Frequently Asked Questions about ACCC® Conductors
These Frequently Asked Questions (FAQs) are intended to clarify, explain, and promote better understanding of ACCC Conductors with the goal of improving the efficiency, reliability and capacity of electrical transmission grids worldwide.
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What is ACCC conductor?
ACCC® conductor (Aluminum Conductor Composite Core) is a high capacity, low sag conductor which consists of a carbon fiber composite core encased in a protective fiberglass sheath that is helically wrapped with conductive aluminum strands. It was developed and patented by CTC Global. ACCC is a registered trademark of CTC. Though CTC produces all ACCC composite core at its manufacturing facility in Southern California, ACCC conductor is stranded by ~two dozen regional conductor manufacturers worldwide. Currently over 43,000 km of ACCC conductor has been installed at over 450 project sites.
What are the advantages of CTC Global ACCC Conductor compared to conventional conductors?
Conventional conductors typically consist of aluminum strands wrapped around steel core wires. The steel core provides strength so supporting structures can be placed further apart. In some cases, steel core wires are not used, but an aluminum alloy is incorporated to improve the strength of the conductor. Special alloys increase electrical resistance which increases line losses. The ACCC conductor offers several advantages compared to conventional conductors with or without steel reinforcement:
- CTC Global’s high-strength composite core allows the incorporation of aluminum strands that provide the greatest conductivity (type 1350-O ≥ 63% IACS*). Various aluminum alloys can decrease conductivity to ≤ 53% IACS (*International Annealed Copper Standard).
- The composite core’s lighter weight (compared to steel core wire) allows the incorporation of ~28% more aluminum without a weight or diameter penalty (using compact trapezoidal strands).
- The composite core’s very low coefficient of thermal expansion enables the ACCC conductor to carry additional electrical current without causing excessive line sag that occurs when conventional conductors heat up under increased electrical load.
- The ACCC conductor’s additional aluminum content (and superior conductivity) substantially reduces line losses compared to any other conductor of the same diameter and weight.
- The ACCC conductor’s non-metallic core also eliminates magnetic hysteresis losses that can be as high as 6% on 3 layer steel core conductor and 20% or more on single layer steel core conductor under high current conditions.
- CTC Global ACCC conductor’s composite core is non-corrosive and will not cause a galvanic effect between the core and aluminum strands that can occur with conventional conductors.
- The ACCC conductor’s composite core – in conjunction with the smooth surface of the trapezoidal shaped aluminum strands – helps dissipate Aeolian vibration more effectively.
- The dissipation of vibration allows the conductor to be installed at higher initial tensions often without the use of dampers (based on project specific analysis) which serves to extend the effective service life of the conductor.
- The high strength, light weight CTC Global composite core enables installation over long spans which can reduce overall project costs by reducing the number (or height) of the required structures on new transmission or distribution projects.
- A reduction of structures can often minimize environmental impact, simplify the permitting process, and effectively reduce construction time.
- CTC Global ACCC conductor’s ability to carry up to twice the current of a conventional conductor makes it ideally suited for increasing the capacity of existing transmission and distribution lines without the need to reinforce or replace existing structures.
- Higher capacity and reduced sag helps improve the overall reliability of the grid.
Why did CTC Global select composite materials for this product?
Carbon & glass fiber hybrid composites offer superior strength to weight ratios (they are much stronger and lighter than steel). Hybrid composite materials do not exhibit the same fatigue failure as metals, nor do they rust, rot, or corrode. Unlike metal alloys, carbon fiber composite materials do not creep over time when subjected to cyclic or continuous high tensile load conditions. They also do not yield (permanently deform) under extreme load conditions. Hybrid carbon and glass fiber composites exhibit elastic behavior and return to their original condition (length) when extreme loads dissipate.
Considering that CTC Global's ACCC composite core is elastic, but the fully annealed aluminum strands yield under relatively low strain conditions, what happens to the aluminum strands when a heavy ice or wind load subsequently diminishes?
