In order to investigate the optical properties of epoxy-mica insulation in ageing process, we conducted multi-factor accelerated ageing treatment on epoxy-mica main insulation of generator stator bars. The blue whiteness of the samples at different voltage and temperature was measured, and the effect of ageing on the optical refractive index and absorption ratio of epoxy-mica insulation material was analyzed. The equation relationship between blue whiteness and ageing time was determined by regression analysis, and the validity of equation was proved by t test. Finally, the insulation life of epoxy- mica main insulation was fitted and analyzed by the variation characteristics of blue whiteness combining with Fallou model. The results show that the blue whiteness value of epoxy-mica insulation decreases constantly with the increase of ageing degree of insulation. Combining Weibull distribution probability with Fallou model, we can realize the life assessment of epoxy-mica insulation.

The traditional insulation reliability evaluation method based on failure life data for solid dielectric need long test time, and the failure data is little and difficult to obtain. In view of this issue, a reliability assessment method based on the test data of accelerated performance degradation was studied in this paper. Firstly, the average charge density was used as the characteristic parameter of performance degradation, and accelerated performance degradation tests were conducted to obtain performance degradation data under 140 kV/mm, 160 kV/mm, and 180 kV/mm of field strength, respectively. Secondly, the pseudo-failure life of XLPE insulation material under different field strengths was calculated, and its statistical distribution was determined. Finally, the pseudo-failure life data was expanded in combination with the virtual augmentation theory, and the expanded data was analyzed by statistical analysis methods. The results show that the performance reliability indicators such as sample reliability, failure distribution density function, and average life can be obtained through using accelerated performance degradation test method to assess the reliability of XLPE insulation. In combination with the accelerated life model theory, the life expectancy of the XLPE insulating material is 15.3 years under 20 kV/mm of stable field strength, and the reliability when reaching the expected life is 0.445 1.

In order to research the influence of processing temperature of master-batch on the properties of HVDC cable insulation, three kinds of master-batch were prepared at different processing temperatures, and then the insulating properties of HVDC cable insulation manufactured by the master-batch were studied. Impurity detection was carried out at first, and then the chemical structure of crystal points detected by impurity detection was analyzed by attenuated total reflectance infrared spectroscopy (ATR-IR). At last, the electrical and mechanical properties of three HVDC cable insulation were compared. The results show that there are more crystal points detected in the HVDC cable insulation prepared by the master-batch under an excessive processing temperature, and part crystal points are light yellow. Compared with semitransparent crystal points, there is no strong absorption peaks in the infrared spectra of light yellow crystal points, but the excessive processing temperature has a greater negative effect on the electrical properties of HVDC cable insulation, while has a smaller effect on its mechanical properties.

In order to apply the frequency domain spectroscopy test technology to nondestructively test of ageing state for large electric machine stator bar insulation, we conducted accelerated ageing tests on the epoxy-mica insulation of stator bar under the combined action of multiple factors such as heat, electricity, and machinery, and tested the frequency domain spectroscopy of samples with different ageing degrees. The Davidson-Cole model parameters of frequency domain dielectric response were extracted as characteristic parameters by least square method, and the ageing degree of epoxy-mica insulation was analyzed according to the parameters. Finally, the relationship between characteristic parameters and ageing time was established by fitting. The results show that the dielectric loss factor (tanδ) of epoxy-mica insulation increases with the increase of ageing degree, especially at the frequency of 10^-3–10^-1 Hz; the characteristic parameters β and τ extracted by the Davidson-Cole model can be used to characterize the ageing degree of insulation. With the increase of ageing time, β decreases linearly, τ decreases exponentially, while Δε increases obviously. A formula for quantitatively characterizing the degree of insulation ageing is obtained through the fitting relationship between τ and ageing time, which can be considered as an empirical formula for evaluating the ageing degree.

In areas with large temperature difference between day and night, negative pressure is easy to form in the expander of oil-immersed inverted current transformers, which will affect the distribution of water in the oil-paper insulation, and then affect the insulating properties of the oil-paper insulation system. In this paper, the saturation solubility of water in insulating oil under different pressure and temperature was measured by indirect method. On the basis of the test results, the relationship between the saturation solubility of water in insulating oil and temperature and voltage in Arrhenius equation was fitted by recursive least square method, and the effect law was studied. The results show that with the increase of negative pressure, the water saturation solubility in insulating oil decreases constantly, and this trend becomes more obvious with the increase of temperature. Under the effect of negative pressure, water in the oil separates out and turns into suspended water in insulating oil or transfers to insulating paper. The translation factor n related to pressure was introduced in this paper, and a mathematical expression involving pressure, temperature, and water saturation solubility in oil was established. The acid value, temperature, and pressure were taken as the comprehensive factors to evaluate the water saturation solubility in insulating oil, and the multi factor influence law was established, in which the influence of acid value and pressure can cancel with each other to some extent. The water distribution and insulation state of the oil-paper insulation system for oil-immersed inverted current transformers can be evaluate more accurately by the above research results.

