The degradation assessment of epoxy/mica insulation under multi-stresses aging

The actual stator bars of 18 kV/300 MW generator were used for accelerated multi-stress aging tests to investigate the degradation of epoxy mica insulation. The stator insulation of large generator was a composition system of epoxy resin, mica and glass fiber. Ultra-wide band PD detecting technique was introduced to measure the PD pulse of the specimens during aging. The data obtained were analyzed using joint time-frequency analysis methods and finally Gabor spectrum was selected to analyze the characteristics of single impulse wave in different aging period. The results of analysis indicate that following increasing aging time, the high-frequency component of single impulse wave continuously increased, while the energy of discharge alternated from low-frequency long time lag to high-frequency short time lag and high-frequency long time lag. Scanning electronic microscope was used to observe the pattern of interface during the aging process, which show that changes in interface is the main reason of aging. The variation of components in insulation was also analyzed by the technique of infrared spectrum and X-ray energy spectrum and the results indicate that epoxy resin creates hydrolytic reaction and the mica undergoes structural change.