Redundant grid keeps Juneau on hydropower
Utility can't repair line until avalanche danger lessens on Thane Road
Persistent avalanche danger is keeping linemen from fixing the Thane-area power transmission line that was downed on Sunday.
Juneau's still powered by the Snettisham Hydroelectric Project, because only one of two redundant lines from Thane to downtown was taken out by the slide.
The aptly named Snowslide Gulch area is known for avalanches, and Alaska Electric Light & Power Co. has two parallel lines that run all the way to Salmon Creek.
Sunday's avalanche took out one of three wires, or phases, on the downhill line. The buried line is undamaged, and can carry the city's entire load.
Nonetheless, AEL&P linemen will fix the line as soon as the area is safe - possibly Wednesday, spokesman Scott Willis said.
"We prefer to have both lines in service. It makes us more secure," he said.
A crew went out to Thane yesterday and cut off the line at the poles. They'll need another half day to splice in a new line.
That's a "fairly straightforward job," Willis said, because crews can work in bucket trucks from the road.
System redundancies
Seven years ago, longtime resident Dean Williams wrote in the Empire that people used to take bets on when Snowslide Gulch would slide, in manner of the Nenana River ice breakup near Fairbanks.
Thane avalanches are an old problem for Juneau - and for AEL&P.
That's why the utility has a second, redundant line in place, and why part of the line in the slide-ridden area is underground.
Beyond Thane, there are several redundancies in Juneau's power generation capacity, both in the transmission lines to town and in the local grid.
Juneau has special avalanche issues. But redundancy also is standard practice for electrical grid designers, Willis said.
In Thane, the avalanches taking out transmission lines in the past have been both natural and manmade. Until a decade ago, the utility had to make a decision each time it heard the Department of Transportation would be blasting snow: Should it turn on the diesel ahead of the blasting, in case the slide took out the line?
"Most of the time when DOT shoots that area, the avalanche comes down in a controlled way," Willis said. "But there's always that risk."
One or both of the parallel Thane lines once went out every five to 10 years, Willis remembered. That was too often. Hence the justification for the underground line, even though it's costlier.
There are other redundancies. Juneauites are by now familiar with the diesel generators that back up power from Snettisham.
There's only one transmission line from Salmon Creek to Auke Bay. But there are backup generators in Auke Bay, so if that line cuts out, Auke Bay homes can still have power.
Most of the electricity distribution system to homes is designed as a loop, so there are generally two ways to feed it.
"That's part of electrical system planning," said Willis.
But there are weaknesses, too. The ends of Juneau's two roads aren't in loops, Willis said.
One last redundancy: The Lake Dorothy Hydroelectric Project is scheduled to come online next fall to supplement Snettisham's power, but it's also a backup if Snettisham goes down.
Lake Dorothy's transmission line will share part of Snettisham's line, so it's not completely independent from Snettisham.
But the Lake Dorothy line will skip the part of the transmission line nearest the Snettisham power plant that avalanches have taken out twice in one year. The part they share, Willis said, is much more reliable.
• Contact reporter Kate Golden at 523-2276 or e-mail kate.golden@juneauempire.com.