Irreversible changes of vacuum circuit breakers' dielectric strength as a result of current breaking operation

Abstract

In the vacuum switching element, current breaking operations cause the changes of the electrodes' surface by means of the following mechanism: 1 — cathode erosion (during the movement of electric arc's cathode spot); 2 — the explosion of the conducting metal bridge during the arc stretching; 3 — condensation of neutral metal vapor on the contacts, and 4 — the emission of micro-particles of the melted electrode surface regions. The first three mechanisms are dominant in the case of diffuse arc (there are more parallel arcs of the small current, up to 100 A, which do not have consolidated the cathode spot). The fourth mechanism is dominant for the constricted arc (cathode spot is consolidated). The diffusion arc has the arc voltage typical value of 30 V, while in case of the consolidated arc, the arc voltage may increase more than 100 V. The transition between the diffusion arc and consolidated arc does not occur instantaneously, but rather, there are some transitional shapes. The mentioned contact topography changes of the vacuum circuit breakers lead to irreversible changes of vacuum insulation through the braking operations. The aim of this paper is to determine the irreversibility rate of the vacuum insulation caused by circuit braking operations thereby evaluating the possible number of these switching operations in the course of the life cycle of one such element. The results of measuring the AC breakdown voltage, pulse breakdown voltage and the voltage whose corresponding prebreaking current equals 10−4 A and 10−5 A will be statistically analyzed for that purpose. The experiments will be carried out on the commercial switching elements with CuCr and CuBi contacts. The experiment parameters will be the current breaking value and interelectrode distance, respectively.