Yunbo Tian

Decay Modes of Anode Surface Temperature After Current Zero in Vacuum Arcs—Part II: Theoretical Study of Dielectric Recovery Strength

Anode surface temperature after current zero has a great impact on the interruption capacity of a vacuum interrupter. The objective of this paper is to theoretically investigate the relation between breakdown voltages and anode surface temperature after current zero. A heat conduction model was adopted to describe the temperature development of the anode, taking account of phase transition and evaporation. The breakdown voltages in certain metal vapor densities were obtained by the Particle-in-Cell/Monte Carlo collision (PIC-MCC) method. Finally, the Paschen curve for copper vapor was obtained using the PIC-MCC method and verified by the theoretical model. Moreover, the minimum breakdown voltage, 30 V, was obtained at a density of 1.3 × 1022/m3 with a gap of 10 mm, which corresponded to a surface temperature of 1983 K. In order to ensure a successful interruption, anode surface temperature should not be higher than 1983 K at current zero, and the melting time should be kept as short as possible.

Anode Erosion Pattern Caused by Blowing Effect in Constricted Vacuum Arcs Subjected to Axial Magnetic Field

The objective of this paper is to investigate the anode erosion pattern caused by the blowing effect in constricted vacuum arcs under an axial magnetic field (AMF). The blowing effect was reflected from the microstructure of the melt layer on the anode. A pair of 42-mm-diameter AMF contacts were installed inside a detachable vacuum chamber. The contact material was CuCr25 (25% Cr). The arc modes were observed with a high-speed charge-coupled device video camera. The anode surface temperature after current zero was measured with a two-color pyrometer. The microstructure of the cross section of the anode surface melt layer was observed by scanning electron microscopy. The experimental arc currents increased from 10.5 to 17.8 kA (rms) with an interval of 2 kA. The results show that the anode erosion pattern in constricted arcs is caused by an interactive process of thermal and mechanical effects. The constricted arc columns melted the anode surface material and blew the molten material from the center to the peripheral region. The redistribution of the anode molten layer caused by this blowing effect formed two erosion regions on the anode surface. The depth of the melt layer in the central region was less than 100 μm, but in the peripheral region it was in the range of 100-400 μm. In these different erosion regions of the anode melt layer, there was considerable variation in both size and shape of the Cr sphere. Moreover, the thermal and mechanical effects of the constricted arc redistributed the Cr content of the molten layer on the anode surface. The Cr content in the central region (Erosion Region I) increased to 31% and in the periphery region (Erosion Region II) it decreased to 18%, respectively, from 25% before the experiments.

Fully kinetic model of breakdown during sheath expansion after interruption of vacuum arcs

Research on sheath expansion is critical to the understanding of the dielectric recovery process in a vacuum interrupter after interruption of vacuum arcs. In this paper, we investigated how residual plasma affects breakdown in the sheath expansion period after the current zero. To simulate sheath expansion and breakdown, we developed a fully kinetic particle-in-cell Monte Carlo collision model with one spatial dimension and three velocity dimensions. The model accounted for various collisions, including ionization, excitation, elastic collisions, charge exchange, and momentum exchange, and we added an external circuit to the model to make the calculations self-consistent. The existence of metal vapor slowed the sheath expansion in the gap and caused high electric field formation in front of the cathode surface. The initial residual plasma, which was at sufficiently low density, seemed to have a limited impact on breakdown, and the metal vapor dominated the breakdown in this case. Additionally, the breakdown probability was sensitive to the initial plasma density if the value exceeded a specific threshold, and plasma at sufficiently high density could mean that breakdown would occur more easily. We found that if the simulation does not take the residual plasma into account, it could overestimate the critical value of the metal vapor density, which is always used to describe the boundary of breakdown after interruption of vacuum arcs. We discussed the breakdown mechanism in sheath expansion, and the breakdown is determined by a combination of metal vapor, residual plasma, and the electric field in front of the cathode surface.

Modelling of crater formation on anode surface by high-current vacuum arcs

Anode melting and crater formation significantly affect interruption of high-current vacuum arcs. The primary objective of this paper is to theoretically investigate the mechanism of anode surface crater formation, caused by the combined effect of surface heating during the vacuum arc and pressure exerted on the molten surface by ions and electrons from the arc plasma. A model of fluid flow and heat transfer in the arc anode is developed and combined with a magnetohydrodynamics model of the vacuum arc plasma. Crater formation is observed in simulation for a peak arcing current higher than 15 kA on 40 mm diam. Cu electrodes spaced 10 mm apart. The flow of liquid metal starts after 4 or 5 ms of arcing, and the maximum velocities are 0.95 m/s and 1.39 m/s for 20 kA and 25 kA arcs, respectively. This flow redistributes thermal energy, and the maximum temperature of the anode surface does not remain in the center. Moreover, the condition for the liquid droplet formation on the anode surfaces is developed. The ...

