Corona Phenomenon
CORONA PHENOMENON AND LOSS As the voltage on a line conductor system is raised beyond a certain limit, a point is reached when a pale violet glow appears on the conductor surface, together with a slight hissing noise and a smell of ozone. This phenomenon is known as corona. For an overhead transmission system the atmospheric air, which is the dielectric medium, behaves practically like a perfect insulator when the potential difference between the conductors is small. As long as the air is subjected to uniform electric field intensity, the flow of current between the two conductors of the line is negligibly small.
But when the electric field intensity reaches a critical value, the air in the immediate vicinity of conductors no more remains a dielectric but it ionizes and becomes conducting. This electric breakdown is accompanied by the following phenomenon:
a. A faint glow appears around the conductors which is visible in the dark.
b. There is a noise.
c. There is a tendency in the conductors to vibrate.
d. Ozone and oxides of nitrogen are produced.
e. There is power loss.
f. There is radio interference.
THEORY OF CORONA FORMATION: The electrons and ions are always present to a small extent in the atmospheric air. When an electric field of uniform intensity is established between the conductors, these ions and free electrons experience forces upon them. Due to this force, the ions and free electrons get accelerated and move in opposite directions. The charged particles during their motion collide with one another and also with the very slow moving uncharged molecules. Experimentally it has been seen that at NTP i.e. 76cm pressure of Hg and 25°C, when the electric field intensity reaches a critical value of about 3000 kV/m, the velocity attained by a charged ion or electron is sufficiently high to tear off one or more electrons from the uncharged molecule with which it makes collision, leaving one or more free electrons and a new positive
But when the electric field intensity reaches a critical value, the air in the immediate vicinity of conductors no more remains a dielectric but it ionizes and becomes conducting. This electric breakdown is accompanied by the following phenomenon:
a. A faint glow appears around the conductors which is visible in the dark.
b. There is a noise.
c. There is a tendency in the conductors to vibrate.
d. Ozone and oxides of nitrogen are produced.
e. There is power loss.
f. There is radio interference.
THEORY OF CORONA FORMATION: The electrons and ions are always present to a small extent in the atmospheric air. When an electric field of uniform intensity is established between the conductors, these ions and free electrons experience forces upon them. Due to this force, the ions and free electrons get accelerated and move in opposite directions. The charged particles during their motion collide with one another and also with the very slow moving uncharged molecules. Experimentally it has been seen that at NTP i.e. 76cm pressure of Hg and 25°C, when the electric field intensity reaches a critical value of about 3000 kV/m, the velocity attained by a charged ion or electron is sufficiently high to tear off one or more electrons from the uncharged molecule with which it makes collision, leaving one or more free electrons and a new positive