Theoretical analyses of arcs in triggered vacuum switches

Abstract

Triggered vacuum switches (TVS) have found fast growing applications in the field of power systems and pulse power. The arc patterns play an important role in the operation of a TVS. Vacuum arcs have two different forms, the diffuse form and the constricted form. A normal vacuum arc is a diffusive arc. By contrast, a constricted arc is an abnormal vacuum arc, which could cause serious electrode erosion and produce a deteriorated dielectric recovery capability, i.e. the TVS may fail to turn off. In order to control the arc, we expect the arc to be in the diffuse form or quickly regain the diffusive nature in case a constricted arc is formed. This work is a theoretical analysis of arcs in TVS, to find the critical conditions in their operation, which would help the optimisation design of TVS. Discussions begin from the development of the arc patterns, the arc initiation, transformation and extinction, and the transition time and recovery time are defined. Analysis shows that, a constricted arc is unavoidable in the initial stage of TVS' operation, but can transform into a diffuse arc within a transition time which depends on the spots diffuse conditions. The arc that has been transformed from constricted form into diffuse form still needs a recovery time before the arc current decreases to its zero. There is a simple expression for spot thermal time constant, and axial magnetic field of suitable magnitude is necessary for a high capacity TVS.