Influence of sand/dust contamination on the breakdown of asymmetrical air gaps under switching impulses

This paper discusses the influence of airborne and surface-adhering sand/dust contamination on the breakdown characteristics of asymmetrical air gaps when subjected to standard switching impulses of both polarities. Extensive measurements of breakdown voltages and breakdown times are carried out under clean and polluted conditions using rod-plane and sphere-plane gaps. When positive switching impulses are applied to gaps ⩽1.8 m, the presence of sand/dust particles ⩽150 μm diameter and ⩽2 g/m³ concentrations have insignificant influence on the breakdown characteristics of the air gaps. However, the presence of such particles can greatly affect the breakdown characteristics for negative switching impulses. Under such conditions, the particles can decrease V₅₀ by ⩽40% for gaps with field factor f ⩽3.5. For gaps with 3.5<f<12, the contamination can increase V₅₀ by ⩽80%. For more nonuniform field gaps, V₅₀ values for clean and contaminated conditions are similar. The particles can 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 particles present in the interelectrode gap or on the anode have very little influence