Aging of dry-type transformer insulation

THERE IS a fundamental difference in the life of insulation containing organic material operated in an insulating liquid as contrasted to the same insulation operated in a gas medium. When operated in an insulating liquid, the liquid tends to replace the material deteriorated and so maintains electrical strength. When operated at a high temperature in a gas medium, as the deterioration progresses, the gas replacing the varnish has a much lower dielectric strength. The base material may or may not deteriorate depending on whether it is organic or inorganic.