Corona effect in Electrical energy

INTRODUCTION :
This type of effect can be noticed that near the overhead lines there exists a hissing noise and sometimes a faint violet glow.
CORONA EFFECT:

When normal alternating voltages is applied across two conductors which conductors are enough spacing in between,then there is no change in the atmospheric conditions surrounding conductors.but if the applied voltage exceeds a particular limits value then the air surrounding the conductors gets ionized due to which hissing noise or faint violet glow appears.

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

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

Theory of corona formation in transmission lines :

The corona effect takes place when applied voltage exceeds the particular value which is called critical disruptive voltage.
As this voltage increases,the glow and hissing noise also increases.If voltage increases up to limits and exceeds then flash over occurs between the conductors.So,we need to change this arrangements ratio of the spacing between conductors to the radius of the conductors is less than 15 then the flash over occurs.

         

In practically this ratio is very high there is a possibility of  a flash over occurs.

Principles of corona :

In the free space surrounding the conductors some ionization is always present due to radioactivity,cosmic rays and ultraviolet rays.Thus the air near the  conductors consists of some free electrons ionized particles along with neutral molecules.when the potential is applied to the conductors,a potential gradient is developed in the air.This potential is maximum around the surface of the conductor.Due to potential gradient the free electrons will start moving with certain velocity which depends on the field of strength.The greater is applied voltage,higher is the potential gradient and the velocity acquired by the free electrons.Ionization of air surrounding the conductors gives rise to corona effect.

Factors Affecting Corona :

(i)Atmosphere : 

As corona is formed due to ionsiation of air surrounding the conductors, therefore, it is affected by the physical state of atmosphere. In the stormy weather, the number of ions is more than normal and as such corona occurs at much less voltage as compared with fair weather.

(ii) Conductor size:

The corona effect depends upon the shape and conditions of the conductors.The rough and irregular surface will give rise to more corona because unevenness of the surface decreases the value of breakdown voltage. Thus a stranded conductor has irregular surface and hence gives rise to more corona that a solid conductor.

(iii) Spacing between conductors:

If the spacing between the conductors is made very large as compared to their diameters, there may not be any corona effect. It is because larger distance between conductors reduces the electro-static stresses at the conductor surface, thus avoiding corona formation.

(iv) Line voltage:

The line voltage greatly affects corona. If it is low, there is no change in the condition of air surrounding the conductors and hence no corona is formed. However, if the line voltage has such a value that electrostatic stresses developed at the conductor surface make the air around the conductor conducting, then corona is formed.

Advantages and Disadvantages of Corona :

Advantages :

(i) Due to corona formation, the air surrounding the conductor becomes conducting and hence virtual diameter of the conductor is increased. The increased diameter reduces the electrostatic stresses between the conductors.

(ii) Corona reduces the effects of transients produced by surges.

Disadvantages

(i) Corona is accompanied by a loss of energy. This affects the transmission efficiency of the line.

(ii) Ozone is produced by corona and may cause corrosion of the conductor due to chemical action.

(iii) The current drawn by the line due to corona is non-sinusoidal and hence non-sinusoidal voltage drop occurs in the line. This may cause inductive interference with neighbouring communication lines.