To address the insulation failure of epoxy resin (EP) used in stator windings of large generator caused by long-term coupling degradation effect of electrical and thermal stress, it is urgent to develop epoxy resin composite dielectrics with both excellent thermal conductivity and mechanical strength. In this study, SiCn@SiCp/EP composite dielectrics with varying SiCn and SiCp contents were prepared by using epoxy resin as the matrix, silicon carbide particles (SiCp) as the nonlinear conductivity base elements, and high aspect-ratio silicon carbide nanowires (SiCn) as the network skeleton. The results show that the multi-scale three-dimensional functional network formed by the mutual matching bridging of SiCn and SiCp provides transmission channels for external stress and internal phonons. The tensile strength, toughness, and thermal conductivity of composite dielectrics reach 64.8 MPa, 1.86 MJ/m³, and 0.326 W/(m·K), respectively. Meanwhile,affected by the percolation network, the composite dielectrics exhibit superior nonlinear conductive characteristics at relatively low SiC content, with a nonlinearity coefficient as high as 7.79 and a threshold field strength of only 2.12 kV/mm. The mutual matching of different morphologies of the same material avoids the complex issues arising from differences in interfacial interactions among multiple heterogeneous fillers in previous studies.

Four types of organic soluble polyimide (PI) resins with semi-cyclic structures were prepared by one-step high-temperature solution polycondensation using alicyclic dianhydride 1S,2R,4S,5R-hydrogenated pyromellitic dianhydride (ccHPMDA) and 1R,2S,4S,5R-hydrogenated pyromellitic dianhydride (ctHPMDA) with two aromatic diamines 9,9-bis[(4-amino-3-methyl)benzene]fluorene (MFDA) and 2,2′-dimethylbenzidine (DMBZ), respectively. Four types of PI films were prepared by dissolving the above four resins in N,N-dimethylacetamide and baking at high temperature. The thermal, optical, and mechanical properties of the PI films were tested. The results show that functional groups including six-membered cyclohexane ring, bulky fluorene substituents, ortho methyl substituent, and rigid biphenyl units endow these PI films with excellent thermal stability. The glass transition temperature of the four films are all higher than 400℃ and the 5% thermal weight loss temperatures are higher than 500℃ in nitrogen. Meanwhile, the four PI films show good optical transparency, with transmittance at 450 nm all surpassing 84.8%, yellow-blue index ranging from 1.09 to 2.15, and haze value ranging from 0.16% to 1.21%. In addition, the DMBZ based PI films exhibit superior mechanical properties, with tensile strength and tensile modulus exceeding 118 MPa and 3.8 GPa, respectively.

To address the problems of poor solution processability, high dielectric constant, and low adhesion of thermoplastic polyimide (TPI) used for adhesive-free flexible copper clad laminates (FCCL), a soluble easter-containing TPI (PI-10) was synthesized via a two-step method involving solution copolycondensation and chemical imidization. Then PI-10 was chemically modified with dicyclopentadiene epoxy resin (HP7200) and fabricated into FCCL. Firstly, the chemical structure of PI-10 was confirmed by FTIR and ¹H-NMR, and then its organic solubility and molecular weight were tested. Subsequently, the curing kinetics behavior and process of HP7200 modified PI-10 (PI-10/HP7200) were investigated by non-isothermal DSC analysis. Finally, the properties of PI-10, PI-10/HP7200, and the prepared FCCL were studied. The results show that PI-10 has good organic solubility and high molecular weight. Compared with PI-10, the tensile strength of PI-10/HP7200 film increases from (97.37±1.68) MPa to (102.25±4.77) MPa, and the dielectric constant (D_k) at 10 GHz decreases from 3.10 to 2.83. However, its glass transition temperature (T_g) decreases from 213.5℃ to 204.2℃, the 5% weight loss temperature (T_d5) decreases from 496.9℃ to 451.8℃, and the coefficient of thermal expansion (CTE) within 50-150℃ increases from 66.20×10^-6℃^-1 to 73.15×10^-6℃^-1, water absorption increases from 2.59% to 3.57%, and the dielectric loss factor (D_f) at 10 GHz increases from 0.009 5 to 0.015 8. In addition, PI-10/HP7200 significantly enhances the 90° peel strength of the prepared FCCL, achieving a maximum increase of about 77% compared to PI-10 without sacrificing other properties, reaching 2.12 N/mm.

