Civil Engineering Reference
In-Depth Information
conductor cross section in mm
2
.
S
¼
b
¼
0.625 for the commonest cables and installation modes.
The electric current density is
I
S
¼
S
b
1
S
0
:
375
A
=
mm
2
δ ¼
a
¼
a
For two typical cables made with conductors having cross sections in the ratio
100-1 (e.g., 150 and 1.5 mm
2
), the ratio between the two values of the current
density is
0
:
375
δ
150
δ
1
:
5
¼
150
1
¼
0
:
17
:
5
This example shows that a given current density cannot be assumed as reference
value to design cable lines, but that voltage drops and continuous operating
temperatures are the main parameters to be considered.
Notice that lines are generally exploited less than they have been designed to,
because a site rarely expands as quickly as foreseen; in consequence, line losses are
much lower than the values given above.
In conclusion, because actual losses are comparatively small, energy-saving
design criteria are not extremely important for the design or the retrofit of the
industrial line.
7.3
Power Factor Control
The power flowing through the electric lines has two components: (1) the active
power (kW), which is the power available for conversion to mechanical, thermal,
chemical, light, or sound energy, and (2) the reactive power (kvar), which is used
for exciting magnetic fields in transformers and electrical machinery or electric
fields in capacitors.
The expressions for the active and reactive power are as follows:
P
¼
√
3
V
I
cos
φ
(kW).
Q
¼
√
3
V
I
sin
φ
(kvar).
The power factor, i.e., the cosine of the angle between voltage and current in a
specific section of an electric circuit, is an index of the ratio between the active and
reactive power flowing in the same section:
tan
φ¼
Q
/
P
cos
φ¼
cos atan
Q
/
P
This relationship can be used to calculate the power factor value in each section
of the network.
