupled to the mains. The synchronous generator is
connected via a DC link or frequency converter to the mains (see Figure 5.38).
In this configuration, the system can operate the generator at a frequency
independent of the mains frequency. Changing the generator frequency varies
the generator speed. Hence it is possible to vary the speed over a wide range
and to run at the optimal speed to obtain the maximum power use depending
on the wind speed.
Figure 5.39 shows clearly that it is possible to take the maximum power
from the wind at low and medium wind speeds by changing the rotor speed.
At high wind speeds it is necessary to limit the power. Two possibilities exist.
In the first solution, the power electronics keeps the frequency and therefore
the speed constant. Then a stall control system can limit the power (see also
Figure 5.14). In the second possibility, an inverter limits the power at high
speeds.
However, neither solution can completely avoid the risk of rotor
overspeeding at very high wind speeds; therefore, an additional power limiting
mechanism is used, for instance by adjusting the pitch of the rotor blades. This
shifts the rotor curves to smaller speeds than shown in Figure 5.39.
Since the frequency converter can achieve other rotor frequencies than that
given by the mains, a gearbox for adjusting the rotor speed to the generator
speed is no longer needed. Gearless wind turbines are produced in high
numbers even in the megawatt class today. Generators for these turbines are
high-pole synchronous generators with 80 or more poles. Besides the reduced
material requirements and resulting cost reductions,
gearless wind power
Rotor
Excitation
Gearbox
SG
Transformer
Generator
Grid
Figure 5.38
Synchronous Generator with DC Link
