Possibly a transformer fire on the outside of the turbine house.
Will update as info becomes available.
Edit:
Another view of it:
Edit 2:
Boulder City confirms Hoover Dam 'isolated fire' is extinguished
The 'isolated fire' as per Doug Hendrix with Reclamation at the Hoover Dam has been extinguished, according to a tweet from the official Boulder City account
www.ktnv.com
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