Power system fundamentals - Wind Power Integration: Connection and System Operational Aspects - page 35

Assuming that the machine has been designed to give sinusoidal voltages, these
three-phase voltages may be depicted by the three phasors shown in Figure 2.13.
This is a balanced, three-phase supply . We will assume balanced operation, except
when considering unbalanced faults. The use of three phases has certain advantages:

Enables better use to be made of the space available for machine windings. ●

Provides the possibility of rotating magnetic fields in electric motors, and
hence simpler starting.

●

Reduces conductor material for a given power transmission capacity. ●

This will now be explained.

2.4.2 Comparison of single- and three-phase systems
Consider supply of a load of x kVA at a distance of l metres from a single-phase
source, as shown in Figure 2.14.
The conductor material requirement is 2k xl , where k is the constant of pro-
portionality relating conductor cross-sectional area to kVA for the system voltage.
Suppose the single-phase system to be replaced by a three-phase system deli-
vering the same apparent power over the same distance. Each phase will supply one
third of the load, as shown in Figure 2.15 for phase A.
The material requirement for phase A is 2k xl /3. Phases B and C will be similar,
giving a total material requirement of 2k xl , as before. However, consider the total

V C

V A

V B

Figure 2.13 Phasor representation of generated voltages

L

V

Load
x kVA

N

l

Figure 2.14 Single-phase supply