Environmental Engineering Reference
In-Depth Information
I
V
ph
V
Figure A.23
Three - phase with four wires
I
V
Figure A.24
Three - phase with three wires
In Figure A.23 the three conductors bunched together in Figure A.22 are merged into one
common return conductor known as the neutral. With the circuits combined in this way, a
voltage can now be measured between any two outer lines. This is known as the line-to-line
voltage or simply the
line voltage
and magnitude-wise is the same no matter which two lines
are chosen.
When discussing three-phase electricity it is normal practice to quote line voltages rather
than phase voltages. Thus, the line voltage in Figure A.23 is indicated by
V
(without a
subscript). Consideration of the geometry of the related phasor diagram shows that (under
balanced conditions)
3
ph
where
V
is the line voltage and
V
ph
is the phase voltage. The four-wire arrangement is very
widely used at the low voltage distribution part of a power system (Figure 1.12); for example
V
= 400 V and
V
ph
= 230 V.
When a three-phase system is perfectly balanced, the currents in the three phases have
equal magnitudes and angles at exactly 120 ° intervals. Thus, the phasor sum of these currents
is zero and so the current in the neutral is also zero. In this case, the neutral can be removed
and the system operated with just three wires as shown in Figure A.24.
In practice, perfect balance of the three phases is not an absolute requirement. A three-wire
system can operate in an unbalanced condition, but this is usually undesirable and is certainly
harder to comprehend. Unbalanced operation is mentioned where necessary, but in general
balanced operation is normally assumed in this topic. The three-wire arrangement shown is
very widely used at 'higher' voltages, i.e. 11 kV and above.
VV
=