The effect of nano-Al₂O₃ on the microscopic properties of vegetable insulating oil was studied by molecular simulation, and its result was verified by tests. The mechanism of surface interaction between nano-Al₂O₃ and vegetable insulating oil molecules was analyzed, models of vegetable insulating oil before and after nano-Al₂O₃ modification were established. The hydrogen bond, radial distribution function (RDF) of oil molecule, and diffusion coefficient of water molecule were studied, and the thermal ageing experiment was conducted on vegetable insulating oils with different concentration of nano-Al₂O₃. The results show that compared with the vegetable insulating oil without modification, there are more hydrogen bonds in the modified vegetable insulating model, the peak value of RDF is greater, and the diffusion coefficient of water molecules is smaller. In the process of thermal ageing, the dielectric loss of the modified oil is smaller than that of the unmodified oil, which shows that the nano-Al₂O₃ modified vegetable insulating oil has excellent stability, and its thermal stability and insulation performance are enhanced.

In the actual manufacturing and field installation, defects such as air, metal impurities, and uneven distribution are easily introduced into the silicone rubber insulation layer of cable joint, which endangers the safe operation of power grid system. At present, the detection of power equipment defects are mostly destructive tests, and the experimental results can only reflect the overall situation of equipment. As a non-destructive testing technology, ultrasonic testing can accurately locate and image defects inside the material, and reflect the changes of local performance inside the device. In this study, a set of ultrasonic testing platform were independently designed and built, and ultrasonic testing was conducted on the artificial silicone rubber samples with different defects. The results show that for flexible materials such as silicone rubber, ultrasonic testing can better image the bubbles, air gaps, and steel needle defects in the sample. Compared with similar X-ray testing, ultrasonic testing has an obvious amplification on the tiny bubbles defects. In addition, the ultrasonic testing technology can reflect the change of stress and density inside the material, and can accurately locate the specific depth of the defect according to the ultrasonic echo. This technology has broad application prospects in the location identification and online detection of internal defects in the silicone rubber insulation layer of cable joints.

With the miniaturization and lightweight of electronic equipment, graphite film materials with high thermal conductivity were widely concerned recently. In this paper, the preparation of polyimide (PI) based graphite film was reviewed, and the influence factors of their performance, which included molecular structure, molecular orientation, and the inducement of other materials, were introduced in detail. The research and patent situation of graphite film composite materials were summarized, and the future research and development direction were suggested and prospected.

The high thermal conductive insulation structure applied in H-class AC high voltage motor was studied. The coil samples with the insulation structure were conducted turn-to-turn impulse, withstand voltage to ground, dielectric loss factor, breakdown voltage, voltage durability (electrical ageing), thermal evaluation and classification (thermal ageing) experiments, and the thermal conductivity of several dry mica tapes were tested and analysed. The results show that the application of dry mica tape with high thermal conductivity could improve the thermal conductivity of insulation structure for motor, and the temperature rise reduces effectively. The insulation structure also show good electrical performance, and the temperature index reaches 181℃, which can meet the requirements of the insulation structure for H-class motor.

Spherical alumina was introduced in drawing and filtering process of graphene nanosheets to build a binary porous structure of “pea-pod-like” alumina-graphene, and an alumina-graphene binary structure reinforced epoxy resin composite was prepared. Its thermal conductivity was tested, and the mechanism of “pea-pod-like” alumina-graphene binary structure enhancing the thermal conductivity of epoxy resin was analyzed. The results show that the horizontal arranged graphene generates partial orientation transformation under the action of spherical alumina, showing a “pea-pod-like” structure, in which graphene provides an efficient heat transfer channel for epoxy composites in plane and out of plane directions and greatly enhances the thermal conductivity of epoxy composites. The thermal conductivity of “pea-pod-like” binary alumina-graphene reinforced epoxy composite is up to 13.3 W/(m·K) and 33.4 W/(m·K) in plane and out of plane directions, respectively with 12.1% of graphene and 42.4% of alumina loading. The “pea-pod-like” alumina-graphene binary structure has a significant effect in improving the thermal conductivity of epoxy resin, which has potential application prospects in the field of electrical packaging.