Following a heavy ice or wind load event (as can be observed in stress-strain testing), the aluminum strands relax around the core which allows the high strength core to carry the majority of the tensile load. While the relaxed strands re-engage under progressively higher tensile load conditions, the advantage of relaxed strands is that the conductor becomes more dimensionally stable under subsequent high current conditions, thereby reducing sag. A secondary advantage is a further improvement in vibration dissipation.
Non-ceramic insulators use a fiberglass composite core. Many of these products had problems with “brittle fracture.” Is CTC Global's ACCC composite core susceptible to brittle fracture?
Several non-ceramic insulator designs (when they were first introduced) utilized a relatively low grade glass fiber that contained boron to reduce manufacturing costs. These products also utilized a relatively low grade resin system that absorbed moisture as the outer silicon / rubber water sheds began to age. Once the sheds aged, moisture was able to wick into the ends of the fiberglass rods, which, when exposed to a highly charged electric field, became acidic. Nitric acid subsequently attacked the boron contained within the glass fibers which caused stress corrosion resulting in brittle fracture. The ACCC conductor’s carbon and glass fiber core uses a hydrophobic epoxy that resists moisture absorption. Wicking does not occur as core ends are encapsulated very deep within sealed dead-ends and splices, and the glass fibers do NOT contain boron. Extensive testing has confirmed that the ACCC conductor core is not susceptible to stress corrosion or brittle fracture. Additionally, there is no electric potential to ground (as exists with insulators), so tracking, voltage puncture, and flashover cannot occur.
How do hybrid carbon fiber composites compare with metal matrix composites?
In the case of the ACCC core, the unidirectional carbon and glass fiber composite utilizes a high grade, high temperature, toughened epoxy matrix. The matrix binds the fibers together which effectively helps transfer and share mechanical loads between them. The generous layer of glass fibers (principally for preventing galvanic corrosion) provides excellent core flexibility and maximizes the structural properties of the high strength, low coefficient of thermal expansion fibers.
In the case of a metal matrix composite (as is utilized in 3M’s ACCR conductor’s core strands), metallic properties dominate and alumina fibers are added to the aluminum matrix to increase stiffness and strength. While metal matrix composites can be exposed to higher temperatures compared with epoxy (polymer) matrix composites (~300°C vs ~200°C), the overall composite still have a higher CTE than ACCC Core, and the vastly different CTE between the metal matrix and its reinforcing fibers limit the number of thermal cycles to which it can be exposed before micro-fractures propagate. The very limited tensile strain (<1% as compared to 2.2% in Carbon fibers and 4.5% in glass fibers) in Nextel fibers makes the ACCR core relatively brittle, requiring very large bending radius to avoid brittle fracture.
How is CTC Global's ACCC core produced?
The ACCC composite core is produced via a pultrusion process where the carbon and glass fibers are impregnated with resin and pulled through a specially heated die to complete curing. The core is made in a continuous process and various lengths are then cut and placed on shipping reels after the entire length has been thoroughly tested.
What types of project applications are ACCC conductors normally selected for?
The ACCC conductor was initially developed as a “High Temperature Low Sag” conductor to mitigate thermal sag on transmission lines that were “capacity constrained” due to sag and clearance limitations that occurred when higher electrical currents caused the conductors to heat up and sag due to their high CTE. The ACCC conductor’s low CTE composite core mitigated thermal sag. It therefore allowed existing transmission lines to be upgraded to carry additional current and is considered to be ideally suited for reconductoring projects.
However, due to the ACCC conductor’s increased aluminum content, greater strength, and excellent self-damping characteristics, the ACCC conductor is now also being utilized on new transmission and distribution lines as it offers increased electrical capacity, decreased line losses, and greater spans between fewer or lower structures. These attributes decrease permitting challenges, simplify tower placement, decrease upfront capital costs, and reduce lifecycle costs. The ACCC conductors improved efficiency and lower line losses can also decrease fuel consumption and associated GHG emissions as certified by SCS Global Services in November, 2016.
How does CTC Global's ACCC compare to conventional conductors?
