Metallic Particle Motion and Breakdown at AC Voltages in CO2/O2 and SF6

This study deals with gaseous insulation contaminated by free moving particles. Two gases were investigated: SF6 (0.45 MPa) and a CO2/O2 gas mixture (0.75 MPa). Video recordings were used to track a free particle moving between a plate and a Rogowski electrode for validation of a 1D particle motion model. The effect of fixed and free particles (4 or 8 mm, Ø 0.9 mm) on the breakdown voltage and the mean time between breakdowns was determined in a concentric set of electrodes. The value of the breakdown voltage for a free particle was between those of a particle fixed to the enclosure and the central electrode. The particle motion in the concentric case could not be observed in the experimental set-up and was therefore simulated using a 1D model. For the 4 mm free particle, the breakdown seemed to be initiated in the inter-electrode gap in CO2 and at the crossing in SF6, while for the 8 mm particle, breakdown occurred at lift-off in both gases. A parameter k describing the width of the time to breakdown distribution was introduced. A low value of k was associated with the breakdown from the particles at the electrodes, while k was larger than 10 when the breakdown was decided during particle flight.