A large number of tests were carried out on XWP2-70 and XP-160 insulators under sunny weather in Wuhan insulator artificial pollution test field, and the dynamic relationship between the first harmonic of leakage current and meteorological factors under different pollution degrees was obtained. The mathematical model of the relationships among the surface salt density of insulators, leakage current, and environment relative temperature under different gray density were established. The relation expression between the first harmonic amplitude of leakage current and the relative humidity under different salt density was predicted by nonlinear regression analysis method, and the quantitative relationship between salt density and the relation expression coefficient was obtained. The results show that the leakage current on insulator surface always reaches the maximum value at 2-6 PM at night, and decreases to the minimum value at 13-17 PM in the day. The overall leakage current curve presents a saddle-shaped curve. The first harmonic of leakage current is positively correlated with humidity and negatively correlated with temperature. Under the same humidity and different temperature, the first harmonics of leakage current are almost the same, but under the same temperature and different humidity, the first harmonics of leakage current change greatly. The leakage current mainly depends on the environment relative humidity, when the relative humidity is higher, the leakage current increases faster with the increase of the relative humidity. Therefore, the influence of humidity should be mainly considered when establishing a pollution prediction model.

The anti-pollution flashover coating in a converter station appears rapid hydrophobicity failure phenomenon in a short time after coating, and there is a large domestic winery within 5 kilometers of it, which belongs to a special operating environment. In order to analyze the reasons for the rapid hydrophobicity failure of the anti-pollution flashover coating in this special environment, two groups of anti-pollution flashover coatings were selected and placed in the converter station to conduct simulation tests, and then the hydrophobicity, roughness, and Fourier transform infrared spectroscopy of the two groups of anti-pollution flashover coatings were analyzed in the field and laboratory. The results show that the hydrophobicity of the anti-pollution flashover coating decreases from grade HC1 to grade HC4-HC5 within 7 months. Affected by the surrounding winery environment, the contamination on anti-pollution flashover coating surface in the converter station contains organic substances such as alcohols and phenols, which affects the migration of small molecular siloxanes, leading to the hydrophobicity failure of the anti-pollution flashover coating, while the coating does not show obvious ageing, and its hydrophobicity can be restored immediately after scrubbing the surface dirt.

In order to study the influence of sand covering at the umbrella skirt of porcelain insulator on the surface electric field and potential distribution, a two-dimensional symmetric electrostatic field model of XP-70 porcelain suspension insulator was established based on the finite element method, and the surface electric field and potential distribution of the insulator under different sand covering conditions were analyzed. The results show that the surface electric field of porcelain insulator shows U-shape distribution, and the electric field and potential are affected by the geometry of porcelain insulator and the medium material. There is a local peak of electric field strength at the junction point between the upper and lower surfaces of the umbrella skirt of each insulator and the fittings. When the insulator umbrella skirt surface is covered with sand, there is a weak effect on the its surface potential, but the field strength at the sand cover of umbrella skirt will decrease, and the field strength decrease amplitude of the insulators near the high voltage side and the low voltage side is greater. The decrease amplitude is not related to the sand layer thickness. With the increase of sand layer thickness on insulator surface, the field strength at the junction between the upper surface of each insulator umbrella skirt and the fittings will increase greatly at first and then decrease to a stable value. When there is no sand belt in the sand layer, the field strength at the no sand belt increases, and the increase amplitude is related to the sand layer thickness and the width of no sand belt.

In this paper, the reasons for the poor oxidation resistance of natural esters were analyzed from their chemical composition, the currently assessing methods and improving techniques of oxidation stability for natural esters were reviewed, and the advantages and shortcomings of different methods were pointed out, which provides a reference for correctly evaluating the oxidation resistance of natural esters.

Aiming at the thermal ageing problem of silicone rubber, a non-linear ultrasonic testing method was introduced to carried out related experimental research. The silicone rubber samples of cable joint were conducted thermal ageing test at 200℃ for different time, and the variation trends of relative non-linear coefficient, micro structure, and surface morphology of the aged samples were tested and analyzed. The results show that with the increase of thermal ageing time, the relative non-linear coefficient of the silicone rubber shows an obvious increasing trend. On the basis of the test results, it is obtained that there is a good linear correlation between the relative non-linear coefficient and the thermal ageing time, and the determination coefficient R² is greater than 0.95. After thermal ageing, the organic content of silicone rubber decreases gradually, the crosslinking degree increases gradually, and the micro defects such as local holes, cracks, and filler precipitation begin to appear on the surface. The microstructure undergoes a certain degree of degradation, which results in the enhancement of ultrasonic non-linear effect.

