Firstly, a natural ester was exposed to the air, and the relationships between its properties and exposure to air duration were measured. Then the oxidation situation of insulating part impregnated with natural ester in air was observed, and the natural ester on the insulating part was removed by traditional gas-phase drying method. Finally, the withstand field strength of solid insulating parts in natural ester transformer was calculated by an electric field analysis software. The results show that the water content of the natural ester increases significantly after exposure to air, but its other characteristics change little. There is no oxide film forming on the surface of insulating parts after exposed to room temperature for two months. The natural ester in insulating parts cannot be removed by gas-phase drying method completely. The withstand field strength of insulating parts in natural ester transformer is 10% higher than that in mineral oil transformer with the same structure.

In order to study the rejuvenating effect difference of rejuvenation fluid on water tree ageing cable when different catalysts were combined with siloxane, we selected a methyl phenyl dimethoxy siloxane (PMDMS) and a long-chain siloxane, tetraisopropyl titanate (TIPT) and long-chain benzene sulfonic acid two catalysts to prepare four rejuvenation fluids with different formula. Water tree aged cables were rejuvenated by injecting the rejuvenation fluid and conducted second ageing, and their electrical properties and microstructure changes were analyzed by the power frequency breakdown test, water tree morphology observation, and FTIR test. The results show that four kinds of rejuvenation fluids can inhibit the growth of water tree, and the breakdown voltage of the water tree aged sample is improved. The rejuvenation fluid composed of long-chain siloxane and long-chain benzene sulfonic acid has the highest retention rate in the insulation layer, and the rejuvenating effect is the best. The rejuvenation fluid composed of PMDMS and long-chain benzene sulfonic acid has the lowest retention rate in the insulation layer, and the rejuvenating effect is the worst. It is proved that when the diffusion speed of siloxane and catalyst in insulation layer matches, the retention rate of the siloxane can be improved, and the long-term rejuvenating effect can be enhanced.

On the basis of radiation spectral characteristics of insulation abnormal discharges, a dual-spectral discharge detection method was proposed. The differences of spectral distribution characteristics under corona discharge, spark discharge, and arc discharge were obtained through experiments, and a dual-spectral sensor based on solid avalanche array was developed. The evolutional relationship between the dual-spectral characteristics and discharge energy was studied using this sensor, and then the discharge hazard quantification criteria based on dual-spectral intensity ratio was summarized. The results show that the light radiation under three kinds of discharge modes has a certain polarity effect. There is a strong correlation between the dual-spectral intensity ratio and the discharge intensity, and there is almost no intersection for the variation range of dual-spectral ratio of low-energy discharge and high-energy discharge, which can be used as an effective criterion to judge partial discharge or arc discharge, and provide a new approach for the optical monitoring and diagnosis of abnormal discharges in equipment.

In order to study the influence of temperature on the nonlinear resistance characteristics of different stress grading materials, a measurement method using the presetting low resistivity tape as the electrode was proposed and applied. The nonlinear volt-ampere characteristics of four stress grading materials under different temperatures were tested. At the same time, the influencing degree of temperature on the nonlinear resistance characteristics of different stress grading materials were studied by function fitting method. The distribution differences of surface electric field along the stress grading structure at stator winding end of 24 kV hydro-generator under the withstand voltage test voltage and the rated phase voltage at different temperatures were further simulated and compared. The results show that the nonlinear volt-ampere characteristics and surface resistivity of four stress grading materials show obvious temperature dependence, with the increase of temperature, both the initial surface resistivity and nonlinear coefficient decrease, there is little difference among the surface electric field distributions along the stress grading structure of 24 kV hydro-generator under different temperatures.

