All ACSS Conductors Are Not Created Equal

The utility industry began adopting Aluminum Conductor, Steel Supported (ACSS) for transmission in the 1970s. ACSS offers performance improvements — primarily with regard to sag and tension — over Aluminum Conductor, Steel Reinforced (ACSR), which has been in use since the early 1900s. However, they can also come with a downside. Line crews often find conventional ACSS difficult to install.

The conventional method of manufacturing ACSS involves annealing the individual strands of aluminum before stranding them around a steel core. Annealing carefully heats and cools the aluminum to decrease tensile strength and increase conductivity. However, the stranding process performed after the annealing causes irregular hardening of the aluminum such that individual strands have different tensile strengths. Additionally, loose stranding is inherent in the process because back-tension on the stranding line must be low to avoid stretching the annealed aluminum strands. When line crews install the conductor, these process flaws can cause installation issues such as birdcaging.

In fact, line crews in the ’70s and early ’80s found the conductors so difficult to work with that they gave them an unflattering nickname. “The acronym used early on for ACSS was SSAC,” said Charles Holcombe, manager of overhead systems and solutions at Southwire Co. “The line crews, many of whom grew up with World War II-era comics, started calling it Sad Sack, for the comic strip character that could never do anything right.”

In the mid-1990s, Southwire developed a manufacturing process that anneals the aluminum strands after they have been stranded onto the steel core. Batch-annealing results in conductors made of strands with uniform tensile strengths that are much easier to install.

Over the years, Southwire and its customers began to notice additional performance benefits of batch-annealed ACSS. The conductors had significantly higher conductivity than the industry standard, but, more importantly, they seemed to experience less thermal sag than conventional, bobbin-annealed ACSS.

Southwire suspected that the batch-annealing process resulted in a lower “thermal knee-point temperature,” which reduces thermal sag. As electrical loading increases, conductors heat up. Since the coefficient of thermal expansion of aluminum is twice that of steel, the aluminum strands expand at twice the rate of the steel core. The thermal knee-point is the temperature at which the aluminum strands have expanded enough that the steel core begins supporting all the tension on the line and thus slows the rate of thermal sag. Because batch annealing results in aluminum strands with uniformly lower tensile strength, conductors made with the process reach the thermal knee-point at lower temperatures, resulting in 10% less thermal sag on average. In application, the reduced sag means reduced costs for transmission projects.

When utility companies plan transmission projects, they select conductors based not only on the amount of transmission capacity needed but also on the site-based needs of the projects given budget constraints. Conductors with greater thermal sag need to be strung between higher towers or a greater number of shorter towers, both of which affect overall project costs.

Since conductors are made in tremendous quantities with an array of performance characteristics, one might assume that the ASTM-compliant conductor of one manufacturer would be equivalent to that of another manufacturer. But that isn’t the case, according to Southwire.

In 2016, an East Coast utility had already decided it needed an ultra-high-strength (MA5) ACSS conductor to meet its design requirements when it received stress-strain test data from Southwire’s lab. The test data showed that Southwire’s batch-annealed, high-strength (MA3) ACSS conductor performed as well as the more expensive ultra-high-strength ACSS conductor. In light of this information, the utility canceled its order for the ACSS/MA5 conductor and ordered Southwire’s batch-annealed ACSS/MA3 instead, ensuring substantial cost savings for the project.

“It’s important for utilities to ask vendors to verify the provenance of the stress-strain and creep coefficients in their wire files,” Holcombe said. Holcombe added that reliable manufacturers can provide test reports showing how they developed the stress-strain and creep coefficients in their WIR files, and he urged utilities to look at them closely before making purchase decisions.

In short, not all ACSS deserves the Sad Sack moniker. Batch annealing has been proven to both resolve the long-term difficulties of pulling wire that wants to birdcage and improve the sag performance affecting the fundamental design parameters of ACSS applications.

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