Modeling Spots on Composite Copper–Chromium Contacts of Vacuum Arcs and their Stability

Abstract

Cathode spots on copper-chromium contacts of vacuum interrupters are simulated by means of a self-consistent space-resolved numerical model of cathode spots in vacuum arcs developed on the basis of the COMSOL Multiphysics software. Attention is focused on spots attached to Cr grains in the Cu matrix in a wide range of values of the ratio of the grain radius to the radius of the spot. In the case where this ratio is close to unity, parameters of spot are strongly different from those operating on both pure-copper and pure-chromium cathodes; in particular, the spot is maintained by Joule heat generation in the cathode body and the net energy flux is directed from the cathode to the plasma and not the other way round. An investigation of stability has shown that stationary spots are stable if current controlled. However, under conditions of highpower circuit breakers, where the near-cathode voltage is not affected by ignition or extinction of separate spots, the spots are unstable and end up either in explosive-like behavior or in destruction by thermal conduction. On the other hand, spots live significantly longer-up to one order of magnitude-if the spot and grain sizes are close; else, typical spot lifetimes are of the order of 10 µs. This result is very interesting theoretically and may explain the changes in grain size occurring in the beginning of the lifetime of contacts of high-power current breakers. A sensitivity study has shown that variations in different aspects of the simulation model produce quantitative changes but do not affect the results qualitatively.

Simulation of cathode spots crater formation on electrodes with micro-scale and nano-scale alloy materials in a vacuum arc

“…Therefore, it is reasonable to assume that the cathode spots are more likely to appear above the grains in case of micro-scale grain alloy electrodes. Since the micro-explosion stage before the appearance of cathode spots is not considered in this simulation as assumption 1 mentioned, it is considered to set the grains as hemispherical particles with a flat upper surface [20][21][22].…”

“…However, there are only few models for Cu-Cr electrodes in the simulation of micro-alloy electrodes. Benilov et al simulated the influence of Cu-Cr alloy electrodes with different chromium grains sizes (Chromium grain radius from 2.5 μm to greater than 20 μm) on cathode spots [20,21]. Wang et al published a direct parameters model on the basis of reference [16] to study the formation and development of cathode spots in Cu-Cr alloy electrodes in the case of much smaller grains (Chromium grain radius from 1.5 μm to 2.5 μm) [22].…”

Ionization features in cathode spot region of vacuum arc

“…At present, two basic ideas have been formed on the nature of the cathode spot: the first model assumes the entry of electrons into the plasma due to thermal and field emission, and the heating of the spot due to ion bombardment [3,4]. In the second model, the emission of electrons and Joule heating is carried out due to the explosion of micro-tips [5,6]. However, the question of the mechanisms of ionization in the cathode plasma, the processes of formation of the space charge of Langmuir remains open.…”