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.

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.

A barium titanate/polyvinylidene fluoride (BT/PVDF) composites was prepared by solution blending method, and the effect of size distribution and content of BT particles on the dielectric properties, electric strength, energy storage properties, and thermal stability of the composites were investigated. The results show that compared with single particle size BT, the synergistic effect of double particle size BT filler makes the composites have more excellent comprehensive properties, and the synergistic effect is the most remarkable when the mass ratio of BT with double size is equal to 1. With the increase of filler content, the dielectric constant and thermal stability of the composites increase, and the dielectric loss factor of the composites maintains at the relative low level. When the mass fraction of BT is 60%, and the mass ratio of BT with double size is 5∶5, the dielectric constant of BT/PVDF composites reaches 46.5 at 100 Hz, which is 5 times bigger than that of pure PVDF; the energy storage density and polarization are 0.18 J/cm³ and 0.011 9 C/m², respectively, which are 176% and 310% higher than those of pure PVDF; the decomposition temperature corresponding to 5% of weight loss reaches 478.4℃, which is 1.3℃ and 30.3℃ higher than that of PVDF composites filled with S-BT and L-BT, respectively.

In order to study the ablation mechanism of buffer layer in cable, we analyzed the change of contact state between buffer layer and aluminum sheath according to the defect phenomenon, and then a circuit model was established to analyze the defect. According to the model, the induced voltage of insulation shielding layer and the main factors affecting the induced voltage were analyzed. A 15 m defective cable sample was intercepted from the breakdown accident line, and a partial discharge detection experimental platform was built with the defective cable sample. The results show that the induced voltage of insulation shielding layer is directly proportional to the number of defects, the resistivity of composite layer, and the thickness of white powder at the defect, and is inversely proportional to the contact area between buffer layer and aluminum sheath, and the discharge signals inside the defect cable have obvious characteristics of poor contact discharge.

Two kinds of nano-fillers nano-tiania (TiO₂) and muti-wall carbon nanotubes (MWCNTS) were selected. Sixteen different epoxy nanocomposite dielectrics were prepared by adding two kinds of nanofiller alone or simultaneously to the epoxy resin. The influence of nanoparticles on the surface flashover characteristics of epoxy nanocomposite dielectrics was explored through the volume resistivity test, surface potential decay and vacuum DC surface flashover experiment. The results show that the surface flashover voltage of epoxy nano-composite is related to the mass fraction of filler, and an appropriate amount of filler can enhance the flashover voltage effectively. Comparing with adding TiO₂ (improving 14.49%) and MWCNTS alone (improving 23.11%), adding TiO₂ and MWCNT simultaneously can further improve the surface flashover voltage up to 44.99 kV (improving 36.06%). The surface trap characteristics of the material are calculated by the surface potential decay curve. By analyzing the relationship between deep trap and surface flashover voltage, it is found that the flashover voltage is linearly correlated with the deep trap energy level, and the deeper the trap energy, the higher the flashover voltage. The depth of surface deep trap energy is improved by adding two kinds of nanoparticle into epoxy composites, thus the electron emission and charge transport processes are suppressed, and the surface flashover voltage increases.

XLPE cable insulation is prone to age under the long-term influence of electrical stress. In order to study the electrical ageing characteristics of XLPE cable, we conducted accelerated electrical ageing experiments on 10 kV XLPE cable, and the physic-chemical properties and electrical properties of the XLPE cable before and after ageing were investigated. The results show that the crystallinity of XLPE decreases significantly after electrical ageing, and the melting peak characteristic temperature of XLPE decreases slightly. The methylene content of XLPE increases, and carbon-carbon double bonds start to appear inside the material. PDC test results show that the polarization and depolarization current of the samples increase after electrical ageing, and the conductivity and low frequency dielectric loss of XLPE increase significantly. This is because the chemical bond break in XLPE molecular chain is broke by high-energy electron bombardment, resulting in the increase of short-chain molecules number and degradation of crystalline region, which ultimately decrease the physic-chemical and electrical properties of XLPE insulation material.

A thermosetting polyester wire enamel with high crosslinking density was prepared by using water-soluble polyester resin as matrix. The adhesion of paint film was regulated by the ratio of hard and soft monomer in polyester resin, the electric strength of paint film was improved by adjusting the branching degree of polyester resin, and the technology of paint film was improved by adding thickening agent. The results show that when the mole fraction of water-soluble functional monomer is 6%, the mole fraction of branched functional monomer is 14%, and the mole ratio of soft and hard chain segments in the remaining monomer is 2∶1, the wire enamel with the water-soluble thermosensitive polyester as matrix has the best comprehensive performance. The optimum mass fraction of curing agent is 3.0% of resin mass. When the mass fraction of thickening agent is 0.4% of the total resin, the elongation at break of the enameled wire is 40%, and the electric strength is 40 kV/mm. The final coating thickness of the enameled wire is 100 μm, its breakdown voltage is 4 kV, and its heat resistance is good, which has potential application value in the new sensor technology and equipment body integrated transformer.

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.

In order to verify the applicability of DGA fault diagnosis methods in oil-filled submarine cable, we chose domestic dodecylbenzene (DDB) insulating oil as research object. First, the decomposition temperature of DDB insulating oil was determined by thermogravimetric test under different heating rates. Subsequently, a test system was built to carry out thermal fault simulation tests of DDB pure oil, oil-paper, and 25^# mineral oil at 150, 250, and 350℃. After test, the percentage of five characteristic gases (H₂, CH₄, C₂H₆, C₂H₄, C₂H₂) dissolved in the fault oils was measured to study the gas production law of DDB insulating oil under thermal fault. Finally, existing DGA diagnostic methods (classic Duval triangle, quadrilateral graphic method) were selected to analyze the gas data. The results show that the proportion of C₂H₄ dissolved in DDB insulating oil under thermal fault increase significantly with the increase of fault temperature, and the proportion of C₂H₄ in mineral oil is at a low level in the whole temperature range. In the thermal fault diagnosis, the classic Duval triangle and quadrilateral graphical method can detect thermal faults by the dissolved gas in DDB insulating oil, but the temperature range of thermal fault cannot be determined accurately, and the fault boundary need to be further modified.

In humid and dirty environment, it is easy for high voltage switchgear to trigger discharge or insulation breakdown fault. To improve the design and operation level of high voltage switchgear, we simulated the electric filed distribution of high voltage switchgear under micro-environment and calculated the minimum discharge distance. Firstly, a multi-physical field finite element simulation model of bus chamber for 40.5 kV high voltage switchgear was established by COMSOL simulation software, and the temperature and humidity distribution in the bus chamber were obtained. The electric field distribution characteristics in the presence or absence of micro-environment were studied by numerical analysis method. Based on the air streamer discharge initiation criterion, the minimum discharge distance in micro-environment was calculated by COMSOL and MATLAB software. Finally, the influence of different insulation partition layout parameters (thickness, location) on the electric field distribution and minimum discharge distance in micro-environment was studied. The results show that the maximum temperature in the bus bar chamber can reach 347.15 K under high temperature and high humidity environment, and the overall temperature presents a gradient distribution of upper and lower. The insulating partition, which is located in the area of low temperature and high humidity, is easy to accumulate wet dirt. In the micro-environment, the electric field at the bus tip and the minimum discharge distance decrease, but the surface electric field increases, and the flashover probability increases.