Environmental Engineering Reference
In-Depth Information
Induction
generator
Figure 4.44
Soft starter
time the contactor at the bottom of the diagram can be closed and the soft starter's job is
complete. The induction generator is usually a 4-pole machine, which therefore has a shaft
speed close to 1500 rpm on a 50 Hz system (1800 rpm on 60 Hz). Thus, a gearbox is
required.
Following the soft starter there is a set of power factor correction (PFC) capacitors. The
induction generator naturally consumes reactive power in setting up its rotating magnetic
fi eld. This reactive power could be imported from the grid, but the network operator would
normally charge for it, making it more economic to create locally. A bank of capacitors is
usually installed in the nacelle or base of the tower to bring the overall power factor close to
unity through the power factor correction principle discussed in the Appendix. Sometimes
the capacitors are divided into sections that can be independently switched so that the reactive
power created can be adjusted to match that consumed by the induction generator as the
operating conditions change.
Lastly, Figure 4.43 shows a transformer that may be in the nacelle or in the base of the
tower and may or may not be included in the turbine manufacturer's scope of supply. The
induction generators in wind turbines usually operate at 690 V and the transformer steps this
up to the voltage of the wind farm power-collection network (typically 11 kV, or 33 kV in
the UK). Large wind farms may have a further transformer to step up the voltage again before
connection to the utility grid.
Two - speed Wind Turbines
The operating speed of an induction machine is based on the synchronous speed, which is
determined by the number of poles, which is in turn determined by the arrangement of the
stator windings. The main generators used in wind turbines often have four poles, which gives
a synchronous speed of 1500 rev/min on a 50 Hz grid. Use of six or eight poles gives 1000
and 750 rev/min respectively. It is possible to change the number of poles in an induction
machine by switching the connections to individual stator windings or by the use of two
generators. The low speed generator, operating at speed
2 in Figure 4.42, is used to improve
energy capture and reduce noise when the average wind speed is low. Since the power avail-
able from the wind is also low, the rating of this generator is between one-quarter and one-
sixth of the main generator. When it is detected that the average wind is above a predetermined
ω
 
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