The strong industry-wide demand for transmission infrastructure with high-capacity lines is well documented, and has prompted manufacturers to develop and bring to the marketplace new conductor technologies to respond to this need.

Chuck Bennett, manager of transmission design and project engineering at CenterPoint Energy (CNP; Houston, Texas, U.S.), guided his design group through the processes of investigating, testing and implementing new conductor technologies. CNP is the first U.S. electric utility to construct a line using a high-temperature low-sag conductor developed by Southwire (Carrollton, Georgia, U.S.) called ACSS HS285.

CNP is using a 1433.6-kcmil aluminum conductor, steel supported/trapezoidal wire (ACSS/TW) HS285 Merrimack conductor as part of the design solution for a new 345-kV transmission project. The project involved designing and constructing 50 miles (80 km) of new two-circuit 345-kV latticed towers, building 18 miles (29 km) of a new circuit on existing 345-kV structures, and designing and constructing a new 345-kV transmission substation in Southeast Texas within the Electric Reliability Council of Texas (ERCOT) system.

THE CHALLENGES

Deregulation of the Texas electric utility industry brought about many changes to CNP and its transmission system. The challenges began in 1998, when new deregulated independent generation projects were announced for completion and full operation from 2000 to 2003. The new generation required constructing transmission-capacity improvements to integrate an additional 8600 MW into the 12,000 MW already connected to the CNP transmission system. CNP quickly realized that old designs and old technologies were not sufficient to achieve the huge transmission system transformation required.

It was during this time period that CNP researched, tested and adopted for use on the utility's transmission system Southwire's then-existing ACSS high-temperature conductor. High operating temperature and, subsequently, high-capacity ACSS conductors were determined to be the only feasible way to meet the required design criteria and construct the capacity increases in time. The use of ACSS to reconductor existing lines allowed for significant capacity upgrades using existing structures and resulted in more than US$100 million in documented savings.

Since that time, hundreds of miles of ACSS high-temperature conductors have been installed on the CNP transmission system, both as replacement conductors on existing structures and original conductors on new structures. By 2005, CNP was recognized as the first U.S. electric utility to install ACSS conductors on a wide-scale basis at 345 kV.

In 2005, CNP faced a new challenge when ERCOT identified a transmission-system constraint that required construction of new high-capacity 345-kV transmission circuits. It required a new two-circuit 2800-MVA, 345-kV transmission line to be located partially in an existing right-of-way and partially in new right-of-way. CNP's unique “opportunity” was that the project needed to be designed, constructed and energized by the summer of 2007. The challenge required CNP transmission engineers to reach into their toolbox and apply not only what they had learned over the previous six years, but also to seek out new technologies. It was during this time that Southwire announced an improvement to its ACSS high-temperature conductor technology and introduced a new ultrahigh-strength product named ACSS HS285.

HIGH-END TECHNOLOGY

Annealed aluminum conductor, supported on steel strands (ACSS) has been in use for many years. It is Southwire's application of other new technologies to the original ACSS concept that brings about the breakthroughs. The result was a high-capacity, high operating temperature, very-high-strength and low-sag ACSS conductor with an economical cost.

The first stage of advancement came with the development and application of Galfan coating, to replace standard galvanizing, for corrosion protection of the steel core wire. The Galfan coating (a zinc, 5% aluminum mischmetal alloy) allows the conductor to operate continuously up to 250°C (482°F) without damaging the steel coating. Standard galvanized coating is limited to 200°C.

The second stage came when Southwire and Davis Wire, a supplier of steel core wires, developed an ultrahigh-strength or HS285 steel wire for use as the supporting core wire for the annealed aluminum conductor. The strength of the core wire is achieved in part through a patenting process and varies according to the wire diameter, but it is approximately 285 ksi, leading to the name HS285. The strength of the core wire allows higher tensions, resulting in even lower sags.

The combination of these two technologies has resulted in a conductor based on the tried and proven performance of ACSR and ACSS technology, with the added benefit of fulfilling today's need for very high current-carrying capacities using the smallest-possible conductor size with low-sag characteristics.

ACCEPTANCE PROCESS

Even though ACSS HS285 was an evolution to the next level by Southwire of a product that CNP had been using successfully for several years, physical plant visits and due-diligence process evaluations and inspections were performed as a first step to accepting and implementing this new transmission conductor product. The due-diligence inspection and audit for the ACSS HS285 included plant visits to three different facilities involved in the production of the finished conductor. CNP had some concerns regarding the long-term performance of the conductor, connectors and other line accessories for ultrahigh-strength application.

ACA Conductor Accessories, a division of AFL Telecommunications (Duncan, South Carolina, U.S.), was selected to design, fabricate and supply connectors for the 1433.6-kcmil ACSS/TW HS285 conductors. Douglas Harms, a licensed professional engineer in Texas with CNP, was assigned to work with the Electric Power Research Institute (EPRI; Palo Alto, California, U.S.) to develop an independent testing protocol for the conductor and line accessories.

A test regimen was developed to validate the strength and stress-strain characteristics of the conductor; test the conductor for corrosion performance; test connectors for both thermal and mechanical performance; and test the vibration performance of the conductor and spacer-damper accessories at elevated temperatures. The testing was performed at EPRI facilities in Haslet, Texas, and at independent laboratories subcontracted by EPRI. Test results showed that the ACSS HS285 conductor should perform as well electrically and physically as the previously evaluated ACSS high-temperature conductors.

