Formative time lag and breakdown in SF6 at small protrusions

Pre-breakdown partial discharges (PBPD) and breakdown in compressed SF₆ at small protrusions in a uniform background field were investigated at pressures of 0.1...0.5 MPa for both polarities. Such geometry is close to technically relevant configurations of compressed gas insulation systems, which are locally disturbed by small defects. Optical and electrical diagnostics allowed us to obtain a consistent picture of the pre-breakdown phase and the breakdown, which occured by stepped leader propagation. It is found that the PBPD activity at both polarities is decisive for the insulation performance of the system. A significant formative time lag of up to several seconds could be observed, depending on polarity and pressure. The role of the dominating pulses during this phase, which are arrested leaders, on the formative time lag and the breakdown will be discussed.