Consideration of corona onset from a water drop as a function of air pressure

Abstract

The paper identifies three different corona onset mechanisms for a water drop attached to the underside of a conductor as a function of air pressure. The three different corona onset mechanisms are a result of two different corona onset processes: water drop instability and ionisation of the surrounding air. Both processes are considered in the paper. A model which determines the corona onset potential as a function of air pressure for a water drop attached to a conductor is presented. The model identifies the three onset mechanisms and accounts for the previously unexplained transition between the ionisation and instability modes. The corona onset potential is determined by modelling both the stability of the drop and the ionisation of the surrounding air as a function of the potential applied to the conductor. This is done over a range of air pressures resulting in an ‘onset versus air pressure’ curve. The entire model is then verified by experimentation. The model accounts for changes in the drop shape with changes in applied potential and the influence of this on the E-field distribution and hence the ionisation processes. Drop volume, conductor surface conditions and applied E-field divergency are accounted for.