Civil Engineering Reference
In-Depth Information
FIGURE 13-7
Resilience in the torque versus speed characteristic of induction generator.
synchronous condition. The magnitude of the restoring power, also known
as the synchronizing power, is highest if the machine is running at no load,
and is zero if it is running at its steady state stability limit.
13.2.2
Synchronous Operation
Once synchronized, the voltage and frequency of the wind system need to
be controlled. When the induction generator is directly connected to the grid,
the grid serves as the frequency reference for the generator output frequency.
The grid also acts as the excitation source supplying the reactive power. Since
the torque versus speed characteristic of the induction generator has a steep
slope near zero slip (
Figure 13-7
),
the speed of the wind turbine remains
approximately constant within a few percentages. Higher load torque is met
by increased slip up to a certain point (Q
), beyond which the generator
becomes unstable. If the load torque is immediately reduced, the generator
will return to the stable operation. From the operating point of view, the
induction generator is softer, as opposed to the relatively stiff operation of
the synchronous generator, which works at an exact constant speed or falls
out of stability.
If the synchronous generator is used, as in wind farms installed in Cali-
fornia in the 1980s, the voltage is controlled by controlling the rotor field
excitation current. The frequency control, however, is not required on a
continuous basis. Once synchronized and connected with the lines, the syn-
chronous generator has an inherent tendency to remain in synchronous lock
with the grid. Only during transients and system faults, the synchronism
can be lost. In such cases the generator must be resynchronized.
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