Corona Effect
Introduction:
One of the phenomena associated with
all energized electrical devices, including high-voltage transm
What is Corona?
§ Electric
transmission lines can generate a small amount of sound energy as a result of
corona.
§ Corona is a phenomenon associated with all transmission
lines. Under certain conditions, the localized electric field near energized
components and conductors can produce a tiny electric discharge or corona that
causes the surrounding air molecules to ionize, or undergo a slight localized
change of electric charge.
§ Utility companies try to reduce the amount of corona because
in addition to the low levels of noise that result, corona is a power loss, and
in extreme cases, it can damage system components over time.
§ Corona occurs on all types of transmission lines, but it
becomes more noticeable at higher voltages (345 kV and higher). Under fair
weather conditions, the audible noise from corona is minor and rarely noticed.
§ During wet and humid conditions, water drops collect on the
conductors and increase corona activity. Under these conditions, a crackling or
humming sound may be heard in the immediate vicinity of the line.
§ Corona results in a power loss. Power losses like corona
result in operating inefficiencies and increase the cost of service for all
ratepayers; a major concern in transmission line design is the reduction of
losses.
Source of Corona:
§ The amount of corona produced by a transmission line is a
function of the voltage of the line, the diameter of the conductors, the
locations of the conductors in relation to each other, the elevation of the
line above sea level, the condition of the conductors and hardware, and the
local weather conditions. Power flow does not affect the amount of corona
produced by a transmission line.
§ The electric field gradient is greatest at the surface of
the conductor. Large-diameter conductors have lower electric field gradients at
the conductor surface and, hence, lower corona than smaller conductors,
everything else being equal. The conductors chosen for the Calumet to the line
were selected to have large diameters and to utilize a two conductor bundle.
This reduces the potential to create audible noise.
§ Irregularities (such as nicks and scrapes on the conductor
surface or sharp edges on suspension hardware) concentrate the electric field
at these locations and thus increase the electric field gradient and the
resulting corona at these spots. Similarly, foreign objects on the conductor
surface, such as dust or insects, can cause irregularities on the surface that
are a source for corona.
§ Corona also increases at higher elevations where the density
of the atmosphere is less than at sea level. Audible noise will vary with
elevation. An increase in 1000 feet of elevation will result in an increase in
audible noise of approximately 1 dB (A). Audible noise at 5000 feet in
elevation will 5 dB (A) higher than the same audible noise at sea level, all
other things being equal. The new Calumet to Comanche 345 kV double circuit
line was modeled with an elevation of 6000 feet.
§ Raindrops, snow, fog, hoarfrost, and condensation
accumulated on the conductor surface are also sources of surface irregularities
that can increase corona. During fair weather, the number of these condensed
water droplets or ice crystals is usually small and the corona effect is also
small.
§ However, during wet weather, the number of these sources
increases (for instance due to rain drops standing on the conductor) and corona
effects are therefore greater.
§ During wet or foul weather conditions, the conductor will
produce the greatest amount of corona noise. However, during heavy rain the
noise generated by the falling rain drops hitting the ground will typically be
greater than the noise generated by corona and thus will mask the audible noise
from the transmission line.
§ Corona produced on a transmission line can be reduced by the
design of the transmission line and the selection of hardware and conductors
used for the construction of the line. For instance the use of conductor
hangers that have rounded rather than sharp edges and no protruding bolts with
sharp edges will reduce corona. The conductors themselves can be made with
larger diameters and handled so that they have smooth surfaces without nicks or
burrs or scrapes in the conductor strands. The transmission lines proposed here
are designed to reduce corona generation.
TYPES OF CORONA:
There are three types of corona.
§ A glow discharge occurs at a gradient of approximately 20 kV
rms/cm. Glow discharge is a light glow off sharp points that does not generate
objectionable RIV/TVI or cause any audible noise.
