Power system fundamentals - Wind Power Integration: Connection and System Operational Aspects - page 34

power VI on the other. This relationship is often referred to as the power triangle ,
for obvious reasons.
The apparent power plays an important role in power system engineering. It
defines, for a given voltage, the physical current flowing in the circuit. Apparent
power is measured in volt-amperes (VA). Power engineering equipment is usually
rated in terms of VA as well as voltage.
From Figure 2.11 it is seen that VI ¼ apparent power S and that

p

P 2 S ¼ þ Q 2

S is measured in volt-amperes (VA, kVA or MVA).

2.4 Introduction to power systems

2.4.1 Three-phase systems
So far, it has been assumed that supply systems consist of a generator supplying
a load through go and return conductors. This is a single-phase system. In fact, AC
transmission invariably uses three phases. The reasons for this will be summarised
below.
Consider the idealised synchronous generator shown in Figure 2.12. A magnetic
field is produced by passing DC through the rotor or field winding. This may be
obtained from a small generator, known as the exciter , on the same shaft. The exci-
ter's windings are arranged in the opposite way to the main machine. Thus its field
winding is on the stator and carries DC. The field will induce alternating voltages in
the rotating coils on the rotor. This AC supply is rectified and fed to the main field
winding. The net effect of this arrangement is that a change of current in the exciter
field will cause the main field current to change in proportion. This in turn will create
a change of synchronous machine-generated voltage - something which is essential
for the smooth running of the power system as consumer demand and current vary.
The voltages induced in the stator coils A, B and C will be displaced by
one third of an electrical cycle from each other, with A leading B and B leading C.

A

C′

B′

Field winding
(direct current)

C

B

A′

Figure 2.12 Idealised synchronous generator