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.

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.

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.

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.

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.

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 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.

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.

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.