Interference with radio and television (RI and TVI) may arise when electrical discharges run on insulators and inject high-frequency currents into associated conductors, just like corona.


The types of discharge which generate interference are: micro sparks between water drops or metal fittings, the latter especially in cases of corrosion; discharges across dry bands on leaky surfaces; surface corona discharges around highly stressed electrodes.
Surface corona discharges are again relatively slow phenomena, incapable of heavy generation at VHF, but principal sources at lower frequencies. Surface discharges may be prevented by hydrophobic treatment. This not only inhibits dry-band formation but also gives good voltage grading, thus removing the over voltages, which cause other types of discharge. It might appear that the installation of a corona ring would smother capacitive over voltages, while avoiding the usual power loss, which follow installation of a corona ring.
Both the above-mentioned types of discharge are, to some extent, weather dependent. Water may cause droplet discharges while suppressing contact discharges by virtue of its high conductivity and permittivity; dry bands do not a fair-weather phenomenon, the existence of which depends on the design of the insulator and the geometry of the insulators of fittings.

Surface corona, if in fair weather, for which reason it has long been the practice to specify higher than normal voltages for corona inception on insulator.


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