Creeping Sparks

In gas insulated high voltage devices usually solid insulation materials are also present, e.g., for mechanical reason like the support of conductors or as enclosure. Even if the gaseous and solid insulation media themselves have sufficient dielectric strength, electric breakdown may still occur because the gas-solid interfaces are usually weaker. Triple points of metal, gaseous, and solid insulation are critical with respect to inception, and insulator surfaces facilitate propagation of discharges, which may lead to surface flashover. Sparks creeping along insulator surfaces are well-known, but the underlying fundamental physics is poorly understood. The improvement of (nowadays empirical) design rules of insulation devices for the prevention of surface flashover requires thus a deeper understanding of the associated physics. In this research work, the streamer-like initial phase of sparking will be investigated. An initial setup has been built that enables to study discharges along the surface of an insulating rod. Some preliminary results indicate that surface discharges do not appear under all circumstances. We implement two measurement techniques that enable us to study the inception and the propagation of the surface discharges. First, we use stroboscopic imaging at gating frequencies up to 1 GHz in order to visualize inception and propagation of the discharge. Secondly, we will use ellipsometry on a crystal exhibiting the Pockels effect in order to determine the electric field at the surface and thus the surface charge on the insulator.