To solve the problem that it is difficult to diagnose the moisture defects of cable joints with the existing methods, this paper proposes a method for diagnosing the moisture defects of cable joints based on frequency modulated continuous wave (FMCW) and time reversal (TR). Firstly, a distributed parameter model of cable was established. The FMCW method was employed to capture multi-frequency reflected signals from impedance mismatches at defects and joints. These inversion signals were injected into a test cable model to derive an energy curve, and the energy curve is served as a diagnostic spectrum for cable defects. Subsequently, the impedance discontinuity point detection was carried out for the simulation cable models with different end-loads and intermediate joints. Finally, joint defect diagnosis was carried out for the real 750 m 10 kV power cable and 2 km 10 kV power cable. The results show that the method proposed in this paper can not only accurately determine the location and characteristic of the impedance discontinuity points in cable, but also improve the distance resolution of the defect location peak. The joint positioning peaks of "positive first and then negative" and "negative first and then positive" can respectively represent normal and damp cable joints. Therefore, the method proposed in this paper can accurately detect the moisture defects of cable joints and has a good engineering application prospect.

A kind of silicone encapsulant with excellent thermal conductivity, self-leveling properties, mechanical properties, and flame retardancy was prepared by employing end vinyl silicone oil with different viscosity as matrix, containing hydrogen silicone oil as cross-linking agent, and spherical alumina as thermal conductive and reinforcement filler. The effects of ratio of end vinyl silicone oils with different viscosity, molar ratio of active hydrogen and vinyl, filler ratio and filling amount on the performance of silicone encapsulant were investigated. The results show that the silicone encapsulant has the best performance when the PDMS-1 and PDMS-2 vinyl end silicone oil are mixed at a mass ratio of 1∶1 as the matrix, the molar ratio of active hydrogen and vinyl is 1.2, and the ratio of spherical alumina with different particle sizes m(5 μm)∶m(15 μm)∶m(50 μm)=2∶5∶3. When the total filling amount of different sizes of spherical alumina reaches 500 parts, the silicone encapsulant has up to 1.50 W/(m·K) of thermal conductivity, up to 1.70 MPa of tensile strength, FV-0 of vertical combustion grade, and excellent operation and processing performance as well as dispersing stability, and its comprehensive performance is excellent.

Multi-scale mining of the spatio-temporal coupling relationship of dissolved gases in oil is helpful to improve the prediction accuracy of dissolved gases in oil and provide a reliable theoretical basis for transformer operation and maintenance decisions. Thereby, a multi-scale fusion prediction method for dissolved gases in transformer oil considering spatio-temporal coupling information was proposed in this study. Firstly, the Res2Net was used to extract the multi-scale time characteristics of the dissolved gas data in oil, and the periodic time feature information of the characteristic gas under different frequencies was captured. Secondly, the implicit relationship between characteristic gases was captured by calculating mutual information, the correlation between different gases was described in the form of topological graphs, and the spatial information features were extracted by using graph convolutional neural network (GCN). Finally, multi-scale temporal information and spatial information were fused and spliced, and temporal convolution network (TCN) was used to predict the dissolved gas in oil. The proposed method was validated using online oil chromatography monitoring data from a 500 kV transformer. The results show that compared with the traditional prediction method, the Res2Net-GCN-TCN model can effectively improve the prediction accuracy of dissolved gas content in oil, and the average prediction accuracy is 98.68%.

Under the high temperature conditions (90, 105, 120, and 135°C), accelerated ageing tests of nitrile rubber (NBR) were conducted in hot air, hot oil, hot air compression, and hot oil compression. The ageing mechanisms were investigated through Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA/DTG), and scanning electron microscopy (SEM), and the effects of ageing temperature, time, and deformation conditions on the ageing behavior of NBR were revealed. The results indicate that crosslinking, oxidation, and chain-breaking reactions occur during the ageing process of NBR, and the crosslinking reaction is the predominant reaction. After ageing at 120°C, grooves appear on the surface of NBR, and after ageing at 135°C, defects such as holes and damages appear on the surface. In the early stage of ageing, transformer oil has a suppressive effect on the increase of permanent deformation under compression. In the later stage of ageing, transformer oil plays a promote role for the decrease of tensile strength, and the higher the temperature, the more obvious the effect.

A flame retardant monomer 6-(2,5-bis ((4-vinylbenzyl) oxy) phenyl) dibenzo [c,e][1,2] phosphono-6-oxide (DOPVB) was synthesized by using 10-(2,5-dihydroxyphenyl)-10-hydro-9-oxy-10-phosphophenanthrene-10-oxide (DOPO-HQ) and 4-chloromethyl styrene (VBC) as raw materials. The DOPVB flame retardant was compounded with epoxy resin (EP) to prepare EP/DOPVB composites with different phosphorus contents, and their thermal, flame retardant, and dielectric properties were tested. The results show that the optimal synthesis conditions for DOPVB is that the molar ratio of reactants DOPO-HQ, VBC, and K₂CO₃ is 1∶2.2∶3.0, and the reaction time is 6 hours, then the yield is 74.55%. The EP/DOPVB cured material with a phosphorus content of 2.5% has the best comprehensive performance, its glass transition temperature is 130.4℃, which is 45.21% higher than that of pure EP. The dielectric constant is 2.98 and the dielectric loss factor is 0.005 03, which is 21.99% and 72.16% lower than that of pure EP, respectively. The vertical combustion test of the composite material prepared from the cured material reaches V-0 level, with a limit oxygen index of 67.4%.

