In order to master the viscosity, thermal conductivity, electrical conductivity, and dielectric properties of transformer oil with different moisture content from normal temperature to extreme cold, we prepared four samples of 45^# transformer oil with different moisture content, and tested their viscosity, thermal conductivity, electrical conductivity and dielectric properties from +30℃ to -50℃. The results show that with the decrease of temperature, the viscosity of transformer oil increases exponentially, the higher the moisture content, the higher the viscosity of transformer oil. The thermal conductivity is less affected by temperature and moisture content. The electrical conductivity and dielectric loss factor increase at first and then decrease with the decrease of temperature, the higher the moisture content, the higher the peak value of conductivity and dielectric loss factor, and the larger the corresponding peak temperature. While the relative dielectric constant increases approximately linearly with the decrease of temperature, the variation trend of relative dielectric constant of transformer oil with different moisture content is the same and the difference is small.

Using bipolar repetitive square wave voltage, with the help of ultra-high frequency testing method, the influence of the voltage amplitude, frequency, duty ratio and rise time of repetitive square wave on the frequency domain energy distribution of inverter-fed motor inter-turn insulation discharge was experimentally studied. The results show that that the frequency domain energy of discharge above 0.50 GHz increases significantly with the increase of duty ratio; the energy of each frequency band increases with the increase of the pulse voltage amplitude, and the high-frequency energy above 1.50 GHz continues to attenuate with the increase of frequency; the rising rate of pulse voltage has a significant impact on the energy of the discharge frequency domain. The high-frequency energy of the discharge pulse at 1.00-2.00 GHz increases with the increase of the voltage rising rate. When designing the UHF sensor for PDIV test of inverter-fed motor insulation under high-frequency and steep pulse voltage, the influence of the above-mentioned pulse parameters on the energy distribution in frequency domain should be fully considered to improve the sensitivity and signal-to-noise ratio of the PDIV test.

The interface pressure between the cable accessories and the body is the key to the stable operation of the cable. In order to study the influence of long-term cooling and heating cycles on the interface pressure of cable accessories, we designed a test system to simulate the operation state of cable using the temperature control mode of cooling and heating cycles. The dynamic thermo-mechanical analysis of silicone rubber materials before and after the experiment was carried out. The results show that the elastic modulus of silicone rubber material under long-term cooling and heating cycles increases significantly at the test temperature. In addition, with the increase of ageing time and ageing temperature, cable accessories deform irreversibly, resulting in the decrease of interface pressure and the sealing performance. The experimental results are verified by finite element simulation of interface pressure based on cable accessory parameters.

In order to study the failure mechanism of water-blocking buffer layer in high voltage power cable, especially the electrochemical corrosion characteristics, we analyzed the composition of the white powders from failed cables, and confirmed the production mechanism of different composition in the white powders combining with theoretical analysis and simulation experiments. The moisture absorption characteristics of the buffer layer was tested, and the influence of the water content and current density of the buffer layer on its electrochemical corrosion characteristics was studied. Finally, based on electrochemical impedance spectroscopy, the influence of electrochemical corrosion on the electrical characteristics of the buffer layer was identified, and the evolution rule of the buffer layer from electrochemical corrosion to final ablation was determined. The results show that moisture ingress due to dampness or mechanical damage of the cable sheath causes electrochemical corrosion of the buffer layer and leads to the generation of white powder (consisting of NaHCO₃ and Al(OH)₃) and results in the final failure of the cable. The corrosion rate is closely related to the current density, while the final corrosion degree mainly depends on the moisture content of the buffer layer.

In this paper, a photoelectric joint detection system composed of a photomultiplier tube and an electric field sensor was used to carry out the discharge tests of ball-plate electrode gap with burr of different gap distances under positive switching impulse voltage. Compared with the discharge tests of the rod-plate and that of ball plate, the 50% discharge voltage(U_50%) and the physical characteristic parameters under the initial streamer were obtained and compared. The results show that in the ball-plate electrode gap with burr defects, the initial delay, the field strength jump value, and the light power jump value of the initial streamer are positively correlated; in the initial stage of streamer, the burr can shorten the initial delay of discharge, especially for short gap. After the initial streamer developing, the development law of initial streamer of ball-plate electrode with burr is similar to that of rod-plate electrode gap, and the burr aggravates the development of discharge. For the whole discharge process, the burr defect will greatly reduce the discharge voltage of the ball-plate electrode, so that it is close to the discharge voltage of the rod-plate electrode gap under the same clearance.

