For decades, new power and telephone lines serving housing developments and office parks, government complexes and educational campuses have been placed underground. However, in most older American neighborhoods, power and communications lines remain suspended from utility poles, making them vulnerable to wind and ice storms. After such storms, people often ask, “Why don’t they put them underground?”
For years, the answer was, “It costs too much.” Indeed, the difference in cost per mile of aerial versus underground construction can seem staggering.
However, with the evolution of trenchless construction methods and factoring in the savings from reduced costs to repair damage caused by excavation to bury cables, the difference between open-cut and trenchless narrows.
Dominion Energy (formerly Dominion Virginia Power) has had active undergrounding programs underway for several years. The company’s Strategic Underground Program targets the most outage-prone overhead tap lines and seeks to place them underground, said Alan Bradshaw, director of electric distribution—underground.
“The goal of the program is to reduce the number of work-repair locations required to restore power outages,” Bradshaw said. “Our data suggest that converting 4,000 miles of the most outage-prone lines could reduce restoration times following severe weather events by up to 50 percent.”
Since July 2014, Dominion Energy has converted 508 miles of outage-prone overhead tap lines to underground, including more than 1,500 individual tap lines across the company’s Virginia service territory.
“We expect to convert 250 miles of outage-prone tap lines in 2017 and, by 2019, anticipate converting 350 miles on an annual basis,” Bradshaw said. “While we install new underground facilities, we are also developing future projects, securing easements and completing designs.”
Dominion Energy has made significant investments in its distribution system over the last decade, and the System Average Interruption Duration Index (SAIDI) has decreased accordingly. However, the system still is susceptible to major storm events. After Hurricane Irene in 2011 and the derecho wind event in 2012, where more than 1 million customers lost power for days, Dominion Energy began to investigate other ways to improve resiliency.
“When we started to look at the data, we found that a significant number of outage events occur on the tap-line portion of our distribution system,” Bradshaw said. “Looking deeper, we found that over 60 percent of the outage events that occur on tap lines actually occur on about 20 percent of our tap lines, many located in backyards with heavy tree canopies, exposing lines to damage from trees.”
These areas are the most difficult and time-consuming to repair, frequently requiring additional crews to hand-carry poles and wires and climb poles while bucket trucks and auger trucks are parked on the roadway because they cannot access the locations where repairs need to be made.
“These labor-intensive repairs can sometimes take days and pull resources from other restoration efforts,” Bradshaw said. “Eliminating these difficult-to-repair locations will lead to a significantly more-efficient restoration.”
Costs of undergrounding
In 2003, Hurricane Isabel left some customers without power for two weeks. The Virginia State Corporation Commission estimated the cost to underground the overhead electric distribution system in Virginia to be $83 billion. Bradshaw said that cost has increased since then, especially when considering the advances in automated equipment currently used by Dominion Energy.
“Tap lines generally have fewer wires, smaller wires and certainly less automation than main feeders,” he said. “So, we shifted our focus from burying all distribution lines to concentrating on those that were most outage-prone. We believe that a full-scale strategic underground program aimed at burying only the most outage-prone tap lines (or approximately 4,000 miles) will cost about $2 billion, or less than 3 percent of the ‘underground everything’ estimate from 2005.”
To select areas to convert, Dominion Energy uses a data-driven process that begins with a review of 10 years of outage data for each tap line. The information is used to develop an events-per-mile metric to ensure the selection of a group of tap lines that have the greatest impact for the lowest cost.
“We take this data and then perform a site review to ensure that the tap lines are indeed good candidates for conversion to underground,” Bradshaw said.
Horizontal directional drilling (HDD) is heavily used.
“Directional drilling minimizes impact to our customers’ properties,” Bradshaw said. “This installation method, as opposed to open trenching, has been key to customer acceptance and willingness to install underground lines in their area. These projects serve established properties where customers have installed sheds or built fences and planted gardens. The ability to safely install facilities using a trenchless technology allows for minimal impact on established properties.”
Construction is being done by five contractors assigned to seven geographic areas throughout the Virginia service territory.
“Each contractor provides design, right-of-way and construction services,” Bradshaw said. “So, while they are installing new underground facilities, they are also securing easements and completing designs for future work. They build their own backlog and in many ways are responsible for their own success. We have been very pleased with our contracted partners.”
According to postconstruction surveys, Dominion Energy customers are “very” satisfied with the underground program, scoring more than 4.5 out of 5.
“We make it a priority to communicate regularly with customers impacted when securing easements for new underground facilities, to review construction schedules, safety, and landscaping once a project is complete,” Bradshaw said. “We use a variety of communication mediums including letters, postcards, telephone calls and door hangers. We have also created a portfolio of reference pieces on topics such as easements, meter base conversions and how directional boring mitigates impact to tree roots. On larger projects, we host community informational meetings to review proposed layouts and discuss project schedules. We have had an overwhelmingly positive response to easement requests, which is critical to the completion of any project.
“Because we rely so heavily on vendors to help us with the program, we are very conscientious about sharing feedback with our contractors and hold them to the same standards we hold our own employees,” he said.
These results have been promising in other ways, as well.
“As expected, tap lines that have been converted have seen significant improvement in their reliability going from an annual SAIDI of 372 minutes prior to underground conversion to less than two minutes after conversion,” Bradshaw said.
As the conversions continue, Bradshaw believes there will be a small reduction in the number of customers that actually experience an outage during these events as a result of the Strategic Underground Program.
“We will have a slight decrease in the number of customers impacted and a significant reduction in the number of work repair locations, and we expect our traditional restoration curve will look a lot different as we complete the program,” he said.
Subject to continued annual regulatory approvals, it is anticipated the Strategic Underground Program will take 10–12 years to complete 4,000 miles of underground conversions.
Undergrounding in Oklahoma
Public Service Company of Oklahoma (PSO) has been converting selected areas from overhead to underground since the early 2000s. Following an ice storm in late 2007 that caused widespread outages, a program was initiated that ultimately converted service laterals in 30 neighborhoods—mostly in the Tulsa area—from aerial to underground.
“Most laterals were in backlots with heavy tree growth, and limited accessibility limitations were replaced with front-lot laterals,” said Stan Whiteford, corporate communications director, PSO.
The conversions left feeder lines on poles and replaced back-lot aerial laterals and service lines with new underground cable at the front of the property. Most underground cable was buried primarily by HDD, which limits the amount of excavation required and reduces surface restoration needed after cable is in the ground.
The program was successful in reducing weather-related outages. PSO has since shifted focus from targeting neighborhoods to employ undergrounding when it is determined to be necessary to address outages or where their are voltage and capacity issues.
“We’ve shifted focus now and employ undergrounding determined to be necessary to address voltage or capacity issues where underground is determined to be the best alternative,” Whiteford said. “A good example is a project started last year for upgrading circuit capacity in a well-established neighborhood from 4 [kilovolts] to 13 [kilovolts]. In this instance, it made sense to abandon backyard services to replace with front yard laterals underground.”
When backyard laterals are replaced, typically a 2-by-2-by-3-foot, pad-mount transformer is placed every three or four houses. Services to each house are directionally drilled. RF meters previously were replaced with smart meters.
“Aesthetics are important, but the focus on undergrounding today is improving service,” Whiteford said.