The Effects of Pollution and Surface Discharges on the Impulse Strength of Line Insulation

The effect of surface discharges on the ability of polluted insulators to withstand lightning and switching surges has long been a matter of conjecture. This paper describes combined power frequency and impulse voltage tests on 66 kV and 132 kV insulator strings. The tests have shown that an appreciable lowering of both lightning and switching surge strength may occur in the presence of pollution layers. The worst lowering of impulse strength (about 50%) has been found in cases where drying of the pollution has reduced the leakage current almost to zero by the production of extensive dry bands on the insulator surface.