It’s a question that inevitably arises after a natural disaster that results in widespread power outages: “Why can’t power lines be buried underground?”
Usually, the simple answer is cost. Buried transmission and distribution lines offer benefits of increased reliability, public safety and a more desirable aesthetic. But underground lines typically involve more expensive materials, greater insulation requirements and higher construction, maintenance and repair costs.
Estimated costs for constructing underground transmission lines range from four to 14 times more expensive than comparable overhead lines. A typical new 69kV overhead single-circuit transmission line costs approximately $285,000 per mile, while a new 69kV underground line costs approximately $1.5 million, according to a 2011 paper published by the Public Service Commission of Wisconsin.
Technical improvements in cable technology, wire placement, conduit sizing, grounding methods, directional boring techniques and other aspects of undergrounding power lines have advanced the reliability of underground power, according to Electric Light & Power. But the cost of construction – especially when it involves trenching through existing developments – remains higher than overhead construction.
In dense urban areas, where overhead lines aren’t an option, underground transmission lines are usually pursued out of necessity.
David Lindsay, senior project manager for New River Electrical Corp. in Cloverdale, Va., told Electrical Contractor that while underground transmission lines remain a rarity compared to their overhead counterparts, demand for underground cable is growing.
“The amount of HV/EHV [high-voltage/extra-high-voltage] cable being installed has continued to increase over the past five to 10 years,” Lindsay said. “Growth in the use of cable is coming from revitalized and growing urban centers, as well as increased use in suburban and extra-urban areas.”
In new construction, developers typically pay the increased cost of underground distribution services and pass those costs to home buyers. It’s more difficult to assign the cost of transmission lines to a specific developer or customer group.
Some communities are opting to shoulder the price tag of underground lines to avoid the construction of overhead transmission lines through established areas. In at least two cases in Florida, a utility and governmental agency agreed to share the cost difference of placing the transmission lines underground, according to Electric Light & Power.
Voters in Palm Beach, Fla., narrowly approved up to $90 million in bonds in 2016 to finance a 10-year project to bury all of the island’s utility lines underground.
Beta Engineering’s high voltage experience involves engineering, procuring and constructing overhead and underground transmission lines. A recent project in California involved constructing a 14 mile 138kV single circuit transmission line. The line, which was broken into overhead and underground sections, included one portion that ran under a major highway.