§ At about 25 kV rms/cm, negative polarity “brush” discharges
occur. So named because the appearance is similar to the round ends of a bottle
brush. The audible noise associated with brush corona is generally a continuous
background type of hissing or frying noise.
§ At a gradient of around 30 kVrms/cm positive polarity plume
corona is generated; so named because of its general resemblance to a plume.
When viewed in the dark it has a concentrated stem that branches and merges
into a violet-colored, tree-like halo. The audible noise associated with plume
corona is a rather intense snapping and hissing sound. Plume corona generates
significant RIV/TVI.
§ These observations are based on fair weather conditions.
Under wet conditions virtually all energized electrodes will be in corona of
one form or another.
§ Many are under the impression that the dielectric strength
of air is greater under dry conditions. That is not true. In fact, the dielectric
strength of air increases with increased moisture up to the dew point when
moisture begins to condense on the surface of insulators and other components
of the line.
Physical Parameters of Corona:
§ Corona is caused by the ionization of the media (air)
surrounding the electrode (conductor)
§ Corona onset is a function of voltage
§ Corona onset is a function of relative air density
§ Corona onset is a function of relative humidity
1. Corona and the Electric Field
§ Corona is NOT solely a function of the Electric Field
§ Corona is a function of the electric field on the surface of
the electrode (conductor)
§ Corona is also a function of the radius of curvature of the
electrode (conductor)
§ Corona is also a function of the rate of decay of the
electric field away from the electrode (conductor)
§ For the preceding reasons, selecting the conductor with the
smallest electric field at its surface is not correct.
2. Corona and the Relative Air
Density
§ Corona has an inverse relationship with air density
§ Standard line designs that perform well at sea level, may
have significant corona issues if used on lines that are installed over
mountainous areas
3. Corona and the Humidity
§ Corona has an inverse relationship with humidity at power
frequencies
§ Fair weather corona is more prevalent in low humidity
environments
4. Corona is Dependent Surface
Condition of the Conductors
§ Corona is enhanced by irregularities on the conductor
surface
§ Irregularities include: dust, insects, burrs and scratches
and water drops present on new conductors
§ Corona will generally be greater on new conductors and will
decrease to a steady-state value over a period of approximately one year
in-service
§ Corona is significantly increased in foul weather.
Methods to reduce Corona Discharge Effect:
§ Corona
can be avoided
1.
By minimizing the voltage
stress and electric field gradient.: This is accomplished by using
utilizing good high voltage design practices, i.e., maximizing the distance
between conductors that have large voltage differentials, using conductors with
large radii, and avoiding parts that have sharp points or sharp edges.
2.
Surface
Treatments: Corona inception voltage can
sometimes be increased by using a surface treatment, such as a semiconductor
layer, high voltage putty or corona dope.
3.
Homogenous
Insulators: Use a good, homogeneous insulator.
Void free solids, such as properly prepared silicone and epoxy potting
materials work well.
4.
If you are
limited to using air as your insulator, then
you are left with geometry as the critical parameter. Finally, ensure that
steps are taken to reduce or eliminate unwanted voltage transients, which can
cause corona to start.
5.
Using
Bundled Conductors: on our 345
kV lines, we have installed multiple conductors per phase. This is a common way
of increasing the effective diameter of the conductor, which in turn results in
less resistance, which in turn reduces losses.
6.
Elimination
of sharp points: electric charges tend to form
on sharp points; therefore when practicable we strive to eliminate sharp points
on transmission line components.
7.
Using
Corona rings: On certain new 345 kV
structures, we are now installing corona rings. These rings have smooth round
surfaces which are designed to distribute charge across a wider area, thereby
reducing the electric field and the resulting corona discharges.
8.
Whether: Corona phenomena much worse in foul weather, high
altitude
9.
New
Conductor: New conductors can lead to
poor corona performance for a while.
10.
By
increasing the spacing between the conductors: Corona Discharge Effect can be reduced by increasing the
clearance spacing between the phases of the transmission lines. However
increase in the phase’s results in heavier metal supports. Cost and Space
requirement increases.
