3/27/2014
Corona Effect on Electrical Transmission Lines | Electrical Engineering
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Corona
Effect
on
Electrical
Transmission Lines
In
power
corona
system
effect
phenomenon
engineering
is
a
which
can
observed in cables
stations,
of
substations
transmission
voltage
special
lines.
be
power
and
For
in
high
lines corona effect is
much more predominant than for
low
voltage
system.
In
ac
transmission lines if there are two
conductors
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whose
Transmission of Electric Power
is
Transmission Line Parameters
to
electrostatic
stresses. This takes place only for
high voltage transmission lines. When
the
applied
potential
difference
across two conductors reaches a
critical value called critical disruptive
voltage then a faint luminous glow
is
much larger than their diameter
then air surrounding the conductor
subjected
spacing
Skin Effect and Proximity Effect
Gas Insulated Substation & Switchgear
Concept of Electrical Busbar
Busbar Protection Schemes
Ferranti Effect in Transmission Lines
of violet color appears surrounding
Corona Effect on Transmission Lines
the conductors. This phenomenon is
What is power Factor?
known as visual corona which is also provided with a hissing sound. In this article we
will discuss about corona effect in detail.
Contents [hide ]
1 What is Corona Effect?
2 Mechanism of corona formation
3 Effects due to Corona
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4 Factors affecting Corona formation
5 Reduction of Corona effect
6 Advantages & Problems of Corona effect
What is Corona Effect?
The
phenomenon of corona formation around conductors in high voltage
transmission lines are accompanied by four parameters which are violet glow, hissing
sound, power loss and ozone gas production. If the potential difference between
two conductors is increased then violet glow, hissing noise and power loss due to
corona will also increase. The phenomenon of corona is much more evident for
transmission lines of 100kV and above. If the conductors are perfectly smooth the
violet glow due to corona will be uniform throughout the entire conductor.
Sometimes the conductor surface is not well polished and it becomes rough at some
points. In this type of conductor the violet glow appears to be bright at rough points
-that is corona effect not uniform throughout the conductor.
Spacing between conductors is an important criterion of corona effect, so if the
spacing is not sufficient then sometimes flashover takes place. This happens
because the corona effect may bridge the conductors (owing to less space between
them) causing luminous glow. Visible glow of violet light due to corona varies in ac
and dc transmission lines. Corona effect occurs when potential difference (p.d)
between two conductors exceeds critical value. One of the conductors is
considered as positive and other as negative. In EHV ac transmission lines the visible
glow of corona is mostly uniform for both the conductors but in case of HVDC
transmission lines the glow is uniform and brighter about positive conductor but
faint and spotty about the negative conductor.
Mechanism of corona formation
The basic principle of corona formation is ionization of air surrounding the
conductors in overhead transmission lines. This ionization of air molecules is
responsible for corona formation and its effects. Some ionized particles are present
in the surrounding air under normal condition. UV rays or cosmic ray produces some
degree of ionization always. Ionized particles include high speed free electrons and
positively charged ions. Air contains ionized particles along with some neutral
molecules.
Corona takes place when a potential difference (p.d) is applied between two
conductors. This p.d sets up a potential gradient in the air which is kept on
increasing. This potential gradient also increases the velocity of free electrons which
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becomes maximum for gradient value of 30kV/cm. High speed free electrons strikes
neutral molecules and electrons are released from those molecules resulting in
formation of ions and more free electrons. This process of ionization continues
until corona forms or flashover takes place.
Effects due to Corona
Corona is a phenomenon which is accompanied by loss of energy. We have already
discussed that corona occurs when the potential difference (p.d) between two
conductors in a transmission line exceeds a critical voltage value. The loss of energy
also increases when the p.d is increased beyond critical voltage. Power loss due to
corona depends upon weather conditions. During humid and moist climatic condition
corona loss is much increased while it is reduced for dry climatic condition. The
corona loss or corona power loss 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
nature. The power loss due to corona is compelled to have some effect on the
efficiency of the transmission line but it does not have any appreciable effect on the
voltage regulation. Different formulas are used to find out corona power loss for
fair weather and for stormy weather condition. Let us see the corona effects at a
glance.
