CORONA IN POWER SYSTEM
Corona is a phenomenon whereby, the air around the conductors is ionized. Consider a single-phase two-wire system, whose spacing is large as compared to the diameter of the wires. When a low alternating voltage is applied across two conductors, there is no change in the condition of the air around. If the applied voltage is gradually increased, it will result in a gradual increase of the voltage gradient. The moment it reaches the maximum value i.e., about 30 kV/cm (peak) or 21.1 kV/cm (R.M.S.), the air surrounding the conductor starts conducting with a hissing sound. In dark, a faint violet glow occurs around the conductor and ozone is produced, and this effect is called Corona. (Fig.1)
THEORY OF CORONA FORMATION (CORONA DISCHARGE)
Due to atmospheric radiation and the presence of ultraviolet rays, the air naturally contains some ionic particles. Along with the neutral atoms, these ionized particles surround the conductor under normal conditions. When a voltage gradient is set up between the two conductors, the free electrons attain greater momentum. Intense electric fields as high as 30 kV/cm in the air, may occur at the surface of the conductors, resulting in the ionization of the neutral molecules as the high momentum free electrons dislodge more electrons from them, thus making the process of ionization cumulative. This aggravation of ions causes the electrical breakdown of the air surrounding the conductor leading to what is known as corona discharge. The corona activity consequently varies markedly with variations in surface and atmospheric conditions.
Corona inception voltage is the lowest voltage at which continuous corona of specified pulse amplitude occurs as the applied voltage is gradually increased. Corona inception voltage decreases as the frequency of the applied voltage increases. Corona can occur in applications as low as 300V.
Corona extinction voltage...