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.

Aiming at the application requirements of rail transit, we studied and compared the performance of A766 aramid insulation paper and the same types of aramid insulation paper at home and abroad. The results show that the composition and structure of A766 aramid insulation paper and the imported aramid insulation paper are basically the same. The temperature index of A766 aramid insulation paper is 223℃, the oxygen index is 30.2%, the smoke density is 1.4, and the smoke toxicity is 0.875, which meets the HL2 and HL2 of R22 and R23 fire rating standards. A766 aramid insulation paper has higher voltage withstand retention rate after immersion in water, the retention rate of 0.05 mm aramid insulation paper is 83.5%, and the retention rate of 0.13 mm aramid insulation paper is 73.8%. The Shawn’s experiment shows that the thermal oxygen ageing property resistance to copper/aluminum metal conductor of A766 aramid insulation paper is the same as that of the imported aramid insulation paper. Therefore, A766 aramid insulation paper can meet the application requirements of rail transit.

In order to study the ageing process of composite insulator silicone rubber in the hot and humid climate in the southern region, we conducted 300 h, 500 h, and 1 000 h salt spray ageing tests on silicone rubber in the laboratory. The concentration of small molecule groups on the surface of silicone rubber with different degrees was detected by time-of-flight secondary ion mass spectrometry (TOF-SIMS), and the ageing mechanism was analyzed combing the hydrophobicity and scanning electron microscope test results. The results show that Si⁺, C₂H₅⁺, SiCH₃⁺, SiC₃H₉⁺, H^-, CH^-, O^-, and OH^- were detected on the surface of silicone rubber. The hydrophobicity of silicone rubber is positively correlated with the content of small alkyl molecules. The content of small alkyl molecules on the surface of silicone rubber is consumed by drip washing of salt spray, which lead to the decrease of hydrophobicity. In the salt spray atmosphere, oxidation and hydrolysis reactions are reacted on the surface of silicone rubber, which results in the broken of Si-O and Si-C bonds, and Si-OH bonds are formed. The hydroxylation lead to the collapse of silicone rubber three-dimensional network structure and the decrease of polymerization degree. The gases such as H₂ and CH₄, generated by the ageing reaction, would form voids on the surface of silicone rubber, which would cause powders and cracks on the surface of silicone rubber.

In order to better guide the application of composite insulation crossarms, the lightning impulse voltage, wet power frequency flashover voltage, pollution flashover voltage, and pollution snow-covered flashover voltage of the square tube composite insulation crossarm and the square bar composite insulation crossarm were compared and studied through experiments. The results show that the 50% dry lightning flashover voltage of the 10 kV square tube composite insulation crossarms is 388.3 kV, which is higher than that of the square bar composite insulation crossarm (370.0 kV). However, the wet power frequency withstand voltage, pollution withstand voltage, and pollution snow-covered flashover voltage of the square tube composite insulation crossarms are lower than that of the square bar composite insulation crossarms. Finally, the different application scheme of the composite insulation crossarms under different operating environment is proposed.

At first, the insulation level of returned cable was analyzed by ultra-low frequency (VLF) dielectric loss test method and microscope observation method. And then, the organic-inorganic composite repair fluid was injected into the cable core through a repair system. After repairing for 24 h, the generated filler was analyzed by scanning electron microscopy (SEM) and infrared spectroscopy (IR), and the insulation level of the cable after repairing was analyzed by VLF dielectric loss test method and breakdown voltage test method. The microscopic test results show that after injecting the organic-inorganic composite repair fluid, the micropores and defects will be filled during the penetration and reaction process of repair fluid in cable insulation layer, both the number and size of micropores in repaired cable insulation are smaller than those in unrepaired sample, at the same time, the hydroxyl absorption peak is reduced significantly. The VLF dielectric loss test results show that the performance indexes of the cable after repairing is enhanced, and the breakdown voltage is improved. It is proved that the organic-inorganic composite repairing technology can improve the insulating properties of field returned cable effectively and prolong the cable insulation life.

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.

Through adding 2-methyimidazolium cobalt (ZIF-67) nano-fillers into 3,3′,4,4′-biphenyltetracarboxylic anhydride (BPDA) - 4,4′-diaminodiphenyl ether (ODA) type polyimide (PI) matrix, we prepared a PI/ZIF-67 three-layer composite films with sandwich structure. The structure of ZIF-67 and PI/ZIF-67 three-layer composite films were characterized by FTIR, XRD, and SEM, and the effect of ZIF-67 content on the thermal stability and dielectric property of the composite films was studied. The results show that when the mass fraction of ZIF-67 is less than 10%, the initial decomposition temperature of composite film is higher than 500℃, which shows good thermal stability. The dielectric constant of PI/ZIF-67 composite films is obviously lower than that of PI. When the mass fraction of ZIF-67 is 10%, the dielectric constant of PI/ZIF-67 composite film decreases by 50%. When the mass fraction of ZIF-67 is 5%, the dielectric constant of PI/ZIF-67 composite film decreases by 71%. When the mass fraction of ZIF-67 is less than 10.0%, the dielectric loss of PI/ZIF-67 composite film is slightly higher than that of pure PI.

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.

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.

Three typical stator defect models were fabricated using the generator stators in actual operation, and then conducted accelerated thermal ageing tests. The difference on partial discharge characteristics of the three typical defects before and after ageing was studied using pulse current method. The results show that after ageing, the partial discharge inception voltage (PDIV) of the model with internal defect increases, while the PDIV of the model with slot discharge and end discharge decrease, this is because there are large number of cracks on the epoxy mica tape insulation surface due to thermal ageing. The phase resolved partial discharge (PRPD) pattern of internal discharge model is symmetrical, and the width of discharge phase becomes narrow after thermal ageing; the PRPD pattern of slot discharge model changes from hill shape to semi ellipse shape after thermal ageing, and the discharge density increases significantly; the PRPD pattern of end discharge model shows the characteristics of corona discharge, and the discharge characteristics at the positive and negative half-cycle of alternating current is obvious asymmetry.