11.
By
increasing the diameter of the conductor: Diameter of the conductor can be increased to reduce the
corona discharge effect. By using hollow conductors corona discharge effect can
be improved.
Sources of Corona and Arcing in Polymer
Insulators:
§ Loose hardware
§ Contamination and surface tracking
§ Missing corona rings
§ Damaged or incorrectly installed corona ring
§ Damaged end fittings or end fitting seal
§ Exposed internal rod due to: Carbonized internal rod by
internal discharges Split sheath due to weathering
Corona Detection:
§ Light Ultraviolet radiation: Corona can be visible in the
form of light, typically a purple glow, as corona generally consists of micro
arcs. Darkening the environment can help to visualize the corona.
§ Sound (hissing, or cracking as caused by explosive gas
expansions): You can often hear corona hissing or cracking Sound.
§ In addition, you can sometimes smell the presence of ozone
that was produced by the corona.
§ Salts, sometimes seen as white powder deposits on Conductor.
§ Mechanical erosion of surfaces by ion bombardment
§ Heat (although generally very little, and primarily in
the insulator)
§ Carbon deposits, thereby creating a path for severe arcing
§ The corona discharges in insulation systems result in
voltage transients. These pulses are superimposed on the applied voltage and
may be detected, which is precisely what corona detection equipment looks for.
In its most basic form, the following diagram is a corona (or partial
discharge) measuring system:
§ It is important that the voltage source and the coupling
capacitor exhibit low noise so as not to obscure the corona. In its simplest
form the pulse detection network is a resistor monitored by an oscilloscope.
Don’t dismiss this simple technique as crude, as we once used this method to
observe the presence of corona in an improperly terminated high voltage
connector, even after a dedicated corona tester failed to find any.
Commercially available corona detectors include electronic types (as above) as
well as ultrasonic types.
Corona Calculations
§ The following corona calculations are from Dielectric Phenomena
in High Voltage Engineering
1. For
Concentric Cylinders in Air:
§ Corona will not form when RO / RI < 2.718. (Arcing will
occur instead when the voltage is too high.)
2. For
Parallel Wires in Air:
§ Corona will not form when X / r < 5.85. (Arcing will
occur instead when the voltage is too high.)
3. For
Equal Spheres in Air:
§ Corona will not form when X / R < 2.04. (Arcing will
occur instead when the voltage is too high.)
§ Arcing difficult to avoid when X / R < 8
Where
§ RO = Radius of outer concentric sphere
§ RI = Radius of inner concentric sphere
§ R = Sphere radius
§ r = wire radius
§ X = Distance between wires or between spheres
Effects of Corona:
(1) Audible Noise
§ During corona activity, transmission lines (primarily
those rated at 345 kV and above) can generate a small amount of sound energy.
This audible noise can increase during foul weather conditions. Water drops may
collect on the surface of the conductors and increase corona activity so that a
crackling or humming sound may be heard near a transmission line. Transmission
line audible noise is measured in decibels using a special weighting scale, the
“A” scale that responds to different sound characteristics similar to the
response of the human ear. Audible noise levels on typical 230 kV lines are
very low and are usually not noticeable. For example, the calculated rainy
weather audible noise for a 230 kV transmission line at the right-of-way edge
is about 25 dBA, which is less than ambient levels in a library and much less
than background noise for wind and rain.
(2)Radios and
Television Interference:
§ Overhead
transmission lines do not, as a general rule, interfere with radio or TV
reception.
§ There are two potential sources for interference: corona and
gap discharges. As described above, corona discharges can sometimes generate
unwanted electrical signals.
§ Corona-generated electrical noise decreases with distance
from a transmission line and also decreases with higher frequencies (when it is
a problem, it is usually for AM radio and not the higher frequencies associated
with TV signals).
