Aiming at the problems of large specific surface area of conventional flake boron nitride and sharp increase in resin viscosity when compounded with epoxy resin, we prepared spherical boron nitride and used it as a filler to compound with epoxy resin to prepare spherical boron nitride/epoxy composites. The preparation process and curing characteristics of spherical boron nitride/epoxy composites were studied, and the influences of the morphology and filling amount of flake/spherical boron nitride fillers on the mechanical and electrical properties of epoxy resin composites were compared. The results show that with the increase of reaction temperature, the curing degree change curve of epoxy resin shows “S” shape, and the curing process can be roughly divided into three stages of “slow-fast-slow”. In terms of mechanical properties, adding a small amount of boron nitride can improve the mechanical properties of the epoxy resin composites; when the filling amount is high, spherical boron nitride/epoxy composites have better mechanical properties than flake boron nitride/epoxy composites. In terms of electrical properties, the relative dielectric constant of the epoxy resin composites increases with the increase of boron nitride content, and the dielectric loss factor is lower than 0.02; compared with the flake boron nitride/epoxy composites, the spherical boron nitride/epoxy composites have less “filler-resin” interface, lower relative dielectric constant and dielectric loss factor; adding an appropriate amount of boron nitride can significantly improve the volume resistivity and electric strength of composites.

Under the background of “carbon peaking and carbon neutrality” development strategy goal, in order to explore the application prospects of green and environment friendly bio-based epoxy resins in electrical equipment, a bio-based resin of maleopimaric-based epoxy resin (MPAER) was prepared using a renewable resource of rosin as raw material. Taking methylhexahydrophthalic anhydride (MHHPA) as the curing agent, we systematically studied the curing characteristics, as well as the thermal, mechanical, and electrical properties of the MPAER/MHHPA system, and the MPAER/MHHPA system was compared with the DGEBA/MHHPA system composed of commercial diglycidyl ether of bisphenol A (DGEBA). The results show that the MPAER/MHHPA system and DGEBA/MHHPA system have equivalent curing reaction activity. The glass transition temperature of the MPAER/MHHPA system is 112.8℃, its mechanical and electrical properties are slightly weaker than those of the DGEBA/MHHPA system, and its electric strength is 9.4% lower than that of the DGEBA/MHHPA system. However, the comprehensive performance of the MPAER/MHHPA system is still good, and its performance can be further improved by optimizing the structure of MPAER or blending with other types of epoxy resins.

The epoxy resin for casting in saturated reactors is prone to ageing and cracking when subjected to electrical stress and mechanical vibration, it is necessary to improve its mechanical and thermal properties on the basis of maintaining its excellent insulating properties and corrosion resistance. In this paper, taking the functional optimization design as principle, a silicon dioxide dielectric layer was formed on the surface of silicon carbide whiskers (SiCw) by heat treatment to obtain SiCw@SiO₂. Then a SiCw@SiO₂/EP composite dielectric was prepared by doping modification, and its dielectric properties, mechanical properties, and thermal properties were tested. The results show that the SiCw@SiO₂/EP composite dielectric has both high dielectric constant and low dielectric loss, and its toughness is also improved. Due to the continuous thermal conductivity network formed by the contact of SiCw, the thermal conductivity of the SiCw@SiO₂/EP composite dielectric increases by 117% than that of pure epoxy resin.

In this paper, BNNS@SiO₂/epoxy composite dielectrics with different filler content were prepared, the chemical characteristics and microscopic morphology of the composite dielectrics interface area were studied, and the trap characteristics and breakdown properties of the composite dielectrics were studied by thermal stimulation depolarization current method and breakdown test. The results show that BNNS@SiO₂ has a core-shell structure and the thickness of the coated SiO₂ is at the nanoscale. There is a clear chemical bonding effect on the surface of BNNS@SiO₂, which can form a strong interface zone with the epoxy matrix, so as to improve its compatibility with the epoxy matrix. A small amount of BNNS@SiO₂ can effectively improve the electric strength of the composite dielectric, and when the mass fraction of BNNS@SiO₂ is 1%, the electric strength of the composite dielectric increases by 52.3%. When the content of BNNS@SiO₂ nanoparticles is small, the deep traps in the epoxy composite dielectric increases. When content of BNNS@SiO₂ nanoparticles is large, the deep traps decrease and the shallow traps increase. In addition, the thermal conductivity of the core-shell structure BNNS@SiO₂/epoxy composite dielectric increases significantly, which is conducive to the heat dissipation of insulation under high electric field. Through the comprehensive effect of deep trap effect and thermal conductivity improvement, the breakdown performance of epoxy composite dielectric is improved significantly.

