Corona Effect

Explaining the phenomena of corona effect , its formation , factors affecting it , its advantages and disadvantages

1. Corona Effect

Definition :
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.
The phenomenon of violet glow, hissing noise and production of ozone gas in an overhead
transmission line is known as corona.
If the conductors are polished and smooth, the corona glow will be uniform throughout the
length of the conductors, otherwise the rough points will appear brighter. With d.c. voltage, there is
difference in the appearance of the two wires. The positive wire has uniform glow about it, while the
negative conductor has spotty glow.


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. When p.d. is applied between the conductors,
potential gradient 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. The greater the applied voltage, the greater the potential gradient and
more is the velocity of free electrons.
When the potential gradient at the conductor surface reaches about 30 kV per cm (max. value),
the velocity acquired by the free electrons is sufficient to strike a neutral molecule with enough force
to dislodge one or more electrons from it. This produces another ion and one or more free electrons,
which is turn are accelerated until they collide with other neutral molecules, thus producing other
ions. Thus, the process of ionisation is cummulative. The result of this ionisation is that either corona
is formed or spark takes place between the conductors.

2. Factors Affecting Corona :
The phenomenon of corona is affected by the physical state of the atmosphere as well as by the
conditions of the line. The following are the factors upon which corona depends :
(i) Atmosphere. As corona is formed due to ionsiation 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.
(ii) 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. Thus a stranded conductor has irregular
surface and hence gives rise to more corona that a solid conductor.
(iii) 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.
(iv) 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.



Advantages and Disadvantages of Corona :
Corona has many advantages and disadvantages. In the correct design of a high voltage overhead
line, a balance should be struck between the advantages and disadvantages.
Advantages
(i) Due to corona formation, the air surrounding the conductor becomes conducting and hence
virtual diameter of the conductor is increased. The increased diameter reduces the electrostatic
stresses between the conductors.
(ii) Corona reduces the effects of transients produced by surges.
(iii)The sound generation effects of corona can be utilized to build high accuracy audio
speakers. The major advantage is that there is zero mass that needs to be moved to create
the sound, so that transient response is improved.
Disadvantages
(i) Corona is accompanied by a loss of energy in different forms as a glowing light, audible sound and
conductor vibration.This affects the transmission efficiency of the
line.
(ii) Ozone is produced by corona and may cause corrosion of the conductor due to chemical
action.
(iii) The current drawn by the line due to corona is non-sinusoidal and hence non-sinusoidal
voltage drop occurs in the line. This may cause inductive interference with neighbouring
communication lines.
(iv) The radio and TV interference occurs on the line because of corona effect.

3. Methods of Reducing Corona Effect :
It has been seen that intense corona effects are observed at a working voltage of 33 kV or above.
Therefore, careful design should be made to avoid corona on the sub-stations or bus-bars rated for 33
kV and higher voltages otherwise highly ionised air may cause flash-over in the insulators or between
the phases, causing considerable damage to the equipment. The corona effects can be reduced by the
following methods :
(i) 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 transmission
lines.
(ii) By increasing conductor spacing. By increasing the spacing between conductors, the voltage
at which corona occurs is raised and hence corona effects can be eliminated. However,
spacing cannot be increased too much otherwise the cost of supporting structure (e.g., bigger
cross arms and supports) may increase to a considerable extent.
(iii) By using bundled conductors. Using a bundled conductor increases the effective diameter of the
conductor. This results in reduction of the corona discharge.
(iv) By using corona rings. The electric field is greater where the conductor curvature is sharp. Therefore,
corona discharge occurs first at the sharp points, edges and corners. To mitigate this, corona rings are
employed at the terminals of very high voltage equipments such as at the bushings of a very high voltage
transformer (Corona discharge also occurs in high voltage equipment). A corona ring is electrically
connected to the high voltage conductor, encircling the points where corona discharge may occur. This
significantly reduces the potential gradient at the surface of the conductor, as the ring distributes the
charge across a wider area due to its smooth round shape.

4. Corona discharges come in two forms: positive and negative. This is determined by the polarity of the electrode used
to produce the corona. Positive and negative coronas are similar but there are some key differentiations. Positive
corona discharges have a much lower free electron density, than negative coronas. However, the electrons in a
positive corona discharge are concentrated and contain more energy. Negative corona discharges appear larger
than positive ones. This is due to the free electrons being more spread out and more abundant.


