April 23, 2021

CONCEPT OF CORONA

The phenomenon of ionisation of surrounding air, hissing noise, and glow of light is told of corona effect.

The corona is halo like purplish glow which appears around conductors when voltage

Passes a critical point. The critical point is usually slightly lower than the voltage which can

Cause flashover. It is more of an electrical leakage than a complete flashover, hence it is called

1.Corona discharge.

Corona is caused by ionization of the air surrounding the conductor. Air is composed of

Molecules which are made up, almost entirely, of atoms of oxygen and nitrogen. When an electric field is set up in the air, such as between two current carrying conductors, the air is agitated or stressed.

If the field strength increases, the agitation will increase until the binding force which holds the oxygen and Nitrogen atoms together break down into charged particles of oxygen and nitrogen which can conduct Electricity. The result is corona.

The corona is affected by

(i) System Voltage

(ii) Surrounding temperature

(iii) Altitude

The probability of corona increases as altitude increases. At sea level, under electrical Stress, some oxygen and nitrogen atoms will become ions, but the density of in-ionized atoms acts as insulators minimizes the corona and hence the flashover. But at higher altitude the availability of these unionized atoms, the insulators, are in minority so the voltage causing corona reduces a large and hence the corona is more.

At very high altitude the availability of oxygen and nitrogen is so poor that very few ion remains in the electric field of conductor, therefore the chances of Corona is again minimised. The density of gaseous medium is inverse to the temperature. As the surrounding air Reaches to a higher temp. The density reduces, considerably the voltage causing corona also Reduces and propagate the corona.

The corona is governed by the formula,

Where

Va – Voltage causing corona

Ea – Voltage gradient (Volts/cm)

ts – Thickness of solid (hard) insulation in cm.

ta – Thickness of air fibre in cm.

Ꜫa – Dielectric constant of air

Ꜫs – Dielectric constant of hard insulation

2. The effects of corona:

The corona causes radio interference i.e. noise and power loss. More important is that

Corona creates ozone & nitrogen anhydrides, one of the strongest oxidising agents known. This Corrosive atmosphere is highly detrimental to the insulators and conducting metals etc.

3. Internal & External Corona:-

For insulator conductor there are two types of corona

(a) Internal Corona &

(b) External Corona

3.1 Internal Corona:

(i) When there is a void in the thickness of the insulation, over the conductor, used to carry the Current at some considerably higher potential. Due to this void insulation divides in three layers in which Middle layer is of gaseous medium, certainly air. The two surfaces of void, perpendicular to the conductors, act as plates of capacitor as change of dielectric medium. This way it forms three capacitors in series.

For any dielectric medium permittivity Ꜫr is always more than 1 there for the capacitance of the capacitor formed by air medium is smaller than the remaining. Hence it bears more voltage than other capacitors. Whenever under normal system voltages or during transients, the voltage reaches to the corona potential, corona occurs which enhances the void. This may lead to an insulation failure.

(ii) In large machines, generally the strip windings with moulded insulation are used.

These conductors are carrying large currents and operating for days together. Due to the

Continuous elevated temperature, thermal cycling and inherent properties of thermo set materials; the insulation loses its elasticity and leaves the surface of insulation which gets filled by the Atmospheric air, containing oxygen and nitrogen. Here also, two capacitors are formed. Whenever the voltage reaches to the corona potential, damages the insulation and lead towards failure

3.2. External Corona or Surface Corona:

When any of the winding leaves the slot in any machine, at the nearest point of core, the

Electrical stresses over the insulation changes.

Fig.

In the slot portion the slot insulation and conductor insulation bear the complete stress,

but as the conductor leaves the slot, in overhang portion, the electrical stress is shared by conductor Insulation and perpendicular thickness, between conductor and the earthed core, of air. Again, here, as the voltage across the air film increases the corona potential, corona occurs.

This is Called surface corona. Surface corona damages the insulation and converts into flash over which may damage a greater part of the winding. To avoid this insulation is enhanced by extending slot insulation to more than 5 mm in 415 V machines and by providing extra insulation at the slot leaving portion.

4.Factor affecting corona

Corona is ionization of the air surrounding the line conductor; it is affected by the physical state of the atmosphere as well as by the condition of the line. The corona is affected by following factors.

4.1.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.

4.2.Conductor

The corona is considerably affected by the size, shape and surface condition 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 conductor, the shape of the ꭓ-section is a series of arcs of a circle each of much smaller diameter than the conductor as a whole. 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.

4.3.Spacing between conductors

With the increase in spacing between the conductors the electro-static stresses are reduce and therefore, the corona effect is reduced. If the spacing between the conductors is made very large as compared with their diameter, there may not be any corona effect.

4.4.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 electro-statics stresses developed at the conductor surface make the atmospheric air surrounding the conductor conducting, corona effect appears.

5.Method of reduce corona effects

The critical voltage can be raised either by increasing the spacing between the conductors or the diameter of conductors. By increase the spacing between the conductor, the voltage at which corona occurs is raised and hence corona effect can be avoided.  However, the spacing cannot be increased to a large extent because it will result in heavier supporting structure and therefore high cost.

The diameter of the conductors can be increased by using hollow conductors with a hemp core. Steel- cored aluminium conductor have a large diameter for a given conductivity and weight, and are thus good from the point of view of corona.

6.Advantage and disadvantage of corona

6.1.Advantage of corona

  1. On the formation of corona the sheath of air surrounding the conductor become conductive and there is a virtual increase in conductor diameter and due to this virtual increase in conductor diameter the maximum potential or maximum electrostatic stress is reduced. Thus probability of flash – over is reduced and system performance is improved.
  2. Effect 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 act as a safety valve  and sometimes lines have been purposely designed to have an operating voltage near to the critical voltage in order to this scheme is that the critical voltage is not fixed for a given line, but may vary considerably with changes in the weather.

6.2.Disadvantage of corona

  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 no-sinusoidal corona current and these may cause some interference with neighbouring communication circuit due to electromagnetic and electrostatic induction effect.
  3. Owing to the formation of corona gas is produced which chemically reacts with the conductor and causes corrosion.

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