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.

The existing test method standards of the compatibility between electrical insulating fluids and structural materials at home and abroad were introduced, and the test method standards of the compatibility between electrical insulating fluids and structural materials in transformers, power capacitors, cable terminals, and oil cooling motors were discussed respectively. The similarities and differences of test conditions and evaluation parameters in each standard were summarized, and the shortcomings of current standards were analyzed. At last, the standard proposal of IEC 63177 Test methods for compatibility of construction materials with electrical insulating liquids proposed by Guangzhou Power Supply Bureau of Guangdong Power Grid Co., Ltd. was introduced compendiously, which will address the shortcomings of existing compatibility test methods.

In this paper, the main ageing forms and influencing factors of oil-paper insulation for traction transformer were analyzed. The ageing mechanism of oil-paper insulation was studied, the current characteristic parameters characterizing the ageing degree of oil-paper insulation and the main residual life prediction methods were summarized, and the advantages and disadvantages of each characteristic parameter and residual life prediction method and the applicable conditions were analyzed. On this basis, the existing main problems in the ageing state assessment and residual life prediction of oil-paper insulation, as well as the future research directions were discussed.

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.

In order to improve the bonding strength of epoxy resin potting materials to metals, the effects of molecular structure, amine curing agent type, and inorganic fillers on the tensile shear strength of epoxy resin potting materials were studied. The results show that the epoxy materials prepared by polyurethane modified epoxy resin and dimer-acid modified epoxy resin have higher tensile shear strength. The tensile shear strength of epoxy materials cured by polyamide, polyether amine, and aromatic amine decrease in turn. With the increase of inorganic filler content, the tensile shear strength of epoxy materials increases at first and then decreases. For the nano gas-phase SiO₂ powder, when its filling mass fraction is 4%, the epoxy material has higher adhesive strength, while for the micro powder, its filling mass fraction needs to reach 40%, the epoxy material can have higher adhesive strength.

Light weight and high insulation composite cross-arm has excellent properties of light weight and excellent insulating properties, which has been widely used in power lines in recent years. In this paper, a composite cross-arm with light weight and high insulation of foam core was prepared, and the cross-arm was conducted water absorption, dye penetration, water diffusion leakage current, electric strength, thermal induction, and vibration ageing tests. The results show that the volume water absorption of the composite cross-arm with light weight and high insulation of core is 0.036%, the water diffusion leakage current is less than 75 μA, the AC electric strength is greater than 71 kV/cm, the vibration ageing resistance is good, and the comprehensive performance is better than that of the cross-arm with traditional solid core, which can meet the need of practical engineering application.

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.

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.

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.

To evaluate the environmental adaptability of insulation system for offshore wind turbine, multi-factor cycling tests including high and low temperature test, humid heat test, salt spray test, and vibration test, were conducted on insulating resin and simulation winding, and their electrical performance changes before and after tests were characterized and compared. The result show that after ageing tests, the performance of the epoxy modified unsaturated polyester impregnating resin does not deteriorate significantly, the normal insulating performance of simulation winding just deteriorate slightly, its residual breakdown voltage is almost no attenuation, and both the key materials and simulation winding exhibit good resistance to marine environments. The insulation system of offshore direct-drive wind turbine still retains stable and excellent dielectric performance after long-term offshore multi-factor cyclic ageing tests, which can meet the requirements of marine environment.

An accelerated thermal ageing test was designed for the main insulation sample of a generator VPI stator bar, and the ultrasonic velocity, ultrasonic spectrum, and ultrasonic defect waveform of samples were analyzed at various ageing temperatures on the basis of the principle of ultrasonic testing. At the same time, artificial hole defects and impurity defects were introduced into the insulation samples to explore the effect of defect introduction on the ultrasonic waveform of the samples. The results show that as the thermal ageing test progresses, the overall ultrasonic velocity in the main insulation of stator bar shows a decreasing trend. The reason is that during the thermal ageing process of the stator bar main insulation sample, the layering of internal structure can cause refraction, reflection, and dissipation of the sound wave during propagation process, resulting in the propagation speed of the sound wave in the insulation sample decrease. The defects can be detected and accurately located through ultrasonic echoes, which indicates that the acoustic performance testing method has good practicality in the diagnosis of stator bar main insulation.

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.

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.

It is found that ablative white spots are often attached to the buffer layer by disassembling high-voltage XLPE fault cables. To study the ablation characteristics of buffer layer, the chemical elements of white spot at different positions of buffer layer were analyzed by using energy dispersive spectrometer (EDS), and the conductivity of the white spots buffer layer were measured. Then on the basis of the measurement results of conductivity, an electrothermal coupling simulation model of high voltage XLPE cable was established, and the heat production characteristics of white spot buffer layer under the action of high frequency overvoltage were calculated. The results show that the white spots in the ablative area of buffer layer are the water-blocking powder precipitated by the buffer layer, and the conductivity of the buffer layer decreases obviously after the white spots are precipitated. Under the action of high frequency overvoltage, the higher the overvoltage frequency, the greater the resistance heat generated in the local periphery of buffer layer white spot, and the higher the temperature rise, which may eventually cause overheating and erosion of the buffer layer.

In order to investigate the effect of alkaline corrosion on the insulating strength of superhydrophobic coatings, superhydrophobic coating specimens with insulating properties were prepared and subjected to artificial alkaline corrosion tests. The insulating strength of the superhydrophobic coating specimens with different corrosion degrees was tested, and the effects of pollution degree, applied voltage amplitude, and wetting degree of surface contaminants on their insulating strength under different temperatures were investigated. The results show that the alkaline environment will destroy the substance and structure of superhydrophobic coating surface, leading to the decrease of its hydrophobicity and self-cleaning capability, which in turn leads to the decrease of its insulating strength. The more serious the surface pollution degree of the alkaline-corroded superhydrophobic coating under wet and polluting environment, the higher the applied voltage, and the shorter the wetting time, the lower the insulating strength of the coating surface. The surface pollution degree of the alkaline-corroded superhydrophobic coatings gradually decreases to a stable value with the increase of wetting time, at this time the surface leakage current of the coating does not change significantly, and the lower the temperature of alkaline corrosion environment, the shorter the wetting time required to reach the above stable state.

On the basis of surface discharge tests and finite element electric field simulation methods, the surface discharge and electric field characteristics of GIS basin insulators with metal ball or wire particle defects were studied. The results show that under different metal particle defects, both the pulse voltage and ultraviolet photon number increase with the increase of voltage. The surface discharge of basin insulators with metal wire defects is more severe, compared to the situation where there are metal ball defects on the high-voltage side, the discharge is more intense when there are metal ball defects on the low-voltage side, and the change trends of simulation results are consistent with the experimental results. When there are metal particle defects on the surface of GIS basin insulators, the surface discharge intensity is positively correlated with the distortion degree of electric field, and the correlation between the distortion degree of electric field and the average pulse amplitude is significant.

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.

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.