In order to study the feasibility of using sapium sebiferum seed oil as the natural ester insulating oil for transformers, sapium sebiferum seed oil was subjected to refining treatments including degumming, deacidification, decolorization, and dehydration, to prepare a sapium sebiferum seed insulating oil. Its physicochemical and electrical properties were compared with those of mineral insulating oil, and accelerated thermal ageing test was carried out on it under 130℃ to study its thermal ageing characteristic. The results show that the main fatty acid component of sapium sebiferum seed insulating oil is polyunsaturated fatty acid, its physicochemical and electrical properties can meet the requirements of relevant standards, and its kinematic viscosity and dielectric loss factor are greater than those of mineral insulating oil, but the ignition point is much higher than that of mineral insulating oil. After thermal ageing at 130℃ for 250 h, 500 h, 750 h, and 1 000 h, the sapium sebiferum seed insulating oil has less color change and higher breakdown voltage than mineral insulating oil, but its increase amplitude of dielectric loss factor is great.

In order to improve the toughness and oil absorption of wet-processed insulating parts such as insulation molding parts and angle rings, strengthen the process operability in the assembly process, and thus improve the operation reliability of transformer, the performance of the cotton pulp composite insulating paper with different raw material ratio was studied. The cotton pulp ratio of the cotton pulp composite insulating paper was determined, and then the finished product was processed and their performance was tested. The results show that the comprehensive performance of the cotton pulp composite insulating paper with 25% volume fraction of cotton pulp is the best, and the corner ring and insulating parts produced by the cotton pulp composite insulating paper have better flexibility and oil absorption.

Natural ester and synthetic ester insulating oils are getting more and more application in transformer industry because of their eco-friendly and excellent fire resistance characteristics. At present, there is no precedent in China for the application of environment friendly ester insulating oil in 220 kV power transformers for offshore substation. According to the characteristics and requirements of offshore substation, the selection of transformer ester oil types and insulation system types were studied in this paper. Taking SZ-150000/220 type transformer as the object, an improved model of the transformer body structure was established for simulation analysis, and the simulation results show that the ester insulating oil meet the requirements of insulation and temperature rise of 220 kV power transformer. The feasibility of ester insulating oil applied in 220 kV power transformer for offshore substation was discussed through comparative analysis and simulation calculation, and an optimized design scheme was proposed to make the equipment adapt the requirements of offshore operating environment, which provides a reference for the application of ester oil transformer in offshore substation.

Vegetable insulating oil is easy to age in electrical equipment and produce polar small molecule substances such as formic acid, acetic acid, and formaldehyde, which reduces the physicochemical properties and electrical properties of insulating oil and affects the safety operation of equipment. In order to solve the quality deterioration problem of insulating oil, the aged vegetable insulating oil can be purifed by adsorption technology to reduce the polar small molecule impurities generated during ageing. In this paper, a 2-methylimidazole zinc salt (ZIF-8) and alumina (Al₂O₃) composite material was prepared, the ZIF-8/Al₂O₃ composite material was used to adsorb and purify the aged vegetable insulating oi, and the physicochemical properties and electrical properties of the vegetable insulating oil after adsorption and purification were tested. The results show that after adsorption and purification by ZIF-8/Al₂O₃ composite material, the acid value and dielectric loss factor of the vegetable insulating oil decrease significantly, the volume resistivity and initial oxidation temperature increase significantly, and the quality is improved significantly. In addition, the molecular simulation results also show that the adsorption capacity of ZIF-8/Al₂O₃ composite material for polar small molecules in vegetable insulating oil is better than that of Al₂O₃.

Methanol is an important characteristic component in the evaluation of oil-paper insulation state of transformer due to its strong stability and high content in the early ageing stage of insulating oil, while water is an indispensable product during the ageing process of oil-paper. In order to clarify the effect of water on the methanol diffusion in vegetable insulating oil, blending models of vegetable insulating oil, water, and methanol with water content of 1.0%, 1.5%, 3.0%, and 5.0% were constructed, and the diffusion trajectory, diffusion coefficient, interaction energy, hydrogen bonding, free volume of the substances in the model were calculated by molecular dynamics method. The mechanism of water promoting methanol diffusion was explained from microscopic perspective. The results show that with the increase of water content, the interaction energy between methanol and vegetable insulating oil decreases, the hydrogen bond stability of the system decreases, and the free volume increases, which ultimately leads to the enhancement of methanol diffusion in vegetable insulating oil.

Under high thermal fault, the internal state change and gas production mechanism of insulation system in super/ultra-high voltage transformer is unclear, which seriously restricts the state analysis and diagnosis of transformer. In view of this problem, the generation path of characteristic gas and decomposition mechanism of oil-paper insulation system in super/ultra-high voltage transformer were studied under high thermal fault by enthalpy theory and simulation method. The gas generation mechanism was obtained according to the enthalpy theory, and the gas generation mechanism and path proposed in this paper were verified by simulation. The results show that the chain alkanes in the oil-paper insulation system are more likely to crack than the cycloalkanes and bicyclic aromatic hydrocarbons under high thermal fault. The generation rate of cracking characteristic gas of various materials in descending order is cellulose, chain alkanes, cycloalkanes, and bicyclic aromatic hydrocarbons. According to the gas generation energy, the characteristic gases CH₄ and C₂H₆ are the easiest to generate, while C₂H₂ is the most difficult to generate. The generation rate of characteristic gases in oil-paper insulation system can be used to judge the severity of high thermal fault.