Thermal and thermo-mechanical aging of epoxy-mica insulated stator bars

Operating generators just above their nameplate rating could enable power utilities to avoid buying high-cost electricity during peak demand and even ultimately to delay building new power plants. However, such increases can have a major impact on the temperature rise of the insulation system and might reduce its lifetime. The aim of this study is to evaluate the effects of temperature on the expected lifetime of power generators using an epoxy-mica insulation system. To evaluate aging of this insulation system in the laboratory, an experiment was designed to reproduce accelerated thermal and thermo-mechanical stresses. The 15 stator bars used in this experiment were thermally aged at three different constant temperatures. In addition, in order to simulate the stress caused by machine starts and stops, sequences of thermal cycling were inserted at specific times during the constant thermal aging. After each completed cycle, dissipation factor and partial discharge levels were measured in order to evaluate the degradation of the insulation system. Overall, the bars were aged for five complete cycles of 2 000 hours for a total of 10 000 hours, of which 75% was done under constant temperature and the rest under cycling. All the bars were subjected to a breakdown voltage test at the end of aging.