Basic Electric Power Engineering Concepts - Renewable Energy in Power Systems - page 273

Additionally, the utility has the legal obligation to supply power to the consumer at a more
or less fi xed voltage. In Chapter 5 it is shown that for transmission lines, with X t R t , the

voltage V L at the consumer terminals is particularly sensitive to changes in Q rather than P .
It should now be obvious that the utility encourages consumers to draw P at minimum Q ,
i.e. with cos

>

as close to unity as possible.
If a capacitor C were to be connected across the load terminals and sized to generate Q C
so that Q C = Q L , then the consumer will appear to the utility as having a unity power factor.
The utility would be delighted with such an arrangement as this will minimize losses in the
transmission line resistance R t and will ensure minimal variation in consumer voltage. This
procedure is known as power factor correction . Such capacitors are used extensively in power
systems to generate or 'inject' reactive power at strategic points of the network.
This discussion also highlights why power system equipment is rated in terms of apparent
power VA rather than P or Q . As an example take the transmission line in Figure A.21. This
line is suspended from insulators that have been designed to withstand (including a safety
factor) the nominal voltage of transmission V L . It is also made of a conductor that has a spe-
cifi c resistance per km and chosen so that when the rated current is carried the heat generated
does not exceed a level at which the expansion of the line produces an illegal sag at the lowest
point of the catenary. With a purely inductive consumer load absorbing the rated current at
nominal voltage the transmission line will be fully loaded in spite of the fact that it transports
no useful power. Similar considerations apply to other power system equipment.

φ

A.15 Three - phase AC
Power systems are almost universally three-phase AC. This gives a signifi cant saving in the
construction of all power system equipment from generators to transmission lines and con-
sumer equipment compared to single-phase [1]. Even more importantly, three-phase very
elegantly provides the rotating magnetic fi eld required in synchronous and induction genera-
tors, as described in Chapter 4.
The three windings of the generator in Figure 4.2 are shown again in Figure A.22, feeding
individual consumers by means of six independent wires. The orientation of the symbols at
120 ° intervals mimics the orientation of the respective voltage phasors. The voltage V ph is a
phase voltage , and the same voltage magnitude could be measured on either of the two other
phases. Such a system would be perfectly capable of producing a rotating magnetic fi eld, but
the use of six wires is highly ineffi cient.

I

V ph

Figure A.22 Three - phase with six wires - three separate circuits