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.

Aiming at the contaminant composition of transmission line insulator in Ningxia Yellow River irrigation area are not clear, and the degree of environmental impact is unknown, we subdivided the environment of Ningxia Yellow River irrigation area into 5 categories: industry, agriculture, urban residential area, desert, and grassland, and insulator contaminants samples in the corresponding environments were taken to conduct microscopic morphology, element content, crystal composition, anion and cation composition and content detection and analysis. Meanwhile, the pollution accumulation characteristics of insulators in irrigated and non-irrigated areas were compared and analyzed. The results show that the soil composition in the Yellow River irrigation area determines the basic composition of insulator contaminants. The industrial and agricultural environment have bigger influence on the micro-structure and different ingredients content of the insulator contaminants, and the urban residential area is followed. The pollution accumulation degree of insulators in irrigated areas was obviously higher than that in non-irrigated areas. In order to further improve the operation reliability of the transmission lines in Ningxia Yellow River irrigation area, some suggestions for anti-pollution flashover operation and maintenance are put forward, which can be use as reference for relate operation and maintenance units.

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.

Electrical tree is a common phenomenon in XLPE cable, and there may be different scales of spike defects during the operation of cables. In this paper, the relationship between electrical tree growth morphology and partial discharge characteristics under different tip curvature radius was studied. The results show that the sample with small tip curvature radius will eventually develop into branch electrical tree under the tree initiation voltage, while the sample with large curvature radius will eventually develop into cluster electrical tree, and the discharge capacity of cluster electrical tree is greater than that of branch electrical tree. The positive and negative half cycle of PRPD diagram of the cluster electrical tree is similar to the “wing shape”. The discharge shape of the positive half cycle of the branch electrical tree is “triangle shape”, and the negative half cycle shape is “wing shape”. The growth rate of the two electrical trees shows the characteristics of “fast-slow-fast”. The growth rate of the branch electrical tree is higher than that of the cluster electrical tree, and the branch electrical tree is more harmful in the actual cable.

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.

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.

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 evaluate the impregnating resin for driving motor of new energy vehicles, epoxy resin and polyester imide resin were selected as research objects. The properties of the two impregnating resins were compared by thermal weight loss and bonding strength tests, and their long-term ageing characteristics were compared emphatically. The results show that the VOC content and long-term thermal resistance of the two impregnating resins meet the requirements. The bonding strength of epoxy resin is higher than that of polyester imide at 25℃, but decreases rapidly at 150℃ and 180℃. Ageing at 200℃ for 2 000 h, high and low temperature impact for 600 cycles, high and low temperature for 8 cycles under ATF oil immersion, high and low temperature for 8 cycles under ATF oil + 0.5% water closed immersion, and ageing at 150℃ for 2 000 h under ATF oil immersion experiments have similar effects on the bonding strength of the two impregnating resins. However, under the condition of ATF oil + 0.5% water closed immersion at 150℃, the bonding strength of epoxy resin is only 16.41%, 13.64%, 4.29%, and 4.29% of the initial value after aged for 500, 1 000, 1 500, and 2 000 h, while the bonding strength of polyester imide is higher than the initial value under the same condition. The structural and elemental changes of epoxy resin before and after its bonding strength decreasing significantly were compared by Fourier transform infrared spectroscopy and X-ray electron spectroscopy, it is inferred that the hydrolysis at high temperature is the main reason for the decline of bonding strength.

In order to study the performance changes of silicone rubber and fluorinated silicone rubber after freezing at extremely cold temperature, the two materials were put into a low-temperature test box to freeze for 0, 150, 300, 450, 600, 750, 900, 1 050 h, and the temperature was set as -50℃. After freezing, the samples were taken out and stood for 2 hours at room temperature, and then the changes of breakdown voltage, maximum tensile force, tensile strength, hardness, and surface topography of the materials with freezing time were measured, and the principle were analyzed. The results show that with the increase of the low temperature freezing time, the breakdown voltage, maximum tensile force, and tensile strength of silicone rubber and fluorinated silicone rubber increase, while the hardness is almost constant. There are cracks on the sample surface after freezing for 300 h, there are obvious cracks and holes after freezing for 750 h, and the crack depth continues to deepen after freezing for 1 050 h. Throughout the freezing process in the extremely cold environment, the performance of fluorinated silicone rubber is better than that of silicone rubber, which indicates that the performance of fluorinated silicone rubber is more stable than that of silicone rubber in extremely cold environment.

In order to accurately evaluate the maximum impulse voltage which corona resistance enameled wire endured, the corona resistance time of enameled wire under different impulse voltage was tested respectively, and the optimal fitting relationship curve equation between them was analyzed. The results show that the corona resistance time decreases exponentially with the increase of impulse voltage. Because the corona resistance time in the fitting equation has practical physical significance, no matter how the corona resistance time changes, it is greater than zero, which can indirectly solve the maximum impulse voltage that the corona resistance enameled wire can endure.