nection with or disconnection from the electric
distribution network, it should be
observed that there is a rotation interval
above the synchronous speed in which the
efficiency is very low. This effect is caused
by the fixed losses related to the low level
of generated power and torque in these low
speeds [12].
Another relevant aspect is the maximum
rotation. At this point, disconnection from
the distribution network should occur, so
that a control system will act as a brake for
the turbine under speed controlled operation.
The disconnection should be performed
for electric safety of the generator in
the case of a faliure in the control brake,
and for the safety of the local electric power
company maintenance teams when the generator
should be disconnected from the distribution
network.
Connection of induction generators to
the distribution network is quite simple
process, as long as the interconnection and
protection guidelines by the local utilities
are followed. Technically, the rotor is put to
rotation in the same direction in which the
magnetic field is rotating, as close as possible
to the synchronous speed to avoid
unnecessary speed and voltage mismatches
[2]. A phenomenon similar to the connection
of motors or transformers to the distribution
network will happen resulting in a
transient exchange of active and reactive
power between generator and load. The
load here considered can be any ordinary
electrical load or a power inverter for interconnection
with the distribution network or
any ac load.
The active power supplied by an induction
generator to the load, similar to what
happens with the synchronous generators,
can be controlled by speed change which is
related to controlling the mechanical primary
power. For the case of stand-alone operation,
the magnetizing current is to be obtained
from the self-excitation process. The mechanical
energy of rotation can only influence the
active component of the current with no
effect on the reactive component.
In the case of an induction generator in
parallel with a synchronous machine, the
excitation depends on the relative speed
between them, so the short circuit current
supplied depends on the voltage drop produced
across the terminals of the synchronous
generator. If the voltage across the
terminals goes to zero, the steady state
short circuit current is zero. The induction
generator dampens oscillations, as long it
does not have to run at the synchronous
speed. All the load variation is followed by
a speed variation and small phase displacement,
much the same with the synchronous
generator [2].