Surface corona development in dielectric covered rod-plane air gaps under impulse voltages

Surface corona development in short rod-plane air gaps with a dielectric covered rod under positive lightning and switching impulse voltages is investigated. Discharge activity was recorded through still photographs and oscillograms of the electric field strength at the earthed plane. The gap length, impulse waveshape and material of the dielectric cover were considered as influencing parameters. The extent of development of the surface corona along the dielectric cover becomes greater with increasing applied voltage and field and with decreasing gap length. Also, it is greater for lightning than switching impulse voltages and occurs at significantly lower applied field for the PTFE than epoxy resin cover. Surface corona development, being greatly dependent on the characteristics of the initial corona emerging in the vicinity of the cover tip, affects the inception of the corona emerging at the dielectric cover upper end, thus also breakdown.