More of Northern Pass could be buried because of cable technology

Last modified: 9/8/2015 12:33:31 AM
Although economics and politics drove the decision to bury an additional 52 miles of the proposed Northern Pass electric-transmission line, making the much-discussed project more tenable to some although still far from guaranteed, the thing that really made it possible is a development only a chemical engineer could love: use of cross-linked polymers to improve insulation of electric cables.

This technology, part of an electric-transmission cabling system known as HVDC Light, to differentiate it from plain old high-voltage DC lines, means that instead of Northern Pass’s original plan to bury four power-transmission cables plus two smaller copper cables alongside them, it could get by with burying two parallel cables.

That’s less cost for cables, less cost for the labor of installing and splicing them together, and much less “civil work,” an engineering term for digging trenches and topping them with concrete slabs to keep people from accidentally uprooting them.

“There’s a significant cost savings” over the cost of burying older technology, said Nathan Scott, an electrical engineer working for Northern Pass, during a recent public session at the Grappone Conference Center in Concord. “Very significant.”

Northern Pass would take hydropower from massive Quebec dams and carry it about 190 miles from the Canadian border south through the White Mountain National Forest and Concord to an existing substation in Deerfield, where it would feed into the regional grid.

If it does get built – which is a big if – Northern Pass would involve the longest underground stretch of HVDC Light in the country, according to Eversource Energy, which co-owns the proposed system with Hydro-Quebec.

HVDC Light is used in a couple of longer runs in the U.S. but they mostly go underwater, including one connecting Long Island to the mainland, while longer underground stretches exist in Europe, which uses much more high-voltage DC than the U.S. to create long-distance power connections.

But there’s a drawback.

HVDC Light can’t carry the total amount of power that Northern Pass originally proposed because even the cross-linked polymer insulation can’t handle all of the heat produced by the high-voltage electricity needed.

The voltage of the current in an electric cable can be considered the equivalent of water pressure in a garden hose. Just as high pressure can break a hose that isn’t designed to handle it, high voltage can melt a cable that isn’t designed for it – especially underground, where dissipating excess heat is more complicated than when power lines are swinging in the wind.

Overhead lines often carry electricity juiced up to as much as 500 kilovolts, but the buried Northern Pass proposal would be limited to about 320 kilovolts.

As a result, the new Northern Pass proposal unveiled last month cuts back the size of the project from 1,200 megawatts to 1,000 megawatts. (A megawatt, or a million watts, is a measure of the total amount of electricity being carried. It is partly but not entirely a function of the voltage.)

That is still a hefty amount, since it equals about 3 percent of the amount of electricity consumed in all of New England on a hot summer afternoon, and is nearly the size of Seabrook Station nuclear power plant, which generates 1,200 megawatts at peak.

ABB, the Swiss company that first developed HVDC Light, says it has developed a new version of the technology with a voltage rating as high as 525 kilovolts, meaning it will become the first underground system “that can transmit approximately the same amount of power as an overhead transmission system” and could, in theory, let Northern Pass bury more lines at a 1,200 megawatt level.

But Sam Johnson, a project manager for Northern Pass, noted during the Grappone session that nobody has actually bought and installed this new technology. Eversource and Hydro-Quebec, joint owners of Northern Pass, aren’t enthusiastic about taking an untested technology and putting it underground, where the cost of future repairs is much, much higher.

While in essence electric cables are just long strands of conductive metal, usually copper, modern power-transmission cables are technical marvels – which is why they’re big (the cable for HVDC Light weighs about 20 pounds per foot, which is actually much less than older technology known as mass-impregnated cables) and expensive.

Even though the cost of burying HVDC Light is much less than burying older, mass-impregnated cables, it is still much greater than running wires overhead on towers. The extra cost is between $5 million and $10 million per mile, on top of the average $3 million per mile that high-voltage DC lines cost to build, according to Eversource CEO Bill Quinlan.

Eversource says this cost difference explains why it hasn’t proposed burying the entire Northern Pass. Its latest proposal, which it calls the ForwardNH Plan, would bury 52 miles of the 192-mile route, mostly through the White Mountains, along with 8 miles already proposed for burial in the North Country. It also includes financial incentives for New Hampshire.

Opponents of Northern Pass like the Society for the Protection of New Hampshire Forests say this isn’t good enough, and point to a U.S. Department of Energy study that says burying the entire line would roughly double the total cost.

As Johnson and Scott explained during a recent Northern Pass session at the Grappone Conference Center, the central core of copper in an HVDC Light line has a core of copper surrounded by several inches of insulation made of cross-linked polymers (known, of course, by an acronym: XLPE), then more insulation and metal screens to protect against stray voltage and other issues with high-powered lines, then fiber-optic lines to carry information about the cables so their performance can be monitored. This is all wrapped in a special sheath to help keep out water, whether submerged in water or encountering New Hampshire’s sometimes damp soils.

Then the cable is run through a PVC pipe for further protection.

Between 1,800 and 2,200 feet of cable can fit on a roll, which means that as many as 20 tons of material must be spooled out at a time. It also means that cable must be spliced together in two or three locations every mile.

Splicing electrical cables together, Scott explained, is tricky and takes specially trained technicians who work inside “splicing vaults,” concrete rooms built underground around every splice. The rooms are up to 10 feet high and 30 feet long, providing work space for technicians and more protection for the sensitive splices.

There’s more to HVDC Light than just cable design, however. ABB and other firms which sell it have a series of other technologies which make the system feasible, such as filters and transformers.

And even though the trench that must be dug for HVDC Light is much smaller than under old, mass-impregnated technology and fewer cables have to be spooled out, burying power cables still takes longer than building towers and stringing cables between them, Scott said.

“You’ve got civil work the entire distance” when burying cable, he said. “With towers, you’ve only got to do foundations and construction where there’s a tower.”

Eversource estimates that it would take 2½ years to build Northern Pass.

The company plans to submit a formal application in October to the state Site Evaluation Committee, the next big step in efforts to build the power lines, which it has been talking about for more than three years.

(David Brooks can be reached at 369-3313,, or on Twitter @GraniteGeek.)

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