Pre-Breakdown Discharges in Highly Non-uniform Fields in Relation to Gas-insulated Systems

In clean SF6 gas-insulated power transmission systems significant pre-breakdown discharges are rarely observed. This is so because the prevailing electric field is only moderately non-uniform. In practical systems, however, conducting particle contamination creates regions of highly non-uniform fields. Initial ionization, under contaminated conditions, takes the form of pre-breakdown discharges. An understanding of pre-breakdown discharges is essential for evaluating the conducting particle effects and the insulation capability of SF6 gas-insulated systems. Rod-to-plane gaps provide a convenient way of studying pre-breakdown discharges in SF6 under highly non-uniform fields under controlled laboratory conditions. The relatively few published reports of investigations of prebreakdown discharges in SF6 are reviewed. Based on the authors´ and other published experimental results, several pre-breakdown regimes (for example, single pulse and multiple pulse pre-discharges and incomplete breakdowns etc.) are identified. The randomness of the time delay of the first pre-discharge current pulse is attributed to the rate of production of initiatory electrons near the anode and the growth of electron avalanches thus generated. The importance of the spatial electric field distribution around the rod electrode (anode) is emphasized. Since in practice transient overvoltages may be superimposed on the system 60 Hz ac voltage, the effect of continuous corona and the space charge due to the 60 Hz ac voltage on the overall insulation performance of SF6 gas-insulated systems under combined ac and impulse voltages also is discussed.