§ Corona interference to radio and television reception is
usually not a design problem for transmission lines rated at 230 kV and lower.
Calculated radio and TV interference levels in fair weather and in rain are
extremely low at the edge of the right-of-way for a 230 kV transmission line.
§ Gap discharges are different from corona. Gap discharges can
develop on power lines at any voltage. They can take place at tiny electrical
separations (gaps) that can develop between mechanically connected metal parts.
A small electric spark discharges across the gap and can create unwanted
electrical noise. The severity of gap discharge interference depends on the
strength and quality of the transmitted radio or TV signal, the quality of the
radio or TV set and antenna system, and the distance between the receiver and
power line. (The large majority of interference complaints are found to be
attributable to sources other than power lines: poor signal quality, poor
antenna, door bells, and appliances such as heating pads, sewing machines,
freezers, ignition systems, aquarium thermostats, fluorescent lights, etc.).
§ Gap discharges can occur on broken or poorly fitting line
hardware, such as insulators, clamps, or brackets. In addition, tiny electrical
arcs can develop on the surface of dirty or contaminated insulators, but this
interference source is less significant than gap discharge.
§ Hardware is designed to be problem-free, but corrosion, wind
motion, gunshot damage, and insufficient maintenance contribute to gap
formation. Generally, interference due to gap discharges is less frequent for
high-voltage transmission lines than lower-voltage lines. The reasons that
transmission lines have fewer problems include: predominate use of steel
structures, fewer structures, greater mechanical load on hardware, and
different design and maintenance standards.
§ Gap discharge interference can be avoided or minimized by
proper design of the transmission line hardware parts, use of electrical bonding
where necessary, and by careful tightening of fastenings during construction.
Individual sources of gap discharge noise can be readily located and corrected.
Arcing on contaminated insulators can be prevented by increasing the insulation
in high contamination areas and with periodic washing of insulator strings.
(3) Gaseous
Effluents
§ Corona activity in the air can
produce very tiny amounts of gaseous effluents: ozone and NOX. Ozone is a
naturally occurring part of the air, with typical rural ambient levels ranging
from about 10 to 30 parts per billion (ppb) at night and peaks at approximately
100 ppb. In urban areas, concentrations exceeding 100 ppb are common. After a
thunderstorm, the air may contain 50 to 150 ppb of ozone, and levels of several
hundred ppb have been recorded in large cities and in commercial airliners.
§ Ozone is also given off by welding equipment, copy machines,
air fresheners, and many household appliances. The National Ambient Air Quality
Standard for Oxidants (ozone is usually 90 to 95 percent of the oxidants in the
air) is 120 ppb, not to be exceeded as a peak concentration on more than one
day a year.
§ In general, the most sensitive ozone measurement
instrumentation can measure about 1 ppb. Typical calculated maximum concentrations
of ozone at ground level for 230 kV transmission lines during heavy rain are
far below levels that the most sensitive instruments can measure and thousands
of times less than ambient levels. Therefore, the proposed transmission lines
would not create any significant adverse effects in the ambient air quality of
the project area.
(4) Induced
Currents
§ Small
electric currents can be induced by electric fields in metallic objects close
to transmission lines. Metallic roofs, vehicles, vineyard trellises, and fences
are examples of objects that can develop a small electric charge in proximity
to high voltage transmission lines. Object characteristics, degree of
grounding, and electric field strength affect the amount of induced charge.
§ An electric current can flow when an object has an
induced charge and a path to ground is presented. The amount of current flow is
determined by the impedance of the object to ground and the voltage induced
between the object and ground.
§ The amount of induced current that can flow is important to
evaluate because of the potential for nuisance shocks to people and the
possibility of other effects such as fuel ignition.
§ The amount of induced current can be used to evaluate the
potential for harmful or other effects. As an example, when an average woman or
man grips an energized conductor, the threshold for perception of an electric
current is 0.73 milli ampere (mA) and 1.1 mA, respectively. If the current is
gradually increased beyond a person’s perception threshold, it becomes bothersome
and possibly startling.
