Environmental Engineering Reference
In-Depth Information
Finally, the
energy gain E
over time
t
(a time period of one year is used in most
cases) becomes:
(5.116)
Mains connection
In most cases, wind power plants are erected in sparsely populated regions
where electric utilities normally operate only weak grids. Before connecting
modern wind generators with powers of more than 1000 kW to the mains, it
must be checked whether the mains has the capacity to take the generated
currents. To keep the transmission losses low, high voltages are chosen for the
transmission of high powers. The voltage ranges are subdivided into:
•
low voltage
0.1-1 kV
•
medium voltage
1-35 kV
•
high voltage
35-230 kV.
Usually wind power stations are connected via a transformer to the medium-
or high-voltage system.
Since both the mains and the wind power station can cause technical
problems, protection equipment must disconnect the wind generator from the
mains in case of error detection, thereby protecting the mains voltage and
frequency. The protection equipment must switch before there is a voltage rise
in one phase of 15 per cent, a voltage drop of 30 per cent or a deviation of
the frequency from the nominal frequency of 2 Hz.
Connecting generators to the mains also produces voltage fluctuations that
should not be greater than 2 per cent. The
maximum rated power S
rG
of the
wind generator is obtained from the short-circuit power
S
SC
at the point of
common coupling and the factor
k
as given in Table 5.9 (VDEW, 1994):
(5.117)
Hence, it is necessary to know the short-circuit power of the mains to calculate
the permissible generator power.
Another aspect that must be considered when connecting a wind generator
to the mains is so-called
flicker
. A series of voltage fluctuations can cause
changes in the luminance of light bulbs. If the generator power is lower than a
thousandth of the short-circuit power of the mains, no further investigations
are necessary. At higher powers, disturbing luminance fluctuations are
possible. To avoid these, a so-called long-term disturbing flicker factor
A
lt
is
estimated. With the relative voltage change
d
(for instance
d
=
S
rG
/
S
SC
) and
the form factor
F
(
F
= 1 for one voltage jump) the flicker after-effect time of
one voltage change becomes:
