Corona free structures for dry type transformers

Dry type transformers arc transformers in which the dielectric medium is atmospheric air or nitrogen. Some are ventilated or free breathing while others are hermetically sealed. In both kinds the principal dielectric medium-air or nitrogen - has a dielectric constant of unity while the available insulation materials have dielectric constants considerably greater. This presents a relatively perplexing problem to the designer of such transformers in that for most electrode configurations encountered considerable dielectric stress is imposed on the air or nitrogen which is by comparison with other common dielectric media mineral oil and Chlorextol a relatively poor insulator.