Study on Relaxation Current Characteristics of Oil-Paper Insulation under DC Electric Field

In this paper, the formation mechanism of the relaxation component in oil-paper insulation is analyzed by the relaxation current variations of insulating paper before and after impregnation with oil, as well as before and after moisture absorption in insulation. The types and influencing factors of the relaxation component in oil-paper insulation are discussed. The results indicate that after impregnation with insulating oil, both polarization and depolarization currents decrease significantly, and the attenuation rate increases significantly. It means that impregnating paper with oil can effectively reduce the dielectric polarization loss of pure paper. After moisture absorption in oilpaper, both polarization and depolarization current increase significantly, indicating a significant increase in dielectric polarization loss due to moisture absorption in oil-paper. There are three types of relaxation polarization in the relaxation current of oilpaper during 1 to $1800 \mathrm{~s}$. Among them, the third type of relaxation time constant $\tau_{3}$, polarization intensity a
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, and trap density show the most significant decrease after impregnation of paper with oil. It may be related to the changes in the interfacial structure resulting from the impregnation of paper fibers with oil. The relaxation time constants $\tau$ and polarization intensities a of the three types of relaxation in moisture-absorbed oil-paper increase significantly. Among them, the increments of time constants $\tau_{2}$ and $\tau_{3}$ are the most significant. It may be related to the formation of numerous new interfacial structures due to moisture absorption in oil-paper.