ABSTRACT
The corona is distinct from the discharges that take place inside gas bubbles within solid and liquid insulation, although the underlying phenomena of gas discharges are the same. The corona discharge is accompanied by a number of observable effects, such as visible light, audible noise, electric current, energy loss, radio interference, mechanical vibrations, and chemical reactions. The discharge process depends on the polarity of the applied voltage. The chapter provides a discussion on the discharge process for each polarity under direct current (DC). The basic difference between AC and DC coronas is the periodic change in direction of the applied field under AC, and its influence on the residual space charge left over from the discharge during preceding half-cycles. Under impulse voltages, the corona starts in an air gap almost clear of any space charge. Positive pulsating corona discharges occur near the onset level as onset streamers and at much higher voltages as pre-breakdown streamers.
