Mechanisms of impulse breakdown in liquid: the role of Joule heating and formation of gas cavities

The impulse dielectric behaviour 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 which could be responsible for dielectric breakdown by sub-microsecond and microsecond impulses. While for short, sub-microsecond impulses ionisation (plasma streamer) is likely to be the main breakdown mechanism, for longer impulses, thermal effects associated with Joule heating start to play an important role. The present paper is provides a theoretical analysis of the latter mechanism in dielectric liquids of different degrees of purity stressed with high voltage impulses with duration sufficient to cause local heating, evaporation and formation of pre-breakdown 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. In order 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 a good agreement when compared with the experimental data available in the literature