Page 54 - Energize April 2022

TECHNICAL

Spectrally selective coating

reduces temperature and

increases capacity of

transmission lines

BY MIKE RYCROFT, ENERGIZE

verhead conductors (OHC) are limited in their current excessive sag which could result in electrical clearance violations
carrying capacity by the maximum allowable conductor and damage to the conductor and/or line hardware.
Otemperature. OHC which operate at lower temperatures The heat generated in the conductor must be dissipated into
increase their current carrying capacity for a given cross sectional the environment to avoid overheating. The mechanisms for heat
area and reduce power losses at a specified ampacity. The transfer out of a conductor are convective cooling (wind) and
temperature of a conductor is dependant on the condition of the radiative cooling (energy radiating from the surface). The most
surface, so coatings could modify this condition to give higher common methods for establishing the relationship between
ampacity for a given conductor size. electrical current and conductor temperature are detailed in
Developments to increase the capacity of OHC by several standards such as IEEE Standard 738. At the core of these standards
percentage points could lead to a greater use of existing routes, is a heat balance equation which states that at steady state, the
and lower costs for new and reconductored routes. An area which heat coming into the conductor must equal the heat flowing
is being researched is the use of coatings to lower conductor out of the conductor. If the heat cannot diffuse fast enough the
temperatures as this promises to provide a major step forward in temperature will rise. This rise in temperature will increase the
OHC savings. amount of heat flowing out of the conductor until equilibrium is
The maximum allowable conductor temperature (MACT) is established, as given in Equation 1 (see Figure 1).
a major limiting factor on overhead transmission lines. MACT is
based on the allowable sag in the conductor and the safe operating P con + P rad = P + I R (1)
2
solar
limits of the conductor. Research into how temperatures could be
reduced for a given load, based on conductor configurations and Where: P con = convective cooling, P rad = radiative cooling,
materials, is ongoing. Benefits include lower line losses, increased P solar = solar heating, I = electrical current, R = electrical resistance
ampacity and lower reconductoring costs. (ignoring magnetic heating).
The prime focus is on the relationship between current flowing
in the cable and the temperature of the cable. This relationship On the left side of Equation 1 is heat energy flowing out of the
limits the current carrying capacity or ampacity of an overhead conductor from both convective and radiative cooling. On the
conductor. right hand side is the heat generated in the conductor by electrical
current and resistance plus the solar heat gain.
Conductor temperature is affected by several factors:
• The source of heat, including resistive heating due to current
flowing in the cable and ambient temperature, as well as direct
sunlight radiation on the cable surface
• Cooling of the cable by radiative emission and conductive cooling

Conductor heat balance equation
As current flowing in a conductor increases, the heat generated
within the conductor increases exponentially. This heat is known
as “I R losses” where “I” is the electrical current and “R” is the
2
electrical resistance of the conductor. Conductors are also heated
during the day by absorbed solar radiation. As the amount of
heating increases, care must be taken to ensure that conductor
temperature does not rise beyond the chosen maximum operating
temperature for the line. The risks from overheating a line include Figure 1: Heat balance in an overhead conductor 5

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