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.

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.

On the basis of the film-making data of the 300 mm wide polyimide casting test line and the 1 200 mm wide polyimide biaxial stretching production line of Guilin Electrical Research Institute Co., Ltd., the effects of formulation, equipment, process, and other factors on the preparation of high thermal conductivity insulating polyimide films were investigated. The results show that compared with the biaxial stretching method, the casting method has advantages such as low trimming loss, simple equipment and process, excellent film-forming properties, lower cost, and good thermal conductivity of prepared films. In terms of improving the thermal conductivity of thermal conductive films, it is necessary to overcome the problems such as difficulty in peeling the casting film from steel belt, increased difficulty in filler dispersion, poor interfacial matching between filler and resin, and decreased film-forming continuity when boron nitride is highly filled.

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.

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.

As a critical insulating dielectric in high-voltage direct current transmission equipment, the comprehensive performance of polypropylene film has an important impact on the operational reliability and service life of power electronic devices. Therefore, it is urgent to establish a quantifiable and comparable comprehensive evaluation method. In this paper, a comprehensive performance evaluation index system of polypropylene film was constructed, and a comprehensive evaluation method combining combination weighting and extension cloud model was proposed. First, the subjective weights of indicators were determined by using the extension analytic hierarchy process, and the subjective weights were obtained by improved entropy weight method. On this basis, the optimization integration of subjective and objective weights were achieved through game theory. Subsequently, the extension cloud model was introduced to calculate the cloud correlation degree to complete the comprehensive performance grade determination of films, and the credibility of the evaluation results was tested in combination with the confidence factor. Finally, the reliability of the model was verified by objective performance characterization experiments. The results show that this method can achieve the grading of comprehensive properties of polypropylene films, and the evaluation results are in good agreement with the test conclusions regarding insulation, mechanical properties, and self-healing performance. The proposed comprehensive evaluation model is scientific and practical, which can provide an effective theoretical basis for the performance evaluation and material optimization design of polypropylene film.

During the laying process, excessive lateral pressure and traction would cause mechanical damage to cables, leading to severe failures such as sheath grounding, overheating, insulation breakdown, and combustion during operation. This paper summarized the research progress on traction and lateral pressure of land cables, including their generation processes and effects on cable insulation structures and materials, calculation methods, monitoring, and online control techniques of traction and lateral pressure. The influence of mechanical stress on microscopic characteristics such as crystal size, crystallinity, and trap number and depth in cable insulation materials was discussed in detail, and the failure mechanisms of cable materials under mechanical stress were analyzed. Regarding the calculation methods for mechanical stress, the shortcomings of current approaches were summarized, and future research directions for cable mechanical stress were outlined.

High thermal conductive insulating materials are key functional materials for achieving efficient heat dissipation in motors, which is of great significance for improving the operational stability and prolonging the service life of motors. This article briefly described the thermal conductive modification mechanisms of insulating materials,reviewed recent research progress on high thermal conductive insulating varnish, mica tapes, films, and potting compounds used in motors, discussed their current application status in the field of motors, and further provided an outlook on future research direction.

Under the "dual carbon" goals, traditional epoxy resins, which are depend on petrochemical feedstocks and are non-recyclable, have severely hindered the current transformation of power systems. Developing low-carbon and environmental friendly epoxy resins has become a research focus. This study adopted a “rigidity-flexibility” balancing strategy to prepare a bio-based epoxy resin with Schiff base dynamic covalent bonds cured directly by an aldehyde-terminated bio-based epoxy crosslinker and a diamine. Its mechanical properties, insulation performance, and property changes before and after multiple recycling processes were investigated. The results show that the novel epoxy resin achieves a tensile strength of 70.1 MPa and an electric strength of 130 kV/mm. This dynamically cross-linked epoxy resin can be effectively reprocessed and recycled via solvent, retaining a tensile strength of 58.8 MPa and an electric strength of 111 kV/mm after multiple reprocessing cycles.

The C₄F₇N/CO₂/O₂ ternary gas mixture exhibits excellent environmental friendliness and insulation performance, but there is still a lack of research on the stability of C₄F₇N/CO₂/O₂ under trace moisture conditions during long-term operation of equipment. In this study, thermal decomposition experiments of C₄F₇N/CO₂/O₂ were conducted at different trace moisture concentrations. At the same time, reactive molecular dynamics simulations based on the ReaxFF force field were employed to explore the influence of moisture on the thermal decomposition characteristics of C₄F₇N/CO₂/O₂ from a microscopic perspective, and the correlation mechanisms between decomposition products and trace moisture was analyzed. The results show that the main thermal decomposition products of the gas mixture are CF₄, C₃F₈, C₃F₆, C₂F₆, and CHF₃. The increase of trace moisture concentration promotes the decomposition of C₄F₇N, leading to an increase in the concentration of decomposition products. However, when the trace moisture concentration further increases, the reaction pathways for generating the main decomposition products are inhibited. This inhibition leads to a downward trend in the concentrations of the main decomposition products, and promotes the formation of more secondary by-products and complex components, causing a shift in the decomposition pathways of the system.