Positive coronas
Properties
A positive corona is manifested as a uniform plasma across the length of a conductor. It can often be seen glowing
blue/white, though many of the emissions are in the ultraviolet. The uniformity of the plasma is caused by the
homogeneous source of secondary avalanche electrons described in the mechanism section, below. With the same
geometry and voltages, it appears a little smaller than the corresponding negative corona, owing to the lack of a non-
ionising plasma region between the inner and outer regions.
A positive corona has much lower density of free electrons compared to a negative corona; perhaps a thousandth of
the electron density, and a hundredth of the total number of electrons. However, the electrons in a positive corona
are concentrated close to the surface of the curved conductor, in a region of high potential gradient (and therefore the
electrons have a high energy), whereas in a negative corona many of the electrons are in the outer, lower-field areas.
Therefore, if electrons are to be used in an application which requires a high activation energy, positive coronas may
support a greater reaction constants than corresponding negative coronas; though the total number of electrons may
be lower, the number of very high energy electrons may be higher.
Coronas are efficient producers of ozone in air. A positive corona generates much less ozone than the corresponding
negative corona, as the reactions which produce ozone are relatively low-energy. Therefore, the greater number of
electrons of a negative corona leads to an increased production.
Beyond the plasma, in the unipolar region, the flow is of low-energy positive ions toward the flat electrode.

Mechanism
As with a negative corona, a positive corona is initiated by an exogenous ionisation event in a region of high
potential gradient. The electrons resulting from the ionisation are attracted toward the curved electrode, and the
positive ions repelled from it. By undergoing inelastic collisions closer and closer to the curved electrode, further
molecules are ionized in an electron avalanche.
In a positive corona, secondary electrons, for further avalanches, are generated predominantly in the fluid itself, in
the region outside the plasma or avalanche region. They are created by ionization caused by the photons emitted
from that plasma in the various de-excitation processes occurring within the plasma after electron collisions, the
thermal energy liberated in those collisions creating photons which are radiated into the gas. The electrons resulting
from the ionisation of a neutral gas molecule are then electrically attracted back toward the curved electrode,
attracted into the plasma, and so begins the process of creating further avalanches inside the plasma.


Negative coronas
Properties
A negative corona is manifested in a non-uniform corona, varying according to the surface features and irregularities
of the curved conductor. It often appears as tufts of corona at sharp edges, the number of tufts altering with the
strength of the field. The form of negative coronas is a result of its source of secondary avalanche electrons (see
below). It appears a little larger than the corresponding positive corona, as electrons are allowed to drift out of the
ionising region, and so the plasma continues some distance beyond it. The total number of electrons, and electron
density is much greater than in the corresponding positive corona. However, they are of a predominantly lower
energy, owing to being in a region of lower potential-gradient. Therefore, whilst for many reactions the increased
electron density will increase the reaction rate, the lower energy of the electrons will mean that reactions which
require a higher electron energy may take place at a lower rate.

5. Mechanism
Negative coronas are more complex than positive coronas in construction. As with positive coronas, the establishing
of a corona begins with an exogenous ionization event generating a primary electron, followed by an electron
avalanche.
Electrons ionized from the neutral gas are not useful in sustaining the negative corona process by generating
secondary electrons for further avalanches, as the general movement of electrons in a negative corona is outward
from the curved electrode. For negative corona, instead, the dominant process generating secondary electrons is
the photoelectric effect, from the surface of the electrode itself. The work function of the electrons (the energy
required to liberate the electrons from the surface) is considerably lower than the ionization energy of air at standard
temperatures and pressures, making it a more liberal source of secondary electrons under these conditions. Again,
the source of energy for the electron-liberation is a high-energy photon from an atom within the plasma body
relaxing after excitation from an earlier collision. The use of ionized neutral gas as a source of ionization is further
diminished in a negative corona by the high-concentration of positive ions clustering around the curved electrode.
Under other conditions, the collision of the positive species with the curved electrode can also cause electron
liberation.
The difference, then, between positive and negative coronas, in the matter of the generation of secondary electron
avalanches, is that in a positive corona they are generated by the gas surrounding the plasma region, the new
secondary electrons travelling inward, whereas in a negative corona they are generated by the curved electrode
itself, the new secondary electrons travelling outward.
A further feature of the structure of negative coronas is that as the electrons drift outwards, they encounter neutral
molecules and, with electronegative molecules (such as oxygenand water vapor), combine to produce negative ions.
These negative ions are then attracted to the positive uncurved electrode, completing the 'circuit'.

