The BPA said that since the shut-offs began on 18 May, 6.7% of electricity generated by wind projects in its region had been locked out from grid access.
The body’s policy of cutting wind supply during times of low demand – typically night time and weekends – was drawn up to counter unusually high levels of hydroelectric generation.
Hydro accounts for 20GW of BPA’s generation capacity, wind 3.5GW and thermal power plants 7GW.
Extra snow-melt coming down from the mountains means that the Columbia Basin runoff this year will exceed runoff in all but two of the last 40 years, with 137% of average flow expected at The Dalles dam on the Columbia River.
Hydroelectric dams generate large amounts of electricity in such conditions because reducing their output would send more water through dam spillways, increasing dissolved gases that can harm fish, and this is prohibited by US environmental law.
"We’ve tried for more than a year to find ways to minimize the impacts of high runoff on wind generation and we’re still looking for solutions from every angle," said BPA administrator Steve Wright.
"We are by no means out of the woods yet, but we’re working to maintain wind’s important contribution to the Northwest power system."
Wind farm owners in the region - including Iberdrola, NextEra Energy Resources, and EDP Renovaveis subsidiary Horizon Wind Energy - have hit back at the curtailments, blasting them as unfair. Last month they took their complaints to the Federal Energy Regulatory Commission (Ferc), citing preferential treatment to other power sources.
However, BPA yesterday claimed the policy of curtailing wind was one of last resort, and that wind had been given preference over thermal plants, which had mostly remained off-line on recent weeks.
BPA added that in the last week only about 2% of scheduled wind generation was curtailed, and for three days in the last week there have been no limits on wind.
