The phenomenon of violet glow, hissing noise and production of Ozone gas in an overhead transmission line is called corona.
Air is not a perfect insulator and even under normal conditions, the air contains many free electrons and ions. When an electric field intensity establishes between the conductors, these ions and free electrons experience forced upon them. Due to this effect, the ions and free electrons get accelerated and moved in the opposite direction.
The charged particles during their motion collide with one another and also with the very slow moving uncharged molecules Thus, the number of charged particles goes on increasing rapidly. This increase the conduction of air between the conductor and breakdown occurs i.e. corona takes place.
Factors affecting corona
The following are the factors affecting the corona;
1.Effect of supply voltage - If the supply voltage is high corona loss is higher in the lines. In low-voltage transmission lines, the corona is negligible, due to the insufficient electric field to maintain ionization.
2.The condition of conductor surface – If the conductor is smooth, the electric field will be more uniform as compared to the rough surface. The roughness of conductor is caused by the deposition of dirt. Thus, rough line increases the corona loss in the transmission lines.
3.Air Density Factor – The corona loss is inversely proportional to air density factor, i.e., corona loss increase with the decrease in density of air. Transmission lines passing through a hilly area may have higher corona loss than that of similar transmission lines in the plains because in a hilly area the density of air is low.
4.Effect of system voltage - Electric field intensity in the space around the conductors depends on the potential difference between the conductors. If the potential difference is high, electric field intensity is also very high, and hence corona is also high. Corona loss, increase with the increase in the voltage.
5.The spacing between conductors -If the distance between conductor is extended beyond certain limits then the corona loss reduces
Disadvantages of corona discharge
The undesirable effects of the corona are:
1.The glow appears across the conductor which shows the power loss occurs on it.
2.The audio noise occurs because of the corona effect which causes the power loss on the conductor.
3.The vibration of conductor occurs because of corona effect.
4.The corona effect generates the ozone gas because of which the conductor becomes corrosive.
5.The corona effect produces the non-sinusoidal signal thus the non-sinusoidal voltage drops occur in the line.
6.The corona power loss reduces the efficiency of the line.
7.The radio and TV interference occurs on the line because of corona effect.
Advantage of Corona:
Due to corona effects, transient effects are reduces. So all the connected equipment to line are protected.
Due to corona formation, the air surrounding the conductor becomes conducting. So virtual diameter of the conductor is increased. The increased diameter reduces the electrostatic stress between the conductors.
Corona decreases the efficiency of transmission lines. Therefore, it is necessary to minimize corona. The following factors may be considered to control corona:
1.Spacing between the conductor - By increasing spacing between the conductors, the corona effect can be reduced.
2.Conductor diameter - For reducing corona loss, this method of increasing conductor diameters is very effective. Diameters of conductors can be increased by using hollow conductors and by using steel-cored aluminum conductor (ACSR).