To investigate the key factors of design for thermo-expandable sheets, we made thermo-expandable sheets from two types of glass mat with different fiber diameter and PA thermoplastic film. Different thermo-expandable sheets were prepared by changing the fiber diameter, resin content and density, and their thermal expansion rate and effect were studied. The results show that the thermal expansion effect of thermo-expandable sheets is codetermined by the elastic potential energy of fiber and the viscous resistance of melting resin. Under the same composition and density condition, the thermal expansion rate of thermo-expandable sheets increases with the increase of fiber diameter. There is a critical resin content for thermo-expandable sheets. When the resin content is below the critical value, the thermal expansion rate increases with the increases of resin content. When the resin content is above the critical value, the thermal expansion rate decreases with the increase of resin content. The critical resin content decreases with the increase of fiber diameter. When the single fiber diameter and resin content is unchanged, the higher the density of thermo-expandable sheets, the greater the thermal expansion rate.

The increasingly developed microelectronic devices have put forward higher requirements on thermal conductive and electrical insulating thermal interface materials, and the silicone rubber composites with high thermal conductivity and electrical insulating can be prepared through construction of micro-nano hybrid heat conduction network. First, a nano-alumina coated graphene oxide was prepared by a simple and green self-assembly strategy, and it was reduced into TRGO@Al₂O₃ nano-hybrid filler by high temperature treatment. Then it was mixed with micron alumina and filled into liquid silicone rubber. The influence of different nano alumina coated amounts on the thermal conductivity and volume resistance of the system was studied by adjusting the ratio of nano alumina to graphene oxide. The synergistic thermal conductive effect of the micro-nano hybrid system was studied by controlling the ratio of nano-hybrid filler to micron alumina. The results show that when the 3% of TRGO@Al₂O₃ and 54% of micron alumina are compounded, the thermal conductivity of the composite reaches about 2.5 W/(m·K), and the volume resistance is greater than or equal to 10⁹ Ω·cm.

The thermal conductivity of insulating varnish could be improved by adding inorganic oxide thermal conductive material into the epoxy modified unsaturated polyester resin. The effect of particle size and surface modification of fillers on the sedimentation of insulating varnish and effect of different filler content on the thermal conductivity and viscosity of insulating varnish were studied. A high thermal conductive and solvent-free insulating varnish was prepared, and its performance was characterized. The results show that the surface modification of filler could reduce its sedimentation rate. When the filler content is 40%, the viscosity of insulating varnish is 72 s, the thermal conductivity reaches 0.46 W/(m·K), and the insulating varnish exhibit no sedimentation at 40℃ after 6 days, which indicates that it has practical application value.

Polyethylene cables would generate crack defects inevitably during long-term operation, which will cause partial discharge faults and threaten the normal operation of power grid. Polyethylene composite materials doped with microcapsules could realize the self-healing of cracks. In order to study the effect of microcapsules on the insulating properties of polyethylene materials before repair, we prepared pure polyethylene samples and polyethylene composite samples doped with different concentrations (0, 0.5%, 1%, 5%, 10%) of microcapsules, and their basic performance, volume resistivity, and AC electric strength were measured. The results show that the volume resistivity of composite samples doped with a small amount of microcapsules (≤1%) is significantly higher than that of pure polyethylene samples. But when the concentration is larger (>1%), the volume resistivity decreases. Compared with the pure polyethylene sample, the AC electric strength of the composite materials decreases, but when the concentration is no greater than 1%, the decrease amplitude is smaller, which could meet the normal operation requirements of cable. This is mainly related to the crystallinity of material, the interface effect between microcapsule and polyethylene, and the characteristics of microcapsule itself. In general, when the doping concentration of microcapsules is no greater than 1%, the insulating properties of the polyethylene composite material can meet the requirements of normal operation for cable.

In order to expand the new energy vehicle market and improve the power transmission efficiency of new energy vehicles, it is necessary to improve the current-carrying capacity of its internal cables. But at the same time, with the increase of load current, the heat dissipation problem of vehicle cable has become prominent. In this study, a finite element simulation model of vehicle cable considering electric field and thermal field was established by COMSOL software, the ampacity was solved by the Nelder-Mead method, and then the effectiveness of Nelder-Mead method was verified by the analytical methods. The improvement effect of the increase of insulation material thermal conductivity on the current-carrying capacity and heat dissipation of the vehicle cable was simulated analyzed. The results show that when a silicone rubber/boron nitride nanosheets (20%) composites with 0.833 W/(m·K) of thermal conductivity was used in vehicle cable, the ampacity of vehicle cable obtained by simulation increases by 5.12%, which can promote heat dissipation more significantly at overload, but it would increase the critical radius of insulation, resulting in a slight increase of conductor temperature when the insulation thickness decreases within the critical radius of insulation.