Creeping sparks

In gas insulated high voltage devices usually solid insulation materials are also present, e.g., for mechanical 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. For instance, triple points of metal, gaseous, and solid insulation can be 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. A setup was built that enables us to study discharges along the surface of an insulating rod. We use stroboscopic imaging at gating frequencies up to 100 MHz to visualize inception and propagation of the discharge. Results indicate that streamers propagate with an increased velocity along the dielectric surface upon contact. Surface discharges, however, do not appear under all circumstances: for some parameters the discharge avoids the surface. The corresponding parameter window for sticking behavior depends on gas composition, pressure, voltage, and repetition frequency