Mar 21, 2024

image credit: AEP Director Technology, CTC Global Dave Bryant is one of the founding members of CTC Global Corporation (formerly Composite Technology Corporation) – the developer and manufacturer of the patented ACCC® Conductor and ancillary... Member since 2012 3 views As demand for electricity in the U.S. grew in the 1950’s and 1960’s, transmission engineers faced the challenge of delivering more power through existing right-of-ways and on new lines. In the 1970’s conductor manufacturers responded by introducing ACSS conductors. This design offered pre-annealed aluminum strands that could be operated at higher temperatures – enabling utilities to push more power through the existing or new sections of the grid. While initial ACSS conductor designs used the same steel core and galvanized coatings found in ACSR conductors (that could not be operated above 93° C without weakening the aluminum strands), later ACSS designs offered higher strength steel cores to offset the overall loss of strength. Drake size ACSR conductor, for example, was rated at 31,500 pounds, while the same size ACSS conductor was rated at 25,900 pounds which reflects the decreased strength of the weaker annealed aluminum strands. The use of high strength (285 ksi) steel raised the overall conductor strength back to par with ACSR. Nevertheless, the coefficient of thermal expansion (CTE) of the steel didn’t change and higher operating temperatures resulted in increased conductor sag. Sag temperature comparison of ACSR, ACSS and ACCC Conductor While the fully annealed aluminum offered a slightly lower thermal knee-point compared to ACSR, the ACSS conductors - quite often - were not capable of delivering their rated capacity before sagging below acceptable limits. Figure 1 compares the sag of ACSR (red line) with ACCC (blue line) and ACSS (orange line). The allowable emergency operating temperature of the ACSR conductor is generally established by manufacturers at 100° C. The graph shows that, under equivalent load (in this case 1,600 amps in Drake size conductors) the ACSS conductor would reach the sag limit of an existing installation after achieving a ~30% gain, far short of its rated capacity. While that gain may have been considered adequate in the past, demand for electricity and the need to mitigate grid congestion, makes its lower performance less than sufficient and far short of today’s ‘future proofing’ requirement. In this lab scenario the ACCC® Conductor was able to deliver much more power with greatly improved clearance and cooler operating temperature. Corrosion of steel core wires Over time, ACSS conductors were introduced with compact trapezoidal strands that could either reduce the conductor’s overall diameter or provide additional conductive aluminum. The added aluminum content was a plus, but the added weight contributed to added sag, so these conductors were not widely deployed. It was also discovered in the 1990’s that trapezoidal designs tended to hold water at the sag belly which contributed to substantial corrosion of steel core strands especially in highly corrosive salt air, agricultural, and polluted industrial environments. In 2004, CTC Global commercialized the ACCC® Conductor that replaced legacy steel cores with a hybrid carbon and glass fiber core that is completely immune to corrosion. Its lighter weight and more compact design allow the incorporation of ~28% more aluminum without a weight or diameter penalty. It’s coefficient of thermal expansion is also ten times lower than steel which virtually eliminates thermal sag. The use of fully annealed aluminum or high temperature aluminum-zirconium alloy enables the ACCC Conductor to operate at 180° C continuously or up to 200° C during ‘emergency’ conditions. The ACCC Conductor’s higher capacity and reduced thermal sag make it ideally suited to help engineers push more power through a modern grid. ACSR/ACSS (L) ACCC (R) Fretting fatigue strand failure of ACSR & ACSS The ACCC® Conductor offers additional benefits over ACSS conductors which include improved resistance to cyclic load fatigue and strand failure caused by Aeolian vibration. As wind crosses transmission lines, it is not uncommon for them to oscillate at high frequencies. This causes the ACSR and ACSS conductor’s round core and aluminum strands to rub against each other which results in fretting. From the resulting divot points, microfractures can propagate causing strand failure. The ACCC® Conductor’s smooth core and substantially increased surface area of its trapezoidal strands, not only dissipate vibration at an order of magnitude better than ACSR or ACSS conductors, they also prevent strand fretting, divoting, microfracture propagation, and strand failure. Testing performed at several laboratories offer proof. The added aluminum content of the ACCC® Conductor compared to ACSR or ACSS conductors of the same diameter and weight reduces the ACCC® Conductor’s electrical resistance. This translates into improved efficiency and reduced line losses. As conductors carry increased amounts of current their electrical resistance causes energy to be lost in the form of heat. Cooler operating temperatures reflect reduced electrical losses. Reduced line losses can reduce fuel consumption and operating costs as well as associated GHG / CO2 emissions. When used to link new renewable or low carbon generation assets, reduced line losses can improve the overall project’s economic viability. In all cases, reduced line losses frees-up generation capacity otherwise wasted, which is particularly important in today’s world where demand continues to grow to support new data centers, the electrification of transportation, and other critical energy and sustainability goals. While the ACCC® Conductor offers far better electrical and mechanical performance compared to ACSR or ACSS conductors – and greatly improved efficiency – it also offers superior reliability and resilience. In addition to greatly reduced sag that can improve clearances to vegetation and underbuilt structures to reduce the risks of sag-trip outages and wildfires, the ACCC® Conductor’s added capacity can be used to reroute power around fire-prone areas during dry and/or heavy wind conditions. In 2012, a firestorm took down several wood structures supporting 431 kcmil Linnet ACCC® Conductor. While several structures had to be rebuilt, the ACCC® Conductor was undamaged ( watch video interview ). Since that time NV Energy has completed more than 30 additional new line and reconductor projects in its service territory with great success. Linemen make fast repairs on ACCC conductor In 2013 an EF5 tornado struck Moore, Oklahoma in the United States. Winds were estimated to reach 210 mph. Based on evidence, a large oil storage drum was blown at high velocity into a newly installed 125-foot tall steel monopole knocking it over by 45 degrees. While the strength of the ACCC® Conductor is credited with not allowing the pole to fall completely, the shock wave this created snapped the aluminum strands of the ACCC® Conductor that was installed along a north-south route to evacuate power from a nearby power plant. Fortunately, the steel pole was quickly replaced and four linemen in two bucket trucks were able to quickly splice in a 20-foot section of new ACCC® Conductor using two ACCC® Splices and put the line back in service quickly. If ACSS conductor had been used the damage would have required replacing several spans due to stretching and/or failure of the non-elastic steel core. When subjected to a heavy ice or wind event, a steel core will plastically deform (‘yield’) and not return to its original length. The ACCC® Conductor’s core is fully elastic and remain unimpacted from such an event. These are just a few examples of the ACCC® Conductor’s improved reliability and resilience compared to ACSS conductors. AEP uses ACCC Conductor on 345 kV transmission Between 1996 and 2010, peak load in Texas in American Electric Power’s service territory grew by over 80% hitting a record 2,378 MW. At that time the load was projected to exceed 3,000 MW by 2020. In spite of the dilemma, load projections and urgency were unable to justify construction of major new lines. AEP approached ERCOT who approved live line ‘energized’ reconductoring of the existing 120 circuit mile Lon Hill-to-North Edinburg and parallel 120 circuit mile Lon Hill-to-Rio Hondo 345-kV transmission lines. AEP selected high capacity, low-sag ACCC® Conductor over ACSS conductor because the ACSS conductors would have required replacement of a majority of the existing structures at a significantly higher cost and much longer timeframes and would have required substantial outages during structure replacement. In addition to saving tens of millions of dollars – and freeing up equipment and linemen to work on other projects – AEP’s decision to use double bundled ACCC® Conductor vs double bundled ACSS conductor reduced line losses by over 300,000 MWh per year, and, based on all combined sources of generation assets at that time, reduced CO2 emission by an estimated 200,000 metric tons (the equivalent of removing 43,000 cars from the road every year). The reduction in line losses also freed-up 34 MW of generation capacity which essentially paid for the ACCC® Conductor from the get go. Link to T&D World magazine article . While the glowing AEP example earned AEP the EEI Edison Award for project of the year in 2016, several other projects have been undertaken by various utilities in the U.S. and 65 other countries where they selected ACCC® Conductor to replace ACSS conductor that had been in service for less than a decade. In July 2020, with the support of PAR Electrical Contractors, Southern California Edison (SCE) completed the first section of their Pardee to Pastoria 220 kV reconductor project near Grapevine Lebec, California. SCE selected 714 kcmil Dove size ACCC® Conductor to replace 666 kcmil Mystic size ACSS conductor that was initially installed in 2012. Its capacity and sag constraints could no longer support the line’s load requirement. ACCC® Conductor has also been selected over ACSS conductor for several transmission lines in the U.S. and internationally. While both conductor types are designed to carry twice the capacity of legacy ACSR conductors, transmission engineers can leverage the ACCC® Conductor’s greater tensile strength (41,200# vs 25,900# standard ACSS or 31,500# high strength ACSS). The added strength and reduced sag allow transmission design engineers the ability to increase spans between fewer and/or shorter structures. This can not only make it easier to position structures in less disruptive locations, it can also reduce environmental and visual impact while reducing project costs and construction timeframes. This advantage is especially useful in mountainous and congested areas where access is difficult and foundation costs comparatively higher. The ACCC® Conductor, thus far deployed to more than 1,250 projects worldwide, has also been selected over ACSR/ACSS alternatives to link renewables and/or communities. In 2006, the City of Kingman, Kansas selected Hawk size ACCC® Conductor for a new 22 circuit mile 34.5 kV transmission line that allowed it to connect with a regional power grid. They chose ACCC® Conductor over ACSS conductor to enable increased spans, reserve capacity for anticipated growth and improved efficiency. While the ACCC® Conductor is more expensive than ACSS conductor on a per foot/meter basis, the value it delivers often reduces upfront capital costs and pays huge dividends over its anticipated 50+ year service life. For more information, please visit www.ctcglobal.com or contact info@ctcglobal.com Connect with CTC Global