Theoretical and experimental studies of air gap breakdown triggered by free spherical conducting particles in DC uniform field

Abstract

In this study, mechanical analysis of the air-gap breakdown with free spherical metal particle is conducted in a DC uniform field to address the problem of metal-particle pollution in DC gas insulation. The criterion and spacing of micro-discharge between the particle and electrode is calculated based on the analysis of electric-field distortion with a lifting particle by using the streamer theory. The gas-breakdown equation that considers the effects of free particle is modified with the new concept of equivalent radius, and the different breakdowns triggered by various sizes of aluminum or copper particles are analyzed in ramp voltages. Different states of particle lifting, direct breakdown, or moving breakdown are also considered. The concept of risky radius is also proposed. The breakdown process is observed using a high-speed camera, and the model is verified through experiments. The breakdown voltage of the moving particle is lower than the still particle's. Three breakdown regimes in the DC uniform field triggered by free metal particles were defined, among which the lifting-voltage breakdown occurs only when the particle is close to the upper electrode for the first time. The breakdown voltage showed a minimum value when the particle moved nearer to the risky radius. The proposed research provides fundamental basis in evaluating the dangerous level of free conducting particles within DC GIL as to improve the insulation design.