Electrical - Wind Energy: Renewable Energy and the Environment - page 138

FIGURE 7.9 Induction generator, 750 kW; stator; and slip rings for transferring power. (Photos by Wade
Weichmann. With permission.)
Some large wind turbines had two generators, one for low wind speeds and the other for high
wind speeds. A common design for newer machines is pole changing, and therefore they are able to
run as a small or large generator, for example, 400 or 2,000 kW, at two different rotational speeds.
The use of one, two, or pole switching generators depends on the energy produced and the extra
cost of each option.
The switching mechanism must not allow the generator to operate below synchronous speed or it
would be a gigantic fan. The control mechanism needs to measure rpm, with some leeway for wind
speeds at the cut-in value, to turn the generator on and off, and at the high wind speed to cut out and
restart the generator after the winds have declined. Some wind turbines use the motor/generator for
start-up, as their blades do not have enough starting torque. When the winds become high enough
(cut-in wind speed), the blades are turned by the motor/generator, and then as rpm increases due to
wind power, and the motor/generator goes past the synchronous speed, it now becomes a generator.
The time delay reduces on-off cycling when the winds are just around those cut-in and cut-out wind
speeds. There was a case where a small (5 kW) downwind wind turbine would start in the upwind
position due to the winds being shifted by 180° from when the turbine shut down. The upwind
position is an unstable condition for a downwind rotor with coning. The control system indicated
start-up; however, the blades were inefficient in that position and the wind turbine used 2 kW of
power—it really was a big fan.
Induction generators (Figure 7.9) are the most common generators for wind turbines from 25 kW
to megawatts because the controls for synchronization to the line are simple, and they are rugged
and mass produced. When there is a failure on the utility grid, they automatically disconnect and
do not present a safety problem. The induction generators decrease the power factor, and correcting
capacitors are installed on individual wind turbines or at the wind farm.
It is possible to have a resonance condition with inductance and capacitance; however, the vari-
ability of the wind ensures that the induction generator output decreases rapidly when there is a fault
on the utility line. Remember, the induction generator is essentially a constant-rpm operation for
the rotor, which is fixed by the frequency of the utility grid. The wind rotor/generator combination
reaches peak efficiency at only one wind speed.
7. 2 . 2 D OUBLY F ED I NDUCTION G ENERATOR , V ARIABLE -RPM O PERATION
There is a gearbox connected to a standard (mostly 1,500 rpm) doubly fed induction generator.
The stator is directly connected to the utility grid and the rotor of the generator is connected to a
converter. An rpm range of 60 to 110% of the rated rpm is sufficient for good energy production. At
wind speeds above the rated wind speed, the blades are pitched to reduce aerodynamic efficiency.
Variable blade pitch is also used for start-up, shut down, and overspeed.