The ACCC conductor’s composite core is lighter and stronger than steel or special alloy core which allows the ACCC conductor to accommodate greater spans, with a lighter more compact design, and also allows approximately 28% more aluminum to be incorporated into any conductor design without a weight or diameter penalty. The added aluminum content decreases electrical resistance, line losses, fuel consumption (or generation requirement) and can help reduce associated emissions. While the ACCC conductor offers the least amount of thermal sag compared to any other high temperature low sag conductor, CTC Global’s ACCC conductor offers higher capacity with reduced losses compared to any other conductor available today.
What types of dead-ends and splices are used with CTC Global's ACCC Conductor?
The ACCC conductor requires specially designed dead-ends and splices. A dead-end assembly consists of a collet housing, collet, and threaded eyebolt. During installation, a lineman removes several inches of the outer aluminum strands to expose the composite core. The collet and collet housing are placed over the core and the threaded eyebolt is inserted into the collet housing (also threaded) and tightened with a pair of crescent wrenches. Tightening the eyebolt into the collet housing tightens the collet and allows it to grip the core. A conventional (though somewhat larger) aluminum sleeve is placed over the conductive aluminum strands and collet / eyebolt assembly and compressed with a conventional 60 ton press using a compression die sized for the particular conductor being installed. The compression sleeve has a jumper pad located adjacent to the eyebolt which allows a jumper to be attached with a standard NEMA four bolt pattern, or other bolt pattern as specified by the customer. Dead-ends are back pressed to prevent conductor birdcaging.
Full tension splices contain two collet assemblies that are installed using the same procedure as is used with dead-ends. However, instead of tightening the collets down with the threaded eyebolt ends, a free rotating threaded coupler is used in this case. Once the collet assemblies have been attached and tightened down, a similar outer aluminum sleeve is placed over the collet assemblies and a 60 ton press is used to crimp the ends of the outer sleeve to the aluminum strands on either side of the inner collet assemblies. It is noteworthy that the added mass of dead-ends and splices allows them to operate at approximately one-half of the temperature of the conductor which helps ensure efficiency, performance, reliability, and longevity.
Can IMPLO technology be used to dead-end or splice CTC Global's ACCC?
Not at this time. However, the technology is currently being evaluated. CTC Global hopes to be able to offer this alternative in the coming months after testing is completed.
Can “back to back” reels of ACCC conductor be pulled in with splices preinstalled?
While it is quite easy to pull in back to back reels of ACCC conductor using back to back Kellum grips or “socks,” CTC Global offers specially designed splice that can be pulled in through sheave wheels. Installation crews have successfully pulled in several 12,500 foot reels, in a single pull.
Can ACCC conductor be used to upgrade an existing line without de-energizing the circuit?
Quanta Services has successfully reconductored several transmission lines with ACCC conductor without shutting down the circuits. The most recent energized project – a 345 kV double-bundled reconductor project which won the EEI Edison Award in 2016, consisted of using over 1,440 miles of ACCC to replace 240 circuit miles of ACSR.
Does CTC Global's ACCC Conductor require dampers?
ACCC conductor dissipates vibration energy more effectively than conventional round wire conductor designs, so in certain cases dampers may be unnecessary. However, the ACCC conductor’s greater tensile strength is often utilized to increase spans under higher tensile loads. It is therefore recommended that designers contact CTC Global or a damper manufacturer to secure recommendations specific to their project. When dampers are recommended for a specific project the exact location of damper placement is specified. Dampers are generally mounted directly on armor rod.
What type of suspension clamps should be used with ACCC conductor?
CTC recommends that AGS Armor Grip® (Preformed Line Products) or similar suspension clamps be utilized. These suspension clamps employ high temperature rated rubber grommets and armor rod. When the angle of the line exceeds 30 degrees, CTC recommends that a double suspension clamp be used in conjunction with a yoke plate. Other suspension clamps may be utilized when span lengths, angles, anticipated ice load, and other factors are considered.
How is ACCC conductor installed? Are there any special requirements?
