MODULE II
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. Electric power transmission practically deals in the bulk transfer of
electrical energy, from generating stations situated many kilometers away from the main
consumption centers or the cities. For this reason the long distance transmission cables are of
utmost necessity for effective power transfer, which in-evidently results in huge losses across the
system. Minimizing those has been a major challenge for power engineers of late and to do that
one should have a clear understanding of the type and nature of losses. One of them being
the corona effect in power system, which has a predominant role in reducing the efficiency of
EHV(extra high voltage lines) which we are going to concentrate on, in this article. When an
alternating current is made to flow across two conductors of the transmission line whose spacing
is large compared to their diameters, then air surrounding the conductors (composed of ions) is
subjected to dielectric stress. At low values of supply end voltage, nothing really occurs as the
stress is too less to ionize the air outside. But when the potential difference is made to increase
beyond some threshold value of around 30 kV known as the critical disruptive voltage, then the
field strength increases and then the air surrounding it experiences stress high enough to be
dissociated into ions making the atmosphere conducting. This results in electric discharge around
the conductors due to the flow of these ions, giving rise to a faint luminescent glow, along with
the hissing sound accompanied by the liberation of ozone, which is readily identified due to its
characteristic odor. This phenomenon of electrical discharge occurring in transmission line for
high values of voltage is known as the corona effect in power system. If the voltage across the
lines is still increased the glow becomes more and more intense along with hissing noise,
inducing very high power loss into the system which must be accounted for.
,Factors Affecting Corona Effect in Power System
As mentioned earlier, the line voltage of the conductor is the main determining factor for corona
in transmission lines, at low values of voltage (lesser than critical disruptive voltage) the stress
on the air is too less to dissociate them, and hence no electrical discharge occurs. Since with
increasing voltage corona effect in a transmission line occurs due to the ionization of
atmospheric air surrounding the cables, it is mainly affected by the conditions of the cable as
well as the physical state of the atmosphere. Let us look into these criterion now with greater
details :
Atmospheric Conditions for Corona in Transmission Lines
It has been physically proven that the voltage gradient for di-electric breakdown of air is directly
proportional to the density of air. Hence in a stormy day, due to continuous air flow the number
of ions present surrounding the conductor is far more than normal, and hence its more likely to
have electrical discharge in transmission lines on such a day, compared to a day with fairly clear
weather. The system has to designed taking those extreme situations into consideration.
Condition of Cables for Corona in Transmission Line.
This particular phenomena depends highly on the conductors and its physical condition. It has an
inverse proportionality relationship with the diameter of the conductors. i.e. with the increase in
diameter, the effect of corona in power system reduces considerably.
Also the presence of dirt or roughness of the conductor reduces the critical breakdown voltage,
making the conductors more prone to corona losses. Hence in most cities and industrial areas
having high pollution, this factor is of reasonable importance to counter the ill effects it has on
the system.
Spacing between Conductors
As already mentioned, for corona to occur effectively the spacing between the lines should be
much higher compared to its diameter, but if the length is increased beyond a certain limit, the
,dielectric stress on the air reduces and consequently the effect of corona reduces as well. If the
spacing is made too large then corona for that region of the transmission line might not occur at
all.
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:
V
g Volts/cm
d
r ln
r
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 g0. If Vc is the phase-neutral potential required under
these conditions, then,
Vc
g0
d
r ln
r
where go = breakdown strength of air at 76 cm of mercury and 25ºC
= 30 kV/cm (max) or 21·2 kV/cm (r.m.s.)
d
∴ Critical disruptive voltage, Vc g 0 r ln
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 δg0 where
3.92
δ = air density factor =
273 t
Under standard conditions, the value of δ = 1.
d
Critical disruptive voltage, Vc g 0r ln
r
Correction must also be made for the surface condition of the conductor. This is accounted for
by multiplying the above expression by irregularity factor mo.
