Dielectric barriers under lightning impulse stress: Breakdown and discharge-dielectric interaction in short non-uniform air gaps

Abstract

Dielectric-air insulation systems can reduce the carbon footprint of medium voltage switchgear, as they can replace SF6-insulated systems. Such replacement is, however, not easy, as the devices are typically compact and complex, with strongly inhomogeneous electric fields. Withstand voltages are typically estimated with empirical models for discharge formation and propagation. Any dielectric surface can change the conditions for such evaluations, especially as the surfaces accumulate charge. The aim of this work was to study the influence of dielectric surfaces on breakdown in non-uniform atmospheric air gaps. Short (≤ 120 mm) rod-plane gaps with a polycarbonate barrier were used throughout. The development of pre-breakdown and breakdown processes were clarified with high-speed imaging, photomultipliers and fast current measurements. It was shown that breakdown around the 50% level is governed by leader-like discharges rather than streamers. Surface potential measurements demonstrated that streamers charge dielectric barriers in a way that is predictable with variations of saturation charge boundary conditions (zero or constant normal electric field at the air side of the interface). Such predictions allow utilization of beneficial surface charging effects in switchgear design. Moreover, it was shown that drift-diffusion models can be used to simulate important discharge behaviour in dielectric-air insulation systems. Such models are computationally heavy, but can become useful for dielectric design of high voltage components as computer capabilities are improving. It was shown that 2D planar streamers charge dielectric surfaces to saturation within tens of ns. The streamer-exposed surface is charged mainly by drifting ions from the streamer channel, but also by electron emission processes from the dielectric surface. The thesis is presented as a collection of seven peer-reviewed articles published in journals and conference proceedings, together with an overview document. The results presented in this thesis contribute to a better understanding on how dielectric barriers influence electrical breakdown in air.