HECO Puts New Composite Conductors to the Test
Utilities are always looking for new ways to extract additional capacity from existing transmission lines. Construction of new lines is riddled with uncertainty about recouping investments and difficulties in obtaining new rights of way. Line upgrades often represent the only viable option for upgrading transmission capacities.
Hawaiian Electric Co. (HECO; Honolulu, Hawaii, U.S.) recognized the benefits of increasing existing transmission line capacity with minimal environmental impact. In 2002, it began investigating a new high-capacity conductor manufactured with a fiber-reinforced aluminum core. This high-performance conductor, developed by 3M (St. Paul, Minnesota, U.S.), offers significant reduction in conductor sag, which enables utilities to increase capacity without changing clearances or increasing structural loads with minimal environmental impact.
HECO agreed to be the first utility to field-test this high-ampacity, low-sag composite conductor on a 46-kV subtransmission line.
| 556 AAC Dahlia | 477 ACCR | ||
|---|---|---|---|
| Maximum current within SAG LIMIT |
Amperes | 820 | 1280 |
| Maximum conductor temperature | °F (°C) | 212 (100) | 464 (240) |
| Everyday tension | lbs (kN) | 1610 (7.2) | 1610 (7.2) |
| Maximum sag (at max. temperature) | ft (m) | 13 (4) | 11.5 (3.5) |
| Maximum tension |
lbs (kN) | 2240 (10) | 2240 (10) |
Composite Conductor
The 3M composite conductor, also known as aluminum conductor composite reinforced (ACCR), relies on a core of fiber-reinforced aluminum wires surrounded by temperature-resistant, aluminum-zirconium wires. This arrangement dramatically increases capacities over those of conventional conductors and the use of lighter weight material, enabling line upgrades without significant tower modifications.
While HECO acknowledged the obvious advantages of using the ACCR, the utility needed to know how the new material would respond to Hawaii's highly corrosive environments. Even the best galvanized steel can severely corrode within two years in coastal regions subjected to salt-laden trade winds and moisture. Experience has shown that steel-reinforced conductors (ACSR) are particularly vulnerable in what metallurgists have dubbed the most corrosive environments in the world, as the north shore is constantly subjected to salt-laden trade winds from the Pacific Ocean.
In the vicinity of the 46-kV test line, the University of Hawaii has built an outdoor corrosion laboratory with conditions that rival some of the world's most adverse environmental test chambers.
In addition to determining corrosion resistance, HECO also needed to understand how ACCR was installed and maintained, the types of accessories, suspensions and deadends used, and the long-term performance of the test circuit. To verify the behavior of ACCR, HECO and 3M developed a pilot project that included installing multiple spans of 3M composite conductors on HECO's subtransmission grid and monitoring the line for two years.
Project Scope
In early 2002, HECO's decision to participate in the field-testing of the composite conductor on a short section of subtransmission line had multiple technical objectives:
Conductor stringing with conventional equipment.
Erection of the conductor with the full range of qualified accessories, including suspensions and terminations.
Circuit operation in the island's most corrosive environment.
Frequent visual inspection of the line.
| Designation | 477-T16 | Metric |
|---|---|---|
| Stranding | 26/7 | |
| Kcmils | 477 kcmil | |
| Diameter | 0.86 in | 21.8 mm |
| Total area | 0.435 in |
281mm |
| Aluminum area | 0.374 in |
241mm |
| Weight | 0.539 lbs/linear ft | 0.802 kg/m |
| Breaking strength | 19,476 lbs | 86.6 kN |
| Thermal elongation | 9.2 × 10 |
16.5 × 10 |
| Resistance | ||
| DC @ 20°C | 0.1832 ohms/mile | 0.1138 ohms/km |
| AC @ 25°C | 0.1875 ohms/mile | 0.1165 ohms/km |
| AC @ 50°C | 0.2061 ohms/mile | 0.1281 ohms/km |
| AC @ 75°C | 0.2247 ohms/mile | 0.1396 ohms/km |
| Geometric mean radius | 0.029 ft | 0.88 cm |
| Reactance (1 ft spacing, 60 Hz) | ||
| Inductive Xa | 0.4296 ohms/mile | 0.2685 ohms/km |
| Capacitive X'a | 0.0988 ohms/mile | 0.0618 ohms/km |
HECO selected a half-mile section of a 46-kV subtransmission line located on the north shore of Oahu for the test because its performance was not critical for the reliability of the network. The existing conductor on the line was 556-kcmil all-aluminum conductor (AAC) to be replaced with a 477-kcmil ACCR.
The composite conductor looks similar to an ACSR except that the core wires are fiber-reinforced aluminum not conventional steel. The test line consisted of two deadend towers, three suspension towers and a single circuit with a ruling span of 430 ft (131 m).
The stringing tension of the composite conductor was calculated to match the sag of the existing 556-kcmil AAC at 70°F (1800 lbs tension); the stringing tension was determined using Alcoa's SAG-10 software.
