Dennis Doss III, P.E., General Cable Corp.
Dennis Doss, III, PE
General Cable Corp.
Transmission line ratings are based on a maximum allowed operating temperature to limit conductor damage and maintain required clearances for public safety. Unfortunately, conductors can reach high operating temperatures when lines overload and we need power the most-peak demand during the summer. What if existing transmission lines could be upgraded to nearly double the line capacity with little or no changes to existing towers?
The high cost and nearly impossible environmental restrictions of constructing new lines have utilities looking for ways to stretch existing systems with minimal capital investment. One critical conductor property makes this a difficult situation. As the operating temperature of conductors increases, thermal expansion causes power lines to sag. This reduces the distance between the ground and energized conductors (vertical line clearance). The National Electric Safety Code (NESC) mandates a specified vertical line clearance for the “maximum conductor temperature for which the line is designed to operate.” This NESC requirement severely limits the current carrying capacity of existing lines. To solve this dilemma, technology and ingenuity gave rise to Aluminum Conductor Steel Supported/ Trapezoidal Wire (ACSS/TW).
The ACSS/TW, a natural evolution in conductor design, marries two proven overhead conductor technologies that have been available separately for over 25 years. ACSS/TW combines the benefits of high operating temperatures of ACSS and reduced diameter of TW. The result is significantly higher transmission capacity without increasing mechanical loads from larger conductors.
ACSS/TW is a composite concentric-lay-stranded cable with two or more layers of trapezoidal-shaped aluminum 1350-O wires surrounding steel strands that form the conductor’s central core. The trapezoidal shape and soft aluminum (fully annealed “O” temper) provide performance advantages and cost savings by solving the problems of sag and limited current carrying capacity.
High temperature capability with less sag
Unlike conventional Aluminum Conductor Steel Reinforced (ACSR) conductors, the “O” temper of ACSS/TW causes most or all of the mechanical load to be transferred to the steel core. This means that ACSS/TW can operate continuously at more than twice the accepted temperature limit of ACSR conductors with significantly less sag. Operating at a higher temperature provides increased current carrying capacity to address higher electrical loads. Also, ACSS/TW is not affected by conductor aging, i.e. additional sag resulting from long-term creep of the aluminum.
More current carrying capacity with trapezoidal shaped strands
The trapezoidal shaped aluminum strands of ACSS/TW create a compact conductor with a smooth outer surface and smaller overall diameter. As compared to a conventional ACSR conductor with the same aluminum cross-sectional area, the ACSS/TW is approximately 10 percent smaller in diameter. The smaller diameter means lower ice and wind loads. In upgrade projects, this may allow higher initial tensions, which reduces final sag and permits higher operating temperatures-providing even more current carrying capacity with little or no structure modifications.
An alternate ACSS/TW design is available with an overall conductor diameter equivalent to that of a conventional ACSR conductor. The equal diameter option produces a 20 to 25 percent increase in the aluminum area. This increased area decreases the conductor AC resistance by 15 to 20 percent and significantly increases the current carrying capacity of the transmission line, providing energy saving advantages for consideration with new and upgrade projects.
Helps solve environmental headaches of new line construction
Because ACSS/TW sags significantly less than conventional conductors at high operating temperatures, there are numerous environment benefits to using ACSS/TW in new line construction. Lower sag enables reduced height and strength requirements for towers and poles, meaning they can be shorter or fewer in number. Also, the phase-to-phase conductor spacing may be reduced, which can lessen right-of-way requirements. In addition, tower height can be further reduced by stringing ACSS/TW at higher tensions to reduce sag.
Conventional aluminum conductors have a long history of use as transmission lines. But as power demands increase, environmental concerns grow and urban distribution systems age, a less-expensive, environmentally friendly solution has become essential for electric utilities to respond to recent trends. ACSS/TW’s higher operating temperatures, reduced ice and wind loading, and increased conductor area nearly doubles the transmission capacity to address emergency electrical loading while reducing sag. Whether it’s reconductoring existing lines with little or no changes to towers or constructing new lines with less environmental impact, ACSS/TW is a viable solution.
Turn to Power Pointers on page 33 for a primer on ACSS/TW overhead conductors.
Doss is director of engineering for General Cable Corp.’s (GCC) Utility Cable Group in Suffern, N.Y. He has 15 years experience as an engineer at an electric utility and 15 years experience as product engineer and now, director at GCC. He may be contacted at email@example.com