new wires for old
Higher-capacity transmission cables aim to get more juice through the bottlenecks.
By Lisa Kosanovic
Electricity
industry experts and decision-makers have known for years that the grid
is overstressed and can't always perform all the tasks it's
being asked to perform. The National Transmission Grid Study, issued by
the Department of Energy in May 2002, warned that the U.S. transmission
system was not designed to satisfy the nation's current level of
demand for electricity. The study concluded that daily transmission bottlenecks
boost electricity costs to consumers, and increase the risk of blackouts.
While the general consensus is that new transmission lines are desperately
needed, adding them is just short of a nightmare for utilities. That's
because the task involves multiple agencies at the state, federal, and
local levels, and because so many parties—all of whom are given
a voice in the siting process—are affected.
Even when a line is approved, which is often not the case, it usually
takes about 10 years to put in, and costs about $1 million per mile.
To avoid the problem, several companies have recently come out with wires
that carry twice the electricity that standard overhead cables carry.
Although these cables cannot solve all of the grid's bottleneck
problems, they can certainly help. "DOE sees it as a very promising
option for the future," said John Stovall, a senior researcher
at Oak Ridge National Laboratory in Tennessee.
For about the past 100 years, electric utilities have been using aluminum-conductor,
steel-reinforced cables, which have a core of steel strands surrounded
by aluminum, to deliver electricity to consumers. The steel supports the
wire as it hangs between two towers, and the aluminum conducts the electricity.
But the steel also causes the cable to conduct electricity poorly, to
sag when heated because of a relatively high thermal expansion coefficient,
and to anneal rapidly and lose its strength when temperatures rise above
120°C.
One easy way to improve the product, Stovall said, is to replace the steel
core with a composite material. Composites can tolerate higher temperatures
without stretching and sagging as much as steel. This means that more
electricity can be put through them, and that the cables can be installed
over rivers and across densely forested areas, installations that have
typically been problematic for utilities. In 1996, for example, sagging
cables caught on nearby trees and short-circuited, causing a major blackout
in the Pacific Northwest.
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| Composite Technology's aluminum cable has a core of carbon and glass fibers. |
Several companies have stepped up with their versions of a composite
core conductor. Composite Technology Corp., for example, makes a cable
with a core of carbon and glass fibers. The cable can conduct up to twice
the electricity that standard wires do, says James Carswell, CTC's
director of investor relations. In company tests, over a 607-meter span,
CTC's aluminum-conductor, composite-core cable sagged 8.3 meters
at 200°C, compared to nearly 27 meters for standard cable.
CTC's product, which is available commercially, can be manufactured
in any size. Standard-size cable costs approximately $4 per foot, Carswell
said.
Sumitomo Electric U.S.A. Inc., which markets three different composite
conductors, offers one that combines a heat-resistant aluminum alloy wire
with a high-strength, galvanized Invar alloy wire. Invar is an alloy of
steel and nickel with a linear expansion coefficient that is nearly invariable
with heat.
The Invar cable also conducts about twice the electricity that standard
cable does. In field tests, Invar cable sagged less than 6 meters over
a 207- meter span at approximately 240°C, compared to nearly 8
meters for aluminum-with-steel cable over the same span.
A company spokesman, Kan Kinoshita, said Sumitomo has sold more than
2,300 miles of the Invar conductor since it began selling it in 1984,
but has not sold any in the United States. The cable is available in all
sizes, according to Kinoshita.
Minneapolis-based 3M also makes a composite core cable, but it is not
yet commercially available. 3M has had help along the way from the Oak
Ridge National Laboratory in developing its conductor, which has a core
of ceramic fiber-reinforced aluminum wires, and an outer layer of an aluminum-zirconium
alloy.
In 1998, the Oak Ridge lab helped 3M with its manufacturing process, under
funding by a one-year grant from the DOE. In April last year, researchers
at Oak Ridge began testing a 1,200-foot section of 3M's cable that
was installed as a sort of outdoor laboratory.
Stovall said that 3M was the first company to approach Oak Ridge's
Powerline Conductor Accelerated Testing facility. According to the lab's
Web site, "The PCAT facility makes possible realistic demonstrations
of advanced technologies under a wide range of operating conditions, without
jeopardizing grid-system reliability."
The lab's goal, he said, is to promote improved conductors by demonstrating
their performance in the field. Otherwise, he said, utilities might be
reluctant to install the wires on heavily loaded lines. "We're
trying to give utilities confidence that these will run," Stovall
said.
The lab's researchers tested 3M's product from April to
October, and found that it met all of 3M's claims, according to
Stovall. One of the tests, which took all of September, involved turning
on current and holding the wire at its maximum rated temperature for one
hour, then turning off the current for a half-hour. By cycling the wire
this way, five to eight times a day for one month, Stovall said, the test
simulated 10 years of service.
Fighting
Corrosion
At its maximum rated temperature of 210°C, the wire sagged 10 feet,
stretched 16 inches, and underwent a tension drop from 3,000 pounds to
1,500 over a 600-foot span, Stovall said. Temperature was measured with
thermocouples, sag at the midsection was measured with a laser, and tension
was measured with load cells.
The tests also confirmed that the product conducts about twice the electricity
that conventional cable can transmit.
Like its counterparts from CTC and Sumitomo, the 3M cable has a low thermal
expansion coefficient, which prevents it from sagging as much as standard
cable. Lower weight also reduces sag.
So far, 3M wires have been tested by utilities in Minnesota, South Dakota,
and Hawaii, but only to determine how they stand up to severe environmental
conditions. Hawaii Electric Co., or HECO, for example, wanted to try composite
conductors because of its corrosive environment.
In April 2002, HECO installed three phases of the wire over 1,800 linear
feet on a 46 kV sub-transmission line, in an area that is surrounded by
salty water. The region is constantly subjected to salt-laden trade winds
from the Pacific Ocean, and even high-quality galvanized steel can corrode
severely there within two years, according to HECO technical services
engineer Sucuma Elliot.
Tests so far have shown no corrosion, according to Elliot, and have confirmed
3M's sag numbers. Moreover, he said, installation was almost the
same for composite conductors as for standard cable.
As demand increases, utilities are doing whatever they can to squeeze
every bit of electricity possible out of existing lines. When possible,
they are adding 10-foot extensions to their towers, and putting in larger
cables if the existing towers are strong enough to support them, according
to Stovall.
Composite conductors, while not the ultimate solution, will likely become
one more method of eking out a little more electricity from existing rights-of-way.
"This is one tool in the toolbox," said Tracy Anderson,
manager of 3M's composite conductor program.
Lisa Kosanovic, a freelance writer specializing
in energy topics, holds a master's degree in mechanical engineering
from the University of Massachusetts and lives in Amherst, Mass.