ACCC conductor is installed using conventional tools, techniques and equipment. While the installation of ACCC dead-ends and splices is slightly different than the installation of conventional ACSR, ACSS or AAAC fittings, the conductors are installed in a similar fashion. As with other types of conductors, it is important to follow IEEE 524 installation guidelines and select appropriately sized sheave wheels based on conductor diameter, pulling tension, and angle of the conductor’s entry in / out of the sheave wheels. As with ACSS conductor, ACCC uses fully annealed aluminum strands that are slightly softer than non-annealed, hardened, or special alloy aluminum. Sheave wheels should be properly aligned so that scuffing of the aluminum strands does not occur and the conductor should not be dragged across the ground that could damage the aluminum strands and induce corona on an energized line. Additionally, as the ACCC conductor’s composite core is essentially non-conductive, care must be exercised such that grounding clamps are placed directly on the aluminum strands.
Is there any advantage to pre-tensioning ACCC conductor during installation?
Pre-tensioning ACCC conductor can lower the conductor’s thermal knee-point to further reduce thermal sag and quickly create an “after load” stable sag and tension condition. The thermal knee point is essentially the apex of the transition period when the aluminum strands thermally expand and relax to the point that they no longer carry any tensile load and all load is then carried by the very dimensionally stable and very strong composite core. While the conductive aluminum used for ACCC conductor yields under very little load, pre-tensioning can be done with very little effort in a very short period of time. Typically ACCC conductors are installed at 15 to 25% of their Rated Tensile Strength. Pre-tensioning the conductor by as little as 5 to 10% for a matter of 30 minutes can effectively relax the aluminum strands such that they no longer carry significant (or very little) tension under normal load conditions. However, should an extremely heavy ice or wind load condition occur in the future, the aluminum strands will reengage which increases the conductor’s overall tensile strength and resistance to sag. Pre-tension also improves the conductor’s self-damping as well as its fatigue resistance, and should be considered when permissible.
Have any installation issues or problems occurred during or after installation?
To date, over 450 projects have been successfully completed using ACCC conductor. During a few of these installations the following problems were encountered and resolved using the methods described below:
- During the installation of dead-ends on earlier projects – where the grounding wires were clamped very closely to the entrance of the dead-ends – birdcaging of the conductor’s strands occurred as a result the compression process where the aluminum strands began to extrude out of the nose of the dead-ends. This is a relatively normal occurrence, however, with the grounding wires clamped very closely to the dead-ends, the aluminum strands were not able to relax uniformly in such a small area. After the grounding wires were removed and the line was energized, the birdcaging dissipated.
- To prevent birdcaging in future installations, CTC Global developed a back pressing technique which has been successfully utilized on over 20,000 dead-ends and splices since the problem was first identified.
- During an early ACCC installation, the tension of the reel control brake was accidentally released which caused the conductor to jump over the side of the bullwheel which broke the conductor.
- CTC Global’s installation support personnel provided feedback, added notes in their Installation Guideline Manual, and discuss the importance of proper reel control during preconstruction meetings and while they remain present during the installation.
- During a 100 circuit kilometer reconductor project (1st project in Poland) using ACCC Stockholm, the original ACSR conductor was used to pull in the new ACCC conductor. One segment worked by one of the sixteen crews (who ignored the guidance from CTC support personnel) ‘rushed 5 km of conductors in a single afternoon’ while encountering an extremely large number of repair sleeves and splices with undersized reel tension brake. As these splices and repair sleeves were pulled very quickly through each sheave wheel (stop and go motion), the tension of the conductors increased and dropped so dramatically that the conductor between the tensioner and the 1st sheave wheel galloped severely, creating a condition of sharp angle of entry for the conductor to the bull wheel around a very small 3” diameter alignment roller, as well as sharp bending angles in the conductor above the sheave wheel and guide wheel area. This resulted in damage to the conductor’s core that did not become apparent until after the line had been installed, energized, and had fallen. All the three breaks were in the same segment installed by the screw.
