Corona Effect:
The phenomenon of violet glow, hissing noise and production of ozone gas in an overhead transmission line.
Reason :
Due to cosmic rays, ultraviolet radiations some ionization is always present in the air. Under normal condition air around the transmission cable contains ionized particles i.e. free electrons, positive ions and also neutral molecules. When the line becomes live means when potential difference applied between the lines, potential gradient is set up in the air and have maximum value at the conductor surfaces. Because of this potential gradient the free electrons acquire greater velocity. With the increase of the line voltage the potential gradient increases and the velocity of the free electrons increase also. When the potential gradient of the conductor surface crosses the maximum limit then the free electrons get enough kinetic energy to strike a neutral molecule and make some more electrons free from that molecule. One electron free then creates one positive ion and some free electrons. Usually 30KV per cm treated as the limit of potential gradient. Those free electrons strike more neutral molecules and keep this process going all like nuclear fission. So this process of ionization is cumulative. The result of this ionization is that either corona is formed or spark takes place between the conductors.
Factors affecting corona are
a) Atmosphere: During stormy or rainy weather corona occurs at much less voltage as compared with fair weather. Because during those cases ion density around the conductors is much more than that of fair weather.
b) Conductor Size: The unevenness of the surface of the conductors decreases the breakdown voltage. Because of this reason with low voltage the insulation will breakdown and create sparks and corona. That’s why solid conductors are used mostly instead of stranded conductors to reduce the corona.
c) Spacing between conductors: Larger the gap between the conductors lesser the corona.
d) Line voltage: Every line voltage has a limit. After that limit breakdown will occur and will create corona and spark. So with the application of small line voltage possibility of occurring corona is lesser.
Advantages:
a) The area around the conductor becomes conductive because of corona formation. It creates a bigger virtual diameter of the conductor. As the diameter increases the electrostatic stress between the conductor decreases.
b) Corona effect reduces the Surge voltage created because of switching and thundering. As greater applied voltage creates corona and sparks, that’s why when surge voltage will become available the corona will form and will absorb the extra power by creating violet glow, noise and sparks.
Disadvantage:
a) It reduces the transmission efficiency.
b) Ozone created by this effect cause corrosion of the conductors.
c) Due to corona effect non-sinusoidal voltage drop occurs across the line. This may cause inductive interference with neighboring communication lines.
Corona effects can be reduced by following methods:
a) Increasing conductor size: If we increase the conductor size the value of potential gradient will increase. To create corona effect large line voltage will be required.
b) Increasing conductor spacing: Corona effect can be eliminated by increasing the spacing between the conductors. Due to increase of the spacing large line voltage will be required to create corona effect.