Corona is accompanied by a loss of energy which increases very rapidly once the visual critical voltage is exceeded. Power loss, due to corona, heavily depends upon weather condition- during humid and moist climate corona loss is much increased,
The energy loss accompanied by the phenomenon, called the corona, is dissipated in the form of light, heat, sound and chemical action. In case of ac system, the current due to corona is non-sinusoidal. In practice, this non-sinusoidal current and the nonsinusoidal voltage drop caused by it may be more important than the power loss.
The effects of corona are summarized as below:
(i) A violet glow is observed around the conductor.
(ii) It produces a hissing noise.
(iii) It produces ozone which can be readily detected by its characteristic odour.
(iv) The glow is maximum over rough and dirty surfaces of the conductor.
(v) It is accompanied by a power loss—the wattmeter connected in the electric circuit will show a reading.
(vi) The charging current under corona condition increases because the corona induces harmonic currents.
FACTORS AFFECTING CORONA
Since corona occurs due to the ionization of the air surrounding the line conductors, it is affected by the physical state of the atmosphere as well by the condition of the line. The corona is affected by the following factors.
(i) Atmosphere:
Corona is caused by the bombardment of molecules, with subsequent dislodging of electrons, by the ionised particles. Corona will thus be affected by the physical state of the atmosphere. The voltage gradient for the breakdown of the air is proportional to its density. In the stormy weather the number of ions may be more than normal, and as such the corona may occur at much less voltage compared with fair weather.
(ii) Conductor:
The corona is considerably affected by the size (diameter), shape (stranded or smooth) and surface condition (dirty or clean) of the conductor. The corona decreases with the increase in diameter of conductor. A stranded conductor gives rise to more corona than a solid conductor, For stranded conductors, the shape of the cross section is a series of arcs of circles each of much smaller diameter than the conductor as a whole. The potential gradient for such a conductor will, therefore, be greater than for the equivalent smooth conductor so the breakdown voltage for such a conductor will be somewhat less than for a smooth conductor, the ratio being between 0.88 and unity. The effect of dirt on the surface is to increase the irregularity and thereby decreasing the breakdown voltage further, an average value for the ratio when both stranding and dirt are taken into account lies between 0.85 and unity.
(iii) Spacing Between Conductors:
With the increase in spacing between the conductors the electrostatic stresses are reduced and therefore, the corona effect is reduced. If the Spacing between the conductors is made very large as compared with their diameters, there may not be any corona effect.
(iv) Line Voltage:
Line voltage largely affects the corona. At low voltage, there is no corona effect, but when the line voltage is increased to such a value that electrostatic stresses developed at the conductor surface make the atmospheric air surrounding the conductor conducting, corona effect appears.
Advantage and Disadvantage of Corona
The advantages of corona are given below.
1. On the formation of corona the sheath of air surrounding the conductor becomes conductive and there is a virtual increase in conductor diameter and due to this virtual increase in conductor diameter the maximum potential gradient or maximum electrostatic stress is reduced. Thus probability of flash-over is reduced and system performance is improved.
2. Effects of transients produced by lightning and other causes are reduced since charges induced on the line by lightning or other causes will be partially dissipated as a corona loss. In this way it acts as a
safety valve and sometimes lines have been purposely designed to have an operating voltage near to the
critical voltage in order to do away with the necessity for, and expense of lightning arrestor gear. An
objection to this scheme is that the critical voltage is not fixed for a given line, but may vary considerably with changes in the weather.
The disadvantages of corona are given below.
1. There is a definite loss of power, although this is not important except under abnormal weather conditions,
2. There is a non-sinusoidal voltage drop due to non-sinusoidal corona current and these may cause some interference with neighboring communication circuits due to electromagnetic and electrostatic induction effects.
3. Owing to the formation of the corona, ozone gas is produced which chemically reacts with the conductor and causes corrosion.