To investigate the effect of vulcanization temperatures on the interface properties of XLPE return insulation for submarine cable flexible joints, XLPE single layer samples and XLPE-XLPE composite insulation interface simulated samples of submarine cable flexible joints were prepared at different vulcanization temperatures(160, 180, 200℃), the conductivity, space charge, gel content, and mechanical properties of the XLPE single layer samples were tested, and the space charge and mechanical properties of the XLPE-XLPE composite interface samples were tested. The results show that with the vulcanization temperature increases, the crosslinking degree of the XLPE single layer samples first increases and then decreases, the mechanical properties continuously improve, and the internal accumulated space charge first decreases and then increases. When the secondary vulcanization temperature of XLPE-XLPE composite interface sample is 180℃, the interface crosslinking property is the best. Under normal temperature and temperaure gradient conditions, the sample exhibits the lowest accumulated space charge amount at the interface and the best mechanical properties. When the secondary vulcanization temperature exceeds 180℃, XLPE undergoes thermal-oxidative ageing at higher vulcanization temperatures, leading to a decline in the mechanical and insulating properties of the interface.

With its excellent insulating properties, silicone rubber has become the core material for key components of power equipment. In order to further improve the insulating performance of silicone rubber, the SiO₂ was hydroxylated by low-temperature plasma and KH570 grafting, and doped into silicone rubber matrix to prepare SiO₂/silicone rubber nanocomposites. The effect laws of different plasma modification conditions (time and frequency) on the surface morphology, surface flashover characteristics, mechanical characteristics, and surface hydrophobicity of SiO₂/silicone rubber nanocomposites were systematically studied. The results show that appropriate plasma treatment time and frequency (10 min and 19 kHz) can effectively improve the surface hydrophobicity, electrical characteristics, and mechanical properties of silicone rubber. Plasma modification can effectively reduces the agglomeration of SiO₂ nanoparticles in silicone rubber matrix, increase the deep trap density in composites and slow down the surface potential decay rate, thereby increasing the pollution flashover voltage of the composites. In addition, plasma modified SiO₂ can significantly improve the tensile strength and hydrophobicity of SiO₂/silicon rubber nanocomposites.

To investigate the effecting mechanism of shallow traps on surface charge and surface flashover characteristics of epoxy/glass fiber composites under AC/DC superimposed voltage, this paper employed ozone surface treatment to regulate the trap parameters of composites, and systematically studied the change laws of surface conductivity, surface charge distribution, and flashover voltage of the composites with different treatment time. The results show that ozone treatment can significantly reduces the shallow trap energy levels of composites (to as low as 0.85  eV) and increases the shallow trap density (up to 2.02×10²⁰  eV^-1m^-3), thereby enhancing the surface conductivity and effectively suppressing surface charge accumulation near the high-voltage electrode. The surface flashover voltage of composites increase significantly with the extension of processing time, after 4 h of ozone exposure, the flashover voltage of the samples under DC and AC/DC superimposed voltage ratios of 1∶5, 1∶3, and 1∶1 increase by 25.02%, 23.98%, 17.60%, and 10.7%, respectively. Analysis reveals that the new shallow traps introduced by ozone treatment can enhance the flashover performance by reducing surface charge accumulation, inhibiting seed electron emission and electron avalanche processes. Especially when the AC/DC superimposed voltage ratios is no more than 1∶3, ozone treatment has a more significant effect on improving the flashover mechanism dominated by charge accumulation in solid layer.

The thermal-oxidative ageing characteristics of cross-linked polyethylene (XLPE) are the critical factor affecting the operational life of cable insulation, but its internal relationship with XLPE cross-linking reaction conditions are still lacking in research. In this paper, seven types of XLPE samples were prepared by precisely controlling the cross-linking reaction temperature and time. Combined with Fourier transform infrared spectroscopy, tensile test, and gel content measurement, the influence law and mechanism of cross-linking reaction conditions on the thermal-oxidative ageing characteristics of XLPE cable insulation were investigated. The results show that the cross-linking reaction conditions directly affect the residual antioxidant content and the cross-linking structure in XLPE, while the thermal-oxidative ageing resistance of XLPE is dominated by the residual antioxidant content, which is also synergistically influenced by the cross-linking structure.When the cross-linking reaction temperature is too low or the time is too short, the cross-linking structure of XLPE is incomplete; when the cross-linking reaction temperature is too high or the time is too long, the cross-linking structure of XLPE will not be significantly improved, while the residual antioxidant content in XLPE will decreases. Both the two cases will lead to a decrease in thermal-oxidative ageing resistance of XLPE. The XLPE prepared under the cross-linking reaction temperature of 170℃ and reaction time of 16 min exhibits excellent thermal-oxidative ageing resistance, which is due to the high residual antioxidant content of XLPE under the cross-linking reaction condition and the perfect cross-linking structure. The research result provides a theoretical support for optimizing thethermal-oxidative ageing resistance of XLPE cable insulation.

