What is Corona?
A Clearly Explained and Illustrated Story About Three Types of
Corona Discharge and Their Relationship to Radio Interference
NOTE: Because Hubbell has a policy of continuous product improvement, we reserve the right to change design and specifications without notice.
®
POWER
SYSTEMS, INC.
Tel 573-682-5521
™
Fax 573-682-8714
®
http://www.hubbellpowersystems.com
®
®
UNITED STATES • 210 N. Allen • Centralia, MO 65240 • Phone: 573-682-5521 • Fax: 573-682-8714 • e-mail: hpsliterature@hps.hubbell.com
CANADA • 870 Brock Road South • Pickering, Ontario L1W 1Z8 • Phone: 905-839-1138 • Fax: 905-831-6353 • e-mail: infohps@hubbellonline.com
MEXICO • Av. Coyoacan No. 1051 • Col. Del Valle • 03100 Mexico, D.F. • Phone: 52-55-9151-9999 • Fax: 52-55-9151-9988 • e-mail: vtasdf@hubbell.com.mx
Bulletin EU1234-H
@2004 Hubbell Power Systems, Inc.
®
What Is Corona?
During the past several years, corona and radio
influence (RI) problems associated with extra-highvoltage transmission have been studied extensively
in the Ohio Brass Voltage Laboratory.
The word “corona”, as well as the definitive
expressions “plume discharge”, “brush discharge”
and “glow discharge” have been used extensively in
connection with this work, under the tacit assumption that everyone is familiar with their meanings.
It is the purpose of this article to explain the
meanings of these expressions as used in such
studies and to describe some of the physical and
electrical properties associated with them.
Definition of Corona
Perhaps the most general definition of corona
possible is as follows: “Corona is a discharge caused
by electrical overstress.” While this definition is
very general, and applies to practically all kinds of
corona, it is still unsatisfactory since it introduces
two new expressions, “electrical overstress” and
“discharge,” which, although they describe the
“cause” and “effect”, must in turn be defined for
various cases.
Actually, corona can appear in solid, liquid or
gaseous insulating materials, and its occurrence
therein is usually associated with the initial phases
of electrical failure of the insulation. In solids, the
occurrence of corona generally results in deterioration of the material, while in liquids and gases,
removal of the electrical overstress eliminates the
discharge, and the material generally recovers its
original insulating properties.
For transmission line studies, the insulating
material in which the discharge occurs is the air
adjacent to conductor or insulator surfaces, when
the electrical stress at these surfaces exceeds the
critical value.
Limiting corona to transmission line conductors
leads to the American Standards Association definition of corona, which reads as follows:
“Corona is a luminous discharge due to ionization
of the air surrounding a conductor around which
exists a voltage gradient exceeding a certain critical value.”
COVER PICTURE
Brush and plume corona discharge on a two-conductor horizontal bundle of Drake ACSR Conductors
spaced 16 inches apart at a test voltage of 240 kV
line-to-ground.
This definition obviously contains the same basic idea given in the above general definition, but
has a rather limited scope. For the purpose of this
article the scope of this definition will be enlarged
to include corona on line hardware and insulators
at high voltages.
Nature of Corona
The corona discharges observed at the surface of
a conductor are due to the formation of electron
avalanches which occur when the intensity of the
electric field at the conductor surface exceeds a
certain critical value.
There are always a few free electrons in the air
as a result of traces of radioactive materials in the
earth’s crust and cosmic ray bombardment of the
earth from outer space. As the conductor becomes
energized on each half cycle of the AC voltage wave,
the electrons in the air near its surface are accelerated by the electrostatic field. These electrons,
having an inherent negative charge, are accelerated toward the conductor on its positive half cycle
and away from the conductor on its negative half
cycle.
The velocity attained by a free electron is dependent upon the intensity of the electric field. If the
intensity of the electric field is not too great, the
collision between an electron and an air molecule
such as oxygen (O2) or nitrogen (N2) is elastic; that
is, the electron bounces off the air molecule with no
transfer of energy to it. On the other hand, if the
intensity of the electric field exceeds a certain
critical value, any free electron in this field will
acquire a sufficient velocity so that its collision
with the air molecule is inelastic; that is, the
electron has sufficient energy to knock one of the
outer orbit electrons clear out of one of the two
atoms of the air molecule. This is the phenomenon
known as ionization, and the molecule with the
missing electron becomes a positive ion.