d
Critical disruptive voltage, Vc g 0m0 r ln kV/phase
r
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
0.3 d
Vv mv g 0r (1 ) ln kV/phase
r r
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:
f 25 r
P 241 10 5 V Vc 2 kw/km/phase
d
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. Electric power transmission practically deals in the bulk transfer of
electrical energy, from generating stations situated many kilometers away from the main
consumption centers or the cities. For this reason the long distance transmission cables are of
utmost necessity for effective power transfer, which in-evidently results in huge losses across the
system. Minimizing those has been a major challenge for power engineers of late and to do that
one should have a clear understanding of the type and nature of losses. One of them being
the corona effect in power system, which has a predominant role in reducing the efficiency of
EHV(extra high voltage lines) which we are going to concentrate on, in this article. When an
alternating current is made to flow across two conductors of the transmission line whose spacing
is large compared to their diameters, then air surrounding the conductors (composed of ions) is
subjected to dielectric stress. At low values of supply end voltage, nothing really occurs as the
stress is too less to ionize the air outside. But when the potential difference is made to increase
beyond some threshold value of around 30 kV known as the critical disruptive voltage, then the
field strength increases and then the air surrounding it experiences stress high enough to be
dissociated into ions making the atmosphere conducting. This results in electric discharge around
the conductors due to the flow of these ions, giving rise to a faint luminescent glow, along with
the hissing sound accompanied by the liberation of ozone, which is readily identified due to its
characteristic odor. This phenomenon of electrical discharge occurring in transmission line for
high values of voltage is known as the corona effect in power system. If the voltage across the
lines is still increased the glow becomes more and more intense along with hissing noise,
inducing very high power loss into the system which must be accounted for.
,Factors Affecting Corona Effect in Power System
As mentioned earlier, the line voltage of the conductor is the main determining factor for corona
in transmission lines, at low values of voltage (lesser than critical disruptive voltage) the stress
on the air is too less to dissociate them, and hence no electrical discharge occurs. Since with
increasing voltage corona effect in a transmission line occurs due to the ionization of
atmospheric air surrounding the cables, it is mainly affected by the conditions of the cable as
well as the physical state of the atmosphere. Let us look into these criterion now with greater
details :
Atmospheric Conditions for Corona in Transmission Lines
It has been physically proven that the voltage gradient for di-electric breakdown of air is directly
proportional to the density of air. Hence in a stormy day, due to continuous air flow the number
of ions present surrounding the conductor is far more than normal, and hence its more likely to
have electrical discharge in transmission lines on such a day, compared to a day with fairly clear
weather. The system has to designed taking those extreme situations into consideration.
Condition of Cables for Corona in Transmission Line.
This particular phenomena depends highly on the conductors and its physical condition. It has an
inverse proportionality relationship with the diameter of the conductors. i.e. with the increase in
diameter, the effect of corona in power system reduces considerably.
Also the presence of dirt or roughness of the conductor reduces the critical breakdown voltage,
making the conductors more prone to corona losses. Hence in most cities and industrial areas
having high pollution, this factor is of reasonable importance to counter the ill effects it has on
the system.
Spacing between Conductors
As already mentioned, for corona to occur effectively the spacing between the lines should be
much higher compared to its diameter, but if the length is increased beyond a certain limit, the
,dielectric stress on the air reduces and consequently the effect of corona reduces as well. If the
spacing is made too large then corona for that region of the transmission line might not occur at
all.
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:
V
g Volts/cm
d
r ln
r
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 g0. If Vc is the phase-neutral potential required under
these conditions, then,
Vc
g0
d
r ln
r
where go = breakdown strength of air at 76 cm of mercury and 25ºC
= 30 kV/cm (max) or 21·2 kV/cm (r.m.s.)
d
∴ Critical disruptive voltage, Vc g 0 r ln
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 δg0 where
3.92
δ = air density factor =
273 t
Under standard conditions, the value of δ = 1.
d
Critical disruptive voltage, Vc g 0r ln
r
Correction must also be made for the surface condition of the conductor. This is accounted for
by multiplying the above expression by irregularity factor mo.
d
Critical disruptive voltage, Vc g 0m0 r ln kV/phase
r
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
0.3 d
Vv mv g 0r (1 ) ln kV/phase
r r
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:
f 25 r
P 241 10 5 V Vc 2 kw/km/phase
d
