In this "Cut the Copper" blog, manufacturers use air as a transformer coolant after their attempts to replace Askarel fluids met with little to no success.

Joe Guentert

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After enduring all of the pain and tremendous costs associated with replacing, rehabilitating, retro-filling and remediating nasty liquid-filled transformers, the electrical industry finally said, more or less in unison, “That’s enough! I’m not going to go through this anguish one more time! No more liquid transformers in my plants ever again. The EPA will never outlaw AIR as a transformer coolant, so from now on, my indoor transformers will be air-cooled.”

Manufacturers like GE and Westinghouse, again, led the charge and developed new dry-type substation transformer designs. The electrical industry adopted these quickly, and dozens of other smaller manufacturers entered the transformer business with new ventilated dry-type transformer designs.

In general, the new dry-types worked well in service, although early adopters immediately complained about how loudly they operated compared to the liquid units they replaced, and how much heat they generated when loaded. The most prevalent early designs used 220 C winding insulation and an allowable winding temperature rise of 150 C over a 40 C ambient. That says that the windings themselves could be operating in completely normal service at temperatures nearly twice the boiling point of water, so it’s no wonder that they felt “warm” (can you spell “i²R winding losses”?)

Later, next generation improvements included things like Vacuum-Pressure Impregnation (VPI) process, that greatly improved uniform quality of the insulation systems, and then lower temperature rise construction (115 C and 80 C ratings), that greatly improved efficiency and also extended useful operating life. 

In general, life was good, again, with many tens of thousands of dry-type transformer installations now in service.