In this study, glass fiber (GF)/phenol formaldehyde resin (PF)/epoxy resin (EP) three-phase electrical insulating composites were fabricated by selective laser sintering (SLS) additive manufacturing technology and subsequent infiltration. In the three-phase composites, glass fibers modified by a silane coupling agent (KH-550) were used as reinforcements, phenol formaldehyde resin acted as the binder and matrix, and infiltrated epoxy resin was the filler. Mechanical and electrical properties such as tensile strength, bending strength, dielectric constant, electrical conductivity, and electric breakdown strength of the GF/PF/EP three-phase composite parts were investigated. The results indicated that after being infiltrated with EP, the bending strength and tensile strength of the GF/PF/EP composites increased by 30% and 42.8%, respectively. Moreover, the flexural strength and tensile strength of the GF/PF/EP composite increased with the increase of the glass fiber content. More importantly, the three-phase composites showed high electrical properties. Significant improvement in the dielectric constant, electric breakdown strength, and resistivity with the increase in the content of glass fiber was observed. This enables the prepared GF/PF/EP composites to form complex structural electrical insulation devices by SLS, which expands the materials and applications of additive manufacturing technology.

When using the frequency domain spectroscopy (FDS) method to evaluate the insulation condition of a transformer, it is essential to calculate the frequency spectroscopy of the oil-immersed paper from the major insulation of transformer with the structural parameters. However, in on-site measurement, the structure of the transformer is often unknown, which greatly limits the application of the FDS method. This article proposed a method for extracting the frequency spectroscopy of oil-immersed paper from the oil–paper insulation without the known insulation structure. First, the frequency spectroscopy of the oil-immersed paper with different moisture content, aging states, and temperatures was measured and analyzed. The results showed that the Havriliak–Negami (H-N) model with hopping conductivity is the most suitable frequency spectroscopy model for the oil-immersed paper in different conditions. Second, the transformer insulation model was constructed, and the frequency spectroscopy of the corresponding oil-immersed paper was calculated. The calculation showed that the error of the structural parameters influences the calculated frequency spectroscopy of oil-immersed paper, especially in the low moisture condition, which leads to the increase of the fitting residual of the H-N model. According to the smallest fitting residual, the structural parameters can be analyzed, and the frequency spectroscopy of oil-immersed paper can be extracted accurately from the oil–paper insulation without the known insulation structure.

The current reliability criterion (RC) of the film capacitor (FC) is usually a fixed value, which does not consider the staged capacitance loss caused by moisture diffusion under high temperature and/or high humidity. The expected lifetime of capacitors would deviate far from the actual one. The moisture diffusion process of the encapsulated FC is analysed based on Fick's second law. In order to investigate the capacitance loss and moisture diffusion characteristics, 4 types of film capacitors are stressed with AC voltage under different temperatures and humidities. Based on the staged analysis of the capacitance and weight gain curves, the effect of moisture diffusion on capacitance loss is revealed. Results indicate that the linear stage of capacitance loss corresponds to the Fickian moisture diffusion stage of weight gain. The effect of voltage on FC aging is the most significant. The capacitance loss rate increases by 301% when the voltage rises by 30%. An RC calculation method of the FC is further proposed, which could also screen the encapsulation quality and shorten the aging test duration by at least 80%.

For metallized film capacitors (MFCs), the pulsed current would lead to the electrode fracture, the process might be accelerated under the repetitive pulsed current, and finally leading to the failure of MFCs. In order to study the mechanism of electrode fracture and its influencing factors, an experiment platform based on metallized film is established to observe electrode fracture by using a microscope. The experimental results show that for electrode fracture of metallized film, the threshold current density (TCD) is $2.20 \times 10^{10}$ A/m ² when the period is 310 $\mu \text{s}$ and the reverse factor is 10%. Theoretical results are also derived based on the analysis of electromagnetic force and Joule heat. The results indicate that, the fracture development is caused by the melting process of the electrode, together with the liquid metal flow driving by the electromagnetic force under the effect of multiple current pulses. Neither the electromagnetic force individually nor the direct vaporization would cause the fracture to develop. With respect to current distortion and heat conversion efficiency, the calculated peak current density is $j_{\mathrm {max}} = 2.17 \times 10^{10}$ A/m ² , which agrees with the TCD in the experiment. Thinner electrode has a larger TCD, when $d_{m}$ decreases from 20 to 4 nm; TCD increases from $1.07 \times 10^{10}$ to $2.25 \times 10^{10}$ A/m ² .

The composite crossarm insulator differs greatly from the suspension insulator in structure and arrangement. This study aims to determine the pollution flashover characteristics of composite crossarm insulators under different voltage grades. Four types of AC composite crossarm insulators with diameters ranging from 100 mm to 450 mm are subjected to artificial pollution test, and then the effects of the surface hydrophobicity state of silicone rubber, core diameter, umbrella structure, arrangement, and insulation distance on the pollution flashover voltage of the composite crossarm insulators are analyzed. Under the pollution grade 0.2/1.0 mg/cm² and voltage grade from 66 kV to 1000 kV, if the silicone rubber surface changes from HC5 to HC6, the pollution flashover voltage of the composite crossarm insulator will increase by 13.5% to 21.0% compared with the hydrophilic surface. If the core diameter changes from 100 mm to 300 mm, the pollution flashover voltage gradient decreases with the increase in core diameter; if the core diameter changes from 300 mm to 450 mm, the pollution flashover voltage gradient increases with core diameter. Under the same insulation height and core diameter, the umbrella structure will have a certain impact on pollution flashover voltage by up to 1.7% to 5.4%. Under the horizontal arrangement, the pollution flashover voltage can increase by 10.5% to 12.1% compared with that under the vertical arrangement. Under the hydrophilic surface and weak hydrophobicity state, the pollution flashover voltage has a linear relationship with the insulation distance. The above results can provide a reference for the structural design and optimization of the composite crossarm insulator.