The initial electron, which lost most of its velocity in the collision, and the electron knocked out of
the air molecule, which also has a low velocity, are
both accelerated by the electric field, and at the
next collision, each electron is capable of ionizing
an air molecule. After the second collision, there
are now four electrons to proceed, and so on, the
number of electrons doubling after each collision.
All this time, the electrons are all advancing toward the positive electrode and after many collisions, their number has grown enormously. This is
the process by which the so-called electron avalanche is built up, each avalanche being initiated
by a single free electron which finds itself in an
intense electrostatic field.
–2–
A 4-conductor bundle of one-inch diameter smooth tubes,
spaced 20 inches apart, used for testing.
Corona discharges on bundled conductors at 400 kV lineto-ground.
The intensity of the electrostatic field around a
conductor is non-uniform. It has its maximum
strength at the surface of the conductor and its
intensity diminishes inversely as the distance from
the center of the conductor. Hence, as the voltage
level in the conductor is raised, the critical field
strength is approached and the initial discharges
occur only at or very near to the conductor surface.
For the positive half cycle, the electron avalanches
move toward the conductor and continue to grow
until they hit the surface. For the negative half
cycle, the electron avalanches move away from the
conductor surfaces toward a weaker field and cease
to advance when the field becomes too weak to
accelerate the electrons to ionizing velocity.
The positive ions left in the wake of the electron
avalanche move toward the negative electrode.
However, they move very slowly because of their
mass, which for air molecules is in the order of
50,000 times the mass of the electron. Having a
positive charge, these ions attract wandering electrons, and whenever one succeeds in capturing a
free electron, it becomes a neutral air molecule
again. The energy level of a neutral molecule is less
than that of the corresponding positive ion, and
hence when a free electron is captured, a quanta or
“chunk” of energy is emitted from the molecule.
This quanta of energy is exactly equal in magnitude to the energy which initially was required to
knock the electron out of the molecule in the first
place. It is radiated as an electro-magnetic wave,
and for air molecules such as oxygen or nitrogen,
this radiation is in the visible light range. Hence,
an observer can see this radiation as a soft violetcolored light, which comes principally from the
recombination of nitrogen ions with free electrons.
–3–
The Manifestations of Corona
The discharges which are produced by electron
avalanches may be observed in the laboratory in
three different ways.
Perhaps the best known manifestation is “visual
corona” which appears as a violet colored light
coming from the regions of electrical overstress
when the test specimen is viewed in the dark. As
described above, this light is produced by the re-
combination of positive nitrogen ions with free
electrons.
The second manifestation of this discharge is
“audible corona”, which appears as a hissing or
frying sound whenever the specimen is energized
above the corona threshold voltage. The sound
waves are produced by the disturbances set up in
air in the vicinity of the discharge, possibly by the
movement of the positive ions as they are suddenly
created in an intense electric field.
The third, and perhaps most serious manifestation of this discharge from the point of view of the
power company, is the electrical effect which causes
radio influence or RI. These avalanches, being
electrons in motion, actually constitute electric
currents, and as such, produce both magnetic and
electrostatic fields in the vicinity. Being formed
very suddenly and being of short duration, these
magnetic and electrostatic fields can induce highfrequency voltage pulses in nearby radio antennas,
and hence may cause RI. These electrical disturbances are usually measured in the laboratory
with a radio noise meter which is closely coupled to
the test specimen by means of the standard NEMA
circuit. This laboratory set-up measures the generated radio noise, and is usually called the “radio
influence voltage” or RIV of the test specimen.
The Aspects of Corona
The three different types or degrees of corona
discharges which are recognized on EHV test specimens in the laboratory are called “plume discharge,”
“brush discharge” and “glow discharge.”
The plume discharge is the most spectacular of
the three, and is so named because of its general
resemblance to a plume. When viewed in the dark,
it has a concentrated stem which may be anywhere
from a fraction of an inch long to several inches in
length, depending upon the voltage level of the
conductor. At its outer end, the stem branches
many times and merges into a violet-colored treelike halo which may range in length from a few
inches at lower voltages to a foot or more at very
high voltages. The audible manifestation associated with plume discharges is generally a rather
intense snapping and hissing sound, readily recognized by the experienced corona observer.
