Corona effect on transmission line.

Corona
When an alternating potential difference is applied across two conductors whose spacing is large as
compared to their diameters, there is no apparent change in the condition of atmospheric air surrounding the wires if the applied voltage is low. However, when the applied voltage exceeds a certain value, called critical disruptive voltage, the conductors are surrounded by a faint violet glow called corona.
The phenomenon of corona is accompanied by a hissing sound, production of ozone, power loss
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 the radio noise. If the applied voltage is
increased to breakdown value, a flash-over will occur between the conductors due to the breakdown
of air insulation.
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.
Some ionisation is always present in air due to cosmic rays, ultraviolet
radiations and radioactivity. Therefore, under normal conditions, the air around the conductors
contains some ionised particles (i.e., free electrons and +ve ions) and neutral molecules. When p.d.
is applied between the conductors, potential gradient is set up in the air which will have maximum
value 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 electrons. When the potential gradient at the conductor surface reaches about 30 kV per cm (max. value), the velocity acquired by the free electrons is sufficient to strike a neutral molecule with enough force to dislodge one or more electrons from it. This produces another ion and one or more free electrons, which is turn are accelerated until they collide with other neutral molecules, thus producing other ions. Thus, the process of ionisation is cummulative. The result of this ionisation is that either corona is formed or spark takes place between the conductors.

Factors Affecting Corona
The phenomenon of corona is affected by the physical state of the atmosphere as well as by the
conditions of the line. The following are the factors upon which corona depends :
(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.

Reference:
Principle of Power System - V. K. Mehta