The Bonneville Power Administration twice ordered Pacific Northwest wind farms to cut production in recent days because it has a surplus of power from hydroelectric dams.
The agency, which manages much of the power grid in the Northwest, confirmed it issued the orders during the early morning hours of Sunday and Monday, when demand is low.
The action rekindles a dispute from last year, when the agency curtailed wind turbines because the water from a large mountain snowpack left the region with more hydropower than the electrical grid could handle.
Michael Milstein, a BPA spokesman, said spring runoff has picked up in the past month or two. "Originally it wasn't looking like that wet of a year, but that has changed," he said.
The agency controls a majority of the region's transmission system and markets power from a system of 31 dams and a nuclear plant in Washington. The nuclear plant reduced its output to 85 percent Monday afternoon in an effort to ease the situation.
Wind-power companies will be reimbursed for lost revenue. But Jan Johnson, a spokeswoman for Iberdrola Renewables, a major wind-power developer, said the shutdowns are unacceptable.
"This is a situation in which Bonneville is unfairly using its control of the federal transmission system to interrupt our generators," she said.
In December, the Federal Energy Regulatory Commission ordered the agency not to discriminate against the wind-power companies if such a situation arose again. Bonneville has asked for a rehearing while trying to reach an agreement with wind interests.
Rachel Shimshak, executive director of Renewable Northwest Project, an advocacy group, said BPA has taken steps to avoid a recurrence of what happened last year, such as selling hydropower energy ahead of an anticipated oversupply, but the weekend shutdown shows it has not done enough.
She said developers of renewable energy will shy away from the region if their transmission contracts with BPA lack certainty and they are subjected to annual curtailments.
Wind power was cut for six hours early Sunday morning and five hours early Monday, said Milstein, the BPA spokesman. He noted there is 750 to 1,000 additional megawatts of wind power in the system than a year ago.
"Which is great," he said. "But it's tough to find a home for all that at three in the morning, when people are asleep."
This being hydropower, another issue is fish. High water can be diverted past turbines through spillways, but that can subject migrating salmon to harmful levels of dissolved gases in the churning water.
Save Our Wild Salmon, a conservation group that filed comments with FERC on behalf on the wind industry, rejects the idea that salmon would be injured by sending more water over spillways. It cites a study showing the opposite.
"BPA has tried to pitch the salmon advocates against the wind advocates as if these are opposing forces, and the reality is wind is very beneficial for salmon," said Amy Baird, a spokeswoman for the group.