Vegetable insulating oil has gained wide attention due to its green and renewable characteristics as well as excellent fire protection properties, while nanoparticles can effectively improve the electrical properties of vegetable insulating oil. In this paper, the effects of nanoparticles on the electrical properties of vegetable insulating oil were summarized from dielectric loss factor and volume resistivity, and the effects of nanoparticle type, concentration, and surface modification on the breakdown properties of vegetable insulating oil were also discussed. Combining with the differences in the effects of nano-modification on the electrical properties of mineral insulating oil and vegetable insulating oil, we summarized the current research difficulties of nano modified vegetable insulating oil, and prospected the research direction and prospects of nano modified vegetable insulating oil.

Thermosetting epoxy resin is widely used in electrical equipment field due to its excellent thermal conductivity and insulating properties. Due to the stable three-dimensional network structure and insolubility of the thermosetting epoxy resin, its recycling and reuse pose challenges. In this paper, we added different mass fractions of boron nitride (BN) into the epoxy (EP)/4-methylhexahydrophthalic anhydride degradation system to prepare high thermal conductivity, high insulating, and degradable BN/EP composites, and studied their thermal conductivity, dielectric properties, and degradability. The results show that taking 2,4,6-tris(dimethylaminomethyl) phenol as the catalyst, the EP/BN composites can achieve atmospheric pressure degradation at 200℃ under the action of ethylene glycol. The thermal conductivity of BN/EP composite with 15% mass fraction of BN is 0.335 W/(m·K), which is 34% higher than that of pure EP resin; the AC electric strength is 101.7 kV/mm, which is 13% higher than that of pure EP resin. BN/EP composites can be degraded through transesterification to obtain EP degradation products (EDP). After the BN/EP composites are compounded with EDP, the thermal conductivity and electric strength of the BN/EP composites is basically unchanged.

An ANF/BTNF paper-based composite film was prepared taking aramid nanofiber (ANF) as the high-temperature resistant matrix and filling with barium titanate nanofiber (BTNF). The effect of different BTNF content on the dielectric properties of the composite films was studied. The results show that the thickness of ANF/BTNF composite film is about 10 μm. When the mass fraction of BTNF is 0%-20%, both the dielectric constant and electric strength of the composite films increase with the increase of BTNF content. When mass fraction of BTNF increases to 40%, the dielectric constant of the composite film increases significantly, but decreases rapidly at high-frequency zone, and the dielectric loss factor increases significantly, at the same time, the electric strength of the composite film decreases slightly. Therefore, the ANF/20BTNF composite film with 20% mass fraction of BTNF has the best comprehensive dielectric properties, the dielectric constant at 1 kHz reaches 4.78, and the electric strength is 8.90 kV/mm.

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.

The mechanical properties of insulation pressboard are key factors in modeling the structure of transformer windings. The operating transformer winding is subjected to periodic electromagnetic forces, and the static stress-strain test results cannot characterize the dynamic mechanical properties of insulation pressboard. In this paper, an asymmetric hysteresis model was proposed to study the dynamic mechanical properties of the insulation pressboard under periodic electromagnetic forces, and the model includes not only linear damping and stiffness coefficients, but also nonlinear dynamic stiffness. In the experiment, a pneumatic vibration exciter was used to simulate the amplitude and frequency of the electromagnetic force, and the stress-strain data of the insulation pressboards under different clamping forces was obtained. A multi-objective optimization algorithm based on NSGA-II algorithm was used to train the model parameters. The results show that the stiffness and damping of the insulation pressboard are closely related to the clamping force, and the oil-immersed pressboard has obvious damping characteristics. As the clamping force increases, the dynamic stiffness of the insulation pressboard decreases during the loading and unloading processes. Compared with the amplitude of periodic electromagnetic force, the frequency of periodic electromagnetic force has more significant effect on the mechanical characteristics of insulation pressboard.

Transformer oil is prone to microbial contamination during storage, utilization, and transportation, which affects the quality and insulating performance of transformer oil. In this study, the effects of temperature and humidity on the microorganism growth in transformer oil were mainly investigated, and the quality changes of transformer oil before and after treatment were analyzed. The functional group structure and organic species of transformer oil were identified, and some effective treatment methods for microbial pollution in transformer oil were proposed. The results show that both the transformer running oil and faulty oil contain bacillus subtilis, bacillus using sugar to produce acid, bacillus using sugar to produce acid and gas, and bacillus producing acid and gas without sugar. The number of microorganisms in faulty oil is much greater than that in running oil, and the number of bacillus subtilis is the largest, while there is no microorganism cultured in new transformer oil. The high temperature could restrict the growth of microorganism in transformer oil. When the temperature is 60℃, the microorganisms will not be completely inactivated, and once the temperature returns to 37℃, they will start to grow and reproduce again. When the mass ratio of oil and water is 99∶1, 49∶1, 19∶1, and 9∶1, four types of bacillus all exist at 37℃, and the number of bacillus increases with the increase of oil-water mass ratio. The regeneration treatment method of high-pressure sterilization+molecular sieve coarse filtration+ultrafiltration can make the contaminated transformer oil clear and transparent, and the microorganisms disappear. The regenerated oil mainly contains O-H, C-H, C=O, and unsaturated bonds. After regeneration treatment, the breakdown voltage and volume resistivity of transformer oil increase from 25.83 kV and 5.70×10¹⁰ Ω·m to 49.50 kV and 4.68×10¹¹ Ω·m, respectively, and the dielectric loss factor decreases from 6.052% to 0.215%, which meet the requirement of DL/T 1419—2015.

An interturn insulation test platform was constructed, and the voltage-withstand tests were conducted on vegetable insulating oil-paper insulation and mineral insulating oil-paper insulation under lightning impulse voltage, switching impulse voltage, and power frequency voltage. The results show that under power frequency voltage, the breakdown voltage of vegetable oil-paper insulation is higher than that of mineral oil-paper insulation. Under switching impulse voltage, the breakdown voltage of vegetable oil-paper insulation is slightly higher than that of mineral oil-paper insulation. Under negative lightning impulse voltage, the breakdown voltage of vegetable oil-paper insulation is significantly lower than that of mineral oil-paper insulation. Considering the differences in breakdown voltage of interturn insulation with different specification of wires, on the basis of test results, the field strength of mineral oil-paper interturn insulation and vegetable oil-paper interturn insulation were simulated and calculated by finite element method so as to guide the design of interturn insulation for vegetable insulating oil transformer. The simulation results show that the maximum field strength of mineral oil-paper interturn insulation is higher than that of vegetable oil-paper interturn insulation under the breakdown voltage.

The partial discharge ultrasonic signal attenuates severely on power transformer equipment shell, resulting in larger differences in the partial discharge signals received by different positions of sensors, which greatly affects its measurement accuracy and detection efficiency in field application. In this paper, a partition model of oil-immersed power transformer was built using finite element simulation software, the partitioning rule of partial discharge ultrasonic signal on the main surface of oil tank was obtained, and the transformer spatial sound pressure and partition sound pressure distribution maps were formed. On the basis of monitoring point optimization method of Pearson correlation coefficient and K-means clustering, the monitoring points with the best sensitivity of spatial sound pressure and partition sound pressure were preferably selected and compared. The results show that the average detection efficiency of the A-level monitoring points is 86.0%, which realizes the efficient detection of transformer partial discharge and provides a new method for selecting the placement points of ultrasonic sensors.