Multilayer ternary composite polypropylene (PP) films with different content of nano-ZrO₂ particles were prepared with a ternary composite PP film constructed by maleic anhydride grafted PP and nano-ZrO₂ particles as the outer layer and neat PP film as the middle layer, and their physical and chemical properties as well as electrical properties were measured. The results show that the nano-ZrO₂ exhibit good dispersibility in ternary composite PP system and there is a distinct interface between ternary composite PP film and neat PP film. The crystallinity of multilayer ternary composite PP films is higher than that of the single layer neat PP film. With the increase of nano-ZrO₂ content, the dielectric constant of the multilayer ternary composite PP film increases, while the DC electric strength increases at first and then decreases. When the mass fraction of nano-ZrO₂ reaches 1.0%, both the DC electric strength and energy storage density achieve the highest value (424.3 kV/mm and 2.14 J/cm³), which are about 10.6% and 31.3% higher than those of the single layer neat PP film. Besides, the dielectric loss factor of multilayer ternary composite PP film remains at the same level of 10^-3 as that of neat PP film. The ternary composite PP and interlayer interface can jointly suppress the migration of heteropolar space charges, which contributes to improve the internal electric field distribution of dielectric, increase the electric strength, and reduce the dielectric loss caused by electron migration.

In this paper, a test platform for partial discharge characteristics and flashover characteristics of insulation molding parts for power transformer was built, and the distribution laws of partial discharge initial field strength, partial discharge initial discharge quantity, partial discharge extinguishing field strength, and surface flashover field strength of different insulation molding parts were obtained. The results show that the partial discharge initial field strength of insulation molding parts decreases with the increase of electrode spacing. When the electrode spacing increases from 10 mm to 25 mm under parallel components, the partial discharge initial field strength decreases by about 40.6%. The partial discharge initial discharge quantity under different components is 8.51-13.70 pC. Under the parallel component, the partial discharge initial field strength of insulating strip and insulating spacer is 2.55-2.85 kV/mm and 1.43-1.78 kV/mm, respectively. Under the vertical component, the partial discharge initial field strength of insulating strip and insulating spacer is 2.65-3.35 kV/mm and 4.31-5.49 kV/mm, respectively. The surface morphology of different insulation molding parts has a significant impact on their surface flashover characteristics. The denser and larger the surface embossed grooves, the better the effect of hindering the development of surface discharge. On the basis of test sample database, a comprehensive performance evaluation method for insulation molding parts based on entropy weight method is proposed. The weight of different performance indicators, which is determined by using four indicators of discharge characteristics of insulation molding parts, as well as the dispersion degree of apparent density and size deviation, is used to evaluate the comprehensive performance of different insulation parts, and the rationality of this evaluation method is verified.

Epoxy composite insulating materials doped with 0%, 1%, and 10% of modified nano Al₂O₃ were prepared and conducted thermal ageing tests at 150℃, and their thermal ageing characteristics and mechanism were studied through dynamic thermal mechanical testing, scanning electron microscopy testing, and linear thermal expansion testing. The results show that an inhomogeneous ageing structure is formed in epoxy composite insulating material during thermal ageing, and the thickness of surface ageing layer is related to the content of nano Al₂O₃. When the doping mass fraction of nano Al₂O₃ is 1%, the thickness of surface ageing layer is the smallest, and the nanoparticles are well dispersed and closely embedded in the epoxy matrix. According to the free volume theory of glass transition, it is found that the free volume fraction of sample is the smallest when the doping mass fraction is 1%, the corresponding α main relaxation time of the epoxy matrix is the longest, and the molecular chain activity is the lowest. Adding an appropriate amount of modified nano Al₂O₃ into the epoxy resin can improve the cross-linking degree of molecular chain and reduce the free volume of system. Thereby the diffusion of oxygen in the material is weakened, and finally the inhomogeneous thermal ageing degree of the epoxy composite insulating material is reduced.

The high power density development and application demand in aerospace field of silicone carbide (SiC) power modules have proposed higher requirements for the high and low temperature resistance of organic silicone encapsulation materials used in modules packaging. In this paper, the temperature resistance of three silicone encapsulants (SEMICOSIL 915HT, Duraseal 1533, R-2188) was compared. The results show that the reaction in the methyl oxidation stage during the thermal degradation process is the same as the early reaction in the thermal oxygen ageing process of silicone encapsulants. The R-2188 encapsulant has 249.5 kJ/mol of highest activation energy for methyl oxidation reaction, 335℃ of initial thermal decomposition temperature, and 5% of lower mass loss during the methyl oxidation stage, which is more suitable for the packaging of SiC power module under high-temperature operating conditions. The SEMICOSIL 915HT encapsulant has -81.1℃ of initial crystallization temperature and 0.006 4 min^-1 of crystallization rate, which is more suitable for the packaging of SiC power module under low-temperature operating conditions.

The natural ester insulating oil transformer has advantages such as green environmental protection and strong overload capacity, which is an important development direction for the high-voltage and large capacity power transformer. The compatibility between insulating oil and insulating paint has a significant impact on the transformer design and operation maintenance. In this paper, the vegetable insulating oil and insulating paint materials were conducted thermal ageing experiments, and the compatibility between insulating oil and six commonly used insulating paint materials for power transformers was studied. The characteristic changes in the kinematic viscosity, moisture content, acid value, dielectric loss, breakdown voltage, and gas chromatography of natural ester insulating oil before and after thermal ageing were observed and analyzed. The results show that the structural integrity and functional stability of the insulating paint sheet remain good. Except for the kinematic viscosity, the physicochemical and electrical properties of the natural ester insulating oil have changed after ageing, but there is no significant deterioration. The dissolved gas content increases significantly after thermal ageing, and the gas chromatography results show good compatibility between insulating oil and insulating paint.

