Aging of dry-type transformer insulating systems

THE METHOD CHOSEN for evaluation of insulation aging should be representative of operating conditions, applicable to a large range of materials, and readily interpretable in terms of expected life of apparatus. A dielectric strength criterion meets these requirements for dry-type insulating systems. In utilizing this criterion the initial strength of each insulation system is determined at the temperature being investigated. Duplicate samples of the insulating systems are then thermally aged at this temperature and periodic, 60-cycle, 1-minute dielectric strength tests are applied at a value of one-half of the initial hot-dielectric strength until failure occurs. Plotting the hours of thermal aging to reduce the dielectric strength to one-half of its initial value against the operating temperature for a range of temperatures gives the thermal aging characteristics for the insulating system.