In order to solve the problem of transformer oil leakage caused by the ageing failure of rubber sealing rings, the rubber sealing rings was conducted life evaluation research. Since there is low oxygen content and a small amount of H₂S retained as an oxidation inhibitor in the transformer insulating oil, and the sealing rubber ages faster in a strong oxidizing medium, we choose H₂S+hot air and hot air environment to conduct ageing tests. The dynamic curve direct method was adopted. According to the relationship between the performance change of rubber and ageing time and the Arrhenius model, accelerated ageing tests were carried out at three different temperatures, and the compression set rate was used as the performance index. The relationship between the performance change rate constant k and the life estimation value t under two environment was obtained to estimate the service life. The results show that the performance of rubber sealing ring degrade in different degrees under hot air and H₂S+hot air environment. With the increase of ageing time and ageing temperature, its hardness increases, and the elastic recovery ability changes to be poor. According to the established life estimation formula, the service life of EPDM rubber sealing ring is predicted to be 33 years and 35 years under hot air and H₂S+hot air environment, respectively.

The insulation defects are likely to occur during operation of on-board transformers due to the harsh operating environment, resulting in the appearance of partial discharges and the degradation of oil-paper insulation, which might affect the safe and stable operation of trains. Therefore, according to the inter-turn insulation structure of on-board transformer, a three-layer oil-paper partial discharge model was designed by using aramid paper, and partial discharge simulation tests were carried out in lab. The partial discharge signal was recorded by partial discharge detector, and the dynamic change rate of discharge statistical parameters was proposed to characterize the development process of partial discharge. The results show that the discharge phase spectrograph of oil-immersed aramid insulating paper is symmetrically distributed, and the discharge magnitude rapidly increases to nC level after initial discharge. The discharge process of oil-immersed aramid insulating paper can be divided into four stages, including starting stages, development stages, dangerous stages, and breakdown stages according to the discharge phase spectrograph and dynamic change rate of discharge statistical parameters. Besides, the dynamic change rate of discharge repetition rate has an inflection point during discharge process, which can be used to evaluate the partial discharge development process of oil-immersed aramid insulating paper.

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.

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.

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.

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.

As the weakest insulation link of gas insulated substation (GIS), the surface defects of basin-type insulator are the key problems in the research of improving the insulation strength of GIS. This paper firstly introduced the generation and development mechanism of common surface defects of basin-type insulator, such as bubble defects, impurity defects, dirty surface, and crack defects. Secondly, the mechanism of flashover induced by surface defects of basin-type insulator was analyzed and summarized, it is explicit that the surface electric field distribution and surface charge accumulation of basin-type insulator are the important factors affecting the flashover of insulator; In addition, the advantages and disadvantages of the existing diagnosis methods of basin-type insulator surface defects were summarized. Finally, In view of the shortcomings of partial discharge detection methods for basin-type insulator surface defects under power frequency voltage excitation, starting with the voltage mode of partial discharge excitation, the current situation of partial discharge detection methods for basin-type insulator surface defects under different voltage excitation was studied and summarized.

Natural ester insulating oil is a kind of environmentally-friendly liquid dielectrics with good insulation properties, and it has been generally applied in oil-filled electrical equipment. Investigating the types and contents of fatty acids in different natural ester insulating oils will be beneficial to grasp the physical and chemical properties of different insulating oils. In this paper, a method of determining the fatty acid composition in natural ester by ¹H-NMR was presented, and the results were compared with that of GC-MS. The results show that this simple and rapid determination of fatty acid content in natural ester insulating oil by ¹H-NMR is feasible and reliable. The maximum error absolute value of fatty acids content measured by ¹H-NMR spectroscopy and GC-MS is 2.95%, which is in the tolerance interval.

In order to explore the influence of high voltage cable oil filled terminal grounding system defects such as lack of copper net and lead seal defects of on the voltage and current of oil filled terminal, we built an RLC equivalent model of oil filled terminal. On the basis of capacitance parameters and test parameters computed by COMSOL, the change of terminal voltage and current characteristic under the defects was analyzed, and then it was verified by actual fault cases. The results show that the loss of copper net leads to the grounding failure of the outer semi-conductive layer, and the voltage of the outer semi-conductive layer increases significantly, results in the discharge. The lead seal fracture leads to the unreliability of the connection between the aluminum sheath and the tail pipe, results in the voltage suspension of terminal aluminum sheath, and causes the discharge between the aluminum sheath and the buffer layer under long-term operation. Defects between the lead seals reduce the contact area between the aluminum sheath and the lead seal, the minimum allowable contact area at the lead seal is about 10%.

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.