Wiring the Evolving Grid: New conductors offer greater capacity with less sag

BPA is set to revolutionize its bulk electric grid through the introduction of more advanced conductors on its transmission system.

More than 15,000 circuit miles of high-voltage conductor, or transmission lines, make up the path for electrons to flow throughout BPA’s service territory. These conductors physically regulate the capacity of the system, limiting how much power can move through lines. BPA’s Transmission Line Engineering group, in partnership with the Transmission Project Management office, have begun the process to analyze and qualify a new conductor type to increase the capacity of BPA’s grid.

The development comes as part of the Evolving Grid program, an initiative designed to increase the reliability and capacity of the federal transmission system and provide more of the infrastructure necessary for BPA’s customers to meet emerging renewable resource portfolio requirements. 

To develop a range of conductors that can help meet this goal, Transmission Services is exploring using more sophisticated conductors, hardware and other transmission materials to increase the capacity of existing transmission corridors. The new conductor comes in an array of diameters and will require different and more robust materials to attach them to towers.

“While new line types are not a new concept at Bonneville, we’ve been using a lot of the same conductor types for many years” said Erich Orth, supervisor and program manager of the Portfolio Delivery Team in the Transmission Project Management office. “Introducing new conductors into our grid is a significant opportunity to maximize the efficiency of existing towers and reduce the demand for new tower construction.” 

BPA’s Transmission Line Engineering team clears conductor options and their accompanying components for use in the agency’s transmission corridors through an extensive qualification process. The evaluation not only determines whether the line is compatible with BPA’s grid and requirements, but also explores what physically attaches to the cable— its hardware— and ensures each piece can meet necessary performance requirements mechanically, electrically and thermally. This is achieved through extensive research, development and physical testing.

In the initial search for advanced conductor options, key requirements included finding a conductor easily obtainable through outside vendors and resistant to high temperatures to prevent line sag. 

“That’s the trouble with putting more power through a line,” Orth explained. “Typically, the higher the temperature of the physical conductor, the more it’s going to sag and elongate, which can cause line clearance concerns.”

Transmission Services has grappled with the physical limitations of BPA’s existing conductors, specifically 2.5-inch expanded aluminum conductor steel-reinforced (ACSR) cable, in its transmission corridors for many years. The Evolving Grid program provided the determining push and capital funding needed to invite choices of modern high-temperature, low-sag conductors with greater capacity potential than the ASCR.

David Atkinson, team lead of the mechanical portion of the Transmission Line Engineering Group, has dedicated many years to validating aluminum conductor, steel-supported, trapezoidal-shaped (ACSS/TW) conductor types for operation within BPA’s conductor library. 

“We are very much in the early stages of identifying which cable sizes to settle on, getting all the cable specified, getting all its supporting hardware identified and qualified,” said Atkinson.

BPA’s current ACSR 2.5-inch expanded conductor has two load-carrying components: the outer strands made of aluminum and a steel core. The materials share the electrical and thermal loads running through a transmission line, allowing the conductor to operate at a maximum temperature of 100 degrees Celsius, or 212 degrees Fahrenheit. 

As a result of the conductor consistently operating at or above 100 degrees Celsius, the aluminum strands in the conductor can soften over time, becoming more pliable and losing residual strength. This process is called annealing. 

“It’s a function of time and temperature,” explained Atkinson. “When we’re looking at the new types of conductors that we’re trying to bring on, we’re looking at conductors that already have fully annealed aluminum.”

One pre-annealed conductor Transmission Engineering is currently qualifying and designing with is called ACSS/TW Plover, an option compatible with BPA’s existing towers and infrastructure that is likely to replace the aging ACSR 2.5-inch expanded conductor.

The difference in electrical and thermal capacities of the new conductor and the ones in use now are vast, Atkinson explained. The standard ACSR line BPA uses can run at just over 2,300 amps when operating at 100 degrees Celsius. With materials about a quarter-of-an-inch smaller in diameter, the ACSS/TW Plover can run up to 200 degrees Celsius, or 392 degrees Fahrenheit, and has a capacity of over 4,300 amps – about an 87% increase in comparison to the ASCR 2.5- inch expanded conductor.

These fully annealed aluminum conductors also offer the added advantage of allowing design teams to anticipate that the steel core will carry all the mechanical load on the line. Essentially, the new conductor can heat to higher temperatures safely while accounting for sag performance, without degrading the core. 

“We try to find the most efficient use of the towers and the conductors that we’re hoping to qualify and approve to use,” Orth said. “There are lots of variables, and really, we’re just trying to find the sweet spot.

The qualification process, however, is not as simple as identifying potential benefits of the conductors themselves. It can take months or years of physical testing and analyses to ensure the new conductor will function as engineered within BPA’s transmission system. With innovation and optimization at the forefront, the partnership between Transmission Line Engineering and Transmission Project Management will allow BPA to bring on these advanced conductors quickly.

“Many hands are helping to make this heavy lift for Bonneville to modernize and transform our bulk electric grid,” said Mike Miller, vice president of Engineering and Technical Services. “In particular, I’d like to acknowledge the Transmission Line Engineering group for their work in exploring the use of advanced ACSS conductors, working in partnership with the Portfolio Delivery Team to bring on new cables to meet the demands of the Evolving Grid program.