The brush discharge is a streamer projecting
radially from the conductor surface. These discharges generally occur all around the periphery of
the conductor. The length of these discharges may
vary from a small fraction of an inch at low voltages
to one or two inches at higher voltages. The name
is suggested by the resemblance which the discharges have to the bristles of a round bottle brush.
The audible manifestation associated with brush
discharges is generally a continuous background
type of hissing or frying sound.
The glow discharge is a very faint, weak light
which appears to hug the conductor surface and
does not project there from as does a brush discharge. It also may appear on critical regions of
insulator surfaces during high humidity conditions. There is generally no sound associated with
glow discharges.
Properties of Corona
On a clean, dry, smooth conductor energized a
little above its critical voltage, only brush discharges occur, and these are generally limited to
the negative half cycle. Fig. 1 shows the corona
which occurs on a smooth conductor when photographed under various conditions. Fig. 1-A shows
the conductor as viewed by ordinary means. Fig. 1B shows it on the negative half cycle only, while Fig.
1-C shows how it looks on the positive half cycle
only. These last two photographs were taken with
a rotating disk located in front of the camera lens.
This disk, which had two 90-degree open segments
and two 90-degree closed segments, was rotated at
1800 rpm with a synchronous motor. The disk was
adjusted on the motor shaft so that the camera
could “see” the conductor only one half the time,
corresponding to either pre-selected half cycle of
the 60-cycle voltage wave applied to the conductor.
A. Both half cycles
B. Negative half cycle
C. Positive half cycle
Fig. 1. Corona discharges on a one-inch diameter clean,
dry, smooth conductor energized at 200 kV line-to-ground.
–4–
As indicated in Fig. 1-B, the corona discharge
appears the same on the negative half cycle as it
does in Fig. 1-A for both half cycles. On the positive
half cycle, the corona is practically absent, except
for one lone incipient plume discharge being present,
probably due to a tiny defect on the conductor
surface.
This difference in the appearance of corona on
the alternate polarities may be readily explained
by the above described discharge mechanism. As
the voltage rises from zero in the positive direction,
electrons in the vicinity of the conductor surface
move towards it into a region of higher field intensity, bumping their way through the molecules of
air. As long as each electron’s velocity at the instant of collision remains below the ionizing velocity, no positive ions are formed, and hence there is
no discharge. This process continues until each
electron in the vicinity finally arrives at and enters
the conductor surface. If the maximum field intensity at the voltage crest is insufficient to cause
ionization, there will be no discharge.
As the voltage rises from zero in the negative
direction, electrons in the vicinity of the conductor
surface move away from it into a region of weaker
field intensity, bumping their way through the
molecules of air. As long as the field intensity is too
low to accelerate any electron to its ionizing velocity between successive collisions, there is no discharge. However, unlike the positive half cycle,
electrons initially present at the start of the negative half cycle are still present as the 60-cycle
voltage wave approaches crest. If the maximum
field intensity at the voltage crest is insufficient to
cause ionization, there will be no discharge.
For a sine wave of voltage, the field intensity is
the same for either crest. Hence, the appearance of
brush discharges on the negative half cycle and no
discharges on the positive half cycles for a smooth
conductor indicates that a higher field intensity is
required for ionization with the conductor surface
positive. This difference is explained by the removal of electrons from the neighborhood of the
conductor surface on the positive half cycle as the
voltage increases at the 60-cycle rate toward its
crest value.
Any defect on the conductor which projects however slightly above the nominal conductor surface,
increases the field intensity in its immediate vicinity. Hence, the defect on the conductor shown in
Fig. 1 projected far enough so that at a test voltage
of 200 kV, it increased the field intensity sufficiently to accelerate to ionizing velocities those free
electrons still remaining in its vicinity, and thus
produced the incipient plume discharge shown in
Fig. 1-C.
