The aramid fiber (AF) reinforced composite has a wide application prospect in the field of electrical insulation, but the product performance is restricted by the poor adhesion between AF and resin matrix. In order to improve their interfacial performance, the plasma etching method and plasma grafting method were used to modify the AF surface. The influence of two methods on the interfacial properties of aramid fibers and AF/epoxy composites, as well as the mechanical properties of the AF/epoxy composites NOL ring were studied. The results show that both two methods improve the shear strength of the AF/epoxy interface. The plasma etching method reduces the tensile strength of the NOL rings, while the plasma grafting method increases the tensile strength of the NOL rings. Compared with the unmodified AF, the shear strength of the AF/epoxy interface which treated with 90 W air dielectric barrier discharge plasma followed by 1% 2,4-toluene diisocyanate increases from 31.7 MPa to 36.4 MPa, the tensile strength increases from 23.6 kN to 25.1 kN, and the correction coefficient K value of tensile strength increases from 62.8% to 72.1%, indicating that the mechanical properties of aramid fiber are made full use.

In high-voltage insulated gate bipolar transistor (IGBT) devices, traditional Si-based semiconductor chips are gradually replaced by emerging wide-bandgap semiconductor chips. High-power-density wide-bandgap semiconductors lead to a sharp rise of operating temperature in power electronic modules. Silicone gel is used as an insulating material for the packaging of high-voltage IGBT devices, and its insulating performance faces the challenges of high temperature and thermal ageing. In order to study the effects of high temperature and thermal ageing on the thermal stability and insulating properties of silicone gel, we studied the effects of high temperature and long-term thermal stress on the dielectric properties, volume conductivity, and breakdown strength of silicone gel by high temperature experiments and thermal ageing experiments of silicone gel materials, and analyzed the high temperature characteristics and thermal ageing characteristics of the silicone gel. The results show that with the increase of temperature, the real part of complex permittivity and breakdown strength of silicone gel decreases, and the dielectric loss and DC conductivity increases. With the increase of thermal ageing time, the real part of complex permittivity of silicone gel increases, the dielectric loss decreases at first and then increases, the DC conductivity increases at first and then decreases, and the breakdown strength decreases.

The transformer fault diagnosis technique based on a single detection method is difficult to identify the same type of defects of oil-paper insulation in detail, which cannot meet the requirements of power system on equipment operation reliability under the background of rapid development of deep offshore wind power. Therefore, an oil-paper insulation defect identification method based on information fusion of phase-resolved partial discharge (PRPD) spectrum and dissolved gas analysis (DGA) data was proposed. Six kinds of electrode models were designed and made to simulate the typical defects of surface discharge in transformers with different electric field inhomogeneity coefficients, and PRPD and DGA data were collected. Then convolutional neural network (CNN) and back propagation neural network (BPNN) were adopted to recognize the patterns of PRPD spectrum and DGA feature vector of six kinds of defects, respectively. Finally, the CNN-BPNN information fusion model based on D-S evidence theory was proposed to realize joint diagnosis based on PRPD spectrum and DGA data. The results show that the CNN-BPNN model based on the D-S evidence theory can effectively correct the wrong output of the single criterion model and reduce the uncertainty of the classification results. When the input dimensions of PRPD spectrum are 8×8, 16×16, and 32×32, the recognition accuracy of the model integrated with the DGA feature vector is 93.21%, 97.53%, and 99.17%, respectively, which is 4.81%, 2.78%, and 0.84% higher than that of PRPD single criterion model. The CNN-BPNN model can effectively integrate the electrical physical information and chemical product information of partial discharge, which not only improves the accuracy of defect identification, but also enhances the confidence of the output results, and reduces the data storage requirements, providing accurate, reliable, and lightweight defect identification methods for intelligent operation and maintenance of transformers.

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 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.

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.

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.

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.

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.

In order to develop an on-line monitoring technology for cable insulation status, we analyzed the harmonic components of resistive current by simulating the metal spike in XLPE cable insulation, which provided theoretical and experimental support for cable insulation diagnosis based on the harmonic components of resistive current. Firstly, the conductivity characteristics of XLPE were simulated and theoretically analyzed based on the jump conductivity model of polymer. Then, a simulation model of metal spike defect in XLPE was established, and the harmonic components of resistive current density under different needle curvature radius and needle plate spacing were simulated and analyzed. Finally, three kinds of XLPE metal spike defect samples with different specifications were made, and confirmatory experiments were conducted under the conditions of 30 μm of needle tip curvature radius and 2 mm of needle plate spacing, 90 μm of needle tip curvature radius and 3 mm of needle plate spacing, and 30 μm of needle tip curvature radius and 3 mm of needle plate spacing. The results show that there is metal spike defect in XLPE insulation, due to the sharp increase of local electric field strength, the conductivity characteristics of the insulation material will repeat between the Ohmic and non Ohmic regions with the change of AC voltage waveform, resulting in the distortion of resistive current waveform and the addition of higher-order harmonic components. When there is metal spike defect in XLPE insulation, there are obvious 3rd and 5th harmonic components in the resistive current, which constitutes the main components of resistive current harmonic component. As the severity of metal spike defect increases, the harmonic distortion rate of resistive current and conductivity distortion rate increase significantly, and the harmonic distortion rate can be used as a characteristic quantity to judge the severity of defect.