Dynamics of arc phenomena at closure of electrical contacts in vacuum circuit breakers

Dynamic phenomena accompanying electrical contact closure in vacuum circuit breakers are considered as consecutive stages, including breakdown, touch, compression, restitution, bridging, and arcing at bouncing. The hybrid mathematical model is elaborated to describe dynamics of the arc and forces acting in the contact gap. The model uses experimental oscillograms of current, voltage and contact displacement and nonlinear equations for arc, anode and cathode temperature fields, and contact motion. Experiments are carried out using a standard vacuum circuit breaker with laser sensors for the measurement of contact gap. The special difference path method is applied to take into account oscillations of a fixed contact. The axisymmetric Stefan problem with two free boundaries corresponding to melting and evaporation interfaces is solved to find dynamics of contact evaporation. It is shown theoretically and confirmed experimentally that in the range of high currents, the force of metallic vapor pressure at arcing in vacuum is comparable with magnetic repulsion force and should be taken into consideration. Two mechanisms of vapor pressure formation and evolution are discussed, which occur due to explosion of micro-asperities at contact touch or due to arcing after bridge rupture at contact bouncing.