Dave Bryant, Composite Technology Corporation, USA
For more than 100 years, electricity has been delivered to consumers using bare overhead conductors that were made up of conductive aluminum strands wrapped around a core that consisted of steel wires. The steel wires were used to augment the strength of the conductive aluminum strands, which, in their most corrosive-resistant alloy, offered reasonable conductivity and modest tensile strength. The added strength provided by the steel core enabled utilities to reduce the number of towers or poles required as they built new transmission lines. This reduction in structures helped reduce up front capital costs. ACSR or ‘aluminum conductor steel reinforced’ became the norm and still represents about 85 per cent of all bare overhead conductors in use today.
As demand for electricity grew and new sources of generation were brought on line, many transmission and distribution lines became thermally constrained. This is due to the fact that, as electrical current increases, the temperature of conductors rises. The increase in conductor temperature is a function of the electrical resistance of the materials utilized.
While copper wires offer improved (reduced) electrical resistance, they are not used extensively for transmission lines because of their higher costs and heavier weight. Aluminum strands, on the other hand, are relatively inexpensive, lightweight, and offer reasonable conductivity. Unfortunately, as aluminum wires are heated above 95ºC, they begin to anneal which causes a substantial loss of strength. In spite of the steel core reinforcement in ACSR conductor, weakened aluminum strands further reduce the integrity of the conductor and cause accelerated creep.
Addressing thermal expansion issues
While creep is considered to be more of a long-term concern, as it typically happens over a prolonged period of time (where the elongation of the conductor results in increased sag), a more immediate concern relates to the relatively high coefficient of thermal expansion (CTE) of the steel core and the even higher CTE of the aluminium strands that allow the conductor to sag as its temperature rises. With clearance limitations between the conductor and the ground/vegetation/electrical under-build and other utility lines well established, sag has become a limiting factor in the ability to increase transmission line capacity.
In consideration of the enormous challenge of increasing transmission line capacity and mitigating thermal sag (and knowing how difficult it was and continues to be to develop new transmission corridors), Composite Technology Corporation (CTC) began developing a new conductor technology using aerospace materials in 2002.
Pioneering hybrid core
The objective was to develop a conductor that could be used to upgrade existing transmission corridors without the requirement of structural modifications. The objective relied on the incorporation of highly evolved aerospace technology and materials science to create a new structural core that could be utilized to reduce thermal sag and allow increased current flow.
As the project evolved, CTC developed a hybrid carbon and glass fibre core, known as the ACCC or aluminum conductor composite core, that resisted degradation from UV light, chemical, thermal, and cyclic load fatigue that had previously been discovered to be the Achilles’ heel of non-ceramic (or polymer) insulators (NCI).
With a clear understanding of how to mitigate these and other environmental challenges, CTC began developing manufacturing techniques and tooling to allow the cost-effective and continuous production of a pultruded composite core that utilized carbon and boron-free glass fibres embedded in a high grade thermoset resin. Very specific grades of carbon, glass and resin materials were selected and developed to provide high strength, minimal thermal expansion and good compatibility.
The glass fibres that surround the ACCC conductor’s carbon fiber core were designed to increase the core’s flexibility and prevent galvanic corrosion that would otherwise occur if the carbon core came in direct (and substantial) physical contact with the aluminum strands in the presence of an electrolyte such as salt water.
High tensile strength
While the carbon and glass fibres provide the composite core’s high tensile strength, the hybrid resin system helps improve the composite core’s flexural strength and effectively balances load sharing between the fibers, so they perform in a unified and composite manner. A composite is defined as ‘the strength of the combined components being greater than the sum of the strength of the individual components’. In other words, when properly combined, a composite component can deliver much more bang for the buck than the individual components when used separately.
Recognizing that higher electrical current can also result in higher operating temperatures, CTC spent a great deal of time and resource developing resin systems that would resist thermal fatigue. The systems that were ultimately commercialized in 2005 – after substantial testing – have proven their ability to perform continuously at temperatures of up to 180 ºC without experiencing thermal oxidation.
Higher temperatures can be easily tolerated for relatively short periods of time – without causing any noticeable loss of strength – but prolonged exposure to temperatures above 200 ºC can impact conductor longevity. Thus the ACCC conductor is rated for continuous operation at 180 ºC and short term ‘emergency’ operation at 200 ºC. CTC recommends that emergency operation be limited to five per cent of the conductors anticipated 50-plus year service life or approximately 20 000 hours.
