The role of microscopic defects in multistress aging of micaceous insulation

The aging mechanism of mica/epoxy insulations under multiple stresses is investigated with both breakdown tests and a scanning electron microscope (SEM). The result of simultaneous multistress testing shows that thermal and mechanical stresses have a synergistic effect on electric life. Similar results are also obtained for residual electrical breakdown characteristics after sequential application of thermal aging and mechanical stress. It is confirmed with SEM pictures that thermal stress causes isolated delaminations at the mica/epoxy interface. Using a newly developed SEM apparatus with servomechanisms, the development of delamination and cracks are observed during mechanical fatigue tests in realtime. A multistress aging model is proposed with relation to the generation and development of the microscopic defects