Stroboscopic imaging of streamers propagating along dielectric surfaces

Summary form only given. Solid insulation materials in gas insulated medium (MV) and high voltage (HV) equipment often act as weak elements in terms of electric breakdown, for instance because the insulator surface may facilitate propagation of discharges. The understanding of the fundamental physics of discharges along surfaces is still poor and has to be improved in order to enable knowledge-based design rules for MV and HV equipment. In this research work, the streamer-like initial phase of electric breakdown along an insulator surface will be investigated.A setup was designed and built that enables studying of discharges along the surface of solid dielectric samples. A tunable high voltage pulse is generated by triggering a spark gap that discharges a charged capacitor. The pulse is supplied to a needle inside a gas-filled vessel, causing inception of streamers. Pressure and gas composition inside the vessel can be controlled. The dielectric samples can be placed at various positions between the needle and a grounded plate, situated 155 mm below the needle. Epoxy-resin material with different fillers are used to vary the dielectric permittivity. An ICCD-camera is used to capture images of the resulting discharges. By gating the intensifier at frequencies up to 100 MHz we are able to stroboscopically visualize propagation of the discharge, providing us with a spatially and temporally resolved velocity profile of the streamers. Results indicate that streamers propagate with an increased velocity along the dielectric surface upon contact. If the gas is air, surface streamers are typically a factor ~2-3 faster than bulk gas streamers for various pressures and pulse voltages. We also found parameter ranges of gas composition, pressure, voltage, pulse repetition frequency and material properties, where discharges do not propagate along the dielectric surface, but rather seem to avoid the surface.