In this paper, binary copolymerized polyimide precursors polyamic acid PAA-A and PAA-B were first synthesized using pyromellitic dianhydride (PMDA) with 4,4′-diaminodiphenyl ether (ODA) and p-phenylenediamine (PPDA), respectively. The corresponding polyimides PI-A and PI-B were then obtained via thermal amide cyclization. Subsequently, block copolymerized polyamide acid PAA-AB was obtained through the reaction of PAA-A and PAA-B, and block copolymerized polyimide PI-AB was also obtained through thermal amide cyclization. Finally, the chemical structure, aggregation structure, and cross-sectional morphology of the three PI were characterized, and their mechanical properties, dielectric properties, electric strength, and electronic structure were tested and analyzed. The results show that the introduction of the third monomer PPDA and ODA does not induce the formation of defects in PI-AB. Furthermore, due to the alteration in chemical structure, the molecular chain spacing of PI-AB slightly increases compared to that of PI-A, which results in a decrease in its glass transition temperature. The electric strength of PI-AB is 6.18% higher than that of PI-A. The dielectric loss factor of PI-AB remains below 0.07 in the frequency range. Moreover, the band gap of PI-AB is wider than that of PI-A, making electron excitation more difficult, which demonstrates that PI-AB has superior insulating properties.

To enhance the energy storage performance of PVDF-based polymer dielectric films, layered structured polymer nanocomposites were prepared by solution casting method. The middle layer utilized poly(methyl methacrylate) (PMMA) with a volume fraction of 30% to suppress energy loss, while the outer layers employed poly(vinylidene fluoride-hexafluoropropylene) (P(VDF-HFP)) to provide high dielectric constant. Inorganic BaTiO₃ (BT) nano-ceramic particles were uniformly dispersed as fillers in the outer layers. By adjusting the filler content, its effects on the dielectric constant, electric strength, and energy density of the composites were investigated, along with the durability and stability of the films’ energy storage performance under different cycle numbers and in different regions. The results show that the addition of a low content of BT increases the dielectric constant, suppresses the dielectric loss, and reduces the leakage current density of the films. The composite with 5% mass fraction of nano-fillers achieves a discharge energy density as high as 15.5 J/cm³ at an electric field of 425 kV/mm, which is higher than the energy storage densities of most reported PVDF and its copolymer-based composites. The variations in discharge energy density after 10 000 cycles and across eight different regions do not exceed 19% and 7%, respectively.

To address the problem of poor interfacial compatibility between polyimide (PI) and poly(vinylidene fluoride) (PVDF) during blending, a fluorinated copolyimide (FPI) was synthesized by introducing -CF₃ groups into the PI main chain via copolymerization, and then FPI/PVDF all-organic composite films were prepared by blending FPI with PVDF. The chemical structure, micromorphology, thermal stability, and mechanical properties of the composite films were characterized, with a particular focus on their dielectric and energy storage performance at high temperature. The results show that after introducing -CF₃ groups into the PI main chain, the interfacial gap between the FPI phase and PVDF phase is reduced to below 100 nm, and the conduction loss of the composite films is significantly suppressed at high temperature. The improved compatibility between PI and PVDF remarkably reduces the dielectric loss factor of the composite films at high temperature and enhances the frequency stability and temperature stability of the dielectric constant. When the mass fraction of PVDF is 5% and the molar fraction of 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) in FPI is 30%, the FPI/PVDF composite films achieve an electric strength of 515.30 kV/mm and a maximum energy storage density of 2.96 J/cm³ at 150℃, which is 85.0% higher than that of the PI/PVDF composite film with the same PVDF content but without fluorinated interfacial modification. Furthermore, the introduction of 6FDA and PVDF contributes to an increase in both the trap density and the deep trap energy level of the composite, thereby enhancing the high-temperature energy storage performance of the composite films.

