Effect of electric field uniformity on positive streamer and breakdown characteristics of transformer liquids

Streamer development in liquid has different dominant mechanisms under uniform and divergent electric fields, respectively initiation and propagation. This paper presents a study on the influence of electric field uniformity on the pre-breakdown and breakdown characteristics of transformer insulating liquids. Streamers were investigated experimentally in different needle-plane non-uniform field and plane-needle-plane semi-uniform field configurations under standard positive lightning impulses. For the same gap distance, streamers under non-uniform field propagate with a more side-branched structure whereas streamer propagation under semi-uniform field is more constrained to the axial direction. Field enhancement factor was used to indicate the degree of inhomogeneity of the field configuration. Streamers in fields with higher field enhancement factors occupy larger areas. As field enhancement factor reduces, the streamer withstanding probability distribution gets closer to the breakdown probability distribution and hence the breakdown happens at a lower normalized voltage level. Furthermore, a transition phase from a propagation-dominated to an initiation-dominated process of streamer development was evident as the field enhancement factor decreases. The field enhancement factor at which this transition happens was quantitatively derived. It is 54.1 for the investigated mineral oil under positive lightning impulses.