INFLUENCE OF SAND DUST CONTAMINATION ON THE BREAKDOWN OF ASYMMETRICALAIR GAPS UNDER SWITCHING IMPULSES

This paper discusses the influence of airborne and surface-adhering sa nd/dust contamination on the breakdown characteristics of asymmetrical air gaps when subjected to standard switching impulses of both polari ties. Extensive measurements of breakdown voltages and breakdown times are carried out under clean and polluted conditions using rod-plane a nd sphere-plane gaps. When positive switching impulses are applied to gaps less-than-or-equal-to 1.8 m, the presence of sand/dust particles less-than-or-equal-to 150 mum diameter and less-than-or-equal-to 2 g/m 3 concentrations have insignificant influence on the breakdown charact eristics of the air gaps. However, the presence of such particles can greatly affect the breakdown characteristics for negative switching im pulses. Under such conditions, the particles can decrease V50 by less- than-or-equal-to 40% for gaps with field factor f less-than-or-equal-t o 3.5. For gaps with 3.5 < f < 12, the contamination can increase V50 by less-than-or-equal-to 80%. For more nonuniform field gaps, V50 valu es for clean and contaminated conditions are similar. The particles ca n also significantly influence the breakdown time lag characteristics under negative switching impulses. The observed effects are primarily caused by sand/dust particles which settle on the cathode, while the p articles present in the interelectrode gap or on the anode have very l ittle influence.