With the rapid development of microelectronics industry, flexible printed circuit board (FPCB) has proposed higher requirements on the dielectric properties of polyimide (PI) films, which is the interlayer insulating materialof FPCB. The conventional PI film cannot meet the requirements of 5G high-frequency communication because of its high dielectric constant. In this paper, the research progress of PI film materials with low dielectric constant in recent years were reviewed from two aspects of changing the intrinsic structure of PI and introducing porous structure, and the development prospect of low dielectric constant PI film materials were proposed.

Free metal particles are usually regarded as one of the most hazardous contaminants in the liquid-phase insulation of converter transformers, which can lead to the partial discharge (PD) initial voltage and breakdown voltage of equipment decrease obviously. In this paper, a transformer oil circulation system was constructed, and DC partial discharge and breakdown experiments were conducted on the transformer oil containing metal particles at different oil flow velocity. The PD characteristic parameters were extracted, and the variation of characteristic breakdown voltage with oil flow velocity was obtained. Then the affecting mechanisms of oil flow velocity on the PD and breakdown characteristics were discussed combined with the particle distribution images between electrodes. The results show that when the oil is static, the partial discharge degree of the transformer oil containing metal particles is highest. With the increase of oil flow velocity, the PD initial voltage increases, and the discharge repetition rate and the accumulation discharge magnitude per unit time decrease. When the oil is static, the breakdown voltage of the transformer oil containing metal particles is lowest. With the increase of oil flow velocity, the Weibull distribution curve of breakdown voltage shifts to the right, and the characteristic breakdown voltage increases, that is, the flowing of oil improves the insulating properties of transformer oil and reduces the breakdown possibility remarkably.

After the ageing of sealant, the moisture will invade the crimping interface from the sealant interface, leading to insulator ageing. To investigate the affecting mechanism of various ageing factors on the interface samples between sealant and armour clamp, immersion tests were conducted on the sealant samples in deionized water, salt solution, nitric acid solution, sulfuric acid solution at room temperature for 28 days, as well as immersion tests in mixed solutions of sulfuric acid or nitric acid with salt at different temperatures. Then the ageing situation of the samples was analyzed by Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) scanning electron microscope (SEM), and energy dispersive spectrometer (EDS). The results show that the sulfuric acid solution has no significant effect on the siloxane matrix of sealant, while the salt solution can cause the breakage of methyl chain of the siloxane side chain, and the nitric acid solution can oxidize the siloxane side chain to generate -OH. The influence of nitric acid and salt mixed solution on the siloxane matrix at room temperature is greater than that at high temperature, and the effect of nitric acid and salt mixed solution on the siloxane matrix is basically equivalent to the superposition of the two solutions acting separately. The influence of sulfuric acid and salt mixed solution on the sealant at high temperature is greater than that at room temperature. The main affecting mechanism of sulfuric acid and salt mixed solution on the siloxane matrix is that the salt solution causes the methyl of silicone side chain to break, making the Si-O-Si main chain expose to sulfuric acid and undergo chain breaking, then the salt solution can continue to affect the deeper methyl groups.

In this paper, a finite element model of ±400 kV DC wall bushing was firstly established, and the electric field and potential distributions of the DC wall bushing were simulated by COMSOL software. Then the effects of ice edge length, ice cover thickness, and rainfall on the surface electric field of the bushing umbrella skirt were studied. The results show that when the ice edge length increases to bridge the umbrella skirt, the electric field distorts seriously. The increase of ice thickness has little influence on the maximum surface electric field of the umbrella skirt. When the rainfall reaches 10 mm, the maximum surface electric field of the umbrella skirt increases sharply, and the electric field distortion points increase. The research results can provide reference for the development and design of UHV and EHV DC wall bushing.

The buffer layer of high-voltage cable will generate gas products during the ablation process. However, the correlation between the gas product concentrations and conductive property of buffer layer during the ablation process is still unclear at present. Therefore, we conducted a simulated ablation experiment on cable buffer layer and studied the evolution law of current density and gas product concentrations during the ablation process of buffer layer. The results show that under both the dry and wet conditions, the current density of buffer layer exhibits a bi-exponential function decay law with the increase of ablation time, and the decay rate of current density shows a trend of decrease rapidly at first and then unchanged basically with the increase of ablation time, only the wet sample exhibits peak current density during the initial stage of ablation. The gas chromatographic test results show that under the wet condition, the gas products during the ablation process of buffer layers are CO₂, H₂, CO, CH₄, C₂H₄, C₂H₆, and C₂H₂. Under both the dry and wet conditions, the gas product concentrations increase rapidly at first and then tend to a stable value with the increase of ablation time, and the increase time of gas concentration is consistent with the decrease time of current density, which indicates the change laws of current density and gas concentration with the ablation time is closely related. It is analyzed that there is a correlative relationship between the current density decay rate and the gas products generation rate during the ablation process of buffer layer, and the variation of gas product concentration can reflect the severity of buffer layer ablation.

