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High-voltage power lines like one in NH could help power the future

Jan 4, 2009 - David Brooks The Telegraph - McClatchy-Tribune Regional News

The future of the nation's power grid -- perhaps the world's power grids -- depends on a technology that Nashua area commuters have been driving past for two decades without noticing: High-voltage DC transmission.

"It was a very big project at the time, quite unusual," said Joe Staszowski of Northeast Utilities, the parent company of Public Service of New Hampshire. "It wasn't like it was serial No. 1, or anything like that, but it was important for it to work right."

Fortunately, it has. And if New England's electric grid is going to enter the 21st century, chances are more lines will be created just like it.

The line in question -- parallel cables on huge, two-armed transmission towers -- runs for more than 800 miles from James Bay, where Quebec owns huge hydropower plants, through Vermont to tiny Monroe, N.H., at what is known as the Comerford terminal. From there it heads southeast, cutting through Bedford, crossing the Merrimack River at the north end of Litchfield and running down through Pelham into Massachusetts, ending at a power station called Sandy Pond in Ayer, Mass.

The first branch, reaching to Comerford, opened in the mid 1980s, the rest opened in 1990. It is capable of carrying 2,000 megawatts of power, enough for several hundred thousand homes, or almost twice the capacity of Seabrook nuclear power station. New England is riddled with large power lines, of course. The unique point of this HVDC line is the last two letters in that description: It carries power via direct current (DC), the sort of power used by batteries, rather than the alternating current (AC) that comes out of electrical sockets and which is used throughout most of the power grid.

What's important about that? Staszowski, who is director of NEPool and ISO relations for Northeast Utilities -- ISO oversees new England's power grid while NEPool is made of participants in the power market -- and a member of a state committee considering expansion of transmission lines in Coos County, explained why.

At the atomic level, DC can be thought of as a stream of electrons, AC as a back-and-forth dance of the subatomic particles. At the everyday world level, each method has advantages and disadvantages; when electric power was first being developed a century ago there was a huge fight between inventors Thomas Edison and Nikola Tesla over which should prevail. Tesla favored AC and won out.

The development of technologies to handle DC power at very high voltages (roughly equivalent to water under very high pressure) has created a major niche for carrying lots of power very long distances. This is partly because big DC transmission is cheaper to build than complicated AC, and partly because DC power travels with much less loss of energy than AC power.

If you want to send a bunch of megawatts from one place, such as dams in northern Quebec, to another place, such as power-hungry Boston, high-voltage DC is the best way to do it.

Another reason HVDC was used for the line from St. James Bay, says Staszowski, is that DC is forgiving.

"We didn't have really another option to get power from Quebec because its system has different characteristics than the New England system. You cannot connect an AC line between Quebec and New England and have it work properly -- it wouldn't stay closed, it wouldn't act well," he said. "That's one of the major advantages of DC: if systems aren't synchronous you need a DC connection to connect the two."

One scenario being studied for our energy future is tapping into even more hydropower that Quebec wants to build and bringing it south via a 1,200- megawatt HVDC line into New Hampshire, which would connect up with the current AC system somewhere in central or southern New Hampshire.

There is, however, a big disadvantage to HVDC: It is very expensive to convert AC power from generators into DC power and then back into the AC power needed in everyday life. This huge cost means that the electricity can only be "off-loaded" from HVDC lines at a few locations.

"It's like a very limited access turnpike: only one place to get on and one place to get off. What if you want to get off in the middle? You can't, or it's very, very expensive to build another ramp," Staszowski said.

So HVDC would never be used to, for example, carry power from a small wood-burning power plant to a bunch of customers. But it could be used to haul big loads of power to Chicago or the East Coast from enormous wind farms in the Midwest, or to Beijing from huge solar-power arrays in the Gobi Desert.

Since wind and solar power is often located where people aren't, such big HVDC lines are likely to become more common around the world. China is planning a score of massive HVDC lines -- some three times the size of the St. James line -- to carry solar and wind power from western deserts to eastern cities.

But we had it first -- well, almost.