The explosion was first reported just after 5 p.m. at a power station on SE Retreat Kanasket Road, according to David Nelson of King County Fire District 20.
Nelson said the explosion sent debris flying through the air, followed by thick smoke that has since been determined to be non-toxic.
Fire crews from King County, Maple Valley, Kent and Skyway responded to the explosion, but decided to let it burn. Nelson said the fire, which is being fueled by mineral oil, will likely burn through the night.
Nearby residents were originally told to shelter in place until officials determined there was no danger of another explosion.
Nobody was injured in the blast and BPA officials are working to switch over and reroute power so crews can get closer. The power station gets power directly from Grand Coulee Dam and distributes it to Puget Sound Energy, Tacoma Power and Snohomish County.
The blast didn't cause an power outages.