/images/TDUpdate.jpg/images/southwire_100x70.jpgHow to Put a Gigawatt Underground - 2004HowToPutAGigawattUnderrgroundJuly2004.htmHow do you get nearly a thousand Megawatts of energy capacity from one side of a major interstate highway to the other – without going overhead?
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How to Put a Gigawatt Underground - 2004

Solid-dielectric high-voltage cable carries two 230kV circuits half a mile under a wetland, highway, and golf course – with help from Forte Power Systems.

How do you get nearly a thousand Megawatts of energy capacity from one side of a major interstate highway to the other – without going overhead? Xcel Energy called on Southwire’s Forte Power Systems to help create a half-mile-long underground high-voltage link.

“The project was the result of a large interstate highway and mass transit expansion, in conflict with an existing overhead line and proposed land development.” says Darrell Sabatka, Project Engineer for Xcel Energy. “Plans called for moving two 230kV circuits from overhead to underground. Southwire’s high-voltage design, manufacturing and installation expertise were a natural fit.”

Xcel Energy knows Forte’s underground HV

Xcel Energy is the fourth-largest combination electricity and natural gas energy company in the U.S., serving over five million customers in 11 Western and Midwestern states. They’re experienced with underground high voltage applications.

For this job, the underground path – 2,700 feet long – included direct burial segments at terminations, 3 X 3 duct banks, and a 610 foot directional boring under a major interstate highway. Xcel Energy provided the infrastructure and chose Forte to design, supply, and install the cable system components.

“Forte’s bid was very competitive, and an earlier 115kV project for Xcel Energy gave us confidence in Forte's technical capabilities and performance,” says Sabatka.

High-voltage cable supports real-time temperature sensing

The two three-phase circuits deliver a total design capacity of 960MVA over six single-conductor 230kV cables. The cables use 2,500kcmil copper conductors, XLPE insulation, corrugated copper sheathing and polyethylene jackets. Forte also supplied 12 cable terminations and equipment for the single point grounded system.

The cables also accommodate sophisticated instrumentation. One cable phase in each circuit carries embedded optical fibers for real-time temperature sensing. Accurate cable temperature data lets the utility get maximum asset use within safe operating limits.

“There were both manufacturing and shipping challenges in the project,” says Randy Denmon, Forte field services manager. “The 4.8" diameter cable weighs about 14 pounds per foot, and Xcel Energy asked for 2,800-foot cable lengths with no splices. The result: cable reels over 13 feet in diameter that weighed up to 45,000 pounds when loaded.”

Project wraps up ahead of schedule

The project carried some schedule pressure because Xcel Energy had committed to the highway project and golf course to remove the overhead lines by May 23. Southwire delivered. The reels were on site ready for installation at the end of March, and a dedicated crew of Forte factory-certified technicians handed over the cable system ahead of the removal deadline. That gave Xcel Energy time to energize the cable well before the overhead lines were due to come down. Southwire service follow-through included a post-project meeting with Xcel Energy to review operations and results.

“Forte, in partnership with Germany-based nkt Cables can also provide extra-high-voltage cable and accessories up to 345kV,” says Denmon. “Turnkey installations can include complete construction management services.”