Firstly, oil-impregnated insulating papers with different ageing states were prepared, and their breakdown voltages under pulsating DC voltage were measured and compared with AC and DC breakdown voltage. The polymerization degree, moisture content, mesopore distribution, and crack characteristics of oil-impregnated insulating papers were measured to explore the possible influencing factors of breakdown voltage under different ageing states. The results show that the AC breakdown voltage is the smallest, and the pulsed DC breakdown voltage (r=1/5) is the largest before failure of oil-impregnated insulating papers. During thermal ageing process, the breakdown voltage fluctuates occasionally or even increases. Compared to AC breakdown voltage, the breakdown voltage under DC and pulsed DC voltages (r=1/5 and 1/3) shows opposite trends with ageing time. The water content and oil absorption capacity of oil-impregnated insulating papers, and the damage to the paper structure are the reasons causing the breakdown voltage to change with ageing time, but the importance of reasons vary with the ageing state.

The degree of partial discharge in transformer is an important basis to reflect the ageing degree of transformer insulating oil and evaluate the service life of transformer oil. In this paper, six kinds of transformer insulating oil with different water content were conducted partial discharge tests by two different test methods and four electrode arrangements. The influence of different test methods and electrode arrangements on the partial discharge initial voltage was analyzed. The results show that the partial discharge initial voltage is not only related to the physical and chemical properties of liquid dielectric, but also affected by the electrode arrangement, water content, and test methods. The test methods affect the dispersion of test results. The water content would change the physical and chemical properties of liquid dielectric, thereby it affects the partial discharge initial voltage from the aspects of macro dielectric property and micro ionization process. With the increase of water content, the electrode arrangement and test methods have less influence on the partial discharge initial voltage. The main factors affecting the partial discharge initial voltage of insulating oil with different water contents change from electrode arrangement and test method to the physical and chemical properties of insulating oil.

Polluted insulators are easy to discharge or even flashover when the surface is wet to a certain extent, which seriously threatens the safety and stability of power grid. In this paper, in view of three kinds of common insoluble contaminates SiO₂, Al₂O₃, Fe₂O₃, we conducted water absorption tests and insulator flashover tests under different environmental humidity, and the influence of environment humidity on the water absorption and flashover properties of insulators polluted with typical insoluble contaminates were analyzed. The results show that the SiO₂ has the best water absorption property, the Al₂O₃ is followed, and the Fe₂O₃ has extremely weak water absorption property or cannot absorb water absolutely. The moisture absorption rate and saturated moisture content of SiO₂ and Al₂O₃ tend to increase with the increase of environment humidity. In the situation of saturated humidity, the surface contamination of insulators polluted by SiO₂ and Al₂O₃ may be washed away with the dripping process of liquid droplets. The pollution flashover voltage gradient of insulators decreases with the increase of environmental humidity, and this phenomenon is more obvious in low humidity.

In order to realize the detection of surface discharge by the temperature sensor inside the composite insulator sheath, we used silicone rubber plate as research object, and established a thermal model to realize the indirect calculation of surface temperature on the silicone rubber plate based on internal temperature. Firstly, according to the heat transfer theory, a thermal model of silicone rubber plate was established. Considering the influence of nonlinear factors in the process of atmospheric convection, the air convection index n was introduced to modify the model, and the solution method of convection heat transfer coefficient h was proposed. Then, temperature rise tests were carried out to simulate the heating condition of silicone rubber plate surface, and the temperature data of the upper and lower surfaces of silicone rubber plate were measured. The measured value of upper surface was compared with the calculated value of model. The results show that the relative error of the steady-state temperature of silicone rubber plate surface calculated by the model based on internal temperature is less than 10%, which meets the accuracy requirement.

The traditional electromagnetic wire appearance defect detection uses manual detection, which is low efficiency and high cost. With the continuous improvement of requirements of motor on the appearance quality of electromagnetic wire, it is necessary to enhance the identification efficiency of electromagnetic wire appearance defects. A visual identity system of electromagnetic wires defects was developed, and the practical application effect of the system was verified. The results show that the defect visual identity system can improve the detection rate of the thin film sintering line appearance defects. By analyzing the statistical types and quantities of defects, the problems of raw materials or the production process problems of electromagnetic wire can be found in time, so as to formulate corrective measures in time and reduce the rejection rate of the thin film sintering line.

