Thermodynamic evaluation of oxidation during brazing process of medium-voltage electrical circuit breakers

Abstract

The present study is a thermodynamic analysis of the different molecular flows occurring during the vacuum brazing of electrical vacuum bottles also called Vacuum In-terrupters (mentioned later as VI). Among the available impurities either coming from the Vacuum Interrupters components materials or from vacuum technology, the analysis is fo-cused on the oxygen since this impurity leads to formation of the more stable compounds, i.e. oxides. During the brazing cycles the resistors of the vacuum furnace and the VI materials including some braze components are the main sources of vapours that partly escape or make deposits on colder parts of the furnace. Vaporization and condensation processes are evaluated, their matter flows quantified and finally their interaction with residual oxygen calculated through a balance between input oxygen from vacuum or neutral gas and the different sources of oxygen coming from the parts under treatment. The enrichment of the VI materials and deposits with oxygen is evaluated as well as the limits required for vacuum conditions. The main conclusion is that present vacuum conditions allow correct working of the brazing sequence but in any other temperature steps or ramps, oxygen is trapped as oxides at the surface of the parts or accumulated in the bulk materials of the furnace.