Effects of electrode size and solid barrier orientation on streamer discharge in transformer oil

Abstract

Geometrical effects of electrodes and solid barriers immersed in transformer oil are investigated on positive streamer initiation, propagation, and transformation to surface flashover using a 2-D axisymmetric model. Electrode radii of curvature in the range of 20 μm to 6.35 mm are selected such that only positive streamers form. Modeling results indicate that the positive electrode size directly determines the streamer initiation voltage, while breakdown voltage and delay are mainly determined by the grounded electrode size. Specifically, sharper positive electrodes require lower voltages to initiate positive streamers and sharper grounded electrodes result in lower delays and higher breakdown voltages. Incorporating perpendicular and parallel orientations of solid barrier interfaces in the electrohydrodynamic model shows that polarization forces from the barrier dielectrics on charge carrying streamers are proportional to the permittivity difference between transformer oil and the solid dielectric. If the barrier permittivity is greater than the oil permittivity, the solid dielectric applies an attractive force on the volume charge that turns the streamer into a surface flashover expanding on the barrier surface. On the other hand, a low permittivity pressboard interface repels the approaching streamer. Theoretical analysis of electric field propagation and charge distribution along the streamers, surface flashovers, and through the interfacial surfaces is presented. Barrier dielectric relative permittivities of 1.1 and 4.4 have been studied while the oil relative permittivity is assumed 2.2.