There are many metal and epoxy resin interfacial structures in power equipments, which can generate interfacial stress concentrations and may induce accidents due to epoxy curing. In this paper, through simplifying interface structure into a coaxial cylindrical model, the formation mechanism of interfacial stress concentration of this interface structure was studied during epoxy resin curing process. The strain and temperature at the interface were measured by the strain and temperature measurement system. Then a model for calculating the interface stress distribution was established, including the heat conduction module, curing dynamics module and curing deformation module. The validity of the model was verified by comparing the calculated results with the measured results. In addition, the air gap defects at the interface, appearing after the curing process, were simulated. The results show that, after the curing process, the first principal stress at the centre of the interface gradually increases from the centre to the edge, reaching 41.06 and 40.20 MPa at the upper and lower edges respectively. The existence of air gap leads to partial discharge at low voltage. The calculation model can be extended to the stress prediction between the metal and epoxy resin in other power equipments.

In vacuum interrupters, anode spot significantly decreases the current interruption capability. Generally, the anode spot is evaluated by the anode spot threshold current. Unfortunately, as a circuit parameter, arc current cannot explain the anode spot formation from the physical point of view. This work proposes the heat flux density as a new parameter to evaluate the anode spot. The objective of this study is to determine the anode input heat flux density as anode spot formation. The heat flux density is numerically obtained by a two-temperature magnetohydrodynamic model. Based on a previous systematic experimental work over a wide range of discharge conditions, the calculated critical anode input heat flux density located in a narrow range from $7.3\times 10^{8}$ to $11.3\times 10^{8}$ W/ $\text{m}^{2}$ . Compared with the arc current, the anode input heat flux density is a key parameter in the interaction between the vacuum arc column and the anode; moreover, the heat flux density has a close relationship with both the arc distribution and the anode activity. Therefore, heat flux density is a reasonable physical parameter to reveal the mechanism of the anode spot formation.

The influence of the applied axial magnetic field on the current density distribution in the arc column and electrodes is intensively studied. However, the previous results only provide a qualitative explanation, which cannot quantitatively explain a recent experimental data on anode current density. The objective of this paper is to quantitatively determine the current constriction subjected to an axial magnetic field in high-current vacuum arcs according to the recent experimental data. A magnetohydrodynamic model is adopted to describe the high current vacuum arcs. The vacuum arc is in a diffuse arc mode with an arc current ranged from 6 kA _rms to 14 kA _rms and an axial magnetic field ranged from 20 mT to 110 mT. By a comparison of the recent experimental work of current density distribution on the anode, the modelling results show that there are two types of current constriction. On one hand, the current on the cathode shows a constriction, and this constriction is termed as the cathode-constriction. On the other hand, the current constricts in the arc column region, and this constriction is termed as the column-constriction. The cathode boundary is of vital importance in a quantitative model. An improved cathode constriction boundary is proposed. Under the improved boundary, the simulation results are in good agreement with the recent experimental data on the anode current density distribution. It is demonstrated that the current density distribution at the anode is sensitive to that at the cathode, so that measurements of the anode current density can be used, in combination with the vacuum arc model, to infer the cathode current density distribution.

High current vacuum arcs are essentially metal vapor arcs, and the contact material has a direct impact on the vacuum arcs. The objective of this paper is to experimentally determine the anode spot threshold current of vacuum arcs subjected to axial magnetic fields for four pure metals in vacuum arcs. The four pure metals are W, Mo, Cr, and Fe, which covers a wide range of thermal and electrical parameters. The experiments were performed in a demountable vacuum chamber, and a pair of Helmholtz coils was set in the chamber to supply uniform axial magnetic fields. The arc current in the experiments was in the range from 2 to 30 kA at 50 Hz. A highspeed camera was used to record the arc evolution through an observing window. The onset of anode spot phenomenon was determined by a combination of arc column images, arc voltage, and anode activity. From this combination criterion, the anode spot threshold current subjected to axial magnetic fields for the four pure metals was determined. The results show that the thermal characteristic T(kρc) ^1/2 of the contact material has an important influence on the threshold current. Three cases of the thermal characteristic are considered. The ordering and proportionality between thermal characteristic and experimental data are studied. The results show that the axial magnetic flux density can significantly increase the threshold current. In addition, the anode spot threshold current for Mo and W increases more rapidly than that for Cr and Fe. Moreover, the anode emission of different metals is discussed.

The vacuum circuit breakers (VCBs) are highly suited for frequent switching of capacitor banks in reactive power compensation systems due to their durability for repetitive operations, environmental friendliness, and maintenance-free arc chambers. Understanding the influence of inrush currents on contact surface conditions provides theoretical support for enhancing the performance of VCBs in switching capacitor banks. In this paper, an experimental system based on the DC dynamic gap measurement method was developed. The effect of varying contact structures on the field emission current during DC prestrike and the consequences of inrush current prestrike arcs on the contact surface condition in vacuum interrupters (VIs) with spiral-type transverse magnetic field contacts during capacitive switching was studied. The contact surface condition is represented by field enhancement factor β. The variation trend of the field enhancement factor β and its relationship with the prestrike gap are obtained. The results indicate that series-connected VIs with spiral-type TMF contacts effectively diminish the prestrike field emission current by a maximum of 60%. The alternating effects of arcing erosion and aging are identified as key factors contributing to the increased variability in contact surface conditions.

As the driving force for the crosslinking of epoxy resin molecular segments to form a three-dimensional network structure, hardener can directly affect the crosslinking degree of composite materials and then affect the formation of the two-phase structure of EP / HTBN composite materials. By preparing epoxy resin (EP) / hydroxyl-terminated butyl nitrile rubber (HTBN) composites with different hardener stoichiometry, the effects of hardener stoichiometry on the two-phase structure and electrical properties of the system are analyzed by SEM cross-section characterization, DC resistivity, and broadband dielectric spectrum. The experimental results show that deviation of the hardener from the standard stoichiometry inhibits the crosslinking behavior, which in turn promotes the precipitation of the rubber phase. The deviation of hardener from the standard stoichiometry causes an increase in DC resistivity, an increase in interfacial polarization strength, and a decrease in epoxy α relaxation strength. The VFT fitting of epoxy resin α relaxation peak frequency versus temperature inverse was performed to obtain the Vogel temperature by Havriliak-Negami (HN) model peak separation results, and the results show that the deviation of the hardener from the standard stoichiometry causes a decrease in the glass transition temperature, which is consistent with the relationship with the crosslinking degree. The above results may provide more effective solution designs for optimizing EP/HTBN composite insulating material curing formulations.