Civil Engineering Reference
In-Depth Information
a 200-mile single-circuit 500-kV, 3-conductor line is approximately $700,000 per
mile [
27
]. The main factors impacting the price of a transmission line are the ter-
rain, the location, the overall distance, the configuration of the conductors, and
any relevant environmental regulations. Wires which shield the conductor bundles
from lightning and grounding are normally installed above the bundles.
11.5.4 Conductor Sag
Because of all the different weather and loading conditions, one can never eliminate
sag in transmission lines and, thus, it must be designed in. The sag is a measurement
of the vertical difference between the line connection point on the adjacent tower
and the lowest vertical point within a particular span of the conductor. There are reg-
ulations as to how low a transmission line can be to the ground. Equation (
11.26
)
shows how to calculate the sag of a parabolic-shaped line [
30
].
wL
2
8
T
cond
S
cond
=
(11.26)
where
S
cond
is the sag in meters,
w
is the conductor weight in N/m,
L
is the hori-
zontal span length in meters, and
T
cond
is the conductor tension in N. In order to
use the above equation, the conductor weight must be provided in N/m. Equation
(
11.27
) shows how to convert the weight from kg/km to N/m.
w
=
w
c
·
9.81
1, 000
[
=
]
N
m
(11.27)
where
w
c
is the weight of the conductor in kg/km.
The sag in a transmission line is directly related to the tension in the line, and
this can be affected by the following variables [
30
]:
• An increase in temperature of the line can lead to thermal expansion of the con-
ductor, thus resulting in an increase in its length, as shown in Eq. (
11.28
).
L
= α
exp
·
T
·
S
(11.28)
where
α
is the coefficient of thermal expansion, T is the temperature increase in
°C, and S is the span length in meters.
• An increase in the wind speed can produce an extra force on the conductor, thus
increasing the tension in the conductor. This can elongate the conductor by way
of elastic stretching, as shown below in Eq. (
11.29
).
L
=
(
T
−
T
o
)
E
·
A
(11.29)
where
T
o
is the initial tension in N,
T
is the final tension in N,
E
is the coeffi-
cient of elasticity, and
A
is the cross section of the conductor in meters.
