Roughness Effects on DC Breakdown and Pre-Discharge in Pressurized Synthetic Air

Abstract

This research investigates the influence of cathode surface roughness on DC breakdown voltages and pre-discharge currents in pressurized synthetic air. A physics-based computational model is presented for predicting breakdown voltages in insulating gases under high-voltage stress. The model combines electron ionization and attachment processes along the discharge path to calculate the evolution of the primary electron avalanche. The model considers gas pressure, gap distance, electrode geometry and electric field distortions due to electron avalanches, and had been validated with a huge number of experimental breakdown measurement series with smooth electrode geometries in synthetic air. In the present work this model is extended to include the influence of electrode surface roughness on breakdown behavior. To accomplish this, cathodes with varying roughness levels were characterized using laser scanning microscopy and corresponding local field enhancements at the surfaces are calculated by numeric simulation. Those, locally non-uniform electric field distributions were used as input for the computational model to predict breakdown voltages. Finally, breakdown and pre-discharge measurements of sphere spark gaps with different electrode surfaces are presented and compared to the model predictions. Validations against these experiments at pressures up to 1.5 MPa show strong agreement between measured and calculated breakdown voltages. The results demonstrate that cathode surface features in the micrometer scale can significantly reduce the insulation strength of pressurized synthetic air and that the proposed model is able to predict this effect.