The electret effect can improve the adsorption capacity of the filter membrane for polar impurities. In this paper, BaTiO₃ doped composite nanofiber filter membrane for transformer insulating oil was prepared by combining electret technology with electrospinning process. The electrospinning conditions for preparing PVDF nanofiber membranes with small diameters and pores without beading were obtained by system study on the relationship between electrospinning parameters and the morphology of fiber membrane. And then the regeneration experiment for transformer insulating oil was carried out. The results show that the filter membrane can remove polar substances, and has high specific surface area and high porosity. After doping BaTiO₃ and corona discharge treatment, the filter membrane can be stably charged. The dielectric loss factor and breakdown voltage of the aged insulating oil are significantly optimized after filtering by the BaTiO₃ doped composite nanofiber filter membrane. Compared with other filter membrane electret-doped composite nanofiber membranes, it has better regeneration effect on transformer insulation oil.

The research status and development tendency of semi-alicyclic colorless and transparent polyimide(CPI) film was reviewed from several aspects, including its developing history, important monomers, resins synthesis technology, film production technology, and the practical applications of CPI films in advanced optoelectronic fields. Emphatically, the current hot-topics in the development of semi-alicyclic CPI films were presented, and the future developing trends of semi-alicyclic CPI films were prospected.

During the normal operation of converter transformer, moisture will cause the decline of the electrical performance of oil-paper insulation and accelerate its ageing, resulting in a decrease of life and insulation breakdown. In order to study the influence of moisture on the space charge characteristics and breakdown characteristics of oil-paper insulation under AC and DC composite voltage, we prepared oil-paper insulation samples with different water content, then we used space charge measurement under AC and DC composite voltage to get the influence of moisture content on the space charge characteristics of oil-paper insulation under the voltage forms of AC, DC, and AC and DC composite voltage, and studied the breakdown characteristics of samples with different moisture contents under different voltage types. The results show that the electric field distortion rate of oil-paper insulation under AC and DC composite voltage is greater than that under DC and under AC electric field; as the DC component increases, the increase amplitude of breakdown voltage of the samples with higher water content is significantly lower than that of samples with lower water content, and the electric strength of samples with low water content is more affected by the DC voltage component than that of samples with high water content.

During the operation of high-voltage DC cables, space charge generates in the insulation layer, it leads to electric field distortion and even insulation breakdown. In order to study the effect of space charge in the actual operation of cable, we obtained the temperature field of high-voltage direct current submarine-land composite cable under the conditions of rated current, maximum steady-state current, and short-term overload current, respectively. According to the simulation results, the temperature difference in the space charge test was set as 10℃, 20℃, and 40℃, respectively, and then the temperature field of 400 kV high-voltage DC cable under different condition was simulated. The results show that the temperature difference between the two sides of XLPE insulation layer is 8.1℃ and 18.7℃under rated current and the maximum steady-state current, respectively; while the temperature difference is 61.1℃ under the short-term overload for 1 hour; and that is 41.1℃ under short-term overload for 36 hours. The experimental results show that the temperature gradient enhances the accumulation of heteropolar charge at the low temperature side of positive pole of XLPE, which leads to the distortion of electric field. Through the correction computation of the electric field distortion rate, it is found that the maximum electric field distortion rate has a linear relationship with the average electric field strength. With the increase of temperature gradient, the electric field distortion rate also increases. The maximum electric field distortion rate is 1.68 under the temperature gradient of 40℃ with the applied electric field of 50 kV/mm.

On the basis of a new type of three-element mixed insulating oil with excellent physichemical and electrical properties, and taking the mineral insulating oil-paper composite system as a reference, the creeping discharge characteristics of the three-element mixed insulating oil-paper composite system were analyzed under needle-plate electrode structure. The electric field distribution of two insulating oil-paper composite systems were simulated under needle-plate electrode structure. On the basis of density functional theory, the energy band structures of natural ester molecule and mineral oil molecule were studied comparatively. The results show that compared with the mineral insulating oil-paper composite system, the three-element mixed insulating oil-paper composite system has smaller partial discharge inception voltage, flashover voltage and larger maximum discharge magnitude, discharge pulse number under creeping discharge fault. There is small difference on electric field distribution between two insulating oil-paper composite systems under needle-plate electrode structure. The energy gap of palm oil molecule and soybean oil molecule in mixed oil are smaller than that of the mineral oil molecule, therefore, the mixed oil molecule is more likely to ionize under strong electric field.