Thermal life evaluation of high temperature insulation systems and hybrid insulation systems in mineral oil

Liquid-immersed power transformers have utilized an insulation system consisting of natural cellulose-based materials and mineral oil for over 100 years. These materials have proven to be reliable and economical when applied within the range of their technical limitations. The bounds of technical limitations for cellulosic materials have been stretched by chemical and physical modifications during the manufacturing process, but they are still present. One of the principal limitations is the maximum temperature which the insulation system can tolerate while still providing an acceptable operating life. A dual temperature test cell has been developed for simultaneous accelerated aging of two different solid insulation materials immersed in mineral oil. This cell was used to derive test data on a cellulose/mineral oil system, a Nomex/mineral oil system and on a hybrid Nomex/cellulose/mineral oil system. The data are presented and discussed.