§ However, before the current flows in a shock situation,
contact must be made, and in the process of establishing contact a small arc
occurs. This causes a withdrawal reaction that, in some cases, may be a hazard
if the involuntary nature of the reaction causes a fall or other accident.
§ The proposed 230 kV transmission lines will have the highest
electric field within the right-of-way, approximately 0.2 to 1.5 kV per meter
(kV/m), and approximately 0.1 to 0.9 kV/m at the edge of the right-of-way.
These fields are less than many other 230 kV transmission lines due to the use
of cross-phasing on the double-circuit lines and higher clearance above ground.
Induced currents have been calculated for common objects for a set of worst-case
theoretical assumptions: the object is perfectly insulated from ground, located
in the highest field, and touched by a perfectly grounded person. Even though
the maximum electric field only occurs on a small portion of the right-of-way,
and perfect insulation and grounding states are not always common, the
calculated induced current values are very low therefore, in most situations,
even in the highest field location, induced currents are below the threshold of
perception and are far below hazardous levels.
§ Agricultural operations can occur on or near a transmission
line right-of-way. Irrigation systems often incorporate long runs of metallic
pipes that can be subject to magnetic field induction when located parallel and
close to transmission lines. Because the irrigation pipes contact moist soil,
electric field induction is generally negligible, but annoying currents could
still be experienced from magnetic field coupling to the pipe. Pipe runs laid
at right angles to the transmission line will minimize magnetically induced
currents, although such a layout may not always be feasible. If there are
induction problems, they can be mitigated by grounding and/or insulating the
pipe runs. Operation of irrigation systems beneath transmission lines presents another
safety concern. If the system uses a high-pressure nozzle to project a stream
of water, the water may make contact with the energized transmission line
conductor. Generally, the water stream consists of solid and broken portions.
If the solid stream contacts an energized conductor, an electric current could
flow down the water stream to someone contacting the high-pressure nozzle.
Transmission line contact by the broken-up part of the water stream is unlikely
to present any hazard.
(5) Fuel
Ignition
§ If
a vehicle were to be refueled under a high-voltage transmission line, a
possible safety concern could be the potential for accidental fuel ignition.
The source of fuel ignition could be a spark discharge into fuel vapors
collected in the filling tube near the top of the gas tank.
§ The spark discharge would be due to current induced in a
vehicle (insulated from ground) by the electric field of the transmission line
and discharged to ground through a metallic refueling container held by a
well-grounded person. Theoretical calculations show that if a number of
unlikely conditions exist simultaneously, a spark could release enough energy
to ignite gasoline vapors. This could not occur if a vehicle were simply driven
or parked under a transmission line. Rather, several specific conditions would
need to be satisfied: A large gasoline-powered vehicle would have to be parked
in an electric field of about 5 kV/m or greater. A person would have to be
refueling the vehicle while standing on damp earth and while the vehicle is on
dry asphalt or gravel. The fuel vapors and air would have to mix in an optimum
proportion. Finally, the pouring spout must be metallic. The chances of having
all the conditions necessary for fuel ignition present at the same time are
extremely small.
§ Very large vehicles (necessary to collect larger amounts of
electric charge) are often diesel-powered, and diesel fuel is less volatile and
more difficult to ignite. The proposed 230 kV transmission
line electric field levels are too low (about 0.2-1.5 kV/m on the right-of-way)
for the minimum energy necessary for fuel ignition under any practical
circumstances.
(6) Cardiac
Pacemakers
§ One
area of concern related to the electric and magnetic fields of transmission
lines has been the possibility of interference with cardiac pacemakers. There
are two general types of pacemakers: asynchronous and synchronous. The
asynchronous pacemaker pulses at a predetermined rate. It is practically immune
to interference because it has no sensing circuitry and is not exceptionally
complex. The synchronous pacemaker, on the other hand, pulses only when its
sensing circuitry determines that pacing is necessary.