Epoxy resin is widely used in the field of motor insulation. The researchers has conducted considerable research on its macroscopic performance changes under thermal oxidative ageing conditions, but the research on the ageing at molecular structure level ageing is limited. This paper systematically investigated the changes in chromophore groups, free radical content, cross-linking network functional groups, and internal micro-defects of epoxy resin samples under different thermal oxidative ageing time using ultraviolet spectroscopy, electron paramagnetic resonance spectroscopy, infrared spectroscopy, and two-dimensional X-ray scattering. The results show that after thermal oxidative ageing, the samples exhibit distinct absorption peaks at ultraviolet wavelengths of 406 nm (corresponding to p-benzoquinone) and 636 nm (corresponding to dibenzoquinone or naphthoquinone derivatives), and the appearance of the samples turns dark brown. The free radical content increases rapidly with ageing time (reaching a high level at 20 h) then gradually decreases, and maintains a low level after 80 h. The destruction of cross-linking network functional groups follows a specific sequence with the increase of thermal oxidative ageing time, and the destruction rate of functional group slows down after 80 h. There are micro-defects inside the sample, and the defect size gradually increases with the increase of thermal oxidative ageing time.

Trace moisture absorption of insulating oils in high-voltage cable terminations during on-site filling or long-term operation may cause electrical performance deterioration and even insulation breakdown. The differences in molecular structure and polarity characteristics among different types of insulating oils will lead to significant differences in the adsorption behaviors of water molecules and the degradation laws of electrical performances. In this paper, the natural water absorption of silicone oil and polyisobutylene insulating oil under different exposure time was studied. The evolution of moisture content in different temperature and humidity environments was determined by Karl Fischer titration method, and the dielectric loss factor, volume resistivity, and power-frequency breakdown voltage of the insulating oil after water absorption were tested. Through molecular dynamics and quantum chemical calculations combined with Gaussian software, the differences in the moisture absorption mechanisms of the two types insulating oils were revealed from the perspectives of microscopic dipole moment, adsorption energy, and Gibbs free energy. The results show that due to the existence of Si-O bonds and polar groups in molecular chain of silicone oil, the dipole moment is 0.69 Debye. The initial absorption rate and total moisture amount are relatively high, and its moisture absorption process conforms to the exponential growth trend, the saturated moisture content is approximately 52×10^-6. The polyisobutylene has a typical non-polar C-H chain structure with a low dipole moment of 0.02 Debye, and the its moisture absorption process shows a linear upward trend. Moisture absorption will lead to a significantly increases in dielectric loss factor, decreases in volume resistivity and electric strength for the two types of insulating oils. Silicone oil fails to meet the national standard requirement for insulation strength after 2 h of exposure, while polyisobutylene maintains good insulation strength within 5 h of exposure.

In recent years, the continuous increase of voltage levels and operating frequency in the aviation power systems have made aviation cables more prone to arc discharges under low-pressure environments. To systematically evaluate the partial discharge inception voltage (PDIV) characteristics of aviation cables under different voltage parameters and low air pressures conditions, taking the sinusoidal voltages ranging from 50-800 Hz and repetitive square wave pulse voltage as excitation sources, the PDIVs of four kinds of cables discharge models in the air pressure of 20-101 kPa was tested in this paper. The results show that the PDIV under square wave voltage is lower than that under the sinusoidal voltage, indicating that the square wave voltage is more likely to trigger partial discharge, which may be attributed to the sharp increase of the electric field intensity due to its steep rising edge, and the harmonic superposition effect aggravates the electron impact ionization process. Under low pressure, the decrease in molecular density and the increase in electron free path enhance the ionization, resulting in a downward trend of PDIV with the reduction of air pressure and a significant increase in the risk of insulation breakdown. However, the frequency has a relatively small impact on PDIV, because the accumulation and dissipation time of air gap charge is much shorter than the power frequency period, and the discharge is mainly determined by the electric field intensity. Among the four kinds of cable discharge models, the PDIV of the twisted-pair cable discharge model is always the highest. This is due to its double insulation, which significantly increases the equivalent thickness and weakens the air gap electric field, thus exhibiting stronger insulation endurance under both sinusoidal and pulse conditions.

