A high toughness composite epoxy resin system was prepared with bisphenol A epoxy resin as matrix, G2019 epoxy resin as the modified resin and hexanediol diglycidyl ether as diluent. The high-performance epoxy resin-based glass fiber reinforced composites for wind turbine blades were fabricated via the pultrusion process, and their mechanical and electrical properties were studied. The results show that the obtained composite epoxy resin system can be used as a high toughness epoxy resin matrix. When the content of G2019 epoxy resin is 30 phr, the impact strength of the epoxy resin composite system casting reaches 25.1 kJ/m², which is 151.5% higher than that of the pure bisphenol A epoxy resin, and the AC and DC electric strength reaches 48 kV/cm and 65 kV/cm, respectively. The pultruded epoxy resin-based glass fiber reinforced composites show outstanding comprehensive mechanical and electrical performances, which can meet the application requirements of wind power fields.

In order to study the influence of crosslinking behavior on the mechanical properties and water tree characteristics of nanocomposite dielectrics, we prepared two nanocomposites of polyethylene/montmorillonite (PE/OMMT) and crosslinked polyethylene/montmorillonite (XLPE/OMMT) by melt blending, and their tensile properties, dynamic mechanical properties, and water tree ageing characteristics were tested. The results show that the OMMT in XLPE plays a role of physical crosslinked point and stress dispersion, which increases the tensile strength and toughness of nanocomposites. The homogeneously dispersed OMMT of lamellar heat resistance and physical support functions improves the storage modulus of nanocomposite at high temperature. The water tree initiation time of the nanocomposites is short, and the OMMT layer is beneficial to buffer the impact of expansion and deformation of water molecules on the molecular chain, so that the growth length of water tree is smaller. The three-dimensional network structure after crosslinking enhances the ability to withstand deformation of nanocomposites, thus the nanocomposites have better resistance to water tree.

The main negative effect of transformer insulation ageing is the decline of mechanical properties. Once the winding coil is impacted by external short circuit, it will probably produce deformation, which will lead to the loss of insulating ability and eventually cause transformer fault. A mechanical analysis model of insulating paper materials was established according to their structural characteristics on the basis of material mechanics. Combined with the ageing mechanism and influencing factors of insulating paper materials, a test platform for the mechanical properties of insulating paper materials was built. The mechanical properties of insulating paper materials under different ageing degrees were studied to obtain the ageing mechanical properties of insulating paper materials. The results show that the plasticity of the insulating paper material basically disappears under repeated cyclic loading, and the insulating paper under different ageing times can be approximately simplified as an elastic material.

The ageing problem of insulating oil under long-term electrical-thermal combined stress seriously threatens the safe and stable operation of oil-immersed transformers. In view of the transformer faults caused by the ageing of insulating oil, it is urgent to research the ageing characteristics, mechanism, and diagnostic methods of insulating oil. Therefore, the development status and key technologies in the electrical thermal ageing characteristics and diagnostic methods of mineral insulating oil and vegetable insulating oil were reviewed. This paper first summarized the physical and chemical properties of mineral oil and vegetable oil for transformers, and their high temperature performance, biodegradability, acid value, and electric strength were compared. Then the two basic forms of electrical and thermal ageing and their products for insulating oil were reviewed. On this basis, the influence of metal materials such as copper on the ageing of oil was emphasized. Finally, the commonly used ageing diagnostic methods of insulating oil were described. The multi-dimensional fusion characterization of feature parameters from different detection methods is expected to improve the diagnostic accuracy, which has a good engineering application prospect.

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 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.

The fault type recognition and condition assessment play the crucial roles in fault diagnosis and maintenance. Different defects will produce partial discharge signals with difference, and the partial discharge signal also changes with the defect severity and the evolution of partial discharge. This situation can be regarded as pattern recognition of different severity levels and evolution stages, and the pattern recognition is a typical classification problem. In this paper, classification problems such as pattern recognition and state assessment were reviewed. Compared to classification results based on mathematical statistics, artificial intelligence has achieved nearly 100% of recognition accuracy. However, there are still some shortcomings in current research, this paper gives some solution strategies and prospects future research direction.

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.

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.