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.

Dodecylbenzene (DDB) is a kind of weak polar dielectric material widely used in self-contained high voltage oil filling cable and its accessories. A domestic dodecylbenzene insulating oil was selected as research object, and its physical and chemical properties, and electrical properties were tested. A test system for simulating different types of electrical faults in oil was established, and the gas production characteristics of the dodecylbenzene under two electrical faults of power frequency breakdown in oil and flashover along the oil-paper surface were studied. The results show that the dodecylbenzene insulating oil has excellent physical and chemical properties, and electrical properties. H₂ and C₂H₂ are the characteristic gases under two electrical faults. According to the dissolved gases analysis (DGA) technology of mineral oil, the three ratio method of IEC is suitable for the fault type diagnosis of dodecylbenzene insulating oil, while the Dewey triangle method is not suitable.

Taking urea and hexagonal boron nitride as raw materials, we prepared an amino modified boron nitride nanosheets (BNNS) by ball milling method, and then prepared a BNNS/epoxy composite by mixing the BNNS before and after modification with epoxy resin. The effect of amino modified BNNS on the surface insulating properties of epoxy resin was studied. The results show that the amino group is grafted on the surface of BNNS successfully by ball milling method, which improves the dispersion of fillers in epoxy composites. When the mass fraction of the modified BNNS is 0.5%, the flashover voltage of the BNNS/epoxy composite increases by 26.9% compared with the pure epoxy resin. In addition, the amino modification reduces the trap energy level on material surface and accelerates the space charge dissipation rate. The dielectric constant and dielectric loss factor of the composites filled with amino modified BNNS increase slightly. The flashover voltage of the composites can be improved by balancing the effects of space charge dissipation and polarization relaxation.

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 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.

The research progresses of different polymers (polyimide, perfluorocyclobutyl-based polymer, benzoxazinyl polymer, benzocyclobutene resin, etc.) with low dielectric constant and low dielectric loss factor in recent five years were reviewed. The influence of different design strategies of polymer structures (chemical composition, molecular structure, pore structure, etc.) on their properties (dielectric properties, water absorption, thermal stability, mechanical properties, etc.) was discussed mainly, and the influencing mechanisms were analyzed briefly. At last, the four common methods (reducing molecular polarity, increasing the free volume of molecular, introducing porous structure, and introducing cross-linking structures) for the design of different low dielectric polymers were summarized, and the development direction of low dielectric polymers in future was prospected.

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.

The dielectric constant (D_k) and dielectric loss factor (D_f) of polyimide under high frequency can be reduced by changing the polarity of polymer and introducing pore structure in summary. In this paper, the research progress of the two modification methods at home and abroad in recent years was summarized. It is indicated that the properties of polyimide can be optimized by adding pore structure on the basis of introducing polar macromolecules. At the same time, the shortcomings of properties characterization in literature were pointed out, and the suggestions were proposed.

Three kinds of solvent-free vacuum pressure impregnated (VPI) insulating varnishes for wind turbine generator were introduced, and their chemical composition and properties were compared with those of the current solvent-containing insulating varnishes. The results show that the solvent-free insulating varnishes have the advantages of low volatilization, environmental protection, and good process applicability, which can meet the technical requirements of current insulating varnish.