Composites on the Line
Electric utilities are ripe for conversion from wood power poles and cross-arms and steel-cored conductive cables to composite constructions, but most must still be persuaded.
#regulation #sustainability #adhesives
The North American electrical infrastructure is aging fast. Steel and, primarily, wood power poles, cross-arms and other structures that support transmission lines throughout the power grid — commonly 40 to 50 years old — have reached the end of their useful lives. At the same time that utilities are deciding how best to replace these items, they also are adding capacity to feed ever-growing power needs. Today, these and other forces are coming to bear on this market, providing the impetus for substantive change. At several key points, composites are the change agent. Composite poles and cross-arms have been commercialized for some time and are readily available in quantity, and composite-reinforced power transmission cabling is in the offing as well.
Life-cycle costs easily favor composites. Well established as moisture- and corrosion-resistant and impervious to animal and insect damage, fiberglass poles last at least three times as long as treated wood poles and require less frequent inspection and maintenance. More enticing to the listening ear of utility purchasing agents is the fact that the installed cost of these poles — the unit cost plus transportation and installation costs — is now frequently competitive with wood and steel. Composite infrastructure components are half as heavy as their wood counterparts and two-thirds (or less) the weight of comparable steel units. Consequently, they can be loaded in greater numbers onto a flatbed truck or railcar for transport, and less costly lifting equipment can be used — especially in remote installations, where smaller helicopters can be used to carry composite poles.
Beyond cost, composites offer timely elimination of creosote and other preservatives now used to treat wood power poles but targeted for eventual regulation as environmental hazards. Glass-reinforced composites also are non-conductive, providing an additional margin of safety for repair personnel.
In cable applications, composite-reinforced cabling reportedly has two to three times the electrical capacity of steel-cored cables of comparable size and weight, which could reduce energy demands on — and, therefore, the pollutants associated with — coal- and gas-fired electrical power plants and enable utilities to significantly increase the supply of power through the existing grid.
Should composites become the material of choice among electrical utilities, market potential for the composites industry is enormous: Assuming a typical composite transmission pole weighs 225 kg/495 lb, a single order for 1,500 poles (a realistic figure, based on recently reported sales) would consume 337 metric tonnes/743,000 lb of composite materials. In the United States alone, conservative predictions foresee the equivalent of 2,000 to 3,000 such orders for replacement poles per year.
Lots of potential, but …
While the potential is great, its realization, so far, is still hit-or-miss. "We have a bright and wonderful future and an ever-so-difficult present," says Spike Tickle, product manager for pultruder Strongwell Corp. (Bristol, Va.). He adds that despite an almost 40-year track record in lighting pole applications, composite transmission poles are still viewed by utilities as new technology.
Bill Arrington, president of composite-reinforced cable developer Composite Technology Corp. (CTC, Irvine, Calif.), characterizes the market as one with few pioneers: "Utilities generally want to be first to be second."
A more upbeat Mark Warren, technical VP for resin manufacturer Resin Systems Inc. (RSI, Edmonton, Alberta, Canada), reports, "Like any company, a utility is looking to improve profitability by reducing costs." He has found many utilities willing to looking at life-cycle costs. Unfortunately, the more common report is just the opposite. "All of us would like to think that utilities would look at installed cost," says Bill Griffin, general manager of Shakespeare Composite Structures (Newberry, S.C.), "but the reality is that most entities look at first cost." Although the numbers improve as the poles get bigger, and escalating steel costs have helped make composites more competitive, the unit cost of a small composite distribution-class pole is still two times that of a wood pole, he explains. Further, transportation and installation are less important factors in easily accessed locations, giving installed cost less appeal except in unusual circumstances. For Griffin's customers, therefore, composite poles and cross-arms are a problem-solving tool. "They're not going to go out and replace all their wood poles with composites," he explains. "But locations with remote access, woodpecker populations, wetlands or other sensitive ecosystems - composites address those problems."
Based on minimum bending strength measured by load, many composite poles produced to date are equivalent to Class 3 (3,000 lb) or Class 4 (2,400 lb) wood poles, two of 16 classifications rated by the American National Standards Institute (ANSI). These classes are typically used for distribution of power from substations to communities. Composites are moving into transmission applications, which carry power from its source to distribution networks, as they broaden into ANSI's Class 1 (4,500 lb) and Class H1 (5,400 lb).
Improvements in both filament winding and pultrusion processes have enabled pole fabricators not only to meet bend strength requirements in these classes, but also to develop uniform strength in all directions sufficient to withstand wind and ice loading, guy and brace forces, and other loads, many of which are transient and nonlinear.
