PTC resistance device, which made by positive temperature coefficient (PTC) material, can be used as a passive component to prevent overcurrent fault. In this paper, four kinds of polymer-based PTC materials were prepared by melting blending method, and the effects of conductive filler concentration, coupling agent, and supplementary conductive filler on their properties were studied. According to the temperature-resistance characteristics and heat transfer equation of polymer PTC composite, its current limiting process was simulated. The experimental results show that increasing the concentration of conductive filler will lead to the decrease of the resistivity of composite at room temperature. The PTC properties of the composites can be improved by using silane coupling agent to modify CB and adjusted by the interaction of various fillers. The simulation results show that under certain assumptions, the PTC resis-tor, as a passive component to prevent overcurrent fault, can limit the short-circuit current of power system in a short time.

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.

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.

Two kinds of modified magnesium hydroxide (MH) were used as flame retardant, and then compounded with low density polyethylene (LDPE) to prepare LDPE/MH flame retardant materials. The effects of the flame retardants on the mechanical properties, flame retardant properties, thermal stability, and electrical properties of the materials were studied. The results show that the flame retardant properties of the materials increase significantly after adding the modified magnesium hydroxide flame retardant, but the mechanical properties, thermal stability, and electrical properties decrease. The flame retardant of the LDPE with amino silane coupling agent modified magnesium hydroxide is better than that of the LDPE with alkyl silane coupling agent modified magnesium hydroxide, but its adverse effects on mechanical properties and electrical properties of materials are more obvious. When the addition amount of alkyl silane coupling agent modified magnesium hydroxide is 70 phr, the comprehensive properties of the material are better.

In order to study the radial temperature field distribution of overhead conductor, firstly, on the basis of heat transfer theory, a 3D model of finite element simulation was established according to the physical structure of conductor, and a simulation experiment was designed. Then the simulation and experimental results were compared. The results show that under natural convection conditions, the maximum radial temperature difference of the conductor can reach 10.4℃, and the maximum surface temperature difference of different position on conductor is 3.4℃. The simulation results coincide with the measured values under natural convection conditions basically, and the relative error between each layer temperature of the conductor is within ±5%, which verifies the reliability of the simulation model. Under forced convection conditions, there are obvious difference between simulation results and measured values, which indicates that the axial heat transfer of the conductor caused by different heat dissipation conditions has significant effect on the radial temperature field distribution of the conductor. For conservative consideration, in practical applications, the core temperature of conductor can be calculated by multiplying the maximum surface temperature of conductor measured from different position of conductor with correction coefficient of 1.05‒1.10.

In the electrical test and operation of generator, the hypotenuse electric field at the end of stator winding is very high, which is a weak part that corona discharge often occurs. In this study, an analytical model and a finite element model of the hypotenuse electric field at the end of stator winding were established. The influencing factors and laws of the hypotenuse electric field were studied, and the influence of the hypotenuse clearance, rotating angle, and dielectric constant of the filling medium on the maximum electric field was analyzed. An effective method to optimize the electric field at the end was presented. The results show that under the in-phase winding, the hypotenuse electric field can be optimized by decreasing the surface electric field of stator bar, the width of bar, and the rotating angle of bar or increasing the hypotenuse clearance; Under the out-of-phase winding, the hypotenuse electric field can be optimized by increasing the dielectric constant of filling medium and the hypotenuse clearance of winding. The finite element model of hypotenuse electric field can accurately analyze the maximum electric field and the distribution trend at the end of winding. The correctness of the model is verified by corona test of UV imaging equipment, which indicate that that the model has good engineering application value.

In order to study the reliability of traction motor insulation system, according to the operation condition of traction motor, we established a reliability verification method with electrothermal ageing, temperature variation, constant damp heat, mechanical vibration, and sand and dust as ageing factors. The comparative tests were conducted on the reference insulation structure of locomotive traction motor and one to be evaluated, and the non-destructive characteristic parameters including insulation resistance, dielectric loss factor, and PDIV were measured. The results show that the reliability verification method can quickly compare the reliability of insulation system. During the test, the insulation performance increases at first and then decreases. Dielectric loss factor and PDIV have more obvious characterization effect on the ageing of insulation system than insulation resistance.

In order to study the influence of metal wire defect on the electric field distribution of basin insulators in GIS, using the actual 1 100 kV basin insulator in GIS as the research object, we simulated and calculated the electric field distribution of basin insulators with metal wire defect using ANSYS by finite element method. The influence of the distribution direction, radial distance, length, and width of the wire on the electric field distribution of basin insulators was studied. The results show that the wire defects in basin insulator can distort the electric field severely, the maximum electric field strength is 10.76 times larger than that of the basin insulator without wire defect, and the transverse distributed wire has greater effect on the electric field of the basin insulator. The distortion degree of electric field of basin insulator is related to the distribution direction, radial distance, and length of wire.

In order to find out the effect of adding nano-SiO₂ on the relative permittivity and conduction characteristic of ethylene propylene diene monomer (EPDM) insulation, we prepared EPDM composite dielectrics with different mass fractions of nano-SiO₂ by melt-blending method. The dispersion of nanoparticles in the EPDM matrix and the bonding properties between nanoparticles and EPDM matrix were analyzed. The relative permittivity and the steady-state current of the EPDM nanocomposite dielectrics at different temperatures and electric fields were measured, and the conductance characteristic were analyzed. The results show that when the mass fraction of nano-SiO₂ is 0.5%, the dispersion of nano-SiO₂ is the best in EPDM. The steric hindrance, which formed by doping of low content, reduces the mobility of the EPDM molecular segments and its side groups, and the interface effect is obvious in the nanocomposite dielectrics, making the relative permittivity and conductance decrease, the threshold electric field strength of space charge injection enhance, the insulating performance improve significantly. With the increase of the mass fraction, the interface effect between nanoparticles and EPDM matrix is weakened. When the mass fraction of nano-SiO₂ is 2.5% or 5.0%, the agglomeration caused by doping makes the relative permittivity and the conductance increase, and the threshold electric field strength of space charge injection decreases. The interface effect caused by the addition of nano-SiO₂ is a key factor affecting the properties of EPDM nanocomposite dielectrics.

Vegetable insulating oil is easy to oxidate during operation and it will affect the service life of transformer. In this study, 2,6-di-tert-butyl-p-cresol (BHT) was chosen as antioxidant, and the characteristic parameters of the vegetable insulating oil were compared by the ageing tests. The effect of BHT antioxidant was analyzed by measuring the acid value, peroxide value, and moisture content. The effect of BTH antioxidant on the insulating properties was studied by detecting the dielectric loss factor, breakdown voltage, and FDS curves. The results show that BHT antioxidant can effectively improve the dielectric constant of vegetable insulating oil. Through the molecular formula analysis on the reaction mechanism between BHT reagent and unsaturated oil in natural ester insulating oil, the oxidation resistance process of BHT in natural ester insulating oil can be reflected more intuitively. When the addition amount of BHT is 0.20%, it has a better effect on improving the insulation performance of vegetable insulating oil.