The heat resistance of ethylene propylene diene monomer (EPDM) decreases significantly after thermo-oxidative ageing, resulting in poorer performance and shorter service life of products made from it. This paper reviews the research progress on improving the thermo-oxidative ageing resistance of EPDM at home and abroad. It introduces three main methods for improving the thermo-oxidative ageing resistance of EPDM, including selecting high thermal-resistant EPDM matrix, compounding with thermal-resistant rubber, and adding antioxidants. And it provides a brief outlook on future research directions for improving the thermo-oxidative ageing resistance of EPDM.

The vacuum surface insulation performance of insulation material used in radiation environment simulation devices will be significantly affected by long-term radiation exposure. By conducting irradiation ageing experiments of different doses of gamma ray on polymethyl methacrylate (PMMA) materials, the variation law of surface microstructure and electrical performance parameters of PMMA materials with gamma ray irradiation dose was studied. The results show that when irradiated under a low-dose (0.1 kGy) gamma rays, the distribution of surface traps on the material is the main factor affecting the vacuum surface withstand voltage characteristics of PMMA the vacuum surface ageing voltage of PMMA increases by 11.7% compared to the unirradiated sample. When irradiated with high doses (greater than 1.0 kGy) of gamma rays, holes appear on the surface of PMMA material, and the local electric field intensity is distorted, becoming a "weak area" of vacuum surface insulation. Appropriate doses of gamma ray irradiation is helpful to improve the vacuum surface insulation performance of PMMA.

In order to realize the recycling and reuse of degraded glass fiber (d-GF), to obtain degraded glass fiber was obtained by chemical degradation method from the selected the 220 kV retired composite insulator core rod, and the mechanical, thermal, and microscopic differences between degraded glass fiber and ordinary glass fiber (GF) were compared. Then, polyethylene (PE) composite materials (d-GF/PE, GF/PF) reinforced with different contents of glass fiber were prepared to explore the reuse prospects of degraded glass fiber reinforced materials. The results show that silane coupling agent can realize the surface modification of degradable glass fiber, and the modification effect is slightly better than that of ordinary glass fiber. The surface of the degraded glass fiber is relatively rough, with scale like damage and a small amount of resin residue in some areas, but there is no obvious erosion or fracture phenomenon on the whole. The fracture stress distribution of degradable glass fiber is relatively discrete, and the average fracture stress is 1 520 MPa, which is 29.95% lower than that of ordinary glass fiber. The interface state between glass fiber and polyethylene matrix in d-GF/PE is good, under the same glass fiber content, the difference in maximum fracture stress between d-GF/PE and GF/PF is only 2.15 MPa.

Composite insulators with a large heating area in a 220 kV operation and maintenance AC line were taken as a sample, the impact of different environmental parameters on the heating and operating performance of the composite insulator was analyzed by conducting power frequency withstand voltage and infrared tests on the heating insulator, as well as physical and chemical property tests on the silicone rubber of the composite insulator. The results show that the abnormal heating of the aged and damp composite insulator sheath is located at the high voltage end. The higher the environmental humidity and the lower the wind speed, the larger the heating range and amplitude. After being dampened, the dielectric constant and dielectric loss of silicone rubber increase significantly, and the heat generation comes from the dielectric loss of the aged sheath on the surface. There are obvious signs of deterioration on the surface silicone rubber of the sheath at the heating point, and the interior of the sheath and the core rod are not affected by the surface heating. The operational performance of the insulator is not affected. In addition, based on the above results, suggestions for on-site maintenance of sheath ageing, moisture, and heat defects were proposed.

The disadvantages of poor thermal conductivity and mechanical properties severely restrict the further application of epoxy resin in electrical equipment insulation. Therefore, Epoxy composite dielectrics (EP/BNNS) with different BNNS contents were prepared by using nitride nanosheets (BNNS) as fillers. The phonon transport, stress distribution, polarization relaxation, trap distribution, and other conditions inside the epoxy composites dielectric were analyzed, and then the mechanisms of BNNS improving the thermal, mechanical, and electrical properties of the epoxy composite dielectric were systematically explored. The results show that when the BNNS filling mass fraction is 5.0%, the thermal conductivity of the epoxy composite dielectric reaches 0.36 W/(m·K), the bending strength and tensile strength reach 120 MPa and 57.4 MPa, respectively, and the electric strength reaches 96.9 kV/mm, while the composites dielectric also have good dielectric properties. The introduction of BNNS can optimize the thermal and mechanical properties of the epoxy composite dielectric without compromising their electrical performance.