A second method of studying corona discharges
–5–
is to observe which polarity produces the higher
readings in the radio noise meter. This is accomplished easily by connecting a cathode ray oscilloscope to the phone jack of the radio noise meter and
superimposing its output upon a 60-cycle reference
voltage wave which is synchronized with the 60cycle test voltage applied to the specimen. A series
of such oscillograms is shown in Fig. 2. The oscillogram of Fig. 2-A is for the clean dry smooth conductor described in Fig. 1. As shown the disturbance
originates on the negative half cycle only, and
always near its crest. The RIV produced by the
single incipient plume discharge on the positive
half cycle, as shown in Fig. 1-C, is negligible compared to that produced by the negative half cycle
brush discharges, and hence is lost in the oscillogram.
Introducing an intentional surface defect of somewhat greater magnitude caused a full-fledged plume
discharge to appear on the conductor surface on the
positive half cycle as is shown by the series of
photographs of Fig. 3. The oscillogram of Fig. 2-B
shows that now the principal RIV occurs on the
positive half cycle, with the negative half cycle
discharges causing only relatively slight disturbances. The one full-fledged plume on the positive
half cycle produced many times as much disturbance as did all of the brush discharges on the
negative half cycle. This difference is shown by the
radio noise meter readings which gave an RIV level
of 3000 microvolts for the brush discharges of Fig.
1 and 25,000-microvolt level for the single plume
discharge of Fig. 3.
As shown in Fig. 3-B, the negative polarity
brush discharge from the intentional defect is much
larger than are those from the smooth portion of
the conductor surface. To the right of the intentional defect there is also a pair of large brush
discharges which apparently came from some unintentional defect on the conductor surface. The
brushes from the defects appear much larger and
brighter than do those coming from the smooth
conductor surface, because they begin to develop at
a somewhat lower voltage on the 60 cycle wave and
the crest field intensity on the conductor surface at
the defects is higher than it is on the smooth
portion of its surface.
Water drops on the conductor surface provide a
multiplicity of projections from which corona discharges can originate. The series of photographs of
Fig. 4 show that many small plumes occur on the
positive half cycle and likewise many brush discharges occur on the negative half cycle. The oscillogram of Fig. 2-C shows that the plumes on the
positive half cycle establish the RIV level with the
wet conductor.
A weathered ACSR conductor generally has a
A.
1-inch smooth conductor energized at 200 kV. Dry,
clean surface.
B. 1-inch smooth conductor energized at 200 kV. Dry,
clean surface, with defect.
C. 1-inch smooth conductor energized at 200 kV. Wet,
clean surface.
D. 0.858-inch stranded conductor energized at 150 kV.
Dry, weathered surface.
Fig. 2. Oscillograms of the output from the phone jack on the Ferris Model 32-B radio noise meter, superimposed upon
a 60-cycle voltage wave synchronized with the high voltage wave applied to the specimen.
multiplicity of tiny surface defects which project
above the nominal surface of the conductor. The
series of photographs of Fig. 5 show that many
small plume discharges occur on the positive half
cycle and that many small brush discharges occur
on the negative half cycle. The oscillograms of Fig.
2-D show that the discharges on the positive half
cycle usually establish the RIV level of the conductor.
The plume and brush discharges shown in Fig.
6 were obtained during a series of tests on a 1.602inch diameter ACSR stranded conductor. The
plumes occur only at locations where one of the
conductor strands was intentionally displaced so
that it protruded about 0.10 of an inch above the
nominal conductor surface. The brush discharges
occurred only on the negative half cycle, while the
plume discharges occurred only on the positive half
cycle.
Corona discharges on a 4-conductor bundle tested
in the laboratory (see page 4) appear to be principally brushes, there being only a few small plumes
apparent. The corona discharges, which appear
only on the outer surface of each conductor, project
radially from the geometric axis of the bundle. This
is the direction of maximum surface gradient.