In this paper, power frequency breakdown tests were conducted on mixed insulating oil containing mineral oil and ethyl laurate. The insulation breakdown characteristics of mineral oil, lauric acid modified ester, and the mixed insulating oil were compared and analyzed. An insulating oil model containing H₂O and H₃O⁺ was established, and the microscopic influence mechanism of ethyl laurate on the power frequency breakdown characteristics of the mixed insulating oil was elucidated through molecular dynamics simulations. The results show that under a 2 mm oil gap, the breakdown voltages of ethyl laurate and the mixed insulating oil are 29.8% and 20.8% higher than that of mineral oil, respectively. The failure field strength distributions of mineral oil, ethyl laurate, and the mixed insulating oil range from 5.62 kV/mm to 12.9 kV/mm, with both ethyl laurate and the mixed insulating oil exhibiting higher failure field strengths than mineral oil. Compared with mineral oil molecules, ethyl laurate forms more hydrogen bonds with H₂O and H₃O⁺, exhibits stronger interaction energy, and has a smaller free volume fraction. In summary, the diffusion motion of impurities such as H₂O and H₃O⁺ in ethyl laurate and the mixed insulating oil is inhibited, making it difficult to form conductive "small bridges",and thus the insulation performance of ethyl laurate and the mixed insulating oil is superior to that of mineral oil.

To investigate the influence of SiC inverter characteristics on the voltage distribution in traction motor windings, this paper first proposed an equivalent circuit model for the high-frequency distributed parameters of motor windings that accounted for the proximity capacitance effect of the conductor. Subsequently, the voltage stress distribution characteristics of the winding under single-phase and three-phase pulse output from the SiC inverter were analyzed, and the correctness of the model analysis results was verified using an experimental test platform. Finally, the effects of the switching frequency, dv/dt, and grounding method of the SiC inverter on the winding voltage stress distribution were analyzed. The results show that under single-phase pulse operation, the maximum voltage stress to ground of the windings occurs on the winding near the inverter of the other two phases. Under three-phase pulse operation, the maximum voltage stress to ground appears on the middle-position winding of the respective phase. The switching frequency does not affect the maximum values of either the voltage stress to ground or the interturn voltage stress of the winding. The dv/dt significantly affects the interturn voltage stress distribution, when dv/dt increases from 2.0 kV/μs to 15.0 kV/μs, the maximum interturn voltage stress increases by about three times. Different grounding methods have a considerable influence on the voltage stress to ground distribution of winding, and the minimum voltage stress to ground is achieved when a 1/2 grounding system is adopted.

The emission spectra of meter-scale long air gap discharge are of great significance for revealing its microscopic physical characteristics and discharge mechanism. In this paper, an experimental platform for a 2 m rod-plane positive polarity long air gap discharge was established. Using a slitless transmission grating and a high-speed camera, the emission spectra of the leader discharge channel in the wavelength range of 400-900 nm were obtained under standard switching impulse voltage. By comparing and analyzing the light intensity distribution and spectral characteristics at six different positions along the main discharge channel, the spectral evolution law before and after air gap breakdown was revealed. Combined with the theory of plasma spectroscopy, the electron temperature at different positions in the discharge channel was further estimated, and its spatiotemporal evolution characteristics were obtained. The results show that the radiation in different wavelength regions of the spectrum corresponds to spectral lines of different ionization states and elements. During the main discharge stage, the ion lines (such as NII) rapidly intensify, while the neutral atomic lines (such as NI and OI) develop more slowly. During the breakdown process, intense radiation simultaneously appears in the visible region (NII lines) and the near-infrared region (NI and OI lines) of the discharge channel, both reaching peak intensities. Based on the Boltzmann plot method, the average electron temperatures of the discharge core channel and the corona sheath layer are estimated using the characteristic spectral line groups of NII and OI, respectively. It is also found that the electron temperature in the discharge channel exhibits a rapid decreasing trend over time.

Thermoplastic polypropylene (PP) cables have been widely used due to their recyclability, simple and efficient manufacturing process, low energy consumption, and good electrical and mechanical properties. However, research on evaluation methods for their insulation ageing state remains relatively scarce. In this paper, three types of PP were subjected to accelerated thermo-oxidative ageing in the laboratory. The changes in carbonyl group content of the aged PP were characterized using infrared spectroscopy (IR) and ultraviolet spectroscopy (UV). Based on the observed change law, the correlation between the carbonyl content and the elongation at break was investigated to evaluate the ageing degree of each PP. The accuracy of the proposed evaluation method was verified using samples aged for 200 days. The results show that the carbonyl index of the three PPs measured by IR exhibits considerable variation with increasing ageing time, and it does not correlate with the elongation at break data, making it difficult to effectively characterize the ageing degree of PP. In contrast, UV spectroscopy reveals that the carbonyl content increases linearly with ageing time during the PP ageing process, based on which a correlation between carbonyl content and elongation at break can be established. Validation using samples aged for 200 days shows that the differences between the actual elongation at break of the three PP samples and the predicted values calculated from the carbonyl content are small. The errors for copolypropylene PPB1 and PPB2 are 5.02% and 6.55%, respectively, while the error for blended polypropylene PPH is slightly larger at 16.54%. It is demonstrated that the proposed method for evaluating the ageing degree of PP using UV spectroscopy possesses high accuracy.

