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 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.

In this paper, thermal ageing tests were conducted on the prepared silicone rubber-based flame-retardant transmission insulator samples at 210℃ for different times by high temperature accelerated ageing method, and their performance were tested by scanning electron microscope (SEM), Fourier infrared spectroscopy (FTIR), and contact angle tester. The results show that with the extension of ageing time, the melamine cyanurate (MCA) flame retardant particles in the sample decompose continuously, a large number of pores are formed on the surface of sample, and the infrared intensity of amino and hydroxyl functional groups continues to decrease. The contact angles measured by different solutions show a downward trend, and the erosion of hydrophobicity by the salt solution and acid solution is more serious. The lifetime of samples under the water condition calculated by Hallberg-Peck model and the contact angle is 16.6 years, the lifetime of samples under acid rain conditions is 10.4 years, and the lifetime of samples under seawater condition is 10.2 years.

Partial discharge would accelerate the ageing of power equipment insulation, which is an important monitoring indicator for the condition assessment of power equipment. The research related to partial discharge mainly include denoising of discharge signals, pattern recognition of defect types, equipment status assessment, and fault location of discharge sources. The artificial intelligence can effectively solve the problems of non-linear fitting and optimal solution in partial discharge detection. This paper introduces the detection methods of partial discharge, summarizes the application of artificial intelligence in both denoising of discharge signals and fault location of discharge sources, and points out the shortcomings and solutions in the current research.

In order to improve the accuracy of relative permittivity measurement by means of non-contact method, we built a corresponding model by electric field finite element analysis and obtained the distribution of electric field and potential of the electrode and around. The edge effect of electrode was analyzed and its influences on the measuring result of relative permittivity were investigated. In addition, the influence of potential factors on the measuring error of non-contact method was analyzed, and then the factors which should be taken into consideration in the correction was confirmed, which were the electrode gap distance, the ratio of sample thickness to electrode gap distance, and the relative permittivity. The results show that the measuring error of relative permittivity increases with the increase of electrode gap distance and relative permittivity of the sample, while decreases with the increase of the ratio of sample thickness to electrode gap distance. Under the model with the electrode gap distance of 0.2-5 mm, the ratio of sample thickness to electrode gap distance of 0.5-1, the relative permittivity of 1-20, 2 380 groups of actual and calculated values of relative permittivity are obtained, and the relative error is -0.37%--41.72%. According to the above relationship among the electrode gap distance, the ratio of sample thickness to electrode gap distance, the measured value and real value of relative permittivity, the measured relative permittivity is then corrected by the 3D interpolation method and matching method. The correction method is verified by simulation and is used to correct the measured relative permittivity of high temperature vulcanized silicone rubber. The results validate the proposed method.

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.

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 the actual manufacturing and field installation, defects such as air, metal impurities, and uneven distribution are easily introduced into the silicone rubber insulation layer of cable joint, which endangers the safe operation of power grid system. At present, the detection of power equipment defects are mostly destructive tests, and the experimental results can only reflect the overall situation of equipment. As a non-destructive testing technology, ultrasonic testing can accurately locate and image defects inside the material, and reflect the changes of local performance inside the device. In this study, a set of ultrasonic testing platform were independently designed and built, and ultrasonic testing was conducted on the artificial silicone rubber samples with different defects. The results show that for flexible materials such as silicone rubber, ultrasonic testing can better image the bubbles, air gaps, and steel needle defects in the sample. Compared with similar X-ray testing, ultrasonic testing has an obvious amplification on the tiny bubbles defects. In addition, the ultrasonic testing technology can reflect the change of stress and density inside the material, and can accurately locate the specific depth of the defect according to the ultrasonic echo. This technology has broad application prospects in the location identification and online detection of internal defects in the silicone rubber insulation layer of cable joints.

In order to investigate the influences of dead time in power electronic pulse width modulation (PWM) algorithm on the properties of insulation for inverter-fed motor, we built a repetitive pulse generator with 2-10 μs of dead time using full-bridge solid-state switch and high speed and real time control technique. Under the bipolar repetitive pulse voltage with 4 kV of peak to peak voltage and 5 kHz of frequency, the effect of repetitive square wave voltage dead time on the partial discharge (PD) statistical characteristics and corona-resistant lifetime of inverter-fed motor insulation were investigated through turn to turn insulation tests and ultra-high frequency test. The results show that the increase of dead time may significantly increase the attenuation effect of surface discharge at the rising edge and falling edge of repetitive square wave, weakens the inhibition of surface charge reverse electric field on discharge in discharge process. The number of discharge increases, and the electrical ageing of insulation accelerates, which leads to the decrease of corona-resistant lifetime of turn to turn insulation for inverter-fed motor. It is concluded that when adding dead time to protect the power device in designing power electric system, we should pay attention to the effect of dead time on the accelerated ageing of insulation system in inverter-fed motor, increase the insulation margin properly.