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The electric breakdown at single and multiple protrusions in SF6 and CO2 is investigated at 0.4 and 0.6 MPa, respectively. Additionally, the breakdown fields at rough surfaces of two different areas were determined. From the measurements, breakdown probability distributions for single protrusions were determined and fitted by Weibull distributions. This allowed the determination of statistical enlargement laws for the 50% breakdown probability fields E50. Such enlargement laws describe, for example, the scaling of breakdown field with electrode area or number of protrusions. The predictions were compared to the experimental data, and both agreement and discrepancies were observed depending on polarity and number of protrusions and gas. Discharge predictions including first electron, streamer inception and crossing, as well as leader propagation, gave further insight to this. It was found that predictions from enlargement laws based on statistical processes may not describe the measured breakdown fields well and that relevant physical breakdown criteria must also be considered.

In high voltage equipment, gases are often used as insulation material. Several parameters can influence the dielectric withstand voltage of gaseous insulation systems, such as the degree of inhomogeneity, gas pressure and type as well as surface roughness. This study focuses on the influence of surface roughness on the withstand voltage of atmospheric pressure air gaps under lightning impulse stresses. For this purpose, surface profiles of different rough metallic surfaces have been characterized using an optical surface scanner with a resolution of 3nm. Based on the extracted features of the surface roughness, axial symmetric random surface profiles have been generated in a multi-physics simulation tool, electric field simulations performed, and streamer criterion applied to estimate the breakdown voltage. To verify the simulation results, lightning impulse breakdown voltage measurements have been carried out in simple configurations (sphere plate and rod plate) for four different degrees of surface roughness of the plate electrode. The experimental results confirm the observed trend in simulations, where the breakdown voltage is only to a limited extend reduced even for considerably rough surface profiles. © 2018 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.