CenterPoint Energy

PROJECT PROFILE

High-Capacity Tie Line Uses Low-Sag Technology

When Houston-based CenterPoint Energy needed to upgrade a tie line between two major generating stations, their choice was a new product from Southwire that delivers high-temperature, low-sag capacity rivaling exotic composite-coredesigns – without the exotic cost. The wire CenterPoint chose puts an ultra-high-strength steel core inside an ACSS/TW conductor architecture.

CenterPoint’s challenge was to bring more power to the greater Houston area. They needed increased transmission capacity between a nuclear plant near Bay City, Texas, and a generating facility in Rosenberg, Texas. The solution was an $80 million, 60-mile, two-circuit 345kV transmission line. The question was how to implement the circuits.

High-Temperature, Low Sag HS285^® Overhead Conductor Won Over Many Options

CenterPoint analyzed 60 different design combinations, factoring in distance between structures, land-use impacts, construction issues and total installed costs. Circuit options included: two conductors using ACSR (aluminum conductor, steel-reinforced); three conductors using standard ACSS/TW (aluminum conductor, steel-supported, trapezoidal wires); and two conductors using Southwire’s new HS285 overhead conductor ultra-high-strength ACSS/TW high-temperature, low-sag design.

CenterPoint found that the new HS285 ACSS/TW conductors would give 55 percent more capacity than the ACSR conductors with a project cost premium of only about four percent. That made the decision clear. Forty miles of the line will use new structures to carry two 1433.6 kcmil ACSS/TW HS285 conductors. That installation was energized in July 2007.

Chuck Bennett, Manager of Transmission Engineering for CenterPoint, says, “We would have used HS285 overhead conductors for the entire project, but testing time and early product availability didn’t quite meet schedule requirements for the first phases. For the same scheduling reasons, we originally designed our new structures for the reduced sag of standard ACSS/TW. Reduced tower height saved several hundreds of thousands of dollars over the ACSR option, but even then we didn’t make full use of HS285 overhead conductor economies. HS285 overhead conductors will reduce maximum sag by yet another three to four feet.”

HS285 overhead conductor rivals composite-core designs

The new HS285 overhead conductor is an enhanced version of Southwire’s ACSS/TW. Like standard ACSS/TW, HS285 overhead conductors are rated for continuous operation at 250°C. An ultra-high-strength steel core puts HS285 overhead conductor sag performance on a par with recently developed composite cores “…at a reasonable cost, not 10 to 30 times the price,” according to Bennett.

The ultra-high-strength steel core material borrows heavily from existing steel technology to develop high tensile strength without loss of elongation, ductility or stress corrosion properties.

The HS285 overhead conductor core is protected by a Galfan coating that contains 95 percent zinc and about five percent aluminum, with a small addition of rare earth elements, primarily cerium and lanthanum. The Galfan coating protects the steel core at operating temperatures that would shorten the life of traditional galvanizing.

In addition to cost advantages, another advantage of HS285 overhead conductors over exotic composite conductors is that it uses the same installation techniques as standard ACSS. HS285 overhead conductors are commercially available with lead times in line with conventional ACSR conductors.

CenterPoint Sees Future Uses

Bennett says, “We have a long history with the ACSS/TW cable architecture. We have been using ACSS/TW since 2000, and have installed over 3,000 miles of it.”

CenterPoint sees HS285 ACSS/TW conductors as an economical solution for increasing capacity needs. The additional mechanical strength will also meet new needs The 2007 National Electrical Safety Code will require higher mechanical loading design criteria, and the Texas PUC may be considering higher hurricane storm-loading criteria. “This is a good option to have in our toolbox,” says Bennett.

If you need increased capacity in a new line, Southwire’s HS285 overhead conductor are the multi-purpose tool you need. Contact us to learn more.