CORONA

Corona phenomenon is the ionisation of air surrounding the power conductor, free electrons are normally present in free space because of radio Activity and cosmic rays. As potential between the conductors is increased, the gradient    around the surface of the conductor increases.Assume that the spacing between the conductors is large as compared with the diameter of the conductor.The free electrons will move with Velocity depending up on field strength.Electrons will collide with Molecules of air and in case the speed is large,They will dis large Electrons from those molecules,There by the number of electrons will increases.The process of Ionisation is Thus Cumulative and ultimately forms on Electron Avalanche. This results in Ionisation of Air surrounding of the conductor In case of the Ratio of the space between conductors to the radius of the conductor is less than 15,Flash over will occur between the conductor before corona phenomenon occurs.Usually for overhead lines this ratio is far more than is number and hence flash over can be regarded as impossible.

Corona phenomena is , therefore,defined as a self sustained electric discharge in which the field intensified ionisation is localised only over a portion  of the distance between the electrodes.When  a voltage Higher than the Critical voltage is applied between two parallel polished wires,The Glow is quite even.After operation for a short time,reddish beads or tufts form along the wire ,while around the surface the wire there is a bluish white glow.If the conductors are examined a stroboscope,so that one wire is always seen when at a given half of the wave , it is noticed that the reddish tufts or beads are formed when the conductor is negative and a smoother bluish glow when the conductor is positive. The AC corona,viewed through a stroboscope has the same appearance as dc corona.As corona phenomenon is initiated, a hissing noise is heard and ozone gas is formed which can be detected by its characteristic odour.