For studying the effect of thermal aging on the dielectric properties of epoxy-mica insulation in frequency domain for generator stator bars

the bar sample is subject to accelerated thermal aging tests at 170 ℃ and the aging time is 0

7

14

21 d

respectively.The frequency domain dielectric spectrum of the sample at different aging times is tested by the frequency domain dielectric spectrum method and the effects of aging time on dielectric loss and relative dielectric constant are analyzed.The micromorphology and chemical structure properties of epoxy-mica insulation system before and after aging were shown by scanning electron microscope（SEM） and Fourier transform infrared（FTIR）.It is shown by the result that the dielectric loss and relative dielectric constant of the insulating system decrease compared with that of the untreated sample due to the strengthening of the cured cross-linking structure and the weakening of polarization strength in the early stage of the aging.With longer aging time and increase of aging degree

the thermal oxygen decomposition of macro-molecular chains of the insulating system occurs.The mica layer also starts cracking

generating a large number of polar free groups

thus causing the increase of dielectric loss and relative dielectric constant.At the same time

the insulation system will be deteriorated by delamination and peeling due to long-term aging.The value of dielectric spectrum of the epoxy mica insulation decreases with the increase of frequency and a relaxation zone will appear in the low frequency range for the unaged and short-term aged samples.The dielectric loss and the imaginary part of the relative dielectric constant are less affected by the degree of aging at high frequency and the difference between the samples at different aging times is not significant.