This paper systematically studies the effect of Fe₃O₄ nanoparticle size on the insulation performance of nanofluid impregnated paper. Three kinds of Fe₃O₄ nanoparticles with different sizes and their nanofluid impregnated papers were prepared. Environmental scanning electron microscopy (ESEM) and infrared spectroscopy were used to analyze the combination of Fe₃O₄ nanoparticles and nanofluid impregnated paper. The effect of nanoparticle size on breakdown voltage and several dielectric characteristics, e.g., permittivity, dielectric loss, of the nanofluid impregnated paper were comparatively investigated. Studies show that the Fe₃O₄ nanoparticles were bound to impregnated paper fibers by O–H bonds, while the relative permittivity and dielectric loss of the nanofluid impregnated papers were increased. Meanwhile, the increase of trap depth, caused by the nanoparticles, can trap the electric charge and improve the breakdown strength. The test results show that the direct current (DC) and alternating current (AC) breakdown voltages of nanofluid impregnated paper increased by 9.1% and 10.0% compared to FR3 nanofluid impregnated paper, respectively.

Polyhedral oligomeric silsesquioxane (POSS) can significantly improve the short-term insulation ability of epoxy materials under high electric field, which makes epoxy/POSS composites show a wide application prospect in high-voltage power equipment. The changes in performance due to aging is the concern of the long-term application of epoxy materials in power equipment. Therefore, the influence of Glycidyl POSS on the insulation performance of epoxy materials in the aging process needs to be investigated. In this article, different mass fractions of epoxy/POSS composites were prepared and the effect of POSS on the dielectric loss of epoxy materials after thermal aging was investigated. The results show that the addition of a small amount of POSS can effectively inhibit the low-frequency dielectric loss of epoxy insulating materials during thermal aging. Combined with the dc volume resistivity test of epoxy composites and the quantum chemical calculation results of POSS molecules, a possible mechanism of POSS inhibiting the dielectric loss of epoxy resin during thermal aging was proposed. The present work provides a reference for further development of POSS in the field of electrical insulation and the application of POSS modified composites in power equipment such as low-frequency transmission and dc transmission equipment.

The epoxy insulators in dc gas-insulated transmission lines (GIL) tend to accumulate surface charges, which causes insulation flashover. Increasing the surface conductivity of epoxy resin, which can restrain the accumulation of surface charges on the epoxy insulator, is a potential method to improve the insulation performance of dc GIL insulators. The conductivity of polymer dielectric is strongly influenced by the charge trap characteristics of the polymer. In this work, we introduce chlorine with strong electronegativity and a larger atomic radius into the epoxy group segment of epoxy resin to improve the distributed energy level structure, which in turn reduces the electron trap depth, to increase the surface conductivity of epoxy insulating material without affecting its intrinsic dielectric strength. Based on the results of quantum chemistry calculation, the modulation laws of introduced chlorine (including the chlorine in the form of a hydrolyzable chlorine atom and a nonhydrolyzable chlorine atom) on distributed energy levels of epoxy resin molecule are anticipated. These laws are explained from the microscopic perspectives of electron energy structure and electron cloud offset. Both the inductive effect of the chlorine atom and the conjugation effect of the 2p electron orbital of the oxygen atom in the epoxy group impact the distributed energy levels by changing the spatial distribution of electron cloud density between and on valence bonds.

Hydrolyzable chlorine is a well-known residue issued from the synthesis of epoxy resin, which may impart specific dielectric properties. In this work, the changes in distributed energy levels and charge trap depths of the molecule containing such defects are investigated using quantum chemical calculations. These changes are analyzed from the microscopic perspectives of electron energy structure and electron cloud offset. The charge transport and charge injection behavior in epoxy materials before and after the hydrolysis are predicted using the analysis results. The volume resistivities and space charges of epoxy materials with three different hydrolyzable chlorine contents are tested at different hygrothermal aging times, which are consistent with the previous predictions. These results are used to explain the ac breakdown strength of these epoxy materials at different hygrothermal aging times. It is indicated that the structural changes (electronegativity and atomic position changes) of the molecule containing hydrolyzable chlorine before and after hydrolysis change the spatial distribution of electron cloud density between and on valence bonds, and in turn, change the energy levels of different molecular segments. This results in a large increase in the overall electron trap depth and a small decrease in the overall hole trap depth of the molecule after hydrolysis.