Corona

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
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.
Important Terms
The phenomenon of corona plays an important role
in the design of an overhead transmission line.
Therefore, it is profitable to consider the following
terms much used in the analysis of corona effects:
(i) Critical disruptive voltage. It is the
minimum phase-neutral voltage at which
corona occurs.
Consider two conductors of radii r cm and spaced d
cm apart. If V is the phase-neutral potential,then
potential gradient at the conductor surface is
given by:
g = __ V__ volts / cm
r loge d/r
The above expression for disruptive voltage is
under standard conditions i.e., at 76 cm of Hg
and
25ºC. However, if these conditions vary, the
air density also changes, thus altering the
value of go.
The value of go is directly proportional to air
density. Thus the breakdown strength of air
at a barometric
pressure of b cm of mercury and temperature
of tºC becomes δ go where
δ = air density factor =3. 92b
273+t
In order that corona is formed, the value of g must
be made equal to the breakdown strength of
air. The breakdown strength of air at 76 cm
pressure and temperature of 25ºC is 30 kV/cm
(max) or 21·2 kV/cm (r.m.s.) and is denoted by
go. If Vc is the phase-neutral potential required
under these
conditions, then, go = Vc
rloge d/r
where go = breakdown strength of air at 76 cm of
mercury and 25ºC

∴ Critical disruptive voltage, Vc = go r loge d/r

= 30 kV/cm (max) or 21·2 kV/cm (r.m.s.)
∴ Critical disruptive voltage, Vc = mo go δ r
loge d/r
kV/phase
where mo = 1 for polished conductors
= 0·98 to 0·92 for dirty conductors
= 0·87 to 0·8 for stranded conductors
 (ii) Visual critical voltage.
It is the minimum phase-neutral voltage at which corona glow
appears all along the line conductors.
It has been seen that in case of parallel conductors, the
corona glow does not begin at the disruptive
voltage Vc but at a higher voltage Vv, called visual critical
voltage. The phase-neutral effective
value of visual critical voltage is given by the following
empirical formula :
Vv = mv go δ r (1 + 0 ⋅3) loged/r kV/phase
δr

where mv is another irregularity factor having a value of 1·0

for polished conductors and 0·72 to 0·82
for rough conductors.
(iii) Power loss due to corona
Formation of corona is always accompanied
by energy loss
which is dissipated in the form of light, heat,
sound and chemical action. When disruptive
voltage is
exceeded, the power loss due to corona is
given by :
Advantages and Disadvantages of Corona
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.
Disadvantages
(i) Corona is accompanied by a loss of energy.
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.
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
(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 thereasons 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.
Example 8.13. A 3-phase line has conductors 2 cm
in diameter spaced equilaterally 1 m apart.
If the dielectric strength of air is 30 kV (max) per cm,
find the disruptive critical voltage for the line.
Take air density factor δ = 0·952 and irregularity factor
mo = 0·9.
Solution.
Conductor radius, r = 2/2 = 1 cm
Conductor spacing, d = 1 m = 100 cm
Dielectric strength of air, go = 30 kV/cm (max.) = 21·2
kV (r.m.s.) per cm
Disruptive critical voltage, Vc = mo go δ r loge (d/r)
kV*/phase (r.m.s. value)

∴ Line voltage (r.m.s.) = 3 × 83·64 = 144·8 kV

= 0·9 × 21·2 × 0·952 × 1 × loge 100/1 = 83·64 kV/phase
A 3-phase, 220 kV, 50 Hz transmission line consists of 1·5 cm
radius conductor spaced 2 metres apart in equilateral
triangular formation. If the temperature is 40ºC and
atmospheric pressure is 76 cm, calculate the corona loss
per km of the line. Take mo = 0·85.