Clear advantages emerge
While CTC’s initial goal of increasing line capacity without exceeding sag limitations was readily achieved, other advantages also became apparent. As with Boeing and Airbus, and other manufacturers that rely on carbon hybrid composites to increasingly greater extents not only for their improved strength to weight characteristics, but especially for their resistance to cyclic load fatigue the ACCC conductor raised the conductor longevity bar substantially.
Resistance to cyclic load fatigue is critical with bare overhead conductors, because everything is cyclic including vibration, tension, and temperature variation. And though, as with any other conductor, aircraft component, or other highly stressed part, physical limitations do exist, the ACCC conductor’s improved strength, dimensional stability, and self-damping characteristics allow transmission line designers greater design flexibility, higher performance, and improved longevity in conditions where other conductor’s service lives would be substantially compromised. The improved resistance to cyclic load fatigue offered by the ACCC conductor has also allowed CTC to warrantee its product for up to ten times the industry standard of one year.
While the ACCC conductor’s greater strength and improved thermal stability can enable longer spans or lower structures on new transmission corridors – which reduces capital costs – the ACCC conductor is also ideally suited for re-conductoring projects, because it is capable of carrying approximately twice the current of a conventional ACSR conductor.
Big upfront capital cost savings
In either case, tremendous up-front capital cost savings can be realized. Additionally, because the composite core is much lighter than its steel counterpart, the ACCC conductor utilizes 25-30 per cent more aluminium (using trapezoidal shaped strands) without a diameter or weight penalty. Not only does the added aluminum content help improve electrical throughput, it also serves to reduce line losses by 30-40 per cent compared to any other conductor type of the same diameter and weight, under equal load conditions.
Comparison of conductors showing differences in conductor sag and temperature under equal load conditions
Reduced line losses offer the utilities huge advantages. Not only can fuel consumption be reduced, which has obvious economic benefits, the reduction of fuel consumption can also dramatically reduce greenhouse gas and other emissions. In the case of a renewable resource, where emissions are not a concern, the utility can deliver more power for the same initial capital investment, which can make the economics of a renewable project much more attractive, or conversely, reduce their initial investment and deliver the same amount of power. Either way, it works.
Considering that the reduction of greenhouse gas and other emissions represents such a significant challenge for the utilities today, re-conductoring an existing line with ACCC conductor is by far the cheapest and fastest way to achieve that objective, especially considering that it doesn’t require special permitting or regulatory approval that can drain resources and delay progress. And, while the simple payback periods for such projects are currently measured in months rather than years as carbon trading grows, these payback periods will ultimately be measured in weeks.
While conserving fuel and reducing emissions has obvious economic, social, and political advantages, saving energy can also help the utilities and their customers conserve money that they would otherwise spend in developing new sources of generation. A megawatt saved is less expensive than a megawatt produced. So profound is this fact that many US utilities and regulators are now referring to a saved megawatt as a ‘negawatt’.
Though many utility CEOs, regulators, and politicians suggest that efficiency is the best fuel, most initiatives, incentives, directives, laws, acts, policies, rebate programmes and marketing campaigns currently revolve around the demand-side of the equation.
Reducing line losses
While the US Energy Policy Act of 2005 does allow utilities to apply for an return on equity (ROE) increase of up to 200 basis points for using new efficient technologies including the ACCC conductor, not many utilities have taken advantage of this, even though progressive utility CEOs, such as James Rogers of Duke Energy, have acknowledged that they typically loose between 8-10 per cent of their energy to line losses.
While reducing line losses by 30-40 per cent or more is easily accomplished by re-conductoring an existing corridor with the ACCC conductor, it’s noteworthy that re-conductoring a single line can also improve overall system efficiency. For example, after a relatively short 11-mile section of a thermally constrained line was re-conductored just outside of San Antonio, Texas in 2006, American Electric Power reported that the ACCC conductor reduced line losses by 0.9 MW on that section of line, and also reduced the overall system losses by an additional 0.2 MW, for a total improvement of 1.1 MW.
In addition to the advantages of increased throughput (revenue), reduced line losses (fuel savings and emission reductions), the added output of renewable energy resources (improved project economics), the lower cost of a saving a ‘negawatt’ compared to producing a megawatt (a risk-free cash saving annuity), the lack of regulatory hurdles (time and money), an increase in ROE (basis point incentives), a decrease in cap-ex costs for new or re-conductoring projects (compared to alternatives), the added system efficiency (for additional savings), and the political advantages (green-message), the ACCC conductor can also mitigate congestion costs by opening up constrained lines to less expensive sources of power.
While it is obvious that not every utility is ready to exploit all of these advantages, as transmission engineers, planners, business managers and public fiduciaries it is nevertheless our job to serve the public’s interest, because if we do not, someone else will.