Current-carrying capacity is an important indicator of cable transmission capacity, which directly affects the operation reliability and economy of HVDC cables. According to the electric field distribution characteristics in the insulation layer of DC cable, an analytical calculation method of the electric field distribution inside and outside the insulation layer based on the equivalent conductivity was proposed. Taking a ±535 kV XLPE insulated DC cable as example, considering the highest operating temperature of cable conductor and the maximum allowable temperature difference of insulation layer, we obtained the load control domains of the HVDC cable under different operating conditions. The results show that the current-carrying capacity and application characteristics of HVDC cables can be analyzed by the electric thermal field decoupling method effectively, and the maximum temperature difference of insulation layer is the core factor limiting the current-carrying capacity of DC cable below critical ambient temperature. Under the critical ambient temperature, raising the highest operating temperature of conductor has limited influence on the current-carrying capacity, while optimizing the electrical resistance of insulating materials and cable structure is the key to improving the current-carrying capacity.

The azodicarbonamide (AC) and aluminum hydroxide (ATH) were used to discuss the effect of their combination on the anti-tracking performance, mechanical properties, and hydrophobicity of silicone rubber composites. The possible reasons for AC and ATH synergistically improving the tracking resistance of the composites were discussed through scanning electron microscopy, contact angle, and other characterization methods. The results show that when the ATH is filled only, the filling amount must reach 100 phr, and the anti-tracking performance of silicone rubber composite can achieve grade 1A4.5. However, when the filling amount of AC and ATH are 3 phr and 80 phr respectively, with the help of the arc extinguishing and gas phase flame retardant effect of a large amount of nitrogen and carbon monoxide generated by the decomposition of AC, the anti-tracking performance of the silicone rubber composite also can achieve grade 1A4.5, and compared with the composite filled with pure ATH, it has better tensile strength and hydrophobicity, which is expected to be applied in the field of composite insulators.

The carbon fiber composite mandrel was conducted thermal oxygen ageing tests at 170、185、200℃ for one month. Through the mechanical properties tests, Fourier infrared spectroscopy (FTIR), and scanning electron microscope (SEM) analysis of the mandrel sample at different ageing stages, the macro properties, micro characteristics, and the relationship between macro and micro properties were studied, and the change laws of mandrel properties with ageing time were obtained. The results show that the accelerated thermal oxygen ageing will further increase the curing degree of the mandrel and enhance its mechanical properties. While during the thermal oxygen ageing process, there are cracks, powders, and holes in the epoxy resin matrix in the outer layer of the mandrel. The interface adhesion between epoxy resin and glass fiber becomes worse, and the epoxy resin matrix becomes brittle, which will damage the mechanical properties. Under the action of these two factors, the mechanical properties of mandrel increase occasionally and then decrease as a whole.

Basin insulator is an important part of gas insulated closed combination switchgear (GIS) device, and its quality has an important impact on the construction and operation of power grid. In this paper, the failure causes of basin insulators were described, and the existing detection methods were compared in common cracking locations. According to the material characteristics of basin-type insulators and field test conditions, the probe frequency, K value, and chip size were calculated theoretically. An ultrasonic water immersion focusing oblique probe device was developed, a method of detecting cracks near the bolt holes of basin insulator by using the low-frequency dual-probe ultrasonic detection technology was proposed, and the test results were verified by laboratory penetration and ray detection. The results show that the ultrasonic refraction transverse wave with high signal-noise ratio can be generated in the basin insulator by using dual probes with 2 MHz 18×18 mm of model size and 24.8°-35.4° of angle (α_L) between the axis of the wafer emission acoustic beam and the normal line of workpiece interface. It is feasible to use special low frequency double probe ultrasonic detection technology to detect the defects of GIS basin insulators.

The effect of thermal oxygen ageing on the properties of irradiated ethylene propylene diene rubber (EPDM) materials were analyzed, and its theoretical service life was evaluated by analyzing the change rule of compression permanent deformation rate with time. The results show that with the increase of ageing time and ageing temperature, the irradiated EPDM is aged more and more serious, the carbonyl content becomes higher and higher, and the compression set and compression stress relaxation become greater and greater. The SEM results show that the EPDM degrade seriously during ageing process. According to Arrhenius equation, the theoretical life of the irradiated EPDM is 7.58 years at 50℃.

