ABSTRACT
This chapter reviews the physical processes leading to the development of corona discharges on the line conductors and presents the current practices in selecting the line conductors. The nonuniform field necessary for the development of corona discharges and the electronegative nature of air favor the formation of negative ions during the discharge development. If the corona onset conditions are met, the secondary emissions are sufficient to trigger new electron avalanches from the cathode and maintain the discharge activity. The movement of ions of both polarities generated by corona discharges, and subjected to the applied field around the line conductors, is the main source of energy loss. The aforementioned corona onset condition emphasizes the great sensitivity of corona activities to the conductor surface condition and, hence, to changes in weather conditions. Three different corona modes—Trichel streamer, negative glow, and negative streamer—can be observed at the cathode with increasing applied field intensities.
