Pros and cons of
undergrounding power lines
By Joe
Naiman
The Alpine Sun
The utility industry’s DistribuTech conference, held at the
San Diego Convention Center in February, included a panel
discussion on the advantages and drawbacks of overhead and
underground utility lines.
“The Real Cost of Overhead vs. Underground Transmission
— It May Not Be What You Think,” was moderated by Michael
Beehler, the associate vice president of Burns and McDonnell.
The panel was comprised of Pacific Gas & Electric senior
consulting engineer Mohan Bhatia, American Electric Power
director of transmission line projects engineering Max Chau, and
Northeast Utilities System transmission project director Anne
Bartosewicz.
The panel noted that neither method was preferred for
all cases. “Each project needs to be evaluated on its individual
merits,” Bhatia said.
Overhead power lines have been the cause of numerous
fires in San Diego County in recent years. In other areas the
only homes lost from overhead transmission lines are by the
eminent domain process when the line is constructed, but ice
storms and hurricanes have led to problems with overhead lines.
“Our industry has seen a lot of outages in years past,”
Beehler said.
The total restoration costs for Hurricanes Katrina,
Rita, Gustav, and Ike totaled approximately $2 billion.
Heavy winds in Ohio during September 2008 caused an
outage, which deprived approximately 2.6 million customers of
power, and the December 2008 ice storms in the Northeast also
cut power to millions of customers. In addition to the utility’s
restoration costs, social costs of outages include lost revenue
and other business disruptions, public safety and security, and
convenience.
The fires and outages have led to calls from
government, the media, and the public for the undergrounding of
utilities. “We as an industry need to give them an answer, not
just for distribution but for transmission as well,” Beehler
said.
Advantages of underground lines include aesthetics,
higher public acceptance, perceived benefits of protection
against electromagnetic field radiation (which is still present
in underground lines), fewer interruptions, and lower
maintenance costs. Decreased right-of-way costs, including
access roads and vegetation, may also benefit the undergrounding
of a distribution line.
The advantages of overhead lines include lower initial
costs, faster and less costly restoration, and easier future
expansion.
Pacific Gas & Electric has a service territory of
approximately 70,000 square miles and has approximately 4.9
million electric accounts and 3.9 million gas accounts. PG&E
serves approximately 14 million customers, or one out of every
22 Americans. The utility currently utilizes approximately
95,500 overhead miles and 23,900 underground miles of electric
lines.
Bhatia focused on PG&E’s Jefferson-Martin project that
serves San Francisco and northern San Mateo County. The
230-kilovolt (kV) line runs from PG&E’s Jefferson substation in
Woodside to the Martin substation just south of the San
Francisco border. The California Public Utilities Commission (CPUC)
granted a permit on Aug. 19, 2004, nearly two years after PG&E
filed its application. Construction began on Jan. 24, 2005, and
the line was released to operation on April 29, 2006.
The final cost for the Jefferson-Martin line was $227.5 million.
The 27.5-mile line consists of 3.12 miles of overhead
transmission and 24.37 miles of underground wires. PG&E
undertook 135 mitigation measures required by the CPUC, which
included noise, traffic, working hour restrictions, and
stormwater constraints.
While undergrounded utilities are normally seven to
eight feet below the surface, the electromagnetic frequency
concern was mitigated by installing the cable 11 feet deep in
residential areas. Undergrounding was also performed in scenic
areas, and barrier fencing was installed to protect endangered
species.
The cost for the 3.12 miles of overhead lines averaged
$3.8 million per mile. The major variables for the overhead work
included terrain, access roads, construction hours, removal of
the 60 kV lines, and the construction sequence.
The underground costs for the five segments averaged between
$5.4 million and $6.8 million per mile. Variables included
easement costs, traffic, and environmental and restoration
expenses.
In some cases, Bhatia noted, the cost of right-of-way
acquisition for overhead lines made underground transmission
more attractive. Another factor is the acceptance of the
policy-making boards. “We as a utility can build only projects
which are permitted,” Bhatia said.
American Electric Power operates in 11 Southwest and
Midwest states and currently has approximately 39,000 miles of
overhead transmission. Between 40 and 50 percent of a project’s
cost is construction labor, between 35 and 40 percent is
construction material, between 10 and 12 percent is engineering,
between 10 and 20 percent is right-of-way, and between 5 and 10
percent is permitting.
Construction labor is becoming scarce, engineering and
other expertise is retiring, and major material items require
time to obtain. Although the recession has dropped copper and
aluminum prices since mid-2008, those commodities increased
considerably in previous years.
“This is extremely susceptible to the market,” Chau
said.
Chau noted that variables include permitting costs,
right-of-way cost and width, terrain and the type of equipment
used, accessibility, soil type, traffic control, and the
presence of other utilities.
“Underground’s less subject to the elements,” Chau
said. “However, to locate a failure of the underground cable is
a lot more challenging.”
The outage rate per mile per year is 0.005 for overhead
and 0.001 for underground and the mean delay time between
failures is 200 days for overhead and 606 days for underground.
The mean repair time, however, is nine hours for overhead and 21
days for underground.
Maintenance is a cost for both. Tree trimming can be
costly for overhead utilities while easements for underground
lines must be cleared for access and visibility.
The most powerful known underground line is a 500 kV
line in China, which covers about 1,500 feet inside a
hydroelectric substation. The highest-power underground lines in
the United States are 345 kV.
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