Shakespeare Composites has been filament winding transmission poles since 1992, transitioning to computer-controlled winding in 1996. Shakespeare favors filament winding because it turns out tapered products without secondary processing. "Mother Nature figured out that a tapered product is a more effective design," quips Griffin. But axial strength (a significant factor in meeting bend strength requirements) was always a challenge in filament winding because of difficulties in positioning fibers axially. Significant technology advances in the intervening decade have enabled the company to wind fibers at low-angles, making it possible to produce its Tuff-Pole brand to Class H1 standards and as tall as 21m/70 ft. In lower classes, poles go as high as 26m/85 ft. Winding consistency has improved as well. According to Griffin, the coefficient of variation on today's poles is better than steel. Currently the company is engaged in finite element analysis to further improve layup and other parameters. Tuff-Poles are wound with Owens Corning (Toledo, Ohio) glass roving wet out with polyester resins from Ashland Specialty Chemical Co. (Columbus, Ohio) and Resolution Specialty Materials (formerly Eastman Chemical Co., Carpentersville, Ill.).
Resin Systems Inc.'s new manufacturing subsidiary RS Technologies delivered its first power poles in late 2003, using a filament winding process with a proprietary (pat. pend.) modification that enables the company to incorporate layers of axially oriented fibers. Initially RSI has focused on 14m/45-ft Class 4 and 18m/60-ft Class 1 poles, which its market research found to be particularly viable niches. Ultimately, the company expects to have a complete complement of pole sizes, with models in all classes. The sectionalized, tapered pole incorporates varying wall thickness, efficiently providing just the strength required at different points along the pole's length. As a result, the company asserts, its poles are lighter and more durable than competing products. RS Technologies uses glass roving manufactured by Fiber Glass Industries Inc. (Amsterdam, N.Y.) wet out with its parent company's Version resin system — a plural-component polyurethane that reportedly offers better strength than polyesters and, unlike the latter, emit no environmentally unfriendly volatile organic compounds (VOCs). (For more on polyurethane resins designed for pultrusion and filament winding processes, see CT June 2004, p. 22.)
For pultruders, whose strength is production of profiles with optimal axial fiber content, advances in resin-delivery and reinforcement-handling technologies have enabled them to produce poles with complex cross-sections and a significant percentage of off-axis fibers. The Powertrusion International (Las Vegas, Nev.) pole, for example, illustrates the complexity that can be designed into a pole to optimize strength, weight and cost. In cross-section, the untapered pole features an octagonal outer wall and cylindrical inner wall, enabling fabrication of poles as tall as 17m/55 ft, with ANSI ratings from Class 5 through Class 1. Axial roving from Fiberex Glass Corp. (Leduc, Alberta, Canada) carries bending loads and Owens Corning multiaxial stitched fabrics and continuous strand mat handle the shear loads, while Precision Fabrics Group's (Greensboro, N.C.) Nexus polyester veil provides UV protection and surface finish. A specially designed material guidance system from Creative Pultrusions Inc. (Alum Bank, Pa.) controls the 1000+ ends of roving and multiple fabrics in the poles, which are fabricated in a single step. The initial matrix resin, Reichhold Inc.'s (Research Triangle Park, N.C.) DION pultrusion-grade isopolyester, was specially formulated for parts with thick cross-sections. Powertrusion continues to develop hybrid resin systems to cost-efficiently meet the demands of each particular application (see "Power pole stands tall," CT Nov./ Dec. 1999, p. 29). It proved its value during the 2003 firestorms outside of San Diego, Calif.: a Powertrusion pole, installed there for demonstration, still stands, suffering only aesthetic damage, in an area where every wood pole and structure burned to the ground.
CTC expects to install its first poles this fall, introducing a hexagonal design. Two three-faced panels of glass mat/polyester with urethane foam core will be pultruded and snapped together to form the six-sided pole. Composites One (Arlington Heights, Ill.) is supplying the fabrics and resins for the poles. CTC's Arrington expects the design to be capable of producing a line of poles from 12.2m to 36.6m (40 ft to 120 ft) tall.