The linear expansion coefficient of cured E51 epoxy resin was tested by using the TMA mode of DMA, and the effects of curing agent types (methyltetrahydrophthalic anhydride (MeTHPA), 4,4-diaminodiphenylmethane (DDM), dicyandiamide (dicy)), and filler powder types and dosage on its linear expansion coefficient were analyzed. The results show that when T<T_g, the linear expansion coefficient of cured E51/Dicy is smaller than that of cured E51/MeTHPA and E51/DDM. When T＞T_g, the linear expansion coefficient of cured E51/dicy is larger than that of cured E51/MeTHPA and E51/DDM. After filling SiO₂, Al(OH)₃, and Al₂O₃ respectively, the linear expansion coefficient of the epoxy composites are significantly lower than that of pure epoxy material, and the linear expansion coefficients of the epoxy composite filled with Al₂O₃ is the smallest. With the increase of inorganic filler content, the linear expansion coefficient of the epoxy composite decreases. However, when the mass fraction of inorganic fillers is 70%–80%, the linear expansion coefficient changes little with the change of filler content.

A aluminum thin-walled cylinder was used to replace the cable to finish the actual setting process of terminal. The interface pressure between the 110 kV prefabricated cable terminal and the stress cone was measured by the electrical measurement method and elastic mechanics theory. By finite element simulation analysis and comparing the calculating result and test result, a reasonable finite element simulation analysis model was established and the terminal setting process was simulated. Several analysis models with different interference were established to study the influence of interference, silicone rubber material parameter, and spring compression on the interface pressure. The results show that the maximum interface pressure is located in the middle of the maximum radius of stress cone. The interface pressure is proportional to the spring compression before the spring compressing to 90 mm, and the interface pressure changes little during the process of the spring compressing from 90 mm to 85 mm. The interfacial pressure is affected by the interference and the cross-sectional area of cable, and it is greatly affected by the elastic modulus of stress cone, and less affected by the elastic modulus of cross-linked polyethylene (XLPE) insulation.

The setting of boundary conditions is a key factor that affects the accuracy of stress calculation for basin insulator. In this paper, an axisymmetric model of basin insulator for miniaturized 252 kV GIS was established. The Mooney-Rivlin function model of rubber sealing ring was introduced to simulate and calculate the stress of basin insulator under different constraint boundary conditions by finite element analysis software, and the result was compared with the measured value of hydraulic test. The results show that the boundary condition using the friction contact is consistent with the engineering practice, which is suitable for the setting of stress simulation boundary condition for basin insulator. The research results can provide reference for the miniaturization and lightweight design of basin insulator of other voltage levels.

This paper summarized the global patent layout of corona resistant PI films, the influencing factors of corona resistance for inorganic nano hybrid films, and introduced the application status of corona resistant PI films at liquid nitrogen temperature, as well as the future development of corona resistant PI films.

The ageing problem of insulating oil under long-term electrical-thermal combined stress seriously threatens the safe and stable operation of oil-immersed transformers. In view of the transformer faults caused by the ageing of insulating oil, it is urgent to research the ageing characteristics, mechanism, and diagnostic methods of insulating oil. Therefore, the development status and key technologies in the electrical thermal ageing characteristics and diagnostic methods of mineral insulating oil and vegetable insulating oil were reviewed. This paper first summarized the physical and chemical properties of mineral oil and vegetable oil for transformers, and their high temperature performance, biodegradability, acid value, and electric strength were compared. Then the two basic forms of electrical and thermal ageing and their products for insulating oil were reviewed. On this basis, the influence of metal materials such as copper on the ageing of oil was emphasized. Finally, the commonly used ageing diagnostic methods of insulating oil were described. The multi-dimensional fusion characterization of feature parameters from different detection methods is expected to improve the diagnostic accuracy, which has a good engineering application prospect.

Compared with mineral oil transformer, vegetable oil transformer has the characteristics of strong overload capacity, high fire safety, and good environmental performance. At the same time, its noise generated during operation is smaller than that of dry-type transformer, and the loss is lower, the size is smaller, the price is lower. So more and more power grid and industrial users began to use vegetable oil transformer. The current research progress of vegetable oil transformer from the aspects of design, material application and research, production and operation maintenance was summarized in this paper, and the research direction and prospect of vegetable oil transformer were prospected.