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Amherst substation key in power delivery

By Staff | Apr 16, 2012

There’s an awful lot of very cool stuff at PSNH’s huge substation in south Amherst, which handles much of the electrical load for Nashua, but maybe the coolest thing can’t even be seen: Coronas.

These electrical discharges occur when there’s so much electricity around that it ionizes air molecules, creating dancing colored lights. Think of St. Elmo’s fire appearing around sailing-ship masts during lightning storms.

Coronas don’t appear in the PSNH substation despite the staggering amounts of current flowing through it because of “corona rings.” Metal doughnuts hanging from the top of many components; these spread the electric field gradient so it isn’t potent enough to ionize the air.

It is still potent enough to create a sort of crackling sound as a background soundtrack.

You’ll have to take my word for that, however, because you’ll never get close enough to hear it.

Electric power substations are the sort of critical infrastructure that is becoming more secure each year. Merely approaching the barbed-wire-topped fencing that surrounds the several-acre substation, which is south of the railroad tracks a few hundred yards from Route 101A, raises a silent alarm.

“As soon as you showed up outside the fence, somebody knew you were here,” said Chuck Christensen, supervisor of substation engineering for PSNH, before giving me and photographer Don Himsel a recent tour of the site.

It took weeks of discussion through PSNH spokesman Martin Murray to set up a tour, due to concerns about safety and security at substations. The situation will only get tighter in this post-9/11 world.

“In five years, this tour might not be possible at all,” said Christensen after giving us a long safety talk (“don’t touch anything; don’t even wave your arms around”), handing over hard hats, goggles and fire-resistant Nomex coveralls, and unlocking the gate.

We visited the site because – well, for no particular reason, really, except curiosity.

Electric substations are “hidden in plain sight” structures, a necessary but overlooked step between power plants like Seabrook Station on the one hand, and the endless power lines that decorate every aspect of modern life on the other.

PSNH has more than 50 substations throughout the state, yet few of us know about them or even notice them – some are right on heavily travelled roads like Broad Street near the Nashua Mall or Canal Street near the Merrimack River.

I’ve driven within a few hundreds yards of the Amherst substation at least a thousand times, but didn’t know it existed.

“That’s the way we like it,” Christensen said.

So what do all these substations do? In a nutshell, they make power usable.

After electricity is produced by power plants using all sorts of fuel – like Seabrook Station (nuclear), Granite Ridge Energy (natural gas), Merrimack Station (coal), Mine Falls (hydropower) and Lempster Mountain (wind) – the electricity gets sent all around New England over high-voltage transmission lines.

The power’s voltage is boosted for the trip because that makes the electricity travel more efficiently.

(Remember the electricity-is-like-water metaphor from high school physics? Voltage is the equivalent of water pressure.)

The power that comes to the Amherst substation, for example, is at 345,000 volts (345 kV, or kilovolts). Nobody can use electricity at that voltage because it would make our homes melt, or something like that, so the voltage has to be damped down. This is where substations come in.

Inside the fence, the power is carried in aluminum tubes (which don’t sway like wires) to enormous circuit breakers, that perform the same role as the switches in the circuit-breaker box in your basement or the surge arrestors next to your computer. If something goes wrong, they try to contain it.

These two parallel sets of equipment – redundancy is good – aren’t little switches, though. Each is the size of a garage, topped by monstrous insulated towers that would be right at home in a Frankenstein movie. The towers have one delightfully low-tech touch: the insulation disks are made of porcelain, just as they have been since the earliest days of the telegraph. “We haven’t found anything better,” Christensen said.

After the circuit-breakers, electricity goes to the transformer, which drops the voltage via inductive coupling. Roughly, the current goes through a coil which creates a magnetic field, and the magnetic field creates a current in a separate coil that is wound differently, which creates a different voltage. (That explanation will draw belly laughs from any electrical engineer, but I’m doing my best.)

The transformer is another entire building so big that it uses 22,000 gallons of highly refined oil for cooling and insulation.

The Amherst substation is unusual because it turn 345 kV power directly into the 34.5 kV current low enough to be handled by the trash can-like transformers atop utility poles along your street. (Those transformers make the final tweak, to the 240 volts that our homes can handle.)

Only five of PSNH substations can do this transformation in one step. Most require several intermediate steps to get down to distribution current. (PSNH also has a half-dozen other substations that handle transmission-only voltages.) Amherst substation was turned on in 1988 with one transformer. A second transformer was added in 2003, adding redundancy and more capacity.

The final step for power is a trip to the control center, an even larger building full of switches and dials, some dating from decades ago and many much newer. It dispenses power in various directions; for this substation, that means mostly east toward Nashua.

Most of this center can be, and usually is, controlled remotely from PSNH’s main offices in Manchester. The utility’s goal is to have as few people as possible on site. For example, they didn’t connect the site to town water and sewer; if construction or long-term repair is necessary, port-a-potties have to be trucked in for workers.

Another really interesting aspect of the station are some long, thin wires, high above the power tubes, that go hither and thither with no obvious pattern. These are lightning interceptors, Christensen explained, and their weird-looking pattern was designed via “rolling sphere analysis.” Look it up; it’s well worth it.

All in all, we didn’t spend much more than an hour inside what might be considered a sort of cathedral to the vision of Faraday and Maxwell, Edison and Tesla, and lots of other folks. There is plenty I don’t understand – after all, people go to school for years to learn this stuff – and chances are it will stay that way, because I doubt that I’ll ever get another chance to make a similar tour.

But I will admit that it makes me think a little bit whenever I mindlessly flip a light switch. And there’s one other thing, too: I’d really like to understand electric coronas.

Time to visit the Boston Museum of Science electricity exhibit again.

Granite Geek appears Mondays in the Telegraph, and online at www.granitegeek.org.

David Brooks can be reached at 594-6531 or dbrooks@nashuatelegraph.com.


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