♣ Violet color glow is observed around the conductor of transmission lines, which is
maximum for rough or dirty conductor surface.
♣ Hissing sound is always produced owing to corona phenomenon.
♣ Ozone gas is produced along with hissing noise and violet glow and the gas can be
detected by characteristic odour.
♣ Power loss or corona loss can be measured by a wattmeter connected in the
electric circuit.
♣ Corona induces harmonic current so the charging current due to corona also
increases.
Factors affecting Corona formation
Corona occurs due to ionization of air surrounding the line conductors, so it is
affected by two conditions – atmospheric state and condition of the transmission
line. Now we will discuss some factors upon which corona formation depends.
These are:
Atmospheric Condition: Corona takes place by the bombardment of molecules by
the ionized particles. This results in dislodging of electrons. Ionized particles include
some free electrons and positive ions. In stormy weather condition the number of
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ions in the surrounding air is much
more than fair weather condition.
Since density of air is more in
stormy
weather
thus
corona
occurs at a much lower voltage.
Conductor:
The
corona
is
considerably affected by the size
(diameter), shape
(stranded or
smooth) and surface condition
(dirty or clean) of the conductor.
A stranded conductor helps in
more
corona
formation
than
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smooth conductor or solid conductor. The potential gradient for stranded conductor
is greater than the smooth conductor. Thus breakdown voltage of such a conductor
is less than a smooth conductor giving rise to more corona formation. Dirty
conductors are also prone to corona formation. These conductors have irregular
surfaces which decreases the breakdown voltage.
Conductor Spacing: Spacing between conductors in transmission lines is an
important parameter for corona formation. With the increase in spacing between
conductors the electrostatic stresses are reduced and thus corona formation
chance is also reduced. If the conductors are placed very large distance apart as their
diameter, there may not be any corona effect.
Line voltage: At low voltage system corona formation is not certain, it takes place
for high voltage system. When line voltage is low, atmospheric condition is
unchanged that is electrostatic stresses are not developed. In this state ionization of
air surrounding the conductor does not takes place. All these conditions lead to
corona formation for high voltages.
Reduction of Corona effect
Corona effects are most prominent for voltage level of 33kV and beyond that. For
very high voltage system air gets highly ionized which may lead to flashover and
cause damage to the equipment. Thus it is desirable to reduce corona effects by
some common methods.
The critical voltage (voltage at which corona occurs) can be raised either by
increasing the spacing between the conductors or the diameter of conductors. By
increasing spacing between the conductors, voltage at which corona takes place is
increased thus eliminating chances of corona effects. The spacing cannot be
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increased to a much greater value because it will lead to high cost of the supporting
structures. Cross arms (insulator support) and support (pole / tower) has to be
bigger in size. The conductor diameter can be increased by using hollow conductors
with a hemp core.
ACSR (Aluminium Conductor Steel Reinforced) conductors are mostly used which
have large diameter and also they have good conductivity and are light weighted. By
using bundled conductors (group of two or more parallel sub conductors) corona
can be avoided because effective diameter of the bundled conductor is much larger
than that of the equivalent single conductor.
Advantages & Problems of Corona effect
So far we have seen corona as an unwanted phenomenon in power system
engineering. Corona involves loss of energy in different forms which is a negative
aspect. But corona can be considered beneficial sometimes owing to following
advantages.
i) On formation of corona the sheath of air surrounding the conductor becomes
conductive and there is virtual increase in conductor diameter. This virtual increase
of diameter reduces maximum potential gradient or maximum electrostatic stresses.
The probability of flash-over is reduced and system performance or efficiency is
improved.
ii) Effects of transients produced by lightning and other causes are reduced due to
corona effect. Lightning causes charges induced on the line which dissipates partially
as corona loss.
Let us also remind you about the problems of corona effect. These are:
i) Owing to corona formation, ozone gas is produced which chemically reacts with
the conductor and causes corrosion.
ii) Corona is always accompanied by loss of energy in form of heat, light, sound, etc.
Transmission line efficiency is affected by this corona loss.
iii) Corona results in non-sinusoidal voltage drop due to non-sinusoidal current flow
in the line. This cause interference with neighbouring communication circuits owing
to electromagnetic and electrostatic induction effects.
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