The influence from the residual magnetic field on the plasma dissipation in the post-arc phase in a vacuum interrupter

Axial magnetic field (AMF) contact structures are often adopted for promoting interruption ability in the vacuum interrupters. After current-zero, there still exists residual magnetic field in the contact gap result from the eddy current. As a consequence, the plasma dissipation process which recovers the dielectric strength of the vacuum interrupter, was influenced immediately in the post-arc phase. The objective of this paper is to investigate the influence from the residual magnetic field on the plasma dissipation in the post-arc phase. A full kinetic 1D3V PIC model was built. From the results, it was found that the influence from the axial magnetic field component Bz on the plasma dissipation could be neglected. While the transverse magnetic field component Bt could trap the residual plasma and decelerate the plasma dissipation process.

Simulation of surface erosion of anode under high-current vacuum arcs

The anode melting and erosion process has a significant effect on the interruption of a high-current vacuum arc. The objective of this paper is to theoretically investigate the mechanism of anode surface erosion caused by a combined effect of vacuum arc heating and the blow effect of arc pressure. A model of fluid flow and heat transfer of an anode region in a vacuum interrupters high-current interruption process is developed. The results show that, under the combined effect of arc heating and arc pressure, an obvious erosion on anode surfaces was seen for peak arcing current of 20 kA. The flow of liquid metal started after 5 ms of arcing, the maximum velocity was 0.95 m/s. This flow of liquid metal on anode surfaces driven by arc plasma may redistribute the thermal energy of molten liquid metal. As a result, the maximum temperature of an anode surface did not stay in the center of an anode surface.

Kinetic model of prestrike arc in vacuum interrupters

During the closing operation of a capacitive load by a vacuum interrupter, there is always a prestrike arc. An inrush current with high frequency and high amplitude flows through the prestrike arc. This prestrike arc with a high energy density destroys contact surfaces locally, resulting in welding between the mated contacts. Consequently, the opened destroyed surfaces, which enhance the local electric fields and bring metal particles into the contact gap, increase the restrike probability of the interrupter under a DC recovery voltage. The objective of this paper is to establish a kinetic model to investigate the characteristics of prestrike arcs and the energy fluxes to the electrodes during closing capacitive loads. The Particle in Cell-Monte Carlo Collision(PIC-MCC) method was adopted in this model to describe this high density plasma within a small gap. The distance between the contacts was set to 1μm. Three different electrode temperatures were set to investigate their influence on results. Electron-atom elastic collision and electron impact ionization were considered in the model. According to the results, the plasma density, current density and energy fluxes to electrodes increased with electrode temperature. The length of arc column also increased with electrode temperature. Electron current was main component of the current. Electron energy flux was less than ion energy flux at cathode, while it was just the reverse at anode.

Kinetic numerical simulation of anode sheath of vacuum arcs

Anode activity is critical in high-current vacuum arcs in vacuum interrupters. If the anode temperature gets too high, an evaporation of metal vapor from an anode melting pool may lead to a formation of anode spots, which would lead to a failure of current interruption of vacuum interrupters. The input energy from an arc column to an anode surface is adjusted by an anode sheath. The objective of this paper is to develop a numerical simulation model of the anode sheath of vacuum arc by using a Particle-in-Cell method. A low-current case and a high-current case of arcing are simulated. The physical parameters of electrons and ions are obtained from magnetohydrodynamic simulations, and are set at the simulation boundary of the current model. The sheath thickness is 2~4 Debye length, and the anode potential fall is about 1.5~3 V for both case. It is found that the acceleration of ions by electric field in the anode sheath is more significant for the high-current case. This also adds to the energy flux into the anode surface.

Dielectric recovery strength after vacuum arc extinctions

Anode surface temperature after interrupting a vacuum arc has a significant impact on the interruption capacity of a vacuum circuit breaker (VCB) because it dominates metal vapor evaporation. The objective of this paper is to theoretically investigate the relationship between breakdown voltages and anode surface temperatures after current zero. A heat transfer model was established to describe the temperature development on an anode surface, taking account of phase transition processes. The contact material was copper. Moreover, PIC-MCC was adopted to simulate the breakdown voltages in a range of metal vapor density. The calculated results verified the two decay modes of anode surface temperature after current zero proposed by our experiments. The metal vapor density evaporated from an anode surface was positive correlated with its temperature. The breakdown voltage was negative correlated with the anode temperature. A higher surface temperature results in a higher probability of breakdown. Thus, it is better to keep the initial surface temperature at current zero below a certain value.

A coupled simulation model of the heating process on an anode under high-current vacuum arcs

Anode activity is critical in a high-current interruption process of a vacuum interrupter. Under a high-current arc anode surface temperature may exceed melting point. Under such condition, an evaporation of metal vapor from an anode melting pool may play a role for a failure of the current interruption. The objective of this paper is to develop a 2D axisymmetric numerical simulation model of heat transfer from a vacuum arc column to an anode region under high-current vacuum arc. The model combined both the magnetohydrodynamic (MHD) model of a vacuum arc column and the heat transfer model of an anode region. The model deals with arc plasma behavior of arc column and heat transfer in the anode region in a coupled way. The temperature distribution, plasma pressure and flow velocity are given. The highest temperature on anode surface is about 1750K and appears at about 7ms. The effect of Lorentz force on the flow of arc plasma was significant. It pushed the arc plasma into the central region and affects the pressure distribution. The results can offer detailed information of high-current vacuum arc and its anode phenomena.