In order to evaluate the application of silicone gels in the packaging of IGBT power modules, three domestic silicone gels were selected for comprehensive comparison, including the appearance, density, viscosity, mixing ratio, and gel time before curing and insulating properties, oil penetration, damping, adhesion force, and high temperature resistance at 200℃ after curing. The differences of the three silicone gels were analyzed, and IGBT power module was encapsulated with the three silicone gels, respectively, then the encapsulated modules were conducted the temperature cycle test. The results show that the initial viscosity and mixing ratio of silicone gels have an impact on the packaging process and equipment, and the penetration, insulating properties, oil penetration, damping, and adhesion after curing have a guiding role in the selection of silicone gel, but the high temperature resistance and temperature cycling resistance are the most important indicators to evaluate whether the sample can be applied to the packaging of IGBT power module.

In order to reveal the ageing characteristics of metallized film capacitor under ultra-high DC field, the capacitor and its dielectric material were studied. On the basis of the practical working conditions of capacitor, the special designed capacitor elements were conducted ageing tests at 60℃ under the DC field of 0E_n, 1.4E_n, 1.5E_n, and 1.6E_n, respectively. The capacity of the capacitor elements during ageing and the base film (biaxially oriented polypropylene (BOPP) film) after ageing failure were tested. The results show that the failure mechanism of the capacitor is related to the ageing electric field, and there is an electric field threshold. When the ageing electric field is lower than the electric field threshold, the capacitor will go through a long-term failure, and its capacitance decreases affected by the performance degradation of film, the electrical strength of the film decreases with the decrease of the activation energy of molecular segment motion. When the ageing electric field is higher than the electric field threshold, the capacitor will go through a short-term failure, and the extremely high electrical stress causes the original weakness to break down rapidly, the self-healing area increases rapidly, so the equipment life is greatly shortened.

Medium frequency transformers have high power density, small volume, complex winding structure, and strong local electrical stress, so the design of transformer must fully consider the electro-thermal stress margin to ensure reliability. In this paper, the method of adding electrostatic ring and angle ring were proposed to suppress the electric field strength at the end of primary winding, and the electric field strength distribution of multi-winding transformer was calculated by electrostatic field finite element analysis method. Combined with the sensitivity analysis method, the influence of the size parameters of electrostatic ring and angle ring on the electric field strength was studied to determine the decisive factors of the maximum electric field strength. The response surface function related to electric field strength was constructed by response surface method to optimize the insulation structure of transformer. The results show that the optimal sizes of electrostatic ring and angle ring are obtained by the above response surface function through taking a 50 kW, 10 kHz multi-winding medium-frequency transformer as an example, which can reduce the maximum electric field strength by 15.4% and 17.49% under short-term power frequency withstand voltage test and rated operating conditions, respectively. It is proved that the response surface method can be applied to the insulation structure design of multi-winding transformer.

In order to study the effect of winding preload on thermal ageing characteristics of insulating pads, four groups of insulating pads were conducted accelerated thermal ageing tests at 135℃ after appling 0, 200, 500, and 1 000 N of preload, respectively, and the mechanical properties of insulating pads were tested. At the same time, in order to simulate the mechanical ageing of insulating pad under the influence of winding vibration in the actual working condition, a sinusoidal cyclic mechanical load with 5 N of amplitude and 2 Hz was applied on the accelerated thermal ageing of insulating pad with 200 N of preloading force. The results show that that thermal ageing causes the stress-strain curve of insulating pad gradually shift to the right under 0 N preload, and its elastic modulus decreases gradually. The elastic modulus of insulating pad decreases to 79.935% of the new insulating pad after accelerated ageing for 336 h at 135℃. Applying preloading force can slow down the thermal ageing of insulating pad, and the greater the preloading force, the lower the ageing rate and ageing degree of insulating pad. But applying cyclic mechanical load will accelerate the ageing of insulating pad. The research results have certain reference and practical application value for the research of insulation pad ageing and transformer vibration.

Biaxially oriented polypropylene (BOPP) film is an important part of capacitor. The key parameters (ash content, isotacticity, molecular weight, melt flow rate, isotactic sequence length) of polypropylene resin used as raw materials for BOPP films and their influence rules were summarized in this paper. At the same time, the existing production technology of BOPP capacitor film was summarized, and the methods to improve the performance of BOPP film were further reviewed. Finally, it is proposed that the future development of BOPP films for capacitors will focus on the ultra-low ash resin raw materials and heat resistant films.