Figure: Sample of large-loss fires at hydroelectric generating facilities.
"Hydroelectric generators have a rather unusual design, with relatively large open areas exposing miles of combustible winding insulation that can propagate a fire. There is an ample fire history involving these machines, especially generators with the older asphalt-based insulation materials. Current winding technology employs epoxy-fiberglass materials that have reduced the fire hazard."
"The protection standards for outdoor oil-filled transformers holding more than 500 gallons are the same as those for conventional electric generating stations (Figure 4). In sum, all transformers must have secondary spill containment and either spatial separation distance between each other (25 feet up to 5,000 gallons and 50 feet for transformers with more than 5,000 gallons of oil) or firewalls."
https://www.powermag.com/fire-safety-in-modern-hydroelectric-stations/