Civil Engineering Reference
In-Depth Information
FIGURE 6-10
Torque versus slip characteristic of induction generator under load.
of rotation of the magnetic flux wave with respect to the rotor. The torsional
stress on the turbine blades and the hub, however, may limit the braking
torque.
The torque-slip characteristic in the generating mode is separately shown
in
Figure 6-10
.
If the generator is loaded at constant load torque T
, it has
L
. Only one of these two pints, P1,
is stable. Any perturbation in speed around point P
two possible points of operation, P
and P
1
2
will produce stabilizing
torque to bring it back to P1. The figure also shows the limit to which the
generator can be loaded. The maximum torque it can support is called the
breakdown torque, which is shown as T
1
. If the generator is loaded under
max
a constant torque above T
, it will become unstable and stall, draw exces-
sive current, and destroy itself thermally if not properly protected.
max
6.2.8
Transients
The induction generator may experience the following three types of tran-
sient currents:
In the grid-connected system, the induction generator
is started as the motor in starting the turbine from rest to the super-synchro-
nous speed. Then only it is switched to the generating mode, feeding power
to the grid. If full voltage is applied during starting, the motor draws high
starting current at zero speed when the slip is one and the rotor resistance
is the least. The starting inrush current can be five to seven times the rated
current, causing overheating problems, particularly in large machines. More-
over, as seen in
Figure 6-11
,
the torque available to accelerate the rotor may
be low, taking a long time to start. This also adds into the heating problem.
For this reason, the large induction machine is often started with a soft-start
circuit, such as the voltage reducing autotransformer or the star-delta starter.
The modern method of starting is to apply reduced voltage of variable
frequency maintaining a constant volts/hertz ratio. This method starts the
machine with the least mechanical and thermal stresses.
Starting Transient:
