In order to study the reliability of nitrile butadiene rubber (NBR) sealing element under the action of extremely low temperature and oil medium, the mechanical properties and microstructure of sealing rings in high-voltage transformers in extremely cold regions were studied. The mechanical characteristics, morphology change, chemical structure characteristics, and thermal response characteristics of the sealing ring after serviced for one year in extremely cold environment were obtained by tensile test, micron indentation test, scanning electron microscope (SEM), infrared spectrum (FTIR), and differential scanning thermal analysis (DSC). The results show that the elastic modulus and hardness of NBR sealing ring decrease in extremely cold environment, which leads to the loss of sealing effect. This is because the surface of sealing ring swell under the joint action of low temperature and oil medium, and micro-cracks appear. Oil immersion in NBR makes the molecular chain spacing increase, and the compliance of main chain of NBR is improved, meanwhile, the molecular structure of NBR changes due to the hydrolysis reaction of cyanide group.

In order to improve the comprehensive performance of bisphenol A epoxy resin (DGEBA), bisphenol A epoxy resin was modified by alicyclic epoxy resin through blending, and the thermal, mechanical and electrical properties of the blending system were comprehensively analyzed. The results show that when the mass fraction of alicyclic epoxy resin 2021P is 10%, the blending system have better thermal, mechanical and electrical properties. Compared with the pure bisphenol A epoxy resin, the glass transition temperature of the blending system increases by 3.18%, and the bending strength and tensile strength increase by 3.95% and 4.65%, respectively. The leakage current decreases by 6.03%, the dielectric loss factor decreases by 24.25%, and the power frequency electric strength increases by 7.16%.

From the perspective of inorganic nanoparticles hybrid polyimide (PI), the research status of dielectric constant, conductivity, electric strength, and corona resistance of PI hybrid materials at home and abroad in recent years was reviewed and commented. The influence of different inorganic nanoparticles doping on the electrical properties of PI materials was expounded, the problems existing in the process of PI hybrid were analyzed, and the direction of PI hybrid materials can be further studied was prospected.

In this paper, the aggregation morphology of copper products in transformer oil and its influence on the ageing performance of transformer oil were investigated. The aggregation morphology of copper products in oil were tested by molecular weight distribution test method, and the ageing properties of oil samples were also analyzed periodically. The results show that the distribution of copper products changes with the ageing time obviously. The proportion of ionic copper decreases gradually with the ageing time, while the proportion of colloidal and suspended copper exhibit a tendency of continuous accumulation. The acid value and dielectric loss factor of the transformer oil increase significantly with ageing time, while the breakdown voltage decreases continuously. The concentration of ionic, colloidal, and suspended copper are positively correlated with the acid value and dielectric loss factor, and negatively correlated with the breakdown voltage.

Using aramid fibrid as reinforcing material and introducing into mica paper-based materials, we manufacture an aramid fibrid/mica composite insulating paper by wet paper with pulp. The morphology, mechanical strength, insulating properties, thermal stability, and glue permeability of the composite paper were systematically studied. The results show that the flake mica and ribbon-like aramid fibrid are orderly stacked to form a “brick-mud” structure. The increase of aramid content makes the composite paper structure more compact, and the mechanical strength also increases. With the increase of aramid content, the gap between mica is filled, and the air permeability of composite paper decreases. The electric strength and thermal stability of the composite paper decrease with the increase of thickness, but the composite paper still maintains good insulating properties and thermal resistance.

High performance glass fiber reinforced epoxy resin composites were prepared by vacuum impregnation and autoclave molding using multifuctional AG80 epoxy resin and phenolic epoxy resin as composite epoxy resin matrix, composite diaminodiphenyl sulfone (DDS) as curing agent and 2-ethyl-4-methylimidazole as accelerator. The results show that the prepared high performance epoxy resin composites have excellent high temperature and high humidity resistance. When the content of DDS is 35 phr, the tensile strength, compressive strength, and bending strength of epoxy resin composites reach 385, 549 and 407 MPa at room temperature, respectively, the insulation resistance reaches 1.0×10⁹ Ω, and the bending strength reaches 210 MPa at 200℃. In addition, the epoxy resin composite still maintains good mechanical properties and insulating properties after high temperature and high pressure hygrothermal test at 205℃ and 140 MPa, which can meet the application requirements of petroleum logging tools in high temperature and high humidity environment.

In order to achieve long-term stable extrusion of insulating materials, a cross-linked polyethylene with different antioxidant content was prepared by melt blending, and the effect of different antioxidant content on the scorch resistance of cross-linked polyethylene was studied. At the same time, the mechanical properties, electric strength, dielectric properties, and thermal properties of cross-linked polyethylene were comprehensively evaluated. The results show that with the increase of antioxidant content, the torque of cross-linked polyethylene decreases gradually, the cross-link time increases slowly, and the scorch resistance of the material is improved. In addition, the temperature of cross-linked polyethylene material is reduced slightly, which ensures the stability of long-term extrusion for material. The conductivity of cross-linked polyethylene does not change significantly with the increase of antioxidant content, while the AC electric strength and dielectric loss increase slightly. The AC electric strength increases by 4.5%, the dielectric loss factor is less than 5.0×10^-4, and the melting and crystallization characteristics do not change significantly.

High voltage power module packaging insulation endures unipolar electric stress, but its space charge phenomenon has never been paid enough attention. The space charge behavior and trap energy distribution of silicone material for packaging were investigated at various temperatures in this paper. The space charge and isothermal relaxation current of silicone samples were tested at 40, 60, and 80℃ under the applied electrical field from 3 to 15 kV/mm by simultaneous measurement system of space charge and trap energy states. The results show that the space charge accumulation is getting worse with the increase of temperature and applied field. There are charges with the same polarity accumulated near the electrode, which distorts the local electrical field distribution. Furthermore, a modified isothermal relaxation current analysis method based on non-negative least-squares iterative algorithm was proposed, and the depolarized relaxation currents at various temperatures were analyzed to provide the trap energy distribution from 0.8 to 1.2 eV, which was beneficial for understanding the mechanism of space charge behavior. The accumulation of space charges results in local field distortion, and the distorted field can cause partial discharge and insulation failure of packaging. Therefore, it is suggested that the space charge problem should be taken into account at packaging design of high voltage power modules.

The faults caused by internal interface defects of composite insulators is a great threat to the safe and stable operation of power grid. In order to study the influence of different shapes of defects and their sizes and moisture immersion on the electric field distribution of insulators, an isosceles triangle defect model including tip curvature radius parameter was proposed. A 3D model of 500 kV composite insulator was established, and the electric field of defects with different shapes and sizes and moisture ratio were simulated and compared by the COMSOL. The results show that the isosceles triangle defect model can better reflect the tendency of defect developing to the low voltage side. The tip curvature radius of defect has no obvious effect on the air gap, but there is a point effect in the water gap. With the increase of moisture content in the defect, the maximum electric field strength increases gradually. Under the action of point effect and moisture immersion, the electric field strength at the tip of low voltage side is distorted severely, which increases the possibility of partial discharge and accelerates its development to the low voltage side.

To investigate the partial defects positioning method of low voltage cable, frequency domain reflectometry (FDR) was used to test the low voltage cable with defects, and the positioning results were analyzed in this paper. Three partial defects of D1, D2, and D3 were made successively in phases A, B, and C of 14.1 m long low voltage cable. The D1 defect was a 20 mm × 2 mm of rectangular insulation (expose cable core) in phase A made at 8.7 m away from the head-end of cable and was paralleled resistors with different resistance to simulate high and low resistance faults. The D2 defect was a longitudinal knife mark defect with 5 mm of length in phase B made at 4.3 m away from the front end of cable, while D3 defect was a V-shaped knife mark defect in phase C made at 11 m away from the head-end of cable. The A phase samples without defects and A, B, and C three phase samples with defects were tested by FDR instrument. Then, the D3 defect was immersed in saturated salt water and the three phases end of cable was applied 1 kV voltage for accelerated ageing. The ageing time was four weeks. After ageing, the three phases A, B, and C were detected by FDR. The waveform characteristics in frequency domain and time domain of different defects were analyzed according to FDR detection results. The results show that the peak amplitude in frequency domain of the sample without defect decreases, but the peak amplitude at the defect of the sample with defects increases and is larger than the waveform amplitude on both sides, and the peak amplitude increases more significantly when there is a low resistance fault. In addition, the time domain waveform of high resistance fault shows a positive wave, but the peak is not significant. The low resistance fault presents a significant negative wave. FDR waveforms in frequency domain and time domain can be used to locate low voltage cable faults effectively, and have higher sensitivity to locate low resistance fault.