Civil Engineering Reference
In-Depth Information
The heat balance for overhead conductors can be shown in Eq. (
11.33
).
q
c
+
q
r
+
mC
p
d
T
c
d
t
=
q
s
+
I
2
R
(
T
c
)
(11.33)
where
q
c
is the convection heat loss,
q
r
is the radiation heat loss,
q
s
is the solar
heat gain,
mC
p
is the total conductor heat capacity, and
R
(
T
c
) is the resistance of
the electrical conductor as a function of temperature [
39
]. The term in this equa-
tion represents the watt losses in the conductor, where all of the power is assumed
to contribute toward resistive heating. However, if the stress in the OHTL is close
to the yield strength of the conductor and electricity is now flowing, there may be
a possibility that some plastic deformation could take place.
Overall, applied electrical power has been proven to have the capability to
greatly reduce a metal's yield strength above a material-specific, electrical power
threshold. Therefore, a portion of the conductor sag, which was originally consid-
ered to be due to heating effects, may actually be due to direct electrical effects
and could be represented by an EEC, specific to the conductor material properties,
the loading conditions, and the electrical power flowing through the line.
To bring the electroplastic effect on OHTL's into perspective, data from [
8
] will
be used to estimate currents which may be above the electrical threshold of materi-
als similar to the materials in the ACSR-DOG conductor. From this reference, the
electrical thresholds for Al6061 and A2 tool steel were found to be 60 and 45 A/
mm
2
, respectively. (Note that these threshold current densities were for compres-
sion and were for specific die speeds. These current densities are just intended to
provide an idea of how much current could be needed to potentially have an effect
on the conductor strength.) For the given diameter of the conductor (14.15 mm),
Eqs. (
11.34
) and (
11.35
) calculate the amount of electrical current where effects
on conductor strength would be notable, based on the electrical thresholds from
[
8
].
A
mm
2
π
4
(
14.15mm
)
2
Amps
Al
=
C
d
·
Area
=
60
·
≈
9,400 A
(11.34)
A
mm
2
π
4
(
14.15mm
)
2
Amps
Steel
=
C
d
·
Area
=
45
·
≈
7,100 A
(11.35)
The large OHTL's coming from the electrical generation stations may be able to
carry this amount of electrical current. However, the EAF experiments explained
throughout this thesis were conducted where the electrical power was supplied as
constant current, with varying voltage. The power in OHTL's is supplied in the
opposite manner, where the electricity in the lines is described in terms of voltage,
since the voltage running through the lines is constant, and the current fluctuates
depending on the power usage of the customers. In order to determine the magni-
tude of the reduction in the strength of the conductor due to direct electrical effects
(i.e., the EEC), specialized experiments would need to be run, which are explained
in the following subsection.