In order to obtain a silicone rubber composite with both good resistivity nonlinear characteristics and high electric breakdown characteristics, a copper calcium titanate (CCTO) ceramic powder was prepared, and the CCTO was mixed with two-component liquid silicone rubber to prepare CCTO/silicone rubber composite (the volume fraction of CCTO ceramic powder was 3%, 5%, and 10%, respectively). Their microstructure, dielectric spectrum characteristics, DC breakdown performance, and DC nonlinear resistivity were tested, and their application performance was compared by establishing a high-voltage DC cable terminal simulation model. The results show that the CCTO ceramic powder has obvious characteristic diffraction peaks, and the powder particle size is about 500 nm; with the increase of CCTO ceramic powder content, the dielectric constant of the CCTO/silicone rubber composite increases, the DC breakdown strength decreases, and the non-linear characteristics of resistivity are significantly enhanced. With the enhancement of the nonlinear characteristics of resistivity, the electric field strength in the reinforced insulation of cable terminal stress cone decreases significantly. When the volume fraction of CCTO ceramic powder in the composite is 5% or 10%, the composite can be used as a reinforced insulating material of stress cone for high voltage DC cable accessory.

The thermal ageing behavior of insulating paper and the evolution of refractive index of cellulose fiber during thermal ageing process were explored. Accelerated thermal ageing experiments were carried out on the oil-paper samples at 120℃ and 130℃, respectively. The degree of polymerization, furfural content, and carbon and oxygen gas content were measured at different ageing periods, and their changing laws were explored. According to the change trend of the crystallinity of insulating paper in the test, a reasonable inference was made about the change of its refractive index, and further analysis was carried out by the Abbe refractometer. On the basis of dispersion staining color method, the evolution rule of refractive index of the oil-paper cellulose fiber during thermal ageing was explored. The results show that as the ageing progresses, the degree of polymerization decreases, while the furfural content and the carbon oxygen gas content increase, the refractive index and crystallinity of the insulating paper increases, and the refractive index of the oil-paper cellulose fiber increases also. The refractive index of cellulose can be considered as a new type of ageing characteristic of insulating paper.

A high temperature resistant insulating varnish for traction motors was prepared by modifying polyester-imide insulating varnish with meta-aramid resin. The effects of meta-aramid resin on the adhesion, electrical properties, flame retardancy, and heat resistance of the insulating coating were studied. The results show that when the content of meta-aramid resin is 15%, the electrical strength, the adhesion, the oxygen index of the insulating coating reaches 65 kV/mm, 12.9 MPa, 28.7%, respectively. After high temperature test at 220℃ for 500 h, the paint film shows slight discoloration without cracking and peeling off, which can meet the requirements of the traction motors.

According to the special requirement of all glass fiber reinforced resin matrix composite (GFRP) transmission tower head on the fastening strength and insulation performance of connecting parts, GFRP insulated fasteners were developed. A medium-high temperature curing epoxy bulk molding compound (BMC) was designed and developed, and its initial viscosity decreases through blending modification of bisphenol A and bisphenol F epoxy resin. The mechanical properties and water absorption of samples with different ratio of Al(OH)₃, talcum powder, and SiO₂ fillers were tested to determine the material system that meets the requirements of molding process. Non-isothermal DSC measurement was conducted on the optimized material system, and T-β extrapolation and orthogonal tests were applied to evaluate, optimize, and determine the forming process parameters. The results show that the failure load, shear strength, and electrical properties of the product prepared by the optimal material system and forming process parameters meet the design requirements, which indicates this material system is suitable for the manufacture of GFRP fasteners with higher electrical performance.

In order to study the influence of insulation ageing of cable joint on the electric field distribution at the typical defects of XLPE/silicone rubber interface, we built a simulation model of 10 kV cable joint by COMSOL Multiphysics simulation software. The electric field distribution in the composite interface with defect of conductive impurities, scratches, moisture, respectively was calculated before and after ageing of insulation. And the growth change of water tree in the cable insulation was analyzed combining with the simulation results. The results show that three kinds of defects can lead to obvious electric field distortion at the defect of composite interface, and the ageing of insulation can increase the electric field strength at the interface, and increase the effect of defects on the electric field distribution at the same time. The electric field distortion at the end of water tree is serious after initiation, with the growth and development of water branches, the electric field strength in the area close to each other decreases, but the local electric field concentration still exists.