Historically, the design of “aluminum conductor steel reinforced” (ACSR) has been employed in overhead high voltage transmission lines. ACSR cables are distinguished by aluminum strands coiled around steel cables. The external aluminum strands serve as conductors of electricity, while the steel core imparts tensile strength to the ACSR cable.
Aluminum possesses ductility, implying that it can deform under tensile stress. The steel core, on the other hand, restricts the aluminum strands from excessive stretching and sagging beyond acceptable limits. Despite their affordability and over a century of use in high voltage transmission, ACSR transmission lines are hindered by their high thermal expansion coefficient. This leads to cable expansion and sagging, and increased resistance with load escalation, resulting in line overheating.
There is a safety limit to how much transmission lines can sag, beyond which they become a public hazard. Moreover, as grid operators transmit more power across the system, the increased resistance on ACSR lines leads to higher transmission losses. These technical shortcomings limit the use of ACSR transmission lines, preventing them from reliably transmitting power beyond their line ratings (under assumed weather conditions) to cater to rising demand. Power loss during transmission, also known as line losses, can vary from zero to over 20% of the transmitted electricity, depending on line and weather conditions. On average, about 8% of the power generated at central stations is lost during transmission, turning into waste heat due to the resistance of the transmission lines. This loss is most pronounced when power is most needed and valuable, such as during peak demand periods in hot, windless weather.
Technologies that enhance the capacity of the transmission network can significantly reduce this loss and boost the efficiency of the existing transmission infrastructure. They do this by enabling lines to carry larger volumes of power without overheating or sagging.
ACCC cables merge high efficiency with an enhanced power transmission capacity, providing a distinct economic benefit over other types of lines, particularly ACSR lines. Furthermore, ACCC cables become more cost-effective than conventional ACSR cables once a specific current limit is exceeded.
How can we enhance the transmission capacity of the power grid network without the need for new rights of way ?
One solution is to substitute ACSR with “aluminum conductor composite core” (ACCC) transmission lines through a process known as “reconductoring.” This involves replacing old cables with new ones using the existing towers and rights of way. However, reconductoring necessitates taking the transmission line out of service during the work, which puts pressure on the rest of the grid and incurs costs for transmission operators. The reconductoring process can be carried out in several sections of the transmission line at the same time to minimize downtime, but the total downtime depends on the length of the transmission line and the size of the crew working on the project.
Reconductoring with ACCC cables can boost the power grid’s transmission capacity without the need for new rights of way. In ACCC lines, aluminum strands carry electricity, while the carbon fiber composite core gives the cable tensile strength. The carbon fiber composite core is up to 25% stronger than a steel core, which significantly reduces the sag of ACCC transmission lines at high temperatures. This means that ACCC cables can transmit more current while sagging less than ACSR cables.
Furthermore, ACCC cables are up to 60% lighter than ACSR cables, which allows them to span longer distances and require fewer and shorter support structures. The reduced number of support structures needed lowers the capital costs of transmission line installation projects. Because they sag less, the electricity flowing through the conductor encounters less resistance, meaning that ACCC can also reduce power transmission losses by 25% to 40%. If transmission losses are reduced, less electricity generation is needed to meet the same load, and greenhouse gas emissions from fossil fuel-based power plants decrease. Lastly, ACCC cables are resistant to degradation from vibrations, corrosion, ultraviolet radiation, corona, chemical and thermal oxidation, and cyclic load fatigue.
To summarize, the advantages of ACCC cables include:
- Enhanced capacity for carrying current and alleviation of transmission congestion;
- Decreased power losses during transmission, which reduces the need for electricity generation;
- Lower levels of electricity generation lead to a reduction in greenhouse gas emissions;
- The process of reconductoring, which involves replacing existing ACSR cables with ACCC cables, can boost the grid’s capacity without the need for additional rights of way;
- ACCC cables require fewer and shorter support structures, which can lower the cost and environmental footprint of transmission projects.