- An assessment of the installation events, conditions, and failure mode confirmed the sequence of events that damaged the core. A decision was made to replace all ACCC conductor installed in the area by the specific crew where these events occurred. During the subsequent conductor replacement, larger hydraulic brakes were added to the reel holders (appropriate for heavier steel reels) and the conductor was directed around the undersized alignment pulley. The smaller ACCC conductors typically have the best flexibility due to its smaller core radius, but it is also more prone to experience larger stress. In addition to securing the support from the utility customer in obtaining commitment from installation crews for following installation best practice in the industry, CTC Global’s installation support staff now offers additional recommendation/training on working with smaller ACCC conductors during preconstruction meetings and while on site during installation. A study sponsored by WAPA, performed by Denver University (M. Kumosa), is available for review. CTC Global had successfully completed another ACCC project in Poland using ACCC Helsinki (one of the smallest ACCC conductor) recently, with three more ACCC projects scheduled to complete within 2011.
Is CTC Global's ACCC composite core flexible?
The ACCC composite core is extremely flexible, but retains kinetic energy when bent, much like a fiberglass fishing pole. Like a fishing pole, the ACCC core exhibits a shape memory characteristic and prefers to be straight. While this characteristic makes it very easy to handle during installation, if the ACCC conductor is bent in a sharp angle (i.e., not around a radius), damage to the aluminum strands or core may occur. Care should be taken so this does not happen. To better understand the ACCC conductor’s bending limitations AEP performed a simple bend test on a Drake size ACCC conductor as shown in Figure 80. After being bent 10 times around a 6 inch (15 cm) radius conduit pipe bender, the only degradation noted to the 3/8 inch (9.5 mm) core was a 20 micron hole within the outer fiberglass shell as identified using fluorescent dye penetrant.
Figure 80 – AEP Tight Radius Bending Test
A technical paper published in China discussed a phenomena they called “bamboo effect.” What is bamboo effect?
The paper described thermal testing of the ‘looped’ ACCC conductor on a table (i.e., conductor is under no tension). At temperatures above 160°C, they observed compressive failure (buckling as in ‘Bamboo’ pattern) on the inner side of the core. Such test is not pertinent to the use of any overhead conductors, where they are always subjected to line tension significant enough to negate the compressive stress experienced in the ‘bamboo’ test. For the application of ACCC conductor as jumper cable, there is still a moderate amount of conductor tension due to its self-weight. The bend radius in the Chinese Bamboo test is significantly tighter than CTC recommended bending radius for ACCC jumper cable applications. When customers follow CTC engineering guidelines for jumper cable installation, there is no risk of any damage to the composite core conductor.
Can the ACCC conductor be used on long spans or on spans subjected to heavy ice loads?
CTC Global, in conjunction with their international stranding partners, offer a wide range of conductor sizes and designs to accommodate a wide range of applications. While extreme span conductor designs are highly specialized, heavy ice load designs fall within CTC’s standard product line. CTC also produces a high modulus, higher strength core known as ULS which can be used to combat extreme ice loads and extreme spans.
The ACCC conductor’s core is made with carbon fiber surrounded by glass fiber. Why is the core not all carbon fiber?
The carbon fiber is surrounded with glass fiber to improve flexibility and toughness, and provide a durable protective layer to prevent galvanic coupling between the carbon fiber and aluminum strands. Galvanic coupling can happen if the aluminum strands are in “direct and substantial physical contact in the presence of an electrolyte within a certain pH range,” so even if the outer glass layer was damaged or cracked, it would be difficult to create direct and substantial physical contact between the aluminum strands and central carbon fiber core. Glass fibers as used in numerous aerospace applications for the same purpose due to their resistance to wear compared to other coatings.
What type of testing has been performed on CTC Global's ACCC Conductor?
During development several R&D tests were performed on the ACCC conductor’s composite core to assess its suitability for this application. The core itself was tested for tensile strength under very high and low temperature conditions. It was also subjected to sustained and cyclic thermo-mechanical load tests, flexural fatigue tests, acid and moisture resistance tests, and dozens of other tests to assess its performance attributes, limitations, and longevity.
