Environmental Engineering Reference
In-Depth Information
10.91
11.01
11.04
11.06
10.92
10.99
11.20
10.93
11.10
11.03
10.92
10.97
11.10
10.93
10.92
10.97
10.92
Figure 6.5
Load fl ow results: voltages (kV)
to correct the power factor towards unity, the voltage rise would have been signifi cantly
higher and perhaps unacceptable. A rerun of the load fl ow analysis would provide a wealth
of information on system voltage sensitivity to a range of renewable energy source locations
and operational conditions.
6.3 Thermal Limits
Power injected by a distributed generator will alter the local network power fl ows. Power
fl ows in individual components may be increased, decreased or even reversed. Power fl ow
(both active and reactive) requires current fl ows and this causes heating in lines, cables and
transformers. Each of these components has a thermal limit, which effectively determines the
maximum current it can carry, and these thermal limits can set an upper limit on the size of
the renewable energy generator that can be accommodated. Load fl ow analysis allows all the
component currents to be calculated.
6.3.1 Overhead Lines and Cables
It may appear surprising that a great many lines and cables are normally operated at well
below their thermal limits, particularly in lower voltage networks (up to say 20 kV). There
are two reasons for this. First, in order to keep customer voltages within range, voltage drops
in conductors must be limited, which often requires use of larger conductors than those dic-
tated by thermal limits. Second, energy losses are signifi cantly less in larger conductors and
a life-cycle cost analysis usually favours such oversizing. As a consequence thermal limits
of lines and cables are rarely reported as a limiting factor in the installation of embedded
generators in 11 kV or similar networks. Voltage and increased fault level considerations
arising from the connection of additional generation, discussed later, usually come into play
fi rst.
At higher voltage levels, i.e. 33 and 132 kV, voltage control is less demanding and thermal
limits of lines and cables may well be the limiting factor. These limits are well understood
and rarely the subject of dispute, but there are a couple of aspects worth noting. First, care
