Background
CenterPoint Energy (CNP) of Houston, Texas, has long used ACSS conductors for new and upgrade projects across their system. An expansion in generation required that the construction of a 2800-MVA, 345-kV line be built on an existing transmission tower line and partially on a new right of way. CNP believed that the existing ACSS conductors available did not quite fit the needs of the project and looked toward the recently developed extension of ACSS capabilities by Southwire with a new core strand coating that withstood higher operating temperatures and a UHS steel core strand.
Conductor Selection
The standard galvanizing coating used on the core strand had limited operation to 200°C. A new corrosion inhibiting coating, Galfan, a zinc, 5% aluminum mischmetal alloy, allowed a continuous operating temperature of 250°C (482°F). With the higher allowable temperatures, the fully annealed aluminum strand conductor could be operated at a higher temperature.
The supporting core strand for the conductor required an increase in tension capability. Southwire has been working with its vendors to develop a UHS steel strand. The new strand has an ultimate strength of 285-ksi—hence its name, HS285. Combined, the new core strand and coating allowed a new range of conductor capabilities, operating at higher temperatures yet developing less sag than conventional ACSS.
CNP now addressed its concerns for the longevity and performance of the new conductor and the accessories that would be used with it. CNP had tests performed, validating tests in several key areas: the strength and stress-strain characteristics of the conductor, its vibration performance, its corrosion performance, and performance of accessories at elevated temperatures, including the suspension system, connectors, and spacer dampers.
Over 60 alternative designs for conductor type and size and structure type were performed to select an appropriate conductor system for the latest grid expansion, which required the equivalent capacity of a two 1590-kcmil Lapwing ACSR conductor bundle. CNP designated a 1433.6-kcmil ACSS/TW HS285 as the best choice. The selection offered a 55% increase in electrical capacity for a nominal cost increase of 4%.
CNP revisited its standard lattice tower families and decided to develop a new family that took full advantage of the sag-tension and loading parameters of the ACSS HS285 conductor. The new family of structures was shorter than the previous standard families.
Construction Aspects
CNP from its long experience with stringing an ACSS conductor also selected a trapezoidal shape for the fully annealed aluminum strands. The TW strands behave better during stringing operations where the pulling of the fully annealed strands through blocks and bull wheels can lead to bird caging if care is not exercised. No problems were experienced during the installation of the new ACSS HS285 conductor.