DESIGN CRITERIA

Sixty alternative project studies were performed in evaluating the numerous combinations of transmission structure and conductor types for the new 345-kV project. The transmission project alternative that was selected and approved by ERCOT and the Texas Public Utility Commission came to be known as the Hillje inter-tie transmission project. CNP's transmission engineers developed a design criteria document as part of the Hillje transmission project process. The document specified electrical and structural criteria necessary to achieve the required lifetime performance for the transmission line.

The ERCOT-approved project required a minimum electrical impedance and capacity requirement equivalent to two 1590-kcmil ACSR conductors per phase. CNP determined that two 1433.6-kcmil ACSS/TW conductors per phase have similar impedance characteristics. The bonus was that this selection would provide a 55% increase in electrical capacity at its continuous rating for a nominal increase (approximately 4%) in cost. The design processes ultimately included combining ACSS HS285 conductor with project-specific latticed steel tower designs. This combination met the electrical, physical and economic requirements of the project.

CNP transmission design engineer William “Billy” Oliver, a licensed professional engineer, used Power Line Systems' PLS-CADD line design software to optimize the transmission line design to meet the established criteria. Oliver developed structural design parameters and worked together with Bob Nickerson and the Thomas & Betts Steel Structures Division to develop two new tower designs that took advantage of the characteristics of the ACSS HS285 conductor. This combination allowed the overall structure height to be reduced in some cases compared to previous designs.

After completion of the designs, Thomas & Betts produced prototype towers for design load testing that were “trial” assembled at steel-tower fabricator Fabrimet's facility, then shipped to the EPRI test facility at Haslet for testing. These tower designs were the last to be tested at the EPRI Haslet test facility before it was closed. (See “Latticed Steel Transmission Tower Testing Still Valuable,” T&D World, May 2006.) Successful testing allowed for the release of new designs for a family of tangent and angle tower structures to be fabricated. The project required 261 tangent and 44 angle and deadend towers to traverse the 50-mile (80-km) distance requiring new structures.

CNP transmission design engineer David Pratt, a licensed professional engineer, was assigned to work on a 20-mile (32-km) section of the project, which would require long spans to overcome challenging terrain features. Among the challenges were roads, creeks, rivers and oil wells. Using the combination of ACSS HS285 and the new structure designs with the PLS-CADD line design software, Pratt was able to meet the terrain challenges and use structure heights that were, on average, 5 ft (1.5 m) shorter than previously designed towers.

THE LINE CONSTRUCTION

Construction of the Hillje project transmission line began in March 2006. As part of the project kickoff, Southwire and ACA Conductor Accessories conducted a training session for the InfraSource Services Inc. line personnel who would be constructing the facilities and installing the ACSS HS285 conductors.

CNP Construction Coordinator Alan Valicek was assigned to oversee the line construction. He had also been an advisor during part of the structure design and testing process. Valicek faced many challenges in managing construction of a transmission line project of this magnitude, including some Southeast Texas-specific challenges such as alligators, snakes, whitetail deer and longhorn cattle.

Line construction proceeded in normal fashion with foundations and tower installation. To facilitate construction of the project, three on-site material yards were established to receive and issue materials. The HS285 wire yard was located near the community of Danevang, Texas.

CONDUCTOR STRINGING

Because of the remote location and terrain conditions of the right-of-way, a helicopter was used extensively while installing insulators and hardware, and installing pulling lines for conductor stringing.

To install the ACSS HS285 conductor, crews followed the standard equipment and stringing techniques recommended in IEEE Guide 524, “Guide to the Installation of Overhead Transmission Line Conductors.” The size of stringing blocks, tensioner bull wheel size and type, and conductor reel and tensioner setup positions require particular attention to prevent bird-caging of the soft, annealed aluminum strands. CNP selected trapezoidal-shaped aluminum strands, rather than round strands, for this ACSS conductor for maximum conductor capacity and better handling during installation.

Southwire Chief Engineer Ridley Thrash was on-site to observe some of the first HS285 wire pulls and review the location of wire pulling and payoff equipment. With the proper equipment and position setup, stringing of the HS285 Merrimack conductor was accomplished using standard wire-stringing procedures. No difficulties were experienced in pulling the ACSS/TW HS285 conductor around large angles.

Compression connectors supplied by ACA Conductor Accessories were installed for in-line and deadend applications. Procedures and tooling for installing the HS285 connectors were found to be the same as with previous ACSS conductors. CNP chose to use spacer dampers for the horizontal conductor configuration. Installation of spacers was accomplished using standard spacer buggy configurations equipped with suspension rollers with neoprene liners.

READY FOR MORE

The Hillje transmission project was completed and released to ERCOT for system operation on July 31, 2007. With the successful installation on the Hillje project, CNP plans to evaluate ACSS/TW HS285 conductors on a project basis for future applications. CNP sees high-temperature low-sag ACSS/TW HS285 conductors as a continuing economical solution for meeting its increasing transmission capacity needs.

---

Douglas P. Harms joined CenterPoint Energy in 1973, originally working in distribution engineering. Since 1980, he has been in transmission operations where he is now a consulting engineer for high-capacity transmission line design, materials, testing and standards. He is chairman of ANSI ASC C119 Standards Committee “Connectors for Electric Utility Application,” and received the EPRI Innovators Award for Creative Testing and Novel Use of ACSS Cable and Hardware. Harms has a BSEE degree from the University of Houston and has been a professional engineer in Texas since 1983. douglas.harms@centerpointenergy.com