The Formation of Brush and Plume Discharges
The reason why brush discharges occur on the
negative half cycle and plume discharges occur on
the positive half cycle may be explained in terms of
the above described corona mechanism.
a. Brush Discharge
As the negative half cycle voltage increases at
the normal 60-cycle rate, the field intensity, which
is always greatest at the conductor surface, increases at the same rate. When the field intensity
reaches the critical value, somewhat below the
voltage crest, free electrons near the conductor
surface are accelerated to ionizing velocities; and
avalanches start to form, moving in the positive
direction of the field, which is away from the
conductor surface. The heavy positive ions left
behind form a positive space charge between the
conductor surface and the tip of the advancing
electron avalanche. This tends to weaken the field
produced by the energized conductors. On the other
hand, the advancing avalanche, being an accumulation of a large number of electrons, produces a
negative space charge just ahead of itself, which
tends to reinforce the intensity of the field due to
the conductor. This increased field causes free
electrons ahead of the advancing tip likewise to
–6–
A. Both half cycles
A. Both half cycles
B. Negative half cycle
B. Negative half cycle
C. Positive half cycle
C. Positive half cycle
Fig. 3. Corona discharges on a one-inch diameter clean,
dry, smooth conductor with an intentional defect, energized at 200 kV line-to-ground.
Fig. 4. Corona discharges on a one-inch diameter wet,
smooth conductor energized at 200 kV line-to-ground.
initiate new avalanches which advance the negative space charge still further. This process is
accumulative, and the brush discharge will continue to grow in length until the net field strength
at the outer tip of the advancing discharge is
insufficient to cause further ionization. Obviously
the length of the streamer will be determined by
the conductor voltage and will achieve its maximum length at crest voltage.
The existence of a brush discharge becomes
apparent by the visible light which is emitted by
each positive ion as it recombines with a free
electron to form a neutral molecule. Thus, the
location and extent of the brush discharge created
by electron avalanches is revealed.
b. Plume Discharge
When the field intensity near the conductor
surface reaches the critical value on the positive
half cycle, electrons in the vicinity of the surface
are accelerated to ionizing velocity and, therefore,
initiate electron avalanches, which advance toward the conductor surface and ultimately strike
it. The heavy ions left behind form a positive space
charge, the tip of which projects outward from the
conductor surface. The increased field intensity at
the tip promotes the formation of additional electron avalanches which move toward it. As a result,
the positive space charge projection continues to
grow outward from the conductor.
Near the conductor surface, avalanches are created principally in front of the advancing positive
–7–
space charge projection, since here the field from
the conductor itself is intense and the combined
field is more or less radial to the conductor. The
further the positive space charge projection advances, the weaker becomes the conductor field
A. Both half cycles
B. Negative half cycle
C. Positive half cycle
Fig. 5. Corona discharges on a weathered 477,000-cm
“Hawk” ACSR stranded conductor with an overall diameter 0.858-inch, energized at 150 kV line-to-ground.
and the resultant field at its tip becomes more
divergent, approaching a hemispherical radial field.
Hence, electron avalanches now can approach the
tip of the advancing positive space charge from
different directions, thus initiating positive ion
projections which branch out in different directions from the initial stem. Many of these branches
continue to grow and again branch out much like
the limbs, branches, and twigs of a tree.
This growth continues until ultimately the plume
reaches such a size that the field intensity at the
tips of the numerous branching positive streamers
is insufficient to cause further ionization.
The plume shown in Fig. 3-C is an excellent
example of a full-fledged plume, while the discharge shown in Fig. 1-C is an example of an
incipient plume which had barely reached the
branching stage.
Radio Influence Associated
with Corona Discharges
Positive polarity plumes are a prolific source of
electrical disturbance, producing RIV levels in the
order of 10 times those of the negative polarity
brush discharges.
In general, it has been found in tests on transmission line conductors above 0.5-inch diameter
that when plumes occur, RIV levels are generously
in excess of 10,000 microvolts.
The observed range of RIV levels for brush
discharges is from about 100 microvolts to 5000
microvolts. The lower figure would apply to just a
few audible, as well as visible, brush discharges on
the conductor just above the visual corona voltage,
while the upper figure applies to higher test voltages, or to voltages just below those at the point
where they occur.
Glow discharges are observed only infrequently
on conductor surfaces, and when they do occur,
they usually persist at voltages below which audible and visual brush discharges have disappeared.
RIV levels associated with glow discharges are
usually under 10 microvolts. These discharges appear to be caused by the tiniest of surface defects,
and can usually be eliminated by cleaning the
conductor surface at the point where they occur.
Fig. 6 Plume and brush discharges on a 1.602-inch diameter ACSR conductor at 300 kV line-to-ground. Each plume
occurs where a strand has been displaced about 0.10 of an inch above the nominal conductor surface.
–8–