Conductor Installation
With a few exceptions, the installation of the ACCR was similar to that of a typical ACSR. When handling the conductor, the bending radius of the conductor must not exceed the breaking limitations of the composite core. To meet this requirement, HECO used 28-inch (71-cm) stringing blocks instead of 20-inch (51-cm) blocks to provide a large safety factor and prevent damage to the core wires. Another significant difference was the accessories designed specifically for the composite conductor. The deadends and suspensions, made by Preformed Line Products (PLP; Cleveland, Ohio, U.S.), were installed like the hardware HECO normally used, except these accessories were heavier and more robust than the hardware for 477-kcmil conductors. PLP accessories are designed to remain cool when the conductor is hot. PLP THERMOLIGN™ deadend and suspension assemblies were used as the hardware for the ACCR.
The electrical connection between the two different sized conductors — the 477-kcmil ACCR and the existing 556-kcmil AAC conductor — was made using the Alcoa-Fujikura (Franklin, Tennessee, U.S.) parallel groove clamp. A standard “sock splice” was used as a conductor grip to pull the ACCR through the stringing blocks. In addition, a swivel was installed between the ACCR and the AAC during this operation to prevent twisting of the ACCR. To assist conductor sagging, a PLP deadend assembly was used to temporarily grip the conductor. Other standard installation equipment used during the stringing operation included a drum puller to pull in the ACCR, a standard reel stand with a friction break to provide back tension, standard chain hoists, cable cutters, grounding clamps and a dynameter.
Representatives from 3M and PLP were on site to witness the line crew in action during the installation procedures, which were completed in three days without problems.
Experience to Date
The 3M composite conductor is expected to behave like a conventional AAC or AAAC in terms of corrosion resistance, because it contains only aluminum and inert aluminum-oxide fibers. The fibers are stable with aluminum and not subject to galvanic corrosion. This test installation was inspected in January 2003 after a six-month operation and visual inspection of the line conductor and accessories confirmed this new material showed no sign of deterioration or corrosion.
Benefits of the New Technology
The 3M composite conductor offers the circuit a transmission capacity (ampacity) of two to three times greater than the conventional AAC and AAAC HECO used. The higher performance coupled with low sag enables line upgrades within existing rights of way without significant tower modifications. Therefore, the use of this technology can relieve projected transmission bottlenecks and provide greater system reliability. For example, an equivalent diameter 477-kcmil ACCR can replace a 556-kcmil AAC with the same tension and sag but deliver a lot more power.
By avoiding tower modifications, conductor upgrades can be completed without extensive environmental studies or heavy construction operations. In some cases, it could even provide an alternative to building new lines by providing additional capacity through line upgrades. This simple solution also reduces service interruption.
What's Next at HECO?
HECO will continue evaluating this technology in the field for the next year. HECO plans to use new technology to upgrade transmission capacities, and to help form and strengthen the transmission grid of the future.
Sucuma Elliot received a BSEE degree from the University of Hawaii and an MBA degree from Hawaii Pacific University. Since joining HECO. Sucuma has held various engineering positions. With 32 years experience in the industry, he is currently a technical services engineer in HECO's engineering department. Sucuma is a registered professional engineer in Hawaii.
SElliot@hei.com
Construction of the 3M Composite Conductor
3M Composite Conductors consist of high-temperature aluminum-zirconium strands covering a stranded core of fiber-reinforced composite wires. Both the composite core and the outer aluminum-zirconium (Al-Zr) strands contribute to the overall conductor strength. The composite core contains metal matrix composite wires made by 3M with diameters ranging from 0.073 to 0.114 inches (0.185 to 0.290 cm). The core wires have the strength and stiffness of steel but with much lower weight and higher conductivity. Each core wire contains many thousand ultra-high-strength micrometer-sized fibers. The fibers are continuous, oriented in the direction of the wire and fully embedded within high-purity aluminum.
Visually, the composite wires appear as traditional aluminum wires, but the exhibit mechanical and physical properties superior to those of aluminum and steel. For example, the composite wire provides nearly eight times the strength of aluminum and three times the stiffness. It weighs less than half of an equivalent segment of steel, with greater conductivity and less than half the thermal expansion of steel.
The outer strands are composed of a temperature-resistant aluminum-zirconium alloy, which permits operation at high temperatures (210°C [410°F] continuous, 240°C [464°F] emergency). The Al-Zr alloy is a hard aluminum alloy with properties and hardness similar to those of standard 1350-H19 aluminum but with a microstructure designed to maintain strength after operating at high temperatures (it resists annealing). In contrast, 1350-H19 wire rapidly anneals and loses strength with excursions above 120°C to 150°C (248°F to 302°F). The temperature-resistant Al-Zr alloy wire has tensile strengths and stress-strain behavior equivalent to those of standard 1350-H19 aluminum wire.
Want to use this article? Click here for options!
© 2008 Penton Media Inc.