Additional tests were subsequently performed on numerous ACCC conductor samples in a wide range of sizes. These tests often included ancillary hardware components such as dead-ends, splices, dampers, spacers, and suspension clamps. In addition to numerous mechanical tests, other systems tests and electrical tests were performed. These tests included lightning strike tests, short circuit tests, corona tests, and other industry standard or specialized tests. In addition to lab tests, several utilities also installed and monitored the ACCC conductor on test spans. The data collected from these tests correlated very well with predicted values based on lab-scale empirical tests and computer modeling. The reports and results of all of these tests are available for customer review. A Summary Technical Report is also available. Please contact CTC Global to gain access to these reports.
Considering ACCC is relatively new, what assurances does CTC Global offer relating to product Longevity?
While the ACCC conductor uses a hybrid carbon and glass fiber composite core that is relatively new to the electrical utility industry, especially as it relates to conductor, the reality is that carbon and glass fiber composites have evolved substantially over the last several decades and have been widely deployed in other demanding application where high strength, light weight, fatigue resistance, and longevity are extremely important. Carbon fiber composites, for instance, are now being used for primary aircraft structures not only for their high strength and light weight, but especially due to their resistance to cyclic thermo-mechanical fatigue. In the application of a conductor, the environmental challenges are numerous, combined, and very cyclic. The hybrid composite core developed and patented by CTC Global, and extensively tested by numerous laboratories and utilities internationally, is ideally suited for this demanding application. This technology allows the ACCC conductor to be operated at any voltage (including UHV) and from sub-zero conditions to a maximum continuous operating temperature of 180°C and up to 200°C for emergency operation. While operation at higher temperatures for relatively short periods of time is acceptable (in high voltage applications that may have thermal constraints – typically below 400kV), and will not void the product’s warrantee (see warrantee discussion below), operation at higher temperatures for prolonged periods of time can reduce the ACCC conductor’s effective service life.
In terms of line design, what guidelines exist?
Through substantial testing, field experience, and engineering interaction, CTC Global and other entities have well quantified the ACCC conductor’s performance as it relates to tensile strength, flexural strength, vibration dissipation, ice load capacity, and electrical performance. The data compiled allows transmission line designers to take full advantage of the ACCC conductor’s high electrical capacity, reduced line losses, high strength, and low sag. The data compiled allows transmission engineers (both electrical and mechanical) to accurately assess ACCC suitability for any project. Industry tools such as PLS CADD™ can be readily utilized to support the engineering process. CTC Global’s application engineers are also available to assist with analysis, comparisons, economic modeling and general support. Available at no charge, they can also provide life cycle cost analysis based on upfront capital costs, line losses / GHG emission reductions, increased capacity, improved longevity and other attributes of the ACCC conductor.
What Quality Assurance procedures does CTC utilize?
Quality is job #1 at CTC Global. CTC Global maintains ISO 9001-2008 certification and follows strict quality assurance procedures from the time it receives raw materials to the time when the finished conductor arrives at the project site. Each conductor core is carefully tested, documented, certified, and recorded. Finished conductor reels are also inspected. Core tests include tensile testing, bending tests (on the entire length of core), and thermal “glass transition” (Tg) tests. Production of the core is highly automated and designed to prevent / identify / remove any potential anomaly. CTC Global also offers one of the best warranties in the industry (up to 10 years).
Does ACCC conductor reduce EMF?
Can CTC Global offer any recommendations regarding outside engineering, installation, or EPC firms?
CTC Global maintains a list of experienced engineering and installation firms. Please contact CTC Global for more information
Can CTC Global provide a list of customer contacts that have installed ACCC conductor?
CTC Global maintains a list of experienced engineering and installation firms. Please contact CTC Global for more information.
Is ACCC conductor RUS Approved?
Is CTC Global's ACCC Conductor Guaranteed?
Yes. The ACCC conductor is delivered with a standard three year warrantee. Other conductor manufacturers typically offer a one year warrantee. For a nominal charge, this warrantee can be increased to up to ten years. Product warrantees may vary regionally, please contact CTC Global for more information
What is a typical production lead time for ACCC conductor and ancillary hardware?
Lead times can vary depending upon project size and current production backlog. A typical project lead time is 8 to 12 weeks. In the case of larger projects, CTC Global can often deliver finished conductor in phases to accommodate project requirements. CTC Global currently has nine international stranding partners who can support product delivery worldwide.