A novel low dielectric loss toughening agent (Si-DABPA) was designed and synthesized, and its structure was characterized, then its toughening effect on bismaleimide (DFE950) was studied. The results show that the Si-DABPA and diallyl bisphenol A (DABPA) have the same reactivity to DFE950. Compared with the DFE950/DABPA cured material, the Thermal degradation resistance of the DFE950/Si-DABPA cured material increases, and the dielectric constant (D_k) and dielectric loss factor (D_f) decrease at 10 GHz. Compared with the DFE950/DABPA/PPE cured material, the tensile strenght of the DFE950/Si-DABPA/PPE cured material increases, while the bending strength and impact strength change little.

The technical standards of insulating oil at home and abroad were summarized and the technical requirements of insulating oil for high voltage instrument transformer were pointed out. The using properties of five kinds of insulating oils commonly used in high voltage instrument transformer in China were compared and analyzed. The results show that No.1 and No.2 insulating oils are negative gassing insulating oil, which are suitable for high voltage instrument transformers. No.3, No.4, and No.5 insulating oils are positive gassing insulating oil, which are not suitable for high voltage instrument transformers. On the balance of aromatics content, gassing properties, gas production properties, sludge solubility, and oxidation stability, No.1 insulating oil has the best comprehensive performance, the lowest gassing, and the best oxidation stability.

In order to improve the strength of composite foam material used for post composite insulator core, the poly(p-phenylene benzobisoxazole) (PBO) fiber was used to reinforce composite foam material, and the fiber content suitable for internal insulation applications was determined by studying the effects of different content of PBO fiber on the thermal, mechanical, and insulating properties of composite foam material. The results show that the addition of PBO fiber endows the composite foam materials with the ability to resist impact damage. When the mass fraction of PBO fiber is 0.4%, the energy absorbed by the composite foam materials in the drop weight impact test reaches the maximum. The addition of PBO fiber can decrease the thermal expansion rate of the material, improve the thermal stability, reduce the potential thermal stress risk, and has little effect on the internal insulation performance of composite foam materials.

Organic-inorganic composite dielectrics are often difficult to have both high dielectric constant and high electric strength. In this paper, a sandwich structured composite film was prepared by using aramid nanofibers (ANF) as high-temperature resistant matrix, boron nitride nanosheets (BNNS) as electrical breakdown enhancer, and barium titanate particles (BT) as dielectric modifier through step-by-step vacuum filtration and hot-pressing method, and the effects of sandwich structure and different modifier content on the microstructure and dielectric properties of the composite films were studied. The results show that the sandwich structured composite films have high flexibility and excellent high-temperature stability. The addition of BT decreases the electric strength of single layer and sandwich structured composite films, while the addition of BNNS increases the electric strength of the composite films. The individual addition of BT or BNNS can increase the dielectric constant of composite films at low frequency, while the simultaneous addition of BT and BNNS can increase the dielectric constant in wide frequency domain. Therefore, a composite film with balanced dielectric properties and electric strength can be obtained by adjusting the sandwich structure and modifier content in each layer. When taking the ANF/BNNS film with 10% mass fraction of BNNS as the outer layer and the ANF/BT film with 10% mass fraction of BT as the core layer, the dielectric constant, dielectric loss factor, and electric strength of the prepared composite film is 1.41 (1 kHz), 0.013 (1 kHz), and 13.3 kV/mm, respectively.

Partial discharge optical measurement method has strong anti-interference ability and high sensitivity, becoming one of the most important and effective methods to evaluate the insulation condition of electrical equipment, whose signal detection capability is closely related to the layout way of fluorescent fiber sensor. An experimental platform for fluorescent fiber PD detection was built in this paper. The influence of the relative distance between sensor and insulation defect on the PD detection effect was studied in linear layout and Archimedean layout, and then compared with the commonly used UHF method. The results show that the sensitivity of the Archimedes layout is better than the linear layout. When the distance between fluorescent fiber and PD defect is less than 45 cm, the two layout methods of fluorescent fiber sensors have higher detection sensitivity than that of the UHF sensors. For the linear layout, increasing horizontal distance has a more detrimental effect on PD detection than increasing vertical distance. For the Archimedean layout, the lateral offset of fluorescent fiber has a greater effect on the detection PD than the horizontal distance. Finally, according to the characteristics of the two layouts and the location of switchgear where PD is likely to occur, recommendations for placing fluorescent fibers in the switchgear were given.