In this article we will discuss about:- 1. Introduction to Corona 2. Factors Affecting Corona 3. Causes 4. Corona Power Loss 5. Methods of Reducing Corona Effects 6. Advantages and Disadvantages.
Contents:
- Introduction to Corona
- Factors Affecting Corona
- Causes of Corona
- Corona Power Loss
- Methods of Reducing Corona Effects
- Advantages and Disadvantages of Corona
1. Introduction to Corona:
When an alternating potential difference is applied across two conductors, whose spacing is large in comparison with their diameters, then the atmospheric air surrounding the conductors is subjected to electrostatic stresses. At low voltage there is no change in the condition of atmospheric air around the conductors.
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However, when the potential difference is gradually increased, a stage arrives when a faint luminous glow of violet colour appears together with a hissing noise. This phenomenon is called “visual corona” and is accompanied by the production of ozone which is readily detected because of its characteristic odour. The glow is due to the fact that the atmospheric air around the conductor becomes conducting due to electrostatic stresses.
If the potential difference is raised still further, the glow and the noise will increase in intensity until eventually a spark-over owing to breakdown of air insulation will take place. The whole phenomenon—the hissing noise, the violet glow, and production of ozone gas is known as the corona. This phenomenon is very much evident in transmission lines of 100 kV and above.
If the conductors are perfectly uniform and smooth the glow will be uniform throughout the length of the conductor, otherwise the rough points will appear brighter. If the spacing between the conductors is not very large as compared with their diameters (interaxial difference is less than 15 times the diameter of the conductors) corona may bridge the conductors and cause flash-over before any luminous glow is observed. It happens due to the fact that the distance between the conductors being smaller, there is no time for the glow to occur.
In EHV ac transmission lines the visible glow of corona is mostly uniform about both the conductors but in case of HVDC transmission the glow is uniform and brighter about the positive conductor but spotty about the negative conductor.
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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 non-sinusoidal 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.
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(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.
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(vi) The charging current under corona condition increases because the corona induces harmonic currents.
2. 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 as by the condition of the line.
The corona is affected by the following factors:
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(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 x-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.
Triple-Harmonic Currents due to Corona:
Corona forms when the voltage of a conductor passes the disruptive critical voltage and disappears when the voltage descends through the same value. This occurs on each conductor every half-cycle, and develops a pulsation in the voltage wave having three times the generator frequency. In an earthed system this triple-frequency voltage causes a triple-frequency current to flow through the capacitance of the system to earth and back through the earthed neutral.
This effect is accentuated by the fact that the effective capacitance of the conductors pulsates at triple frequency due to the increase and decrease of effective diameter of conductors caused by the corona. Because of corona triple frequency currents flow through the ground in case of a non-earthed system, the neutral has a voltage to earth of triple frequency.
3. Causes of Corona:
i. Dielectric Strength of Air Causing Corona:
The value of potential gradient at which complete disruption of air occurs, is called the disruptive strength or dielectric strength of air. The breakdown strength of air at a barometric pressure of 760 mm, and temperature of 25° C is 30 kV/cm (maximum) or 21.1. kV/cm (rms) and is denoted by g0.
The value of dielectric strength of air (i.e. g0) depends upon the density of air—g0 is proportional to density of air over a wide range and thus directly proportional to the barometric pressure, and inversely proportional to the absolute temperature. Thus breakdown strength of air at a barometric pressure of b cm of mercury and temperature of t° C becomes δ g0 where-
ii. Disruptive Critical Voltage Causing Corona:
Let us consider the two-wire line shown in Fig. 7.1, where r is the radius of the line conductors and d is the distance between their centres, where d is very large as compared to r. If a positive charge of q coulombs per metre of conductor length is given to conductor A, then a negative charge of q coulombs per metre of conductor length will be induced on conductor B.
Consider point P at a distance of x metres from conductor A.
Electric field intensity at point P due to charge on conductor A = q/2 ε0 x acting towards B as shown.
Electric field intensity at point P due to induced charge on conductor B = q/2 ε0 (d-x) acting towards B as shown.
Resultant electric field intensity at point P,
PD between conductors A and B
Now since r is very small as compared to d, d – r ≃ d and, therefore,
Now gradient at any point x from the centre of the conductor A is given by,
Substituting for q from Eq. (7.2) in above equation, we have,
where V is voltage between two conductors.
where V is the line to neutral voltage of the system.
From the expression for the potential gradient it is clear that for a given transmission system, the potential gradient increases as x decreases i.e., the potential gradient is maximum when x = r, the surface of the conductor, and this value is given by,
where r is the radius of conductors in cm, d is the spacing in cm and V’ is the value of voltage of the conductors to neutral, both gmax and V being expressed in rms values.
When the disruptive gradient of air is reached at the conductor surface,
In practice, corrections have to be applied to the above formula for air density and surface conditions of the conductor and thus the complete formula becomes,
where δ is the air density factor [Refer to Eq. (7.1)] and m0 is the conductor surface condition factor.
The value of m0 may be taken as unity, 0.98 – 0.93 and 0.87 – 0.80 for polished, roughened (or weathered) and stranded conductors respectively.