Polyimide (PI) has been widely used as a kind of special engineering plastic. However, the further industrialization development of PI is limited due to its poor melting property. Thermoplastic polyimide (TPI) is developed by modifying the thermosetting PI, and its melting property and processability are improved qualitatively compared to thermosetting PI, so that it can be applied to flexible copper clad laminate, 3D printing and other fields. In this paper, the synthetic method, thermal, mechanical, and processing properties of TPI were summarized, and the application of TPI in flexible printed circuit and other important industrial fields were concluded. The future research directions of TPI were proposed.

Natural ester insulating oil has higher ignition point and flash point, excellent natural degradation ability and good insulation performance, and it is a green substitute for mineral insulating oil. Due to the physical and chemical characteristics of natural ester insulating oil, it is still facing many challenges to widely used in transformer equipment. In this paper, the preparation process of natural ester insulating oil was reviewed combining with its shortcomings. The research results of natural ester insulating oil were expounded from the aspects of additive modification, mixed modification, nano modification, and chemical modification. Finally, the future research direction was prospected.

The effect of nano-Al₂O₃ on the microscopic properties of vegetable insulating oil was studied by molecular simulation, and its result was verified by tests. The mechanism of surface interaction between nano-Al₂O₃ and vegetable insulating oil molecules was analyzed, models of vegetable insulating oil before and after nano-Al₂O₃ modification were established. The hydrogen bond, radial distribution function (RDF) of oil molecule, and diffusion coefficient of water molecule were studied, and the thermal ageing experiment was conducted on vegetable insulating oils with different concentration of nano-Al₂O₃. The results show that compared with the vegetable insulating oil without modification, there are more hydrogen bonds in the modified vegetable insulating model, the peak value of RDF is greater, and the diffusion coefficient of water molecules is smaller. In the process of thermal ageing, the dielectric loss of the modified oil is smaller than that of the unmodified oil, which shows that the nano-Al₂O₃ modified vegetable insulating oil has excellent stability, and its thermal stability and insulation performance are enhanced.

In order to discover the bonding mechanism of self-adhesive silicone rubber tape, we prepared a self-adhesive silicone rubber tape through the reaction of boric acid and methyl-vinyl silicone rubber. The variation of bonding force at interface of self-adhesive silicone rubber tape with temperature and soaking time was traced by tensile tests, and the self-fusion effect was verified. A physical model of self-fusion effect was proposed with the help of infrared spectrum test. The results show that when the content of boric acid in the reaction system is 2.5%, the self-adhesive tape has the highest initial bonding force. When silicone rubber has the molecular structure of short chain boron-containing siloxanes imbedded in long chain siloxane network, the well contacted silicone rubber interfaces can bond through complexation between boron and oxygen atoms in the boron-containing siloxanes, and the bonding force is stronger than the cohesion among polysiloxane molecules. The complexation can not complete instantly, and the higher the temperature of interface, the shorter the time of complexation. Under 60℃, the complexation strength can reach the optimum value within 12 hours.

An aramid 1313 fibrid was prepared by precipitation method, and its morphology structure and properties were characterized by fiber length test, thermogravimetric analysis (TGA), X-Ray diffraction (XRD), scanning electrical microscopy (SEM). The dewaterability of aramid 1313 fibrids and chopped fibers in the papermaking process was evaluated by dewatering time, and the mechanical properties and electrical insulation properties of the paper were studied. The results show that with the increase of molecular weight of aramid 1313 fibrid, the average length of fibrids increases, and the tensile strength, elongation at break, tearability, and insulating properties of the paper increase. The fibrid is non-crystalline structure. With the increase of molecular weight, it becomes to film structure, and the thermostability increases. However, the thermostability of chopped meta-aramid fiber is still higher than that of aramid 1313 fibrids.