Strongwell was the first into the market with a tall transmission pole (as high as 24m/80 ft) that meets the equivalent of Class 1 and H1 ground-line moment capacities (the pole's bending strength at ground level). The company's S28 pultruded pole is a tapered structure assembled from six longitudinal sections to form, in cross-section, a 12-sided polygon. To fabricate the pole, the company pultrudes three identical profiles, each featuring three adjacent, angled panels, with flat surfaces on what will be the pole's faces and longitudinal strengthening ribs on the inner faces. Each profile has a large middle panel roughly twice the width of the two wing panels. After the profiles are pultruded to full pole length, the middle panel of each is center-cut lengthwise at a slight diagonal. The result is six sections, each with one rectangular panel and one trapezoidal panel. When joined the sections are arranged so that trapezoidal and rectangular faces alternate, with the narrower ends of the trapezoidal panels at the top of the pole. Using patented tongue-and-groove-style joints, the six sections are adhesively bonded with Pliogrip two-part urethane adhesive from Ashland Specialty Chemical Co. (Columbus, Ohio). To further improve bending strength, the poles also are filled with urethane foam. Strongwell has sold several hundred poles to date. Major customers include Bristol Tennessee Electric System, Southern California Edison, Tennessee Valley Authority (TVA), Bristol Virginia Utility Board and Allegheny Power.
Panels are pultruded using vinyl ester resin from either Ashland or Dow Chemical Co. (Midland, Mich.), multiaxial glass fabrics and rovings from Owens Corning and Saint-Gobain Vetrotex America (Valley Forge, Pa.), rovings from PPG Industries (Pittsburgh, Pa.) and surfacing veil from Xamax Industries Inc. (Seymour, Conn.). Resin pigment is supplied by Plasticolors Inc. (Ashtabula, Ohio). The pole's surface coating is a two-part urethane from Tnemec (North Kansas City, Mo.).
Newmark International (Birmingham, Ala.), a distributor of Strongwell poles for Class 1/H1 applications, also produces its own line of poles using centrifugal casting. In this process, polyester surfacing veil and glass fabric are wrapped around a mandrel and inserted into a mold. The mandrel is then removed and the mold is spun at 1,300 to 1,500 rpm. Resins and catalysts are injected into the mold as it rotates. Centrifugal forces help the resin wet out the layup, conform the layup to the mold's inside surface and consolidate the laminate. The part is then cured in place.
Cross-arms hold up
Composite cross-arms seem to be more easily accepted by the industry than poles, maintains Danny Lonergan, Powertrusion's VP of engineering, because utilities find these smaller, less expensive components "easier to justify."
Powertrusion rolled out its cross-arm line in 2002, selling it as the "lightest weight cross-arm available." Powertrusion cross-arms mimic the geometry of wood cross-arms, with rectangular pultruded beams filled with closed-cell urethane foam. Unlike wood cross-arms, he adds, the stronger composite cross-arms need no braces. Instead, the ready-to-install assemblies mount on poles with a simple bracket that reduces customers' material requirements and installation labor. Sales are "picking up well," Lonergan notes, adding that "the poles and cross-arms complement each other to provide a 'total' engineered system for power distribution."
Now owned by Shakespeare, the Lewtex line of pultruded glass/polyester cross-arms brought the first foam-filled design to this application in 1992, eliminating intrusion of water and nesting pests and enabling in-field drilling. While wood cross-arms are rated at up to 5,000 lb for "deadend" applications (where the power line ends), Lewtex 2.4m/8-ft cross-arms, for example, can handle up to 15,000 lb and last three times as long as wood counterparts. As a result, in some applications that require doubled wood cross-arms, a single composite cross-arm may suffice. Griffin attributes the cross-arms' success in part to their sale as a pre-drilled system with connecting hardware in place, cutting field labor costs significantly and making them more cost-effective than wood.
After a disappointing response to its original Omega cross-arm, Britt Engineering (Birmingham, Ala.) is redesigning its product. The original departed from established composite cross-arm designs, using a unique combination of pultruded glass/polyester with a thermoplastic inner core and outer casing. According to company president Frank Britt, the arm had the highest strength-to-weight ratio and greatest horizontal pin rating strength (at 3,000 lb) of any in the industry and was designed to enable field drilling and installation with standard hardware. (Pin rating measures the cross-arm's ability to withstand loads when the lines are not perpendicular to the cross-arm from a bird's eye view.) Despite the cross-arm's performance and life-cycle-cost advantages, Britt attempted to sell the cross-arm to electrical utilities for about three years with no success. "Pricing was killing us," he reports. "The people who do the buying are not the ones who look at long-term economics."
Britt anticipates that the new Omega cross-arm, currently undergoing developmental testing, will offer nearly the same performance as the old design but with a unit cost reduction of 16 to 20 percent. He is working with an undisclosed composites manufacturer, which has a proprietary process that combines a thermoset structural section with a thermoplastic extrusion.
