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.

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.

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.

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.

In this paper, the modeling methods of three-capacitance model, conductance model, and plasma model for the numerical simulation of partial discharges under DC voltage and their advantages and disadvantages were introduced, and the recent research achievements of experts and scholars at home and abroad using these models were reviewed. The researches on partial discharge testing under DC voltage in the recent years was summarized, and the impacts of temperature, insulation material, voltage harmonics, atmospheric pressure, and defect on the partial discharge charactristic under DC voltage were summarized. Finally, the limitations of current numerical simulation studies were discussed, and the potential areas for further research were prospected.

To explore the moisture characteristics of the cold shrink intermediate joint for distribution cables at the current stage, undamped joints samples were made firstly in this paper. Accelerated moisture ageing platform was used to treat the samples with moisture. Then the polarization and depolarization current (PDC) test system was used to test the polarization and depolarization currents of the samples which damped for 0, 2, 4, 6, and 8 weeks. At last, their DC conductivity was calculated, and the time constant of three branches was obatined by branch identification. The experimental results show that the DC conductivity of intermediate joint will change only after the joint has damped to a certain extent. The third branch time constant decreases with the increase of moisture degree. This is because the moisture reduces the interface charge migration resistance, which would reduce the interface polarization time. Therefore, the third branch time constant can sensitively reflect the moisture degree of the cable intermediate joint, which can be used as the characteristic parameter to judge the moisture degree of the cable intermediate joint.

Currently, the dielectric of plastic film capacitors is usually linear dielectric polymers with a low dielectric constant, leading to a relatively low energy storage density of capacitors. High-energy-density dielectric polymers under research often suffer from excessive dielectric loss, limiting their practical utilization. Poly(methyl methacrylate) (PMMA) has been commonly employed to enhance the mechanical and breakdown properties of high-energy-density fluoropolymer beceause of their good compatibility between them. However, the commercially available PMMA also exhibits too high intrinsic dielectric loss. In order to decrease the dielectric loss of PMMA, a copolymer named MS was synthesized from methyl methacrylate (MMA) and styrene (St). Subsequently, a series of composites were fabricated by incorporating low content of MS into bulk polymerized PMMA, and their dielectric properties, energy storage characteristics, and insulating peoperties were investigated. The results show that the composites can significantly reduce the dielectric loss of PMMA, making it more suitable as the modified material of high-energy-density polymer compared to PMMA. Under 5 500 kV/cm of electric field, the discharge energy density of the dielectric film composed of 10% MS achieves 5 J/cm³, and its charge-discharge efficiency can attain 83%.

To meet the growing demand for high thermal conductive and electrically insulating composites, a silver nanoparticles (AgNPs) modified nonwoven fabric (AgNPs@NWF) was prepared by polydopamine (PDA) modification and in situ reduction process using PET nonwoven fabric (NWF) as a template. The continuous boron nitride nanosheets (BNNS) thermal conductive network (BNNS@NWF) and AgNPs/BNNS synergistic thermal conductive network (AgNPs/BNNS@NWF) were constructed by adsorbing BNNS on the surfaces of NWF and AgNPs@NWF through the dispersion and interfacial bonding of nanocellulose through a cyclic impregnation adsorption and layer-by-layer assembly process. BNNS-AgNPs/BNNS-BNNS sandwich-structured thermal conductive composite films were prepared by hot pressing process with BNNS@NWF as the surface layer and AgNPs/BNNS@NWF as the intermediate layer, and their microstructure, thermal conductivity, insulating properties, mechanical properties, and actual thermal management performance is characterized and tested. The results show that a synergistic three-dimensional thermal conductive network of AgNPs/BNNS was constructed in the composite films, at the same time the insulating properties are ensured. When the mass fraction of BNNS and AgNPs is 34.8% and 3.3%, the in-plane thermal conductivity of the composite films reaches 7.56 W/(m·K), the volume resistivity reaches 3.54×10¹³ Ω·cm, and the mechanical properties is good. The actual application test show that the composite films have good thermal management performance.

In order to improve the toughness and solubility of bismaleimide resin and improve its application in the field of high frequency and high speed copper clad laminate, a silicone modified bismaleimide resin (Si-D936) was designed and synthesized from silicone and D936 type bismaleimide in this paper. The characteristic structure of Si-D936 and silicon-D936 monomer were characterized by IR, NMR, and gel permeation chromatography. Additionally, the effects of different silicon contents on the solubility, heat resistance, mechanical properties, and dielectric properties at 10 GHz of the Si-D936 were explored. The results show that compared with the modified D936, the solubility of Si-D936 in butanone is obviously improved, and the highest solubility can reach 30% (at room temperature). However, with the increase of Si content, the molecular weight of Si-D936 increases, and the solubility decreases slightly. The Si-D936 modified resin has good thermal properties after curing, but the heat resistance and residual carbon rate decrease gradually with the increase of Si content. The silicone modification can improve the mechanical properties of D936 resin, in which the impact strength and tensile strength of Si-D936-1 (the molar ratio of D936 to HMM is 3:1) increase by 35% and 24.6%, respectively. However, with the further increase of Si content, the cross-linking degree of the modified resin is affected, and the impact strength, bending strength, and tensile strength of the modified resin decrease. After silicone modification, the dielectric properties of the modified resin are improved, and the dielectric loss is reduced from 0.009 4 to 0.007 2 (10 GHz).

Low-frequency cable is a key equipment in the flexible low-frequency AC transmission system, and the characteristics of insulation medium under low-frequency voltage are of great significance to the design and operation of cable. In order to study the growth and partial discharge characteristics of electrical tree in cross-linked polyethylene under low-frequency voltage, a real-time observation system for electrical tree growth at different frequencies and a synchronous measurement system for partial discharge were designed and constructed. The initiation, growth, and partial discharge characteristics of electrical tree in XLPE samples were investigated at four frequencies of 20, 30, 40, and 50 Hz. The results show that the influence of voltage frequency on the growth and partial discharge characteristics of electrical tree in XLPE has obvious rules. In the range of 20-50 Hz, with the decrease of voltage frequency, the tree starting voltage of XLPE increases slightly, but the growth rate of electrical tree is accelerated, the damage area increases, the amount and number of partial discharge increase, and the partial discharge phase is basically unchanged.