A novel film capacitor was prepared by using a mass-produced surface modified high dielectric polypropylene film, and the power density, energy storage properties, capacity retention ratio, and the cycle life under large current of the capacitors were studied. The results show that the maximum energy storage density of the new capacitors with the surface modification polypropylene film can reach to 3.2 J/cm³, and the capacitor shows excellent comprehensive performances.

A thermal-mechanical simulation model of cable insulation fused joints was established, and then the effect of the difference in fluid temperature and flow rate between water-cooled and air-cooled cooling methods on the internal stress distribution of fused joints was analyzed. The effect of the material performance difference between the new cross-linked polyethylene and the old cross-linked polyethylene on the stress distribution was also studied. The results show that the change of water-cooling parameters has a great influence on the stress of the insulation layer of the fusion joint, and the stress distribution is concentrated near the interface of the new and old insulation layer, and the difference of the material properties leads to the similar stress distribution.

Metal particles in gas insulated metal enclosed switchgear (GIS) and gas insulated enclosed transmission line (GIL) can bounce near the insulator under the electric field, which intensifies the charge accumulation and may cause surface flashover in serious cases. In this paper, a 126 kV real size disc insulator was selected as the test sample. A sealed chamber and coaxial structure electrodes were designed, and the effects of bouncing metal particles on the surface charge accumulation behaviors of insulator at different temperatures (23, 40 and 60℃) were studied by using Kelvin electrostatic probe. The results show that the charge accumulation on the nonplanar region is closely related to the motion mode of metal particles at room temperature. And the charge accumulation on the insulator surface is positively correlated with the temperature. As the temperature rise to 60℃, the metal particles moving to the vertical surface of the ground electrode are easy to cause surface flashover, and a large number of heteropolar charges would be accumulated on the non-plane region in the same radial direction as the flashover.

The broadband dielectric response of oil-paper insulation was studied, and broadband dielectric response tests were carried out on laboratory samples of oil-paper insulation with different moisture contents. According to the relaxation polarization part of dielectric spectrum, the spectral parameters characterizing the moisture degree were extracted, and their relationship with the moisture content in the oil-paper insulation was analyzed. Further moisture tests for the oil-immersed bushing scaled-down model were designed, and the application of the spectral parameters in the moisture diagnosis of bushing was discussed. The results show that the low-frequency polarization process in the relaxation polarization spectrum is more sensitive to the moisture content, and its loss peak value can be used as a characteristic parameter to characterize the moisture content; under different moisture types of the bushing, the change patterns of the loss peaks and characteristic frequencies of the low-frequency and high-frequency relaxation polarization are different, which can be used as a preliminary criterion for the moisture type of the bushing.

Three kinds of soluble polyimide (PI) resins were prepared from 4,4′-hexafluoroisopropylidene)bisphthalic anhydride (6FDA) and three kinds of romatic diamine monomer containing rigid-rod amide bonds in the molecular chains by one-step high temperature solution polycondensation reaction, respectively. And then three kinds of transparent PI films were prepared by the N,N-dimethylacetamide (DMAc) solution of the above resins. The films were tested by attenuated total reflection Fourier transform infrared spectroscopy, thermogravimetric analysis, dynamic mechanical analysis, thermomechanical analysis, and UV-visible spectrum, and their yellowness index was tested. The results show that the PI films exhibit good optical transparency, their ultraviolet cutoff wavelength is below 380 nm and the optical transmittance is over 80% at the wavelength of 500 nm. In addition, the PI films exhibit good thermal stability with the glass transition temperatures (T_g) higher than 320℃ and the 5% weight loss temperatures (T_5%) over 520℃. The linear thermal expansion coefficients (CTE) of the PI films are all lower than 50×10^-6/K, indicating that the introducing of rigid amide bonds can efficiently enhance the high-temperature dimensional stability of the solution-processable fluoro-containing PI films.