Page 56 - Energize April 2022

TECHNICAL

is an uncontrolled and uncontrollable process which depends
on temperature, time and air quality, which alters the surface
condition of the conductors by the formation of a surface layer
of oxide by the normal process of surface oxidation, or some
other compound, depending on the level and type of pollutants
in the air. Laboratory tests on sections of weathered cable reveal
that the effect of weathering is to increase the emissivity of the
cable, and thus reduce the temperature for a given ampacity
level.
Improvement in air quality has resulted in lower levels of
pollution, which has the effect of retarding the rate of weathering 2
on conductors; this means that this phenomenon can no Figure 4: Effect of applying SSC coating to OHC
longer be relied on to improve the performance of overhead Two products have been tested in the market: General
transmission lines. 5 Cable’s TransPowr E3X Technology and the photonic coating
developed by Assetcool. Surface coatings can be applied to all
Spectrally selective coatings overhead conductors (e.g., all aluminium (AAC), aluminium
The situation could be improved by artificially weathering or steel reinforced (ACSR) and aluminium, steel supported (ACSS)).
altering the conductor surface to have controllable values of The resulting conductors maintain the same benefits of the
emissivity and absorptivity prior to installation, and ensuring that underlying technology but with improved heat dissipation for lower
these values remain constant during the lifetime of the conductor. temperature and higher ampacity.
This has been achieved by the use of spectrally selective coatings Field tests show a 20% to 30% reduction in temperature for
(SSC) which both increase and stabilise the emissivity and reduce the coated conductor when compared to uncoated conductors
or eliminate the increase in the absorptivity of the shiny new operating at the same ampacity and environmental conditions. In
conductor with time. The coatings bond chemically with the addition, a coated conductor affords approximately a 20% increase
conductor and are mechanically stable. in ampacity when operating at the same temperature as an
Emissivity and absorptivity are equivalent at the same uncoated conductor. Since the coating is inorganic and stable in the
wavelength. However, overhead conductors emit thermal radiation environment, the technology is expected to provide a benefit for
at substantially different wavelengths than the sun does. SSC the lifetime of the conductor. Figure 4 shows the effects on ε and α
technology takes advantage of this scientific phenomena by values when a SSC coating is applied.
implementing a spectrally selective surface coating which radiates One of the side benefits is that the coating protects the
heat (i.e., has high emissivity) in the far infrared region - this is conductor from the effects of weather and pollution. Several
where conductors radiate - and blocks heat absorbtion in the products subjected to extensive laboratory and field testing have
visible spectrum where the sun radiates (Figure 3). shown the advantages of the method.
This development is based upon recent advances in
nanotechnology, allowing the coating to simultaneously reflect Field tests in the USA
over 90% of incident solar radiation, which prevents the line from Field tests were conducted in 2016 at the Oak Ridge National
heating up, whilst emitting large amounts of infrared, which cools Laboratory’s powerline conductor accelerated test (PCAT) facility,
it down. SSC is a well-developed technology which finds application using General Cable’s Transpower cables with E 3X technology
mainly in the solar energy and window glass field. coating. The laboratory consists of two 600 foot (180 m) aerial
spans where conductors can be subjected to a variety of situations
while the performance is closely monitored.
Figure 5 shows the basic test setup.
Drake ACSS cable with a nominal rating of 900 A was used
for the test. A 2,25 MW controllable DC power source provided
current to the test conductor. The power source is fed by a
13,8 kV/4160 V transformer. The power supply increases the
current from zero to 5000 A DC and voltage from zero to 400 V
DC. A manual limit can be set to prevent the power supply from
operating above a specified current. The conductor temperature
limit, based on the real-time measured conductor temperature,
can be set to prevent damage to the conductor and accessories.
Tests were conducted at current levels ranging from 650 A to
1500 A. Table 2 shows the results of the test, and Figures 6 and 7

Figure 3: Spectrum comparison of solar and conductor thermal radiation. show the temperature differences at the minimum and maximum
(Magnitude expressed as a percentage of peak magnitude) 4 currents.

energize | April 2022 | 54