Frequency domain dielectric spectroscopy characteristics of oil-paper insulation under accelerated thermal aging

To investigate the impact of aging condition and temperature on the frequency domain spectroscopy (FDS) test results of oil-paper insulation, accelerated thermal aging experiments were performed on oil-impregnated pressboard sample in this work. FDS tests ranging from 1 mHz to 1 kHz were carried out during the aging process, and for the same aging condition, FDS tests were repeated at different temperatures. The variations of dissipation factor (tanδ), real capacitance (C'), imaginary capacitance (C″) and moisture content caused by aging time and measurement temperature were analyzed. Experimental results show that both tanδ and C″ of oil-impregnated pressboard increase with the increasing measurement temperature and thermal aging time at lower frequencies, while have no obvious change at higher frequencies. When temperature and thermal aging time increase, the frequency point where C' changes rapidly shifts to higher frequencies. The moisture content of pressboard sample obtained from FDS test results increases with the increment of thermal aging time in the earlier stage and shows no obvious change in later stage. The estimated moisture content is also influenced by temperature. When the temperature of sample is higher, the value of estimated moisture content is obviously larger.