In order to deeply grasp the breakdown characteristics of AC 500 kV cross-linked polyethylene (XLPE) submarine cable insulation materials under electrical-thermal stress and establish an electrical-thermal combined lifetime model, we conducted electrical-thermal breakdown experiments on the XLPE material at 25, 40, 55, 70℃ under step stress firstly. The AC electric strength and voltage duration time were analyzed by Weibull distribution to obtain the equivalent AC electric strength and voltage duration time at different temperatures. Then the FALLOU, SIMONI, CRINE models were established by multiple linear regression method, and their error was analyzed. Finally, an E-T lifetime model for the AC 500 kV XLPE material was constructed. The results show that at the same temperature, the equivalent electric strength decreases with the increase of voltage duration time of each voltage stage. At the same voltage duration time of each stage, with the increase of temperature, the equivalent AC electric strength and voltage duration time both firstly increase and then decrease. The analysis on the electrical-thermal lifetime models indicate that the fitting error of FALLOU, SIMONI, CRINE models is large, and their fitting goodness do not meet the accuracy requirements. An improved electric-thermal combined ageing lifetime model is obtained by using stepwise regression to calculate the significance and correlation between electric-thermal variables and lifetime, and the error analysis show that it has better fitting accuracy.

In this paper, the current fire protection standards of transformer oil for rail transit were introduced in detail. The necessity of synthetic ester as transformer oil for rail transit was described mainly from the aspects of fire safety, environmental protection, moisture resistance, and later oil change maintenance.

DGEBA/OSC cross-linked epoxy resin blend systems with different proportions were constructed by molecular dynamics (MD) simulation using methyl tetrahydrophthalic anhydride (MTHPA) as curing agent, and the blend system samples were prepared. The thermodynamic properties of the blending systems were studied by simulation and experiment. The simulation and test results show that with the increase of the OSC proportion, the glass transition temperature (T_g) of the blend systems increases, when the molar ratio of DGEBA to OSC is 6∶4, the T_g increases by 30.7 K. The elastic modulus and strength of the systems increase at first and then decrease, and reach the optimum at 8∶2 of molar ratio. The addition of OSC can reduce the fractional free volume (FFV) and mean square displacement (MSD) of the systems, and weak the molecular segment movement ability. The calculation results of synergy rotational energy barrier and cohesive energy density (CED) show that the blend system with stronger molecular segment rigidity and larger interaction force between segments has better thermodynamic properties.

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.

The surface corrosion rate of superhydrophobic coating during chemical corrosion was evaluated by using mass change representation method, and a modified superhydrophobic coating by adding fluorine-containing compound to the final coating was proposed. Under room temperature, acid, alkali, and salt corrosion resistance experiments were conducted on glass substrates coated with fluorine-containing superhydrophobic coatings, fluorine-free superhydrophobic coatings, and RTV coatings, respectively, and different corrosion time and corrosive medium concentration were set. The results show that the acid and salt resistance of superhydrophobic coating is stronger than the alkali resistance. The addition of fluorine-containing compounds can significantly improve the chemical resistance of superhydrophobic coating. The acid and alkali resistance of the fluorine-containing superhydrophobic coatings is similar to that of the RTV coatings, but its salt corrosion resistance is significantly better than that of the RTV coatings.

In order to study the influence rules of repetitive square wave frequency on the corona-resistant lifetime of polyimide, a test platform with bipolar square wave was constructed to test the corona-resistant lifetime and partial discharge statistical characteristics of polyimide at different frequencies. The results show that with the increase of frequency, the corona-resistant lifetime of polyimide decreases， and the trend becomes less steep gradually. This is because with the increase of repetitive square wave frequency, the discharge amplitude and phase at the rising edge and the falling edge reduce gradually, which leads to the weakening of electrical ageing effect by discharge in a single cycle. Therefore, when the insulation was accelerated aged by increasing the square wave voltage frequency, the above characteristics need to be considered.

In order to explore the effect of compound anhydride curing agent on the cured product of epoxy resin, we built crosslinking models of methyl tetrahydrophthalic anhydride (MTHPA)/phthalic anhydride (PA) (their compounding ratio was 10:0, 9:1, 7:3, 5:5, 0:10, respectively) and bisphenol A-type glycidyl ether (DGEBA) by molecular dynamics method. The effect of different compounding ratios on the structure, thermal properties, and mechanical properties of the crosslinking epoxy resin with the same crosslinking degree was studied. The results show that when the compounding ratio of MTHPA and PA is 10:0, the free volume proportion of system reaches maximum, and the motion ability of molecular chains is the strongest. When the compounding ratio is 7:3, the comprehensive mechanical properties of system reach the maximum. When the compounding ratio is 9:1 and 7:3, the glass transition temperature of system reaches the highest and the lowest, respectively.

In order to study the influence of phase change on the space charge characteristic and electric strength of polylactic acid (PLA), we prepared PLA samples and studied their physical and chemical properties, conduction current, space charge, and electric strength. The results show that the phase change (glass transition) temperature of PLA is 59℃, and before and after the phase change, the PLA nucleates and the unite cell grows, respectively. As a result, the conductivity of PLA first increases then decreases and increases again with the increase of temperature. More shallow interface traps are introduced in the phase transition process, resulting in the average trap depth of PLA decreases with the increase of temperature. Before and after phase change, PLA is in glass state and rubber state, respectively. When the temperature increases by 20℃, the electric strength of PLA in glass state decreases by 12%, while the electric strength of PLA in rubber state decreases by 40%.

In order to improve the mechanical properties and thermal stability of cellulose insulating paper, the nano-SiO₂, surface modified by KH550, was doped into cellulose insulating paper. The nano-SiO₂/cellulose composite models with different grafting density were established through molecular simulation, and their mechanical properties, mean square displacement, cohesive energy density, solubility parameter, and radial distribution function were calculated and analyzed. The results show that when the grafting density of KH550 is 12.5%, the nano-SiO₂/cellulose composite system has optimum deformation resistance, large cohesive energy density and solubility parameter.