Are they? That seems an overly broad generalization. It is probably much more accurate to suggest that dry type transformers are the most common customer owned transformers with low voltage windings. Saves dealing with oil and oil containment.
Dry types are pretty good for lower powers ie 2-3 MW size, and you can incorporate them into a substation type packaged substation anywhere you need. No oil risks and no real fire hazards, usually the tranny if it does go bang wont have enough of an explosive bang to destroy the surroundings. It can be put into a building anywhere and pretty much located near to the load. Cuts losses and can be cheaper. No big copper requirements in feeding the load. Just run your 11kV to the point of use.
You need to cool oil down and that requires heat exchangers and such, a cast resin just pump in cool air and keep it cool.
For step up it generally involves larger powers say >10MW
Heat losses require better disposal of the heat. Also Power generators generally have a lot of switching operations going on and dry types dont really like being switched on when connected - both step up and step down.
Just from what Ive heard and seen but then again I may be deluded !
Thanks Ruggedscot's & Davidbeach's, According to IEEE dry type Transformers are usually used for step down. However I think that the price of dry type is higher than the oil type, but you are right for the hazardous point of view.
