Surface Flashover Over a Microprofiled Cylinder in Air

Abstract

Increasing the lightning impulse (LI) flashover voltage of insulators can require significant changes to the geometry and material properties of the insulator or electrodes. In some cases, such changes can be difficult to implement as they may require more volume or may compromise e.g. mechanical properties of the insulating parts. In this work, a new micro-scale surface profile which inhibits streamer discharge propagation without introducing major changes to the insulation system geometry or materials is demonstrated. Polyoxymethylene (POM) cylinders with 25 % glass fibre in air were machined with 0.5x0.4 mm rectangular grooves spaced 0.6 mm apart and stressed with LIs. A non-profiled cylinder and a cylinder with a larger, semicircular grooves were tested for comparison. The cylinders were tested in a 48.5 mm atmospheric air gap with an exposed triple junction as the positive electrode. The rectangular surface profiles increased the LI flashover voltage (U 50% ) from 70.0 to 96.5 kV peak . High-speed imaging showed positive surface streamer discharges stopping on the surface profiles. Discharge inception calculations were used to explain the influence of the surface profile on the flashover process. The results are relevant for technological applications with electrically stressed solid insulators in gases.

Flashover of Smooth and Knurled Dielectric Surfaces in Dry Air

“…Due to the weakening of the electric field, this will require a higher applied voltage being required to initiate flashover, than across a non-profiled surface. This effect of positive streamer development has been seen in previous research on profiled dielectric surfaces tested under positive polarity impulse voltages, [26], where discharges were observed by a high-speed camera to propagate only partially across insulator surfaces. For the formation of negative streamers, however, they do not rely as much on adsorbed electrons from the gassolid interface, where any electrons which are adsorbed by the surface will have little effect of the field at the front of the streamer as the electrons emanate from the streamer head in the gas/solid interface, manifesting in a lower applied voltage to initiate flashover.…”

“…To explain the higher positive breakdown voltages in the case of knurled dielectric spacers, as compared with smooth spacers and open-air breakdown voltages, the following hypothesis is proposed. Due to the plasma streamer(s) propagating some distance over the dielectric surface, similar to as shown in [26], the electric field in the whole system can be re-distributed resulting in a decrease in the field on the electrode edge. A 2dimentional electrostatic model was created using the QuickField finite element solver and the field distribution in the current setup has been obtained in two different cases.…”

“…The development of positive streamers is further impeded by the knurled surface with increasing pressure, due to the aforementioned mechanisms of positive discharges near dielectric surfaces, decreasing the field at the head of the streamer. However, as negative discharges are wider and more diffuse, as discussed in [26], and with electrons emanating from the streamer head, there is minimal effect of the field at the front of the negative streamer, so therefore, the pressure, and the initial location of the discharge at the TJP has little effect on the flashover voltage.…”

A kinetic Monte Carlo study of positive streamer interaction with complex dielectric surfaces

“…For example, Meyer et al [7,8] have presented a combined computational and experimental study for streamers propagating over non-planar surfaces consisting of planar dielectrics with circular and square channels running orthogonal to the streamer propagation direction. Meyer et al [8] used high-speed imaging and performed computer simulations, and an experimental follow-up study [9] demonstrated the relevance of these effects for high-voltage technology (albeit in a different field configuration). Tangentially connected computational studies have been presented by Wang et al [10] and Konina et al [11], the latter focusing on streamer interaction with pores and droplets.…”