The Tennessee Valley Authority (TVA) is no stranger to 500-kV transmission lines or the relentless need to modify existing power lines to meet the energy demands of the region. But, when the utility needed to increase the ampacity rating of its 500-kV Pin Hook-Wilson transmission line to meet load growth, the engineering department was forced to think outside the box.

To achieve the increased capacity, design engineers at TVA needed to increase the maximum operating temperature of the conductors to 100°C (212°F). Increasing the operating temperature of the line made the existing conductors sag lower and created clearance violations that usually would be handled by increasing the conductor attachment height, lowering distribution crossings or modifying conductor tensions.

A 1-mile (1.6-km) section of the line crossed over John Percy Priest Lake and Long Hunter State Park. The U.S. Army Corps of Engineers (USACE) controls the lake, and the Tennessee State Park Service maintains the surrounding property. The lower sag from the uprate project on the 2,530-ft (771-m) span over the lake violated a 1970 USACE permit by 4 ft (1.2 m). However, when TVA contacted the USACE to modify the existing permit, the updated permit was issued with a minimum conductor elevation that was 14 ft (4.3 m) higher than the original permit. This created an 18-ft (5.5-m) clearance violation over the lake.

TVA, ACCR conductor

Existing Conditions

The existing 500-kV transmission line was a three-bundle-per-phase 954,000-cmil 54/7 aluminum conductor steel reinforced (ACSR) conductor supported by two 250-ft (76-m)-tall lattice suspension towers and two 170-ft (52-m)-tall dead-end lattice towers. Depending on lake levels, all or part of the suspension tower foundations are located inside the lake. Environmental concerns over Osprey and Indiana bats, as well as special clearing agreements with the state park, complicated the construction efforts and threatened to delay the project.

Although both of the suspension towers are more than 200 ft (61 m) tall and normally would require aeronautical lighting and marking, the utility was able to avoid the aeronautical requirements because taller towers on adjacent lines had aeronautical lighting and marking.

Evaluating Options

TVA evaluated several options to meet the new permit height requirements, including tower modifications, installing new towers, retensioning the existing conductor and installing new conductor.

The tower modification option would add a 20-ft (6.1-m) lattice extension to the existing 250-ft (76-m)-tall towers. Since this would make the towers taller than the adjacent lines, this option also would require both suspension towers be equipped with aeronautical warning lights, solar panels, batteries, monitoring systems and hazard spheres, and the towers would need to be painted with aviation orange and white paint. Installing the extensions would require a 300-ft (91-m)-tall crane to be located on barges inside the lake. The crane would have to be trucked in and assembled on site.

The option of designing and constructing a new river crossing tower that would be located near one of the existing towers had complications, too. The new tower would be 300-ft tall and require a taller crane. New foundations would need to be constructed in the lake. The same aeronautical marking modifications as the previous option would be needed. Then the old tower would need to be removed. Environmental permitting for installing foundations inside the lake also was evaluated as part of this option.

Another option, retensioning the existing conductor, was analyzed, but the tension required to achieve the needed clearance far exceeded the National Electrical Safety Code allowable tensions for the existing conductors. The additional tension also would have overloaded the dead-end structures, requiring modifications to those structures and possibly their foundations.

For the reconductor option, all ACSR wires failed to provide the required clearance and ampacity, and required major modifications to the structures. Use of a conductor option with annealed aluminum was found to meet the sag and ampacity requirements, but this option was eliminated because of the risk of corona damage on this 500-kV application.

Although TVA had never used aluminum conductor composite reinforced (ACCR) on its transmission system, TVA engineers explored ACCR as an option. An 824,000-cmil 24/19 ACCR operating at 112°C (234°F) was found to meet the minimum sag and ampacity requirements. It could be installed at a lower tension than the existing conductor. Smaller ACCR conductors operating at elevated temperatures would have to meet the ampacity requirements, but corona concerns dictated the conductor diameter be 1 inch (25 mm) or larger. 

TVA, ACCR conductor