To improve the oil-paper insulation performance in oil-immersed transformers, this paper utilized hexamethyldisilazane coupled with dielectric barrier discharge (DBD) plasma to modify cellulose insulation paper. The physical morphology and chemical composition of the surface and cross-section of the modified insulation paper were characterized, and the influence of the medium flow rate on the insulation paper properties and oil-paper insulation performance was investigated. The results show that at a medium flow rate of 15 mL/min, the oil-paper insulation performance is significantly enhanced. The dielectric constant of the modified insulation paper decreases to 2.5, the flashover voltage and electric strength increase by 33.8% and 25.0%, respectively, the water contact angle increases by 194.3%, and the water absorption decreases by 44.9%. Silicon-containing thin films are deposited on the surface of the insulation paper and penetrate into the interior of the fibers, which can introduce deep traps to restrict charge migration and fill the fiber pores with hydrophobic substances. The introduction of the low-polarity film reduces the dielectric constant of the insulation paper, mitigates the electric field distortion at the oil-paper interface, and improves the interfacial bonding ability of the oil-paper interface, thereby enhancing the oil-paper insulation performance.

To address the problems of low time-frequency resolution and cross-term interference in the identification and localization of subway cable defects using time-frequency domain reflectometry (TFDR), this paper proposed a parameterized resampling time-frequency transform (PRTF) method as a time-frequency analysis approach for TFDR test signals. The PRTF method employed a Gaussian-envelope linear frequency modulation signal as the test signal, combined it with a dynamic resampling operator and an adaptive time-frequency atomic decomposition technique, and further introduced a normalized time-frequency cross-correlation function. By extracting time-delay information from local peaks, accurate defect localization was achieved. Simulation experiments were conducted on a flexible control cable in flame-retardant class B1 for subways, and a defect was preset at a distance of 3 900 m from the head end to verify the effectiveness of the proposed method. The results show that compared with the traditional Wigner-Ville distribution (WVD), short-time Fourier transform (STFT), and continuous wavelet transform (CWT) methods, the proposed method achieves a localization error of only 5.5 m, and its time-frequency cross-correlation function curve is free of interference terms. In contrast, the WVD method suffers from significant interference terms, while the localization errors of the STFT and CWT methods both exceed 16 m. The proposed method significantly improves the detection sensitivity and localization reliability of weak local defects in subway cables.

To improve the accuracy of locating internal partial discharge sources in transformers, this paper proposed an acoustic-electric joint localization method for transformer partial discharge based on the snake optimization (SO) algorithm. A three-dimensional model of the partial discharge source inside a transformer was established, and the localization performance of different methods for typical partial discharge source positions was tested through simulations using ultra-high frequency (UHF) sensors and ultrasonic sensors. The results show that compared with traditional localization algorithms, the SO algorithm exhibits stronger global search capability and faster convergence performance, and can better escape local optimum traps. Using a transformer partial discharge localization experimental platform to simulate the partial discharge process in a real transformer, it is found that the comprehensive localization error of the proposed acoustic-electric joint localization method based on the SO algorithm is consistently below 50 mm, with high localization stability.

The intermediate joint of power cables is a key node in power transmission and distribution system, the interface pressure of the joint directly impacts its insulation performance and current-carrying capacity. However, a technical bottleneck remains in the non-destructive quantitative measurement of interface pressure in multi-layer heterogeneous materials. This paper proposed a non-destructive testing method for interface pressure in 10 kV cold-shrinkable cable accessories based on ultrasonic nonlinear effects. First, samples with different interface pressures were prepared, and reference values were obtained using built-in sensors. Then, a mechanical pressurization device was designed to expand the experimental pressure range. Based on a nonlinear ultrasonic testing platform, time-frequency domain characteristics of the echo signals from three types of composite interfaces of the 10 kV cold-shrinkable cable joint were analyzed. Finally, a quantitative mapping model between interface pressure and the nonlinear ultrasonic coefficient was established and verified. The results show that in the interface pressure range of 0-0.40 MPa, the amplitudes of the fundamental wave and the second harmonic wave generally decrease with increasing interface pressure, while the nonlinear coefficient first increases and then decreases. The use of the pressurization module improves the accuracy of the pressure measurement results. The application of silicone grease on the XLPE interface can fill the microscopic air gaps and weaken the nonlinear effect in that area, but has little influence on the nonlinear behavior of the interface and does not affect the pressure measurement results. It is demonstrated that the relative nonlinear coefficient can serve as an effective characterization index for the interface pressure of cold-shrinkable cable accessories, providing a reliable non-destructive testing technical path for cable operation condition monitoring and life assessment.

