"The phenomenon of violet glow, hissing noise and production of ozone gas in an overhead transmission line is known as 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.
If the conductors are polished and smooth, the corona glow will be uniform throughout the length the conductors, otherwise the ro points will appear brighter.
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.
Radioactivity refers to the process in which an unstable nucleus will decompose spontaneously, or decay, into a more stable configuration by emitting certain particles or certain forms of electromagnetic energy. results due to nuclear instability, because the nucleus experiences the conflict between the two strongest forces in nature.
There are many n isotopes which are unstable and emit some kind of radiation. The mos common types of radiation are called alpha, beta, and gamma radiation.
Cosmic rays are immensely high energy radiation, mainly originating outside the Solar System. They may produce showers of secondary particles that penetrate and impact the Earth's atmosphere and sometimes even reach the surface.
Ultraviolet (UV) light is an electromagnetic radiation with a wavelength from 400 nm to 100 nm, shorter than that of visible light but longer than rays. It is present in sunlight.
The greater the applied voltage, the greater the potential gradient and more is the velocity of free electrons.
- When potential difference is applied between the conductors, potential gradient (or Electric Field Intensity) 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.
Electric field intensity is defined as the strength of an electric field at any point. The unit of measurement is the volt/meter. Potential gradient or electric field intensity, E =-(dV/dx)
Factors Affecting the Corona :
The following are the factors upon which corona depends :
- Atmosphere : *As corona is formed due to ionisation 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.
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. a *Thus a stranded conductor has irregular surface and hence gives rise to more corona than a solid conductor.
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.
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.