Influence of barriers on the lightning and switching impulse strength of mean air gaps in point/plane arrangements

The influence of clean and polluted barriers on the breakdown voltage of point/plane mean air gaps (30 to 200 cm) under lightning and switching voltages is addressed. The conditions of existence of flashover around the barrier without breakdown and without surface discharge are determined. It is shown that a clean insulating barrier situated near the electrode point, particularly at about 20% of the gap, leads to a significant increase in the breakdown voltage of the air gap. This increase is especially due to channel elongation of the disruptive discharge. A polluted barrier with surface conductivity higher than 1.6 μS leads, similar to a metallic barrier, to a decrease in the breakdown voltage as compared with that without a barrier. A simplified model to estimate the optimal position of the barrier, considering the surface charge densities on both of its sides, is presented. It gives results in good agreement with experimental ones