In this paper, the heterogeneous non-naphthenic transformer oil based on natural gas-to-liquid technology and traditional naphthenic transformer oil were used as research object. The streamer branching patterns in oil samples in the 25 mm point-to-plate electrode system was observed under positive lightning impulse voltage. The streamer discharge images were conducted multifractal analysis to obtain the multifractal parameters as the classification and evaluation index of streamer branching morphology. The results show that when the applied voltage is lower than the accelerated voltage, the streamer branching process in the two oil samples can be classified as a two-stage model dominated by the steady growth of side branch and main branch. When the applied voltage is higher than the accelerated voltage, the streamer branching process in naphthenic oil is a three-stage model, each stage dominated by steady growth of side branch, steady growth of main branch, and rapid growth of main branch, respectively, and the streamer branching process in gas-to-liquid transformer oil only includes two stages, which are steady growth stage of main branch and rapid growth stage of main branch. Further theoretical analysis shows that the competition between space charge and primary electric field is distorted due to the higher ionization energy of gas-to-liquid transformer oil components, which is the main reason for the abrupt change of streamer branching model in gas-to-liquid transformer oil under accelerating voltage.

Silicone rubber composite insulators would produce powders on their surface after long-term operation in high humidity environment. In order to study the effect of water ingress into on-site aged silicone rubber on its properties, we used the composite insulator sheds which have operated over ten years in the southern coastal areas as research object. Deionized water immersion tests were conducted on the silicone rubber samples which have been removed their chalking layers to study the effect of water ingress into samples on their mass and physicochemical properties, and the influence of water ingress on the secondary chalking phenomenon of the silicone rubber was discussed. The results show that the silicone rubber samples which have removed their chalking layers could generate new chalking layers after deionized water immersion, whose microstructures and physicochemical properties were close to the naturally formed chalking layers. It is determined that water is the main influence factor which aged silicone rubbers produce powders on their surface again by changing the water temperature of tests, and high temperature can accelerate the generation of chalking layers.

The cable joint interface is easy to discharge along the surface, which would lead to insulation failure, making it become the weakest link in the power cable system. In this paper, the effects of different interface roughness, pressure, and temperature on the AC breakdown voltage of the silicone rubber/polyethylene double-layer dielectric interface were investigated, and the interface discharge channels after breakdown were analyzed. The results show that the interface breakdown voltage at room temperature is related to the interface roughness. The higher the interface smooth degree, the higher the interface breakdown voltage, and the smaller the interface carbonization area. The bigger the interface pressure, the higher the interface breakdown voltage, while the interface carbonization area increases at first and then decreases. With the increase of temperature, the interface breakdown voltage decreases, but the breakdown voltage at higher temperature does not decrease significantly. In addition, the interface carbonization area increases at first and then decreases with the increase of temperature, and the change of interface contact state at different temperatures is one of the main reasons affecting its properties. The interface state of cable accessories has a significant impact on their interface breakdown characteristics, which should be paid attention to during cable operation and maintenance.

The ethylene propylene rubber for wind turbine twisting cable insulation needs to have high electrical properties and mechanical strength simultaneously to meet the demanding operating conditions. Different morphologies of zinc oxide nanoparticles doped ethylene propylene rubber samples were prepared to investigate the influence of zinc oxide morphology on the mechanical, electrical, and thermal conductive properties of ethylene propylene rubber. The results show that the larger the specific surface area of zinc oxide nanoparticles, the bigger the crosslinking density of ethylene propylene rubber, while the crystallinity remaines unchanged basically. In terms of mechanical properties, the excessive crosslinking caused by high specific surface area of zinc oxide would slightly decrease the elongation at break and tensile strength of ethylene propylene rubber. In terms of electrical properties, the specific surface area of zinc oxide has little effect on the resistivity, dielectric loss, and breakdown strength at room temperature, but the dielectric constant and breakdown strength at 90℃ increase slightly with the increase of specific surface area of zinc oxide. There is no apparent variation rule in the regulation of thermal conductivity of ethylene propylene rubber by zinc oxide nanoparticles.

With the rapid development of offshore wind power, high-voltage XLPE submarine cables are palying critical role in transmitting electricity in the marine transmission system. In order to investigate the effect of thermal oxygen ageing on the frequency domain dielectric properties of high-voltage XLPE submarine cables, XLPE samples were conducted accelerated ageing experiments at 135℃, and the frequency domain dielectric properties of XLPE samples with different ageing degree were tested. When analyzing the dielectric properties of measured samples, we found that a large amount of conductance loss and polarization loss were superimposed in the low and medium frequency domains, which seriously affected the accuracy of applying frequency domain dielectric properties to assess the ageing state of high-voltage XLPE submarine cable insulation. To slove this problem, a method for separating conductance loss and polarization loss in dielectric response data was deduced. Finally, the effect of conductance loss on the accuracy of model parameter identification was explored, and a characteristic parameter to characterize the ageing degree was proposed. The results show that the frequency domain dielectric curves of high-voltage XLPE submarine cables shift towards high frequency as the ageing progresses, and the low-frequency dispersion phenomenon becomes more apparent. The accuracy of H-N (Havriliak-Negami) model parameter identification can be improved by removing the conductance loss in the imaginary part of complex dielectric constant. The relaxation strength ∆ε and high-frequency dielectric constant ε_∞ could characterize the insulation ageing state of high-voltage XLPE submarine cables, which can realize the assessment of insulation ageing for high-voltage XLPE submarine cables.