Surface profile effect on streamer propagation and breakdown in air

In a uniform field arrangement, under direct voltage, positive streamer propagation and breakdown are investigated along cylindrical insulators with different profiles, inserted perpendicularly between two parallel plane electrodes. The basic properties of streamer propagation and breakdown, namely the electric field required for a stable propagation together with the associated velocity and the breakdown field together with time to breakdown, are measured as influenced by the pulse voltage amplitude used for the streamer initiation and by the insulator profile. It is shown that a strong relation between streamer propagation and breakdown exists, because the insulator profile exerts a similar influence on the breakdown and propagation fields. The effect of a shed on an insulating surface, forming an 'obstruction' to streamer progress, is to increase the stability for propagation and breakdown fields, and to reduce the propagation velocity at all applied fields compared with those in the case of a smooth insulator. Along the surface of a smooth insulator, a streamer system propagates with a 'surface' and an 'air' component; however, a shed on an insulating surface modifies this system, resulting in only one component reaching the cathode.

I. Introduction

Surface flashover has been studied extensively, because its understanding is essential for the design of insulating systems. Valuable contributions can be found in reviews referring to flashover in compressed gases [1] or in vacuum [2]. Another important study is described in [3], where an attempt to model the discharge along smooth insulators in a non-uniform. field was made. In [4] a model for streamer propagation along smooth insulators in a uniform field was proposed. This model took into account the effects of the pulse voltage used for streamer initiation, of the ambient electric field and atmospheric conditions, and of the nature of the insulating material.