Concept of Corona

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Corona:

When a normal alternating voltage is applied across two conductors with enough sparing in between them,then there is no change in atmospheric conditions. But if the voltage applied exceeds a particular limiting value then the air surrounding the conductor gets ionised due to which hissing noise or a faint glow appears.

Thus, the phenomenon of hissing noise, faint glow and production of ozone gas surrounding the overhead lines, due to ionisation of air is called Corona.

The complete disruption in dielectric strength or insulation of an insulating material (air) near the surface of the conductor at certain point is called concept of Corona. Corona occurs only when the electric field intensity is greater than dielectric strength of air.

Fig. Corona Effect


In general Corona occurs around the power conductor due to two reasons:

I.) Electrical power transmission is at higher operating voltage.

II.) Number of free electrons in the space surrounding the power conductor are more due to radio activity.

Critical Disruptive Voltage :-

--The voltage at which self field intensified localised ionisation of air takes place is called as critical Disruptive Voltage.

Visual Critical Voltage:-

--The voltage at which the visual corona begins is known as visual Critical Voltage. Visual glow of corona occurs at a voltage higher than critical Disruptive Voltage.

Factor affecting Corona and corona loss:

I.) Atmospheric conditions

II.) Size of conductor

III.) Space between conductor

IV.) Line voltage

V.) Surface conditions

VI.) No. Of Conductor per phase

VII.) Clearance from ground

VIII.) Electrical factor

Advantages of Corona :

I.) Due to Corona, the air surrounding the conductor is ionized and becomes conducting. This increases the virtual diameter of the Corona. Hence the maximum gradient or maximum electrostatic potential stress is increase.

II.) The Corona will act as a safety wall to the transmission line  conductors against lightening strock, in which the peak magnitude of lightening surge will dissipate in the form of corona loss.

Disadvantages of Corona:

I.) There is certain real Power loss in the form of corona loss.

II.) Corona causes an interference with the neighbouring communication lines.

III.) The ozone gas formed due to corona chemically reacts with the conductor and causes corrosion.

IV.) The third harmonic components produced due to Corona makes the current non- sinusoidal and these are occurs non- sinusoidal Voltage. Thus increases the corona loss.

Method to Reduce Corona:

I.) By use of large size of conductor.

II.) Use Bundle conductor

III.) Use Hallow conductor


Remember:

DC Corona loss = 1/3 * ( AC Corona loss)

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