The phenomenon of Corona :
If we increase the voltage, there is a corresponding increase in the electric field intensity. If air is subjected to a uniform electric field intensity of peak value less than 30 kV/cm, the flowing current between conductors is negligible.
But when electric field intensity increases and reaches this critical value of 30 kV/cm air ionizes and becomes conducting. This breakdown is accompanied by the following phenomenon.
(1) Hissing Noise is produced.
(2) A faint pale yellow glow appears which is visible in the dark.
(3) Production of Ozone gas.
(4) Radio Interference occurs,
(5) There is a loss of power.
If all these phenomena occur then we say corona produced in a transmission line.
Theory of Corona Formation :
The phenomenon of the corona is the Ionization of air surrounding the power conductor. Due to cosmic rays, free electrons are normally present in free space.
When the potential difference between the conductors has increased gradient around the surface of the conductor increases. If the spacing between conductors to be large as compared to the diameter of the conductor, free electrons move with a certain velocity depending on strength of the field. This means the greater the applied voltage, the greater the potential gradient, more is the velocity of free electrons.
Due to the large velocity of electrons, electrons have collisions with the molecules of air and dislodge the electrons from these molecules. This produces another ion and one or more free electrons. This process being commutative, will lead to an avalanche of electrons. Eventually, there will be ionization of air surrounding the conductor.
If the spacing of the conductor is less than 15 times the diameter of the conductor, flashover will take place between the conductor, and either corona is formed or spark takes place between the conductors.
Factors Affecting Corona :
(1) Atmosphere :
Corona formation is due to the ionization of air surrounding the conductors, So corona will be affected by the physical state of the atmosphere. The voltage gradient for a breakdown of air is proportional to its density.
If the weather is stormy, then the number of ions may be more than normal, and corona may occur at much less voltage compared to fair weather.
(2) Conductor Size :
Corona is affected by the shape and conditions of conductors and also by the size of the conductor. Corona decrease with increases in diameter of the conductor.
A stranded conductor has more corona than a solid conductor because unevenness of surface decreases the value of breakdown voltage so in the stranded conductor which has an irregular surface corona effect is more pronounced.
The effect of dirt on the surface is to increase the irregularity and by this breakdown voltage decreases so corona increases.
(3) Line Voltage :
Corona is directly affected by line voltage. At low voltage, there is no corona effect, but when line voltage increases then electro-static stresses developed at the conductor surface make the air surrounding the conductor conducting, which leads to corona formation.
(4) Spacing between conductors :
Spacing between conductors affects corona formation. If the spacing between conductors is increased, then electrostatic stresses are reduced and the corona effect is reduced.
If the spacing between conductors is made very large as compared with their diameters, then there may not be any corona effect due to the reduction of electrostatic stresses.
Methods of Reducing corona :
It has been seen that intense corona effects are observed at a working voltage of 33 kV or above careful design should be made to avoid corona on the sub-station or bus bar rated for 33 kV and higher voltage otherwise highly ionized air may cause flash-over in the insulator or between the phases, causing considerable damage to the equipment the corona effect can be reduced by the following method:
(a) By increasing conductor size:
By increasing conductor size, the voltage at which corona occurs is raised and hence corona effects are considerably reduced. This is one of the reasons that ACSR conductors which have a larger cross-sectional area are used in the transmission line.
(b) By increasing conductor space:
By increasing the spacing between conductors the voltage at which corona occurs is raised and hence corona effect can be eliminated.
Advantages of Corona :
A few years ago, the corona was considered as something to be avoided because of energy losses in it and distortion of the waveform. But nowadays, the corona is considered advantageous because of the following benefits:
(1) Owing to corona formation, the air surrounding the conductor becomes conductive and there is a virtual increase in conductor diameter and due to this, electrostatic stress is reduced between the conductors so the probability of flashover is reduced and system performance is improved.
(2) Effects of transients produced by lightning surges and other causes are reduced. Charge induced on the line by surges will be partially dissipated as a corona loss. In this way, it acts as a safety valve.
Disadvantages of corona are given below:
(1) There is a loss of power more pronounced at abnormal weather conditions which affect the transmission efficiency of the line.
(2) Due to corona formation, Ozone gas is produced which chemically reacts with conductor and causes corrosion
(3) Due to non-sinusoidal corona current there is a non-sinusoidal voltage drop which causes some interference with neighboring communication circuits due to electrostatic and electromagnetic induction.
4) Owing to distortions of the waveform, harmonics are introduced into the transmission line. These harmonics increase corona loss.
What is Auto-Recloser In Transmission System ? How its Works ?
What is Aviation Light ? Why it is used ?