Civil Engineering Reference
In-Depth Information
• If ice forms on the conductor, it will increase the overall diameter of the con-
ductor and will also add weight to the line. This will increase the tension and
can lead to elastic stretching as was calculated in the equation above.
• As the transmission lines age, they go through countless cycles of loading and
unloading and heating and cooling, which can weaken the overall strength of
the line.
11.5.5 Effect of Temperature on Transmission Line Longevity
The heating of the transmission lines is currently the most critical attribute respon-
sible for the line sag and the longevity of the lines. The heating effect is due to the
large power running through the lines. Initially, the lines are sized for an estimated
amount of electricity, but as more is run through, their temperature increases sig-
nificantly. Standard operating temperatures of OHTL's may be around 30-70 °C;
however, lines have been known to reach up to 175 °C or more when overloaded
[
31
]. In the case of aluminum conductors, the degradation in the aluminum
strength due to heat is cumulative and irreversible.
In [
31
], the effect of high-temperature cyclic loadings on an ACSR conductor with
compression dead-ends and full tension compression splices was examined though
evaluation of the tensile strength, hardness, and metallurgical changes. Cyclic tests at
250, 500, 750, and 1,000 thermal cycles were run at temperatures of 100 and 175 °C.
The average hardness values of the conductors decreased at both temperatures;
however, they significantly decreased for the 175 °C tests, so much so that they
could not even be read using the Rockwell H scale for the 500- and 100-cycle
tests.
The same conductors were tested in tension in compliance with ANSI C119.4
class 1 full tension. In this case, the conductor is required to withstand 95 % of its
rated breaking strength (RBS), which was 4,066 lb in this case. The tension force
decreased as the number of cycles increased for each respective temperature. For
the 100 °C tests, the 1,000-cycle test produced a tensile strength below the required
95 % RSB. For the 175 °C tests, the 250-, 500-, and 1,000-cycle tests all produced
tensile strengths below the 95 % RSB. Hence, this indicates that the cycling of
high temperatures has a notable effect on the tensile strength of the conductor.
The metallurgical evaluation in this work involved lightly magnifying the con-
ductors to try to evaluate them. No differences could be seen at this level of mag-
nification; however, grain size should be evaluated. It is expected that there will
be differences in the pre-test and post-test samples, since there was a significant
effect on the hardness and tensile strength.
This specific work supports the statement that the temperature of the conductor
can dictate its integrity. Some ideas to improve the conductor life would be to mini-
mize the temperature and duration for which the conductor is at a high temperature.
To this end, new technology has produced wireless sensors that can be attached to
the conductor and can be used to monitor the temperature in real time [
32
].
