Civil Engineering Reference
In-Depth Information
11.5.6 Applying EAF Modeling to OHTL Sag Calculations
Of note, from all the research that has been collected on transmission line design
and the sag in transmission lines, the main reason for the increase in the sag is due
to the temperature increase by way of thermal expansion when excess electricity is
supplied through a transmission line [
33
]. However, what if there were actual effects
due to the electroplastic effect as well? For example, in the EAF modeling strategy
of this thesis, the applied electricity is separated into the portion that assists deforma-
tion and the portion that assists resistive heating. The resistive heating portion of the
applied electricity is all that is accounted for in the thermal expansion calculation.
For example purposes, a “DOG” ACSR conductor will be examined with the
following specifications [
34
]:
• Overall line diameter of 14.15 mm
• Line weight of 3.89 N/m
• Calculated breaking load (CBL)
=
32.68 kN
• Initially set tension in the cable
=
20 % CBL
=
6.536 kN
• Tower span
=
275 m
Using Eq. (
11.26
) above, the conductor sag can be calculated, as shown Eq. (
11.30
):
Sag
=
3.89
m
·
(
275 m
)
2
8
·
(
6,536 N
)
=
5.6m
(11.30)
The typical amount of sag in an OHTL can be from about 3 to 10 m, depending on
the electrical conditions. Aside from the sag on the line, it is also important to ana-
lyze the tension, and corresponding stress in the line. The stress can be calculated
from Eq. (
11.31
):
Tension
Area
=
[N]
Cable Stress
=
=
MPa
(11.31)
m
2
The stress on this particular OHTL is shown in Eq. (
11.32
):
6,536 N
Cable Stress
Dog_Conductor
=
4
(
0.01415m
)
2
=
41.56MPa
(11.32)
π
Although the stress in the ACSR is below the yield strength limit for high-strength
steel [
35
,
36
], it may already be higher than the yield strength limit for 1xxx elec-
trical aluminums depending on the temperature of the conductor [
37
]. In addition,
the lines could potentially be rated for carrying several hundred or even thousands
of megawatts (MW) of power through them. Experimental works have shown that
EAF has the ability to significantly reduce the stress needed to plastic deforma-
tion in both aluminum and steel metals [
8
,
19
,
20
,
23
,
38
]. With this being said,
the direct electrical effects may lower the plastic deformation strength of the lines
even further than what the heating can attribute toward.
