Using insulation in dry-type and gas-filled transformers

Summary form only given. Ever since 1935, when their development was announced by Westinghouse, dry-type power transformers have become increasingly important in the distribution of electric power. From the common beginning of ventilated dry-type transformers, there has evolved a wide variety of related lines of equipment. The best known of these is the open dry-type transformer, which uses class B and C solid insulations along with ambient air as the coolant. Related to these ventilated units are sealed dry-type transformers which use class H insulations and nitrogen as the coolant. Also related are the ventilated class H units which have become more common during the past few years.