A problem with the biggest hydro turbine at the Whitehorse dam knocked out power to most of the territory for a few hours on Feb. 18.
Yukon Energy said an issue with one of the electrical pieces of the turbine referred to as the fourth wheel caused the afternoon shutdown.
The fourth wheel is responsible for half of all the power generated by all four turbines at the dam.
When it failed, the other hydro generators in the territory weren’t able to make up for the sudden spike in demand.
“Because it’s such a big unit, once we lose it it’s like the other units can’t keep up,” said Yukon Energy spokesperson Janet Patterson.
“So they try to take on the load of the one that we just lost, but they just can’t, so that’s why you see that cascading effect.”
The company’s director of operations, Guy Morgan, said the fourth wheel was generating about a third of all the power in the territory when it went down.
The lights went out in almost all Yukon communities on the power grid. Because of the way the system’s protections are built, Mayo, Keno and Stewart Crossing were spared, Patterson said.
Old Crow and Watson Lake, which are not on the grid, likewise never lost power.
Dawson was restored in about 15 minutes with back-up diesel. Faro came back online after about 25 to 30 minutes, again with back-up diesel.
Power in Whitehorse was gradually restored starting around 5 p.m., with the last pockets of downtown coming back online after 6 p.m.
Carla Howard, a spokesperson with ATCO Electric Yukon, said a few customers had to wait longer. Two transformers blew a fuse after the power came on. About 10 customers in Porter Creek were without power until 8:45 p.m. and about 30 customers in Takhini were back up and running by 7:30 p.m.
The last Yukon Energy customer to be restored was the Minto mine, at 6:46 p.m.
While working to restore the system, Yukon Energy used diesel generation, Patterson said. That’s because it’s faster to get up and running than generators that run on liquefied natural gas, she said.
“Once everyone was back on the system, because our number 4 unit was still offline, we didn’t have enough hydro to meet the full load,” she said. “So we used LNG to supplement.”
LNG ran “for a few hours” on Saturday night, Patterson said.
By the next morning the broken piece of the fourth wheel was fixed. It was functioning by 9:30 a.m.
There have been discussions about doing a major overhaul of the hydro generator this summer, Patterson said. The broken piece was part of a component that is scheduled to be completely replaced this April.
There are ways to prevent this kind of cascading failure when a turbine goes down. But Morgan said they would cost too much to be worthwhile.
It would mean running other generators in the territory at very low output so that there would be a “spinning reserve” available to kick in as soon as another system went down, he said.
Running equipment inefficiently just in case the backup reserve is needed for something the size of the fourth wheel means “you’re burning fossil fuels, you’re burning LNG. There’s wear and tear on the equipment,” Morgan said. “Your power bill would be huge.”
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