§ Interference resulting from the transmission line electric
or magnetic field can cause a spurious signal in the pacemaker’s sensing
circuitry. However, when these pacemakers detect a spurious signal, such as a
60 hertz (Hz) signal, they are programmed to revert to an asynchronous or fixed
pacing mode of operation and return to synchronous operation within a specified
time after the signal is no longer detected. The potential for pacer
interference depends on the manufacturer, model, and implantation method, among
other factors.
§ Studies have determined thresholds for interference of the
most sensitive units to be about 2,000 to 12,000 milli gauss (mG) for magnetic
fields and about 1.5 to 2.0 kV/m for electric fields. The electric and magnetic
fields at the right-of-way edge are below these values, and on the
right-of-way, only the lower bound electric field value of 1.5 kV/m is reached.
Therefore, the potential impact would not be significant.
(7) Computer
Interference
§ Personal computer monitors can be
susceptible to 60 Hz magnetic field interference. Magnetic field interference
results in disturbances to the image displayed on the monitor, often described
as screen distortion, “jitter,” or other visual defects. In most cases it is
annoying, and at its worst, it can prevent use of the monitor. Magnetic fields
occur in the normal operation of the electric power system.
§ This type of interference is a recognized problem by the
video monitor industry. As a result, there are manufacturers who specialize in
monitor interference solutions and shielding equipment. Possible solutions to
this problem include: relocation of the monitor, use of magnetic shield
enclosures, software programs, and replacement of cathode ray tube monitors
with liquid crystal displays that are not susceptible to 60 Hz magnetic field
interference. Because these solutions are widely available to computer users,
potential impacts would be less than significant
CORONA RING:
§ The ring, which surrounds the energized end of the
transformer bushing, serves two functions.
§ It is a corona ring that is intended to electrically shield
the bushing terminal and connections. It does so by reducing the voltage
gradient to a level well below the ionizing gradient of the surrounding air at
the maximum transformer output voltage.
§ It’s also a grading ring, which helps electrically grade the
external voltage on the bushing from line to ground (at the bushing flange).
The bushing is likely a condenser bushing, which contains a capacitance-graded
core to grade the voltage radically from 100% at the central conductor to
ground at the flange and, axially from ground to the top and bottom ends of the
core.
§ Grounding the test transformer following a circuit breaker
test is necessary for safety but you are grounding the entire test circuit; not
just the corona ring. I suspect the corona ring just happens to be a convenient
attachment point for the hook on your ground stick.
§ Die cast are usually 380, sand and permanent mold 356 or
A356, and fabricated rings are usually made from 6061 thin wall tubing or pipe
that is formed and welded; with appropriate brackets and other mounting
provisions.
§ Corona grading ring should be designed to reduce the
critical dielectric voltage gradient (typ. 20 to 30 kVrms/cm) to prevent corona
effect, internal discharge and reduce E-field in live parts and fitting that
cause radio/ TV interference (RIV), audio noise and losses. Corona ring could
also help to smooth the voltage profile distributing the voltage more uniform
along the insulator preventing concentration of over stresses.
§ For porcelain post insulators, some manufacturer recommends
one corona ring and for 500 kV and above two rings. However, for composite
insulator the corona ring is recommended for 220/230 kV. Most equipment
manufacturer provide corona ring base on testing such surge arrester, switches,
CT’s/PTs, etc.
Difference between Arcing Horn Gap and Corona
Ring:
§ At transmission line voltage the arcing horns, when the
breaker is closed normally have nothing except corona from the tips and arc
marks, the instant the breaker begins to open an arc is established across the
gap between the arc horns, when the gap is long enough the arc breaks. The plan
is to keep the sliding contacts from getting arc metal removal so the contacts
maintain low resistance, arcing horns are sacrificial.
§ At switchgear voltage, there are arc chutes and usually
puffers to extinