/images/buttons/swirl_60.jpg/images/southwire_100x70.jpgTurn-key Underground HV Puts Substation On-Line - 2009turnKeyUndergroundHV.htmUnderground high-voltage installations call for specialized engineering and skill, which Southwire supplies.
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Turn-key Underground HV Puts Substation On-Line - 2009

Underground high-voltage installations call for specialized engineering and skill, which Southwire supplies.


When Georgia Power (GP) needed to accommodate load growth near a large, upscale shopping mall north of Atlanta, the solution was a new substation. GP turned to Southwire for help with underground high-voltage feeds to the new facility.

"Running the 230 kV underground helped us maintain a natural buffer from a heavily-traveled arterial, and avoided a drastic change to the surrounding environment. Those were important considerations in this high-density area," says Mike Hazelton, GP manager of project management. Lewis Waters, Southwire senior project manager, says, "Southwire handled the whole underground high-voltage project, from civil design and construction management to electrical design and installation to terminations ready for the 230 kV overhead lines."

Civil engineering design has specialized requirements
"The underground feeds had to come in on a tight schedule, from design to operation" says Mark Smith, GP transmission line design supervisor. "Southwire pulled it off."

For service reliability, GP wanted two parallel 230 kV feeds, routed separately. A complicating factor was that one of the 230 kV feeds had to cross under the nearby busy thoroughfare.

The feed crossing the road required boring a 48-inch (1.2 meter) diameter hole that ran 510 feet from end to end. The hole was lined with a smooth pipe, and a bank of PVC conduits of various sizes went into the pipe.

With the conduits in place, the pipe was filled with a special grout mix formulated for good heat-transfer properties. "Heat transfer is very important in underground HV," says Waters. "We took samples of the grout about every 50 cubic yards (38 cubic meters) to be sure the heat-transfer properties were consistent."

Conductors were designed for the application
After studying GP's requirements, Southwire engineers developed an application-specific cable design. The solid-dielectric 230 kV cables carry 3,000 kcmil copper conductors. The conductors are insulated with 920 mils of high-purity cross-linked polyethylene (XLPE). A corrugated copper sheath provides high fault-current capacity. Over the sheath is a two-layer jacket consisting of 120 mils of linear low-density polyethylene (LLDPE) covered with 10 mils of semi-conducting polyethylene (SCPE). The assembled cable is heavy--it weighs about 16 pounds per foot (23.9 kg/m).

Southwire project management goes smoothly
With Southwire engineering and project management, the turn-key underground installation went smoothly. Work on the duct banks started in November of 2007. By late April of 2008, the cables had been installed and tested. The circuits were energized in early May of 2008. "We were very pleased with the collaboration," Smith says. Now that the project has reached its first anniversary, the circuit continues to provide much-needed power to the urban area.

Southwire has supplied millions of feet of HV cable to customers in North America. Complete underground high-voltage solutions from Southwire can include feasibility studies, conceptual engineering, ampacity studies, specialized underground system design, cable design and manufacture, and complete project management services. Waters concludes, "With turn-key solutions, we can assure that every step of the project meets Southwire quality standards."