Cable completes composite circuit
Composite-reinforced power transmission cables are a more recent development than their support structures. Standard Aluminum Core Steel Reinforced (ACSR) cable consists of aluminum conductor wires wrapped around a load-bearing core of solid steel wire. Two cable developers, CTC and 3M Co. (St. Paul, Minn.), have introduced designs that replace the steel core with a composite, providing several significant advantages. The composite-core alternatives can operate at higher thermal loading (at least 200°C/392I°F compared to 75°C/167°F for steel-cored cable). As a result, composite-cored cables sag less in extreme heat, allowing utilities to place transmission poles further apart in new installations and still maintain required minimum ground clearance. Compared to conventional steel-cored cable (in which the steel, though conductive, has greater resistance than aluminum), composite-cored products boost electrical transmission capacity — as much as a two-fold increase with CTC's cable (with a nonconductive core) and three-fold or better with 3M's conductive metal-matrix core. CTC's design decreases line losses by as much as 28 percent through the use of conducting wires with trapezoidal rather than round cross-sections, which allows more aluminum mass and, as a result, greater capacity. The cumulative effect of these improvements is a significant increase in power transmission capability without the need to rebuild or replace existing structures or erect additional new support structures. This is especially significant in light of predictions that the North American power grid will have to be expanded significantly in the next 20 years. Composite-cored cable could significantly mitigate the otherwise enormous expense and legal obstacles utilities will face to gain new rights-of-way for traditional cables.
In CTC's Aluminum Conductor Composite Core (ACCC) cable, the steel core is replaced with a pultruded carbon/glass hybrid composite, using a proprietary resin designed for the application. Testing and analysis is being provided by the M.C. Gill Composites Center at the University of Southern California (Los Angeles, Calif.). The lightweight composite allows CTC to wrap more aluminum around the core compared to a similar size of ACSR.
3M's Aluminum Conductor Composite Reinforced (ACCR) cable features a core of metal-matrix composite, overwrapped with aluminum-zirconium conducting wires. The replacement consists of continuous, oriented alumina fibers embedded in an aluminum matrix, a composite material that offers the same strength and stiffness as steel cores but at a lighter weight and higher conductivity.
CTC and 3M report that their cable products are under consideration by a number of utility companies. Meanwhile field testing continues. Both companies report successful testing at a number of sites under a variety of electrical, mechanical and thermal conditions over the past several years. 3M currently has a test line in place near Phoenix, Ariz. Meanwhile, CTC has provided cable and poles for a 33.8 km/21-mile demonstration project in Kingman, Kan., U.S.A. (see CT June 2004, p. 10).
Up to the challenge
One significant issue is the lack of design standards for composite cable components in the electrical industry. Neither the National Electric Safety Code nor the ANSI standards currently address composites. Pole manufacturers use "wood-equivalent" values for the ANSI classifications, but Shakespeare's Griffin points out that composites meet at least the minimum strength requirement while wood poles only average that minimum strength, distorting side-by-side comparisons. Safety codes pose an additional problem: some utilities demand that composite poles provide the same overload factor as wood — four times the computed total wind loading — while others recognize composites as an engineered material that, like steel, needs an overload factor of only 2.5. While manufacturers can continue to build to their customers' specifications, they believe that a standard code for composites could help propel them further into this market.
As Strongwell's Tickle's "wonderful future" beckons just out of reach, composites fabricators in the power transmission market's "difficult present" wear multiple hats, improving their products, educating potential customers and lobbying for standardization. "Our operation with respect to poles is evolving, requiring a missionary effort," says Powertrusion's Lonergan. "We're getting the product qualified, which ultimately will lend itself to usage." As composites purveyors get the word out to the utilities market, resistance should continue to diminish. Longeran typifies a pervasive if cautious optimism: "It's an unbelievable market — once we fully tap into it."
- Bouvier Kelly
- Britt Engineering Inc.
- Chromaflo Technologies Corp.
- Composites One
- Creative Pultrusions Inc.
- Fiber Glass Industries Inc.
- Olin Epoxy
- Owens Corning Composite Solutions Business
- PPG Industries Inc.
- Reichhold LLC2
- Shakespeare Composite Structures
- Xamax Industries Inc.
Powerhouse manufacturer’s high-pressure compression molding process forms prepregged CFRP components with forged-metal properties.
There are numerous methods for fabricating composite components. Selection of a method for a particular part, therefore, will depend on the materials, the part design and end-use or application. Here's a guide to selection.
Oven-cured, vacuum-bagged prepregs show promise in production primary structures.