Analysis of microdischarge threshold conditions between a conducting sphere and plane

Abstract

Geometrical coefficients for the determination of the potential difference across the grounded plane and an arbitrarily charged conducting sphere in the presence/absence of an electric field are analysed and calculated numerically. An estimation method of microdischarge thesshold conditions in a gap between the free sphere and the plane using the Paschen curve and the coefficients is proposed and is applied to two cases: a free conducting sphere placed in a parallel plane gap in SF 6 gas; water drops dripping from a high voltage conductor and approaching the grounded plane. It is found from the applications that the influence of gas pressure is less pronounced in the case of microdischarge threshold field strength in SF 6 as compared with the breakdown field stregth of the main gap, and the critical radius of the sphere for reducing the breakdown field strength of the main gap is nearly the same as that for the microdischarge threshold. Moreover, the experimental minimum charge on the water drop to produce microdischarge near the grounded plane agrees well with the present estimated value.