In this paper, the influence laws of radial and axial dimensions of air gap on the internal electric field of motor insulation were studied by constructing the insulation structure and its internal air gap model. The quantitative relationship between the change of air gap size and electric field were revealed, and the mathematical model was established based on the air gap size parameters. At the same time, to simulate the air gap generated in actual operation, the cold-hot shock cycling test was conducted on models. By comparing the changes of partial discharge inception voltage (PDIV), dielectric loss factor, and electrothermal ageing life of the model before and after the test, the effect of the air gap on long-term reliability of the insulation structure was comprehensively evaluated. The results show that when the axial dimension of air gap is constant, the gap electric field decreases exponentially with the increase of the radial dimension of air gap. When the radial dimension of the air gap is constant, the air gap electric field increases exponentially with the increase of the axial dimension of air gap. As the cycle period of cold-hot shock increases, the overall PDIV of the model shows a downward trend, while the dielectric loss factor shows an upward trend. The electrothermal ageing life of the samples before and after cold-hot shock cycleing test are quite different, the ageing life of the sample after 10 cycles of cold-hot shock is only about 1/30 of that of the control sample.

In this paper, a very fast transient overvoltage (VFTO) discharge simulation experiment platform was established. The effect of VFTO on power equipment was simulated by multiple lightning shock waves, and the effect of discharge amplitude on the electrical insulation performance of ethylene propylene diene monomer (EPDM) was studied. The results show that after 200 times of pulsed discharges, carboxylate salt groups are formed on the EPDM surface, and the dielectric constant and dielectric loss factor increase, while the AC electric strength and surface water contact angle decrease significantly. The degradation degree of above performance increases with the increase of discharge electric field strength. When the field strength reaches 40 kV/mm, the AC electric strength of the EPDM sample decreases from 42.28 kV/mm to 37.16 kV/mm after 200 times of pulse discharges, with a decrease of 12.11%, and the water contact angle decreases from 100.53° to 88.63°.

Silicone rubber (SIR) is widely used in outdoor power equipment due to its excellent electrical insulation and weather resistance. However, during long-term operation, silicone rubber inevitably suffers from the impacts of external natural environments. In this paper, the surface ageing characteristics of SIR under long-term out door environments and its influence mechanism on surface flashover performance were systematically studied by three typical environmental simulation tests of thermal ageing, ultraviolet ageing, and salt spray ageing. Combined with scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and flashover voltage tests, the microscopic morphology evolution, chemical group changes, and flashover performance change laws of silicone rubber under different ageing conditions were comprehensively analyzed. The results show that in the early stage of thermal ageing (0-500 h), the high temperature induces re-crosslinking reaction, which reduces the number of surface defects of silicone rubber. In the later stage of thermal ageing (1 000 h), the molecular chains break, and the surface resistivity of the sample drops to 78% of the initial value, and the flashover voltage decreases by 8%. Ultraviolet ageing destroys the Si-O and Si-C bonds in silicone rubber due to high-energy rays, resulting in the precipitation of particles on the surface and the formation of deep traps. After 1 000 h of ultraviolet ageing, the flashover voltage of the sample decreases by 10.3%. Due to the combined effect of Cl^- corrosion and solution penetration in salt spray ageing, significant bulges and corrosion marks appear on the surface of the sample, and the relative dielectric constant of the samples increases by 23%, while the flashover voltage decreases by 31%. Though the quantitative analysis of the entropy weight method, the weights percentages of the effects of salt spray, ultraviolet, and thermal ageing on flashover voltage are 37.1%, 35.7%, and 27.2%, respectively. This suggests that cable accessories in coastal areas with high salt spray and high ultraviolet radiation are more prone to causing insulation failure due to environmental ageing.