In the quest to meet increasing electricity demands and enhance grid reliability, transmission engineers have continuously sought innovative conductor solutions. In response to these challenges, the 1970s saw the introduction of ACSS (Aluminum Conductor Steel Supported) conductors that were designed to operate at higher temperatures to enable increased capacity over thermally limited ACSR (Aluminum Conductor Steel Reinforced) conductors that were introduced in the early 1900s. In the early 2000s, CTC Global introduced the ACCC® Conductor which uses a hybrid carbon fiber core that offers several advantages over legacy ACSR and ACSS conductors.
While the use of fully annealed aluminum strands in ACSS conductors did allow higher operating temperatures, unfortunately, it also reduced the conductor’s overall strength. As an example, a 795 kcmil Drake size ACSR conductor is rated at 31,500 lbs., while a 795 kcmil ACSS Drake size conductor is rated at 25,900 lbs. Reduced conductor strength generally requires shorter spans between additional structures which can increase siting challenges and lead to additional environmental impact.
Over time, a number of manufacturers introduced higher strength steel core wires to improve the ACSS conductor’s overall strength. Newer versions of 795 kcmil ACSS Drake size conductors – using ‘ultra-high strength’ steel cores – are now rated up to 32,600 lbs. In contrast, CTC Global’s Drake size ACCC® Conductor of the same diameter and weight is rated at 41,200 lbs. – a substantial improvement.
For reference, ‘standard strength’ steel conductor core wires have a rated strength of 210 to 230 kpsi, while ‘high strength’ steel core wires are rated at 235 to 250 kpsi, and ‘ultra-high strength’ steel core wires range from 265 to 285 kpsi. In comparison, the ACCC® Conductor’s core is rated at 313 to 375 kpsi or higher.
While increased strength enables longer spans between fewer and/or shorter structures – which improves design flexibility, reduces environmental impact, and shortens construction timeframes – the difference in these material’s ‘coefficients of thermal expansion’ (CTE) is even more significant. The ACCC® Conductor’s composite core offers a CTE around ten times lower than steel and around twenty times lower than aluminum. The lower CTE of the ACCC® Conductor offers substantial benefits – especially at higher operating temperatures that can cause conventional ACSR and ACSS conductors to exceed sag limitations – often before they reach their potential ampacity.
ACSR conductors are generally designed to operate at a maximum continuous operating temperature of 75° C. Higher temperatures may be allowed during high demand periods but exceeding 93° C for extended periods of time can weaken the aluminum through annealing and change the conductor’s sag profile. Higher temperature operation for sustained periods can also compromise the integrity of dead-ends and splices.
While ‘enhanced’ versions of ACSS conductors – that use improved anti-corrosion coatings to enable operating temperatures up to 250° C, their unchanged (still very high) coefficient of thermal expansion does nothing to mitigate their propensity to sag. While higher thermal limits and specially designed hardware theoretically allow higher ampacity, the structures that support these conductors must be close enough and tall enough to accommodate the additional thermal sag that substantially exceeds that of a typical ACSR conductor under its design limits. In other words, it is unlikely that an ACSS conductor could be used to replace an existing ACSR conductor without raising or replacing existing structures. It is also noteworthy that IEEE, Cigre, and other researchers have discovered that the ‘slightly conductive’ steel core temperatures in ACSS conductors can be substantially hotter (+50° C or more) than the outer aluminum strands – which further exacerbates sag issues.
As pointed out in many recent papers, and laid out by the U.S. DOE and FERC, reconductoring with Advanced Conductors is expected to help our nation reach its sustainability goals quickly and cost effectively. While higher strength steel and improved corrosion resistance are steps in the right direction for ACSS conductors, their enhancements fail to achieve the performance advantages of Advanced Conductors on every level.
The ACCC® Conductor’s composite core is not only >50% stronger than steel core and impervious to corrosion, it’s also 70% lighter. The lighter weight allows the incorporation of ~30% more aluminum without a weight or diameter penalty. While there are benefits to installing larger heavier conductors of any type (as greater aluminum content helps carry more electrons with lower electrical resistance), the ACCC® Conductor offers higher capacity and improved efficiency without the need to replace or modify existing structures.
While ACSS conductors can operate at temperature’s ranging from 210° to 250® C, the ACCC® Conductor can deliver more power with lower losses at much cooler temperatures. A test performed by Hydro One at Kinectrics Lab showed that a Drake size ACSS conductor under a 1,600 amp load (with no wind) reached nearly 250° C, while an ACCC® Drake size conductor under the same conditions carried the same current at 182° C. The cooler operating temperature of the ACCC® Conductor underscores its improved efficiency and substantially lower line losses.
While engineers strive to interconnect more renewable generation assets to reach sustainability goals and deliver more power to accommodate growing demand from datacenters, electric vehicles, and heat pumps, we should remember that conserving energy is far less expensive than creating it. While offering several benefits above and beyond ACSS conductors, the ACCC® Conductor’s improved efficiency should be considered a paramount advantage that underscores the major differences between ‘enhanced’ and ‘advanced.’
For more information please visit www.ctcglobal.com or contact CTC Global at [email protected]
