The effects of repetitive pulse voltage on the surface potential of the inverter-fed motor rotor bar insulation with different length of extension part and fixed metal plate were discussed under different rise times. Then the changing mechanism of the insulation surface potential was discussed combined with charge relaxation theory and finite element simulation. The results show that the surface potential peak-to-peak value of bar decreases with the increase of repetitive pulse rise time. When the pulse rise time reaches 800 ns, the potential peak-to-peak value at the end of the conductive armor tape is close to twice of the applied voltage peak-to-peak value. Under working conditions, when the insulation surface temperature exceeds 150℃, white smoke will be generated and accompanied by partial discharge phenomenon. It is analyzed that the jump voltage with short rise time is an important parameter affecting the electrical and thermal properties of bar. Finally, according to the experimental and simulation results, some reasonable improvement suggestions are proposed for the insulation design of inverter-fed motor bars.

In order to explore the effect of very fast transient overvoltage (VFTO) on the breakdown properties of epoxy resin, epoxy resin samples were prepared and conducted 1 000 times of VFTO ageing tests at 20 kV and 40 kV, respectively. The effects of VFTO ageing on the chemical structure, micromorphology, dielectric parameters, and breakdown properties of the epoxy resins were studied by infrared spectroscopy, scanning electron microscopy, dielectric spectroscopy, and breakdown experiment. The results show that after VFTO ageing, the contents of C-O-C bond and C=O bond in epoxy resin decrease, the internal defect increases, the relaxation behavior enhances, and the electric strength decreases. After VFTO ageing at 20 kV and 40 kV, the electric strength of epoxy resin decreases by 14.89% and 25.67%, respectively. Through analysis, it is found that after the VFTO ageing, the internal macromolecular chains of the epoxy resin were broken, leading to an increase of defects, which is the main reason for the decrease of its electric strength.

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.

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 power frequency breakdown voltages of a new eco-friendly insulation CF₃SO₂F/N₂ and CF₃SO₂F/CO₂ mixtures were measured under sphere-sphere and needle-plate electrodes. The effects of pressure, electrode spacing, and electric field non-uniformity on the power frequency breakdown characteristics of the CF₃SO₂F mixtures were analyzed and compared with SF₆. The results show that under slightly non-uniform electric field, the power frequency breakdown voltage of CF₃SO₂F mixtures is linearly related to pressure, and the breakdown voltage show weak saturation trend with the increase of electrode spacing. Under extremely non-uniform electric field, the power frequency breakdown voltages of CF₃SO₂Fmixtures exhibits "hump" curves of "rising-falling-rising" as the pressure increases, and the corresponding pressure of "hump" peak is between 0.2 MPa and 0.35 MPa. The overall power frequency breakdown strength of the CF₃SO₂F/N₂ mixture is greater than that of the CF₃SO₂F/CO₂ mixture. At 0.3 MPa and above, CF₃SO₂F/N₂ mixtures can maintain a highly insulation level relative to SF₆ under extremely non-uniform electric fields, which has good application potential.

The flexible low-frequency transmission system is superior in enhancing transmission capacity, reducing line charging reactive power, and improving voltage quality at the end of transmission channels, which can effectively meet the urgent demand for efficient aggregation and transmission of medium- and long-distance offshore wind power. In order to study the operating characteristics of submarine cables under low frequency condition, an electric-magnetic-thermal-flow coupled finite element simulation model of 220 kV cross-linked polyethylene was built considering the influence of external laying environment, and the steady-state ampacity and temperature field distribution of submarine cables operating at 50 Hz and 20 Hz in different laying sections were analyzed. The finite element simulation was verified by building a steady-state thermal path model of submarine cable on the basis of IEC 60287:1995 and previous simulation results of thermal field distributions in submarine cable under low frequency. The results show that under different laying environments of land section, sea section, and submarine section, the relative errors between the ampacity and temperature field distribution calculation results of simulation model and the analytical equation of IEC are within 3%, indicating that the temperature field simulation model of 220 kV cross-linked cable proposed in this paper has good accuracy and high efficiency. Meanwhile, it is found that the frequency reduction can reduce the AC resistance of cable conductor, improve the current distribution in the cable conductor, and reduce the operating loss of each part of cable, thereby the overall operating temperature of cable is reduced eventually and the cable transmission capacity is improved favorably.

In the long-term operation process, due to the ageing of transformer oil for offshore wind power, the moisture and other impurities will be produced, which may cause transformer insulation failure, resulting in economic losses and safety accidents. Thus, it is urgent to propose an effective method of transformer oil filteration and judgment to improve the transformer oil performance and evaluate its ageing status. In this paper, the oil filter film was conducted superhydrophobic modification, and the effects of the number of oil filtration, the type of film, and the superhydrophobic modification on the properties of oil before and after filtration were investigated. The health classification model of transformer oil was established by support vector machine algorithm. In addition, a new method based on machine learning was proposed to evaluate the oil filtering effect of superhydrophobic film. The results show that the oil filtering performance of the film after superhydrophobic treatment is improved greatly. Especially after three times of modified organic film filtration, the comprehensive performance of transformer oil has significantly improved, meeting the applicable standard of transformer oil. Compared with several algorithms, the model built by support vector algorithm has the highest accuracy of 84.8%.

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 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.