Composite insulators in long-term outdoor operation undergo ageing due to the coupled effects of environmental factors. Accurately assessing the performance and ageing status of insulators that have been in service for many years is of great significance for the operation and maintenance of transmission lines. In this paper, a composite insulator that has been in service for 16 years on a 220 kV transmission line in Linyi area was investigated. By conducting multi-scale characterization analyses of the silicone rubber materials of different positions of the composite insulator umbrella sheds, including surface morphology, thermal properties, mechanical properties, electrical properties, and chemical structure, the spatial distribution law of the ageing degree of the silicone rubber material was investigated. The results show that after long-term service, the ageing degree of the composite insulator material exhibits spatial distribution characteristics. In the silicone rubber umbrella sheds of the composite insulator, the ageing degree of the large umbrella sheds is higher than that of the small umbrella sheds, the ageing degree at the high-voltage end is higher than that at the low-voltage end, and the ageing degree on the outer side is higher than that on the inner side. In addition, the differences in heat flow and thermogravimetric loss characteristics of the silicone rubber material at different umbrella shed positions are relatively small, while key parameters such as microstructure, hydrophobicity, chemical groups, electric strength, and creep compliance can effectively characterize the ageing degree of the umbrella sheds and significantly reflect the spatial ageing differences.

To improve the electrical performance of transformer oil under high electric field strength and extreme operating conditions, in this paper, modified transformer oils containing different concentrations of SiO₂ and ZnO nanoparticles were prepared, the terahertz time-domain spectra of different oil samples were measured experimentally, and the differences in their terahertz time and frequency domain spectral characteristics were analyzed. The results show that the absorption coefficients of different oil samples exhibit similar variation trends in the lower frequency range of 0.2-1.0 THz, while absorption is enhanced in the higher frequency range of 1.0-2.0 THz, where the differences in absorption characteristics among the oil samples are significant. Compared with pure transformer oil, the refractive index and permittivity of the nano-modified transformer oils in the terahertz frequency range increase significantly. In the frequency range of 0.2-2.0 THz, as the nanoparticle concentration increases, the average dielectric constant of both types of nano-modified transformer oils first increases and then decreases slightly. Meanwhile, the average dielectric loss factor of the ZnO nano-oil exhibits a fluctuating downward trend, whereas that of the SiO₂ nano-oil first decreases and then increases.

Aiming at the thermal ageing problem of composite insulator silicone rubber sheds, relevant research using nonlinear ultrasonic detection technology was conducted in this paper. First, accelerated thermal ageing tests were conducted on the silicone rubber sheds of composite insulators, and the material parameters of samples with different thermal ageing time were measured. Then, a simulation model for nonlinear ultrasonic detection of composite insulator silicone rubber was established, and a nonlinear ultrasonic detection platform was built to carry out ultrasonic detection experiments. Finally, the relationship between the thermal ageing time of the composite insulator silicone rubber material and the ultrasonic nonlinear coefficient obtained from both simulations and experiments was verified. The results show that as thermal ageing time increases, both the fundamental and second harmonic amplitudes of the thermally aged samples exhibit a decreasing trend, while the relative nonlinear coefficient increases significantly. The linear fitting degree between the relative nonlinear coefficient measured in the experiment and the thermal ageing time is high. Thermal ageing leads to changes in the macroscopic mechanical properties of the material, manifested as decreased density and increased elastic modulus, which causes attenuation of acoustic wave energy, while the change in internal structure of material enhances the nonlinear effect. The second-order relative nonlinear coefficient can effectively characterize the thermal ageing degree of composite insulator silicone rubber materials, providing a new method for the non-destructive testing of the thermal ageing state of composite insulators.