Basic Principle or phenomenon of Corona

Corona

Introduction

It can be noticed that near the overhead lines there exists a hissing noise and sometimes a faint violet glow. The effect due to which such phenomenon exists surrounding the overhead lines, is called corona effect.

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

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

The corona effect takes place when applied voltage exceeds a particular value which is called critical disruptive voltage. As this voltage increases, the glow and hissing noise also increases. If the voltage increases upto breakdown value then the flash over occurs between the conductors, due to breakdown of air between the conductors. Similarly if the ratio of spacing between the conductors to the radius of the conductor is less than 15 then the flash over occurs before corona. In practice this ratio is very high and there is no possibility of a flash over. When a voltage more than the critical disruptive voltage is applied, the faint violet glow is even between the two parallel polished conductors and can be seen all along the length of the conductors. At the rough points, it appears little bit brighter. In case of d.c. voltage, reddish beads are formed near negative conductor while smoother bluish white uniform glow is formed near positive conductor. The hissing noise can be easily heard while the formation of ozone gas can be detected from its odour.

1. Basic Principle 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, ionised particles alongwith the neutral molecules. When the potential is applied to the conductors, a potential gradient is developed in the air. This potential gradient is maximum around the surface of the conductors. Due to the potential gradient, the free electrons will start moving with certain velocity which depends on the field strength. The greater is the applied voltage, higher is the potential gradient and the velocity acquired by the free electrons. These electrons moving with high speed collide with the neutral molecules in the air. If the potential gradient is about 30 kV/cm (maximum) then striking free electron dislodge the electrons from the neutral molecules. This increases the number of free electrons which also acquire high velocities and start colliding with other neutral molecules. The process of ionization is cumulative which finally results into an electron avalanche. This ionization of air surrounding the conductors gives rise to corona effect.

Review Question

1. Explain the phenomenon of corona.

AU : Dec -03, 12, 16, May-15, Marks 4