This paper concerns theoretical and experimental investigations of the effect of a conductor coating on the onset voltage of a corona on overhead transmission-line conductors. The onset voltage for a self-maintained discharge on the basis of discharge physics is evaluated. This calls first for accurate calculation of the electrical field in the vicinity of a coated conductor and its correlation to the field values near a bare conductor of the same radius. The well-known charge simulation technique is used for field calculation. The calculated electrical field values are utilized in evaluating the onset voltage of positive and negative coronas on bare and coated conductors. The onset voltage increases with a conductor coating. The calculated onset-voltage values agreed satisfactorily with those measured for laboratory models and full-scale test lines.

This paper is aimed at investigating how the characteristics of corona and silent discharges including ozone generation are influenced by geometry of the discharge reactor. The corona discharges have been generated in coaxial wire-cylinder and wire-duct reactors stressed by dc and ac voltages. The silent discharges have been generated in the same reactors in addition to tubular reactors under ac voltage after pasting a dielectric barrier on the ground electrode of the reactors. The reactors were fed by dry air flowing at atmospheric pressure and temperature. The pulse characteristics of silent discharges are compared with those of ac corona discharges. The current-voltage and the ozone generation characteristics of silent and ac discharges are recorded. These characteristics depend significantly on the geometry of the reactor irrespective of the discharge type. NomenclatureC ozone concentration, ppm d width of the discharge gap, mm dV /dt time derivative of voltage V , kV s −1 P discharge power, W Q air flow-rate through the reactor, litre min −1 r radius of the discharge wire, mm STP standard temperature and pressure conditions V o onset voltage of the discharge (peak value), kV V p applied voltage (peak value), kV

The relative importance of atmospheric negative ions in corona formation and breakdown in a radial electric field has been investigated, with special reference to effects occurring at an insulating surface of a cycloaliphatic polyurethane with dolomite filler. The inception times, corona charge, light emission (by UV photography) and sparkover voltages under lightning impulses have been recorded under natural atmospheric conditions and under the influence of negative ions introduced from an auxiliary corona. The presence of excess negative ions on the surface is shown to increase both the charge injected and the mean radius of the corona, attributable to an augmentation of streamer discharges. In similar experiments in air, the excess ions cause a transition to a `glow' discharge. Effects of negative ions on sparkover are not significant on the insulator surface, but they cause a small increase in air. Comparisons between the two cases lead to the conclusion that the most important effect of the ions on the surface is to provide `seed' electrons for streamer propagation, following ion detachment in the field of advancing streamer tips.

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This paper is aimed at investigating the inception voltage of the positive corona in point-to-plane gaps in atmospheric air as influenced by a surrounding dielectric enclosure. First, the method of calculation of the inception voltage is explained. It is based on the criterion of self-sustained growth of onset streamers. This calls for calculating the electric field in the vicinity of the stressed point with and without the surrounding enclosure. Then, the calculated inception voltages are compared with those measured experimentally.