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

Increasing the load of generators in service above nameplate rating could allow power utilities to delay or avoid building new power plants, or prevent buying electricity at high cost during peak demand. 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 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, a sequence of thermal cycling was introduced 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. The results revealed that after two complete cycles of 2000 hours, there was delamination of bars aged at high temperature. Up to now, all the bars were aged for two complete cycles for a total of 4000 hours.