Civil Engineering Reference
In-Depth Information
(
)
η= − ⋅
12
RR
+
(6-12)
1
2
=
0.02 perunit. The efficiency is then simply 1-2(0.02+0.02) = 0.92 perunit, or
92 percent. Eight percent of the input power in this machine is lost in the
conversion process.
The losses generated in the machine are removed by providing adequate
cooling. Small machines are generally air-cooled. Large generators located
inside the nacelle can be difficult to cool by air. Water cooling, being much
more effective than air-cooling, can be advantageous in three ways:
For example, a machine with R
and R
each 2 percent, we write R
= R
1
2
1
2
•
for the same machine rating, the water cooling reduces the gener-
ator weight on the nacelle, thus benefiting the structural design of
the tower.
•
it absorbs and thus reduces the noise and vibrations.
•
it eliminates the nacelle opening by mounting the weather-air heat
exchanger outside, making the nacelle more weather-proof.
•
overall, it reduces the maintenance requirement, a significant ben-
efit in large machines usually sitting on tall towers.
6.2.6
Self-Excitation Capacitance
As the generator, the induction machine has one drawback of requiring
reactive power for excitation. The exciting power can be provided by an
external capacitor connected to the generator terminals (
Figure 6-7
)
. No sep-
arate AC supply is needed in this case. In the grid-connected generator, the
reactive power is supplied from the synchronous generators working at the
other end of the grid. Where the grid capacity of supplying the reactive
power is limited, local capacitors can be used to partly supply the needed
reactive power.
The induction generator will self-excite using the external capacitor only
if the rotor has an adequate remnant magnetic field. In the self-excited mode,
the generator output frequency and voltage are affected by the speed, the
FIGURE 6-7
Self-excited induction generator with external capacitor.
