CORONA

 When an alternating potential difference is applied across two conductors whose spacing is large as compared to diameters, there is no apparent change in the condition of atmospheric air surroundings the wires if applied voltage is low. However, when applied voltage exceeds a certain value, called critical disruptive voltage, the conductor are surrounded by a faint violet glow called corona.

The phenomenon of corona is accompanied by a hissing sound, production of ozone, powerless and radio interference. The

higher the voltage is raised, the larger and higher the luminous envelope becomes, and greater are the sound, the power loss and radio noise. If the applied voltage is increased to breakdown value, a flash over will occur between the conductors dude to breakdown of air insulation.

The phenomenon of violet glow, hissing noise, and production of ozone gas in an over head transmission line is known as corona. 

If the conductors are polished and smooth, the corona glow will be uniform throughout the length of conductors, otherwise the rough points will be brighter. With D. C voltage there is difference in the appearance of two wires. The positive wires has uniform glow about it, while the negative conductor has spotty glow. 

Theory of corona formation :

Some ionization is always present in air due to cosmic rays, the ultraviolet radiation and radioactivity. Therefore, under normal condition, the air around the conductors contain some ionized particles (i.e.,free electron and positive ions) and neutral malocules. When P. d is applied between the conductors, the potential gradient is setup in the air which will have a maximum at the conductor surfaces. Under the influence of potential gradient, the existing free electrons acquire greater velocities. The greater the applied voltage, the greater the potential gradient and more is the velocity of free electron.

Factors affecting the Corona :

The phenomenon of corona is affected by physical state of atmosphere as well as the condition of the line, the following are the factor upon which corona depends. 

(1).Atmosphere.

(2).Conductor size

(3).Spacing between the conductor

(4).line voltage