Mechanisms of Impulse Breakdown in Liquid: The Role of Joule Heating and Formation of Gas Cavities

The impulse dielectric behavior of insulating liquids is of significant interest for researchers and engineers working in the field of design, construction, and operation of pulsed power systems. Analysis of the literature data on transformer oils shows that potentially there are several different physical processes that could be responsible for dielectric breakdown by submicrosecond and microsecond impulses. While for short submicrosecond impulses ionization (plasma streamer) is likely to be the main breakdown mechanism, for longer impulses, the thermal effects associated with Joule heating start to play an important role. This paper provides a theoretical analysis of the latter mechanism in dielectric liquids of different degrees of purity stressed with high-voltage (HV) impulses with duration sufficient to cause local heating, evaporation, and formation of prebreakdown gas bubbles. The proposed model is based on the assumption that dielectric breakdown is developed through percolation channels of gas bubbles, and the criterion of formation of these percolation chains is obtained. To test the developed model, the breakdown field-time characteristics have been calculated for the liquid with chemical composition close to that of transformer oils but with known thermodynamic characteristics (n-hexane). Its dielectric strength has been obtained as a function of externally applied pressure and temperature. The analytical results show good agreement when compared with the experimental data available in the literature.