Environmental Engineering Reference
In-Depth Information
values mentioned above) and the currents, throughout the system. The almost illegible
numbers in Figure 5.12 are the values of these variables, which have been calculated by load
fl ow analysis. How such calculations are performed will now be considered.
Consider the transmission line running across the middle of the diagram from node
Brennimelur
to node
Hrauneyjarfoss
. The power transfer over this transmission line will
obey the mathematical analysis presented earlier in this unit. Indeed, the power transfer
over every individual transmission line in the whole system must obey the equations pre-
sented earlier. The challenge is that, in general, the equations require to be solved
simultaneously.
While the mathematical techniques and the software packages for performing load fl ow
analysis are readily available, obtaining suitable input data can be much more diffi cult. The
required input data can be divided into network data and load/generation data.
5.6.3 Network Data
The impedances of the individual lines are usually calculated from knowledge of the line
type and the length of the line. In practice, particularly in lower voltage networks, these are
not always known with absolute certainly. The line may have been installed fi fty years ago
and could have been modifi ed since. Utilities do not always have perfect records. Also, par-
ticularly with underground cables, the details of the installation can signifi cantly affect the
characteristics; for example the dampness of the ground and the proximity of other cables
can have a signifi cant effect.
The ratios and impedances of the transformers must also be known and, again, records can
lack details. Any tap-changing mechanisms on transformers must also be included in the
model.
5.6.4 Load/Generation Data
Time Dependence
One load fl ow analysis solution provides the results for one operational instant. If any of the
loads or generator outputs are changed, then the analysis must be run again. With modern
computers, re-running the analysis is trivial; the challenge is in deciding what data should be
used for loads and generators, given that they are continuously varying.
A common approach is to identify the worst cases, and typical choices are the winter
maximum peak demand and the summer minimum, except in the US where the maximum is
due to summer air-conditioning. At the winter peak, the system is stretched to its limit and
it is necessary to ensure that no equipment is overloaded or any node voltage is below the
minimum permitted. During the summer minimum, because of the low loading of transmis-
sion lines and cables, there may be an excessive reactive power available that would manifest
itself as high voltage at nodes.
In a transmission system, the loads and generation normally vary smoothly and the relevant
maximum and minimum fi gures can readily be identifi ed. In lower voltage networks, the
loads are much more variable and the worst case conditions can be very diffi cult to
identify.
