| Summary: | Polarization and traps determine the electrical property of oil-paper insulation, but most attention has been paid to the modification of insulating oil with nanoparticles, so there are is little research about oil-impregnated paper, and the origin for performance variation is not understood yet. In this paper, spherical nanoscale titanium dioxide was prepared by the hydrolysis method and nanofluid-impregnated paper (NP) was fabricated through oil-impregnation. The frequency domain spectrum was measured for polarization analysis, and both thermally stimulated depolarization current (TSDC) and isothermal surface potential decay (ISPD) methods were used to reveal trap parameters. Results show that NP’s low frequency permittivity is much larger, and another peak appears in the spectrum even though the content of nanoparticles is very low. With the addition of TiO<sub>2</sub> nanoparticles, TSDC’s amplitude and peak temperature increase, and the trap energy becomes shallower. TiO<sub>2</sub> nanoparticles’ strong polarization and high activation energy contribute to NP’s larger interface polarization intensity and activation energy. Furthermore, because of oxygen vacancies, TiO<sub>2</sub> nanoparticles offer a transfer site for holes and electrons to escape from deep traps; thus, the trap energy is greatly reduced.
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