An Alaska electrical cooperative that serves the Valdez-Cordova area is planning to move a four-mile stretch of a critical 138-kV transmission line out of what is called the most active avalanche area in North America.
“The [original planners] didn’t find out about the true potential to avalanche exposure until it was too late,” Chris Botulinski, Copper Valley Electric Cooperative’s director of transmission and distribution, told TransmissionHub on July 16.
The 106-mile long Solomon Gulch line that runs from the Copper River Basin to Valdez, Alaska, was energized in 1981, “and it chugged along quite nicely until 1988 when we had what we refer to as ‘the big avalanche’,” he said. That avalanche damaged seven towers, took the line out of service for 273 days, and cost $1.8m to repair.
“Because of the overall structure of the line, the conductors were stronger than the support towers and it just pulled the structures down in front and in back” of the tower that took the direct hit, Botulinski said.
Following that incident, the line was redesigned so that the conductor would break before pulling down adjacent towers.
The utility got a 12-year reprieve after the 1988 incident, but avalanches again took the line out of service in 2000, 2003, 2006, and 2009. With the winter of 2012 marking the end of another three-year cycle, “We go into it just waiting,” Botulinski said.
The most vulnerable stretch of the line runs along the south side of the Richardson Highway in the Thompson Pass area, about 35 miles east of Valdez. Since 1981, that portion of the line has been hit or reached by 33 of 51 avalanches in the area.
Two relocation options presented
The Solomon Gulch line is critical because the coop is electrically isolated.
“That transmission line is Copper Valley Electric’s only interconnection between our two main communities,” Botulinski said. “We are not interconnected to any other grid in the entire state, so we are self-sufficient on every aspect of generation, transmission, and distribution,”
Accordingly, the system plans for an N-3 contingency: the outage of the transmission line as well as the two largest generation units.
“One of the major reasons we want to move it out of there is overall system reliability,” he said.
“As far as I’m concerned, the best mitigation is avoidance,” Botulinski said. “That’s what we’ve proposed to the Department of Natural Resources (DNR). We’ve done our due diligence and studied multiple options, and here are the two best.”
The two options being presented would relocate the line to the north side of the highway, reducing its vulnerability to future avalanches.
However, the relocation would place the line between motorists and the Worthington Glacier. The possibility of an intrusion into that viewscape has roused some opponents, who would prefer to see the line placed underground, according to Botulinski.
Underground lines problematic
In Alaska, undergrounding is even more difficult and expensive than in other areas of the country, Botulinski said.
“Our terrain is not very forgiving when it comes to underground cable like that,” he said. “In that area alone, that particular stretch gets probably some of the greatest amount of snowfall in the state of Alaska, averaging over 500 inches of snow a year.”
In addition, expertise is an issue.
“We do not own or operate any 138-kV underground transmission line, and it’s not a readily accepted construction technique in Alaska,” he said. “Knowing that there’s not the expertise in this state to do that, it’s a pretty scary thing to consider.”
An underground line would also be expensive – costing $8m to $10m to construct that four mile portion – but the costs don’t end there. Because the system’s load is small, peaking at approximately 12 MW, placing a stretch of line underground would increase the system’s capacitive reactance.
“That creates some significant issues for us trying to energize that line,” Botulinksi said. “If we put it underground, the additional capacitive reactance of that four-mile section would make it such that we wouldn’t be able to energize that line” without adding another substation or modifying an existing substation for an additional $1.5m to $2m, he said.
The co-op still needs to obtain approval from the Federal Aviation Administration (FAA) because the line will parallel the runway at the Thompson Pass airport. Following FAA approval, the utility expects to obtain permission to proceed from the DNR in mid- to late September, Botulinksi said. The bulk of the work will have to wait until after the winter season.
Because Copper Valley Electric is not economically regulated, the DNR is the ultimate authorizing agency; the utility is not required to present this type of situation to the Regulatory Commission of Alaska for its approval.
Total cost of the project is estimated at $2.1m to $2.8m, which is less than the $2.9m required to rebuild the line after the 1988, 2000, 2003, 2006, and 2009 avalanches.