In the case of stranded conductors the variations in m0 is due to the number of strands employed, and the varying ratio between the strand diameter and overall diameter in different sizes of conductors. It also depends on the surface conditions of the conductor, often improving after a conductor has been in service for a long time, and become weathered and the roughness and abrasions oxidised away. Any mutilation of the conductor during construction or stringing lowers m0 considerably.
The value of 
given by Eq. (7.6) is known as disruptive critical voltage, and is defined as the minimum phase to neutral voltage at which corona occurs and holds good for fair- weather conditions. Its value is considerably reduced during bad atmospheric conditions, such as fog, sleet, rain and snowstorms.
iii. Visual Critical Voltage Causing Corona:
In the case of parallel wires it is found that visual corona does not begin at the voltage 
at which the disruptive gradient of air g0 is reached, but at a higher voltage 
called the visual critical voltage. Visual critical voltage is defined as the minimum phase to neutral voltage at which glow appears all along the line conductors.
Thus when corona begins, the potential gradient gv at the conductor surface is higher than the disruptive gradient g0. Contrary to what might be expected, gv, the apparent strength of air is not constant but depends on the size of the conductors, air being apparently stronger at the surface of small conductors than large ones.
Peek states that the disruptive critical voltage must be so exceeded that the stress is greater than the breakdown value up to a distance of 0.3√δr cm from the conductor. Thus visual corona will occur when the breakdown value is attained at the distance r + 0.3√δr from the axis, instead of at the distance r. This requires that the voltage to neutral be 1 + (0.3)/√δr times the disruptive critical voltage. Thus the visual critical voltage is,
where mv is a roughness factor, which is unity for smooth conductors. When the wire is stranded or rough, mv is less than unity. mv may be taken as 0.98 – 0.93 for rough conductor exposed to atmospheric severities and 0.72 for local corona on stranded conductors.
Because of irregular surface of the conductor, the corona does not start simultaneously on the whole surface but it takes place at different points of the conductor which are pointed and this is known as local corona. For this mv = 0.72. For decided or general corona along the length of the conductor mv may be taken as 0.82.
4. Corona Power Loss:
The formation of corona is associated with a loss of power, which will have some effect on the efficiency of the transmission line, but will not be of much importance to have any appreciable effect on the voltage regulation.
As in the case of the critical voltage for corona, the power loss is affected both by atmospheric and line conditions.
Under the fair weather conditions, the following empirical formula for the corona power loss has been ascertained due to Peek:
where Vph is voltage to neutral in kV, 
is the disruptive critical voltage to neutral in kV and f is the supply frequency in Hz.
Under the stormy weather conditions, 
is taken to be 0.8 times its fair weather value and power loss due to corona is given by,
As a matter of fact, with perfectly smooth and cylindrical conductors no loss occurs until the visual critical voltage is reached, when the loss suddenly takes a definite value as calculated by the Eq. (7.8) or Eq. (7.9). It then follows the quadratic law for higher voltages. The empirical relation as derived by Peek has certain limitations and holds good only under certain conditions (i.e. the supply frequency lies between 25 and 120 Hz, the conductor radius is greater than 2.5 mm and ratio Vph/Vd0 exceeds 1.8). Also a small error in m0, the irregularity factor, will lead to wrong results when using the above formula.
When the ratio Vph/Vd0 is less than 1.8 Peterson’s formula is to be applied for determining the corona losses and is given as:
where K is a factor which varies with the ratio of Vph/Vd0 as given below:
5. Methods of Reducing Corona Effects:
The critical voltage can be raised either by increasing the spacing between the conductors or the diameter of conductors. By increasing the spacing between the conductors, the voltage at which corona occurs is raised and hence corona effects can be avoided.
However, the spacing cannot be increased to a large extent because it will result in heavier supporting structures (e.g., the bigger cross-arms and supports) and therefore high cost. The diameter of the conductors can be increased by using hollow conductors with a hemp core. Steel-cored aluminium conductors have a large diameter for a given conductivity and weight, and are thus good from the point of view of corona.
By using bundled conductors also corona effects can be avoided because effective diameter of the bundled conductor is much larger than that of the equivalent single conductor.
Voltage Limitations of Lines:
The basis for the design of a transmission line is essentially financial, the most economical line being the most acceptable. It is because power loss due to corona is of no great importance. It is considered satisfactory to design a line for operation at a voltage just below the disruptive critical voltage for fair weather (taking δ = 1). It is economical to have a small corona loss in bad weather, i.e., for a fraction of the year (storms are experienced at intervals) rather than have large conductors and heavy supporting structures to avoid corona entirely.
6. Advantages and Disadvantages of Corona:
In the past, the corona was considered as something to be avoided because of the energy loss associated with it and the distortion of the waveform.
But now the corona is considered beneficial because of the following advantages:
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.
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 neighbouring communication circuits due to electromagnetic and electrostatic induction effects.
3. Owing to the formation of corona, ozone gas is produced which chemically reacts with the conductor and causes corrosion.