6. The mechanical design of overhead lines is determined by :
– Minimum distances of the conductors to earth, to other conductors and to the
tower
– Minimum clearances and minimum length of insulators, depending on the
pollution class
– Sag, taking account of conductor and ambient conditions
– Tensile stress of the conductor
– Wind forces on the conductor, insulators and towers.

Minimum distances for nominal system voltage U n ≥ 110 kV as indicated in
Table 9.4 are defined in IEC 60071 - 1 (VDE 0111 - 1). The length of the insulators
is determined by the minimum length of insulators and the specific creepage dis-
tance for different pollution classes according to IEC 60071 - 2 (VDE 0111 - 2).
The contamination classes are defined as follows below, whereas the specific
creepage distance refers to the highest voltage for equipment Um .
– Class 1 Lightly polluted areas without industry and with spread settlement
(houses with exhaust from heating devices to be considered); areas with small
industrial density or small populated areas, which are exposed to frequent wind
and rain; areas far from sea shores or on large heights above sea level. The
specifi c creepage distance can be kept at 1.6 cm kV− 1 .
– Class 2 Medium polluted area, industrial areas without any particular emissions,
areas with medium population density with exhaust from heating devices to be
considered; areas with high population density and/or industrial areas, which
are exposed to frequent wind and rain; areas which are more than 1 km distant
from the sea shore. The specific creepage distance shall be 2.0 cm kV− 1 .
– Class 3 Heavily polluted areas with high industrial density and suburbs of
larger cities with considerable exhaust gases from heating; areas near sea shores which are exposed to relatively
strong wind from the sea. The specific creepage distance shall be 2.5 cm kV− 1 .
– Class 4 Very heavily polluted areas of limited extent, including areas which are
exposed to thick, conductive deposits; areas of limited extent which are very
close to the sea shore with occurrence of strong salty wind. The specific creepage
distance shall be 3.1 cm kV− 1 .

7. The classification of air pollution with respect to exhaust from heating devices
should be done with care, since the increased use of gas for heating reduces the
relevant emissions, with a corresponding effect on the creepage distance. Classi-
fication standards are lacking in some countries.
The minimal conductor – earth distance is determined by the maximal conductor
sag and the maximal permissible continuous tensile stress of the conductor.
According to EN 50341 - 1 and EN 50423 - 1 (VDE 0201), the following shall
apply:
– The continuous tensile stress of the conductor must not be exceeded under any
of the following condition: − 5 ° C with triple the normal load or double the
increased additional load; − 5 ° C with normal additional load and wind load; or
− 5 ° C with increased additional load and wind load.
– The horizontal component of the line tensile strength shall not exceed the values
given in VDE 0201 Table 3 at the mean yearly average temperature (in Germany:
+10 ° C) without wind load.
– In order to calculate the maximal conductor sag, either +40 ° C without additional
load or − 5 ° C with normal or increased additional load are to be applied. The
smallest sag always arises from − 20 ° C without additional load. In countries
facing high current loading of the conductor during summer, higher temperatures
have to be taken into account.
– Normal additional load in newtons per meter of conductor length is to be taken
as 5 + 0.1 d N m− 1 , where d is the conductor diameter. For insulators, 50 newtons
per meter of insulator chain should be used.
– Increased additional load can amount to a multiple of the normal additional
load and its consideration should be based on observations spanning many years
as well as on topographical and meteorological characteristics. Such additional
load mainly has to be considered in cold climates because of ice and snow
deposits.
– Additional load due to warning bowls or radar reflectors is to be considered as
normal additional load with an additional 1 cm layer of ice on the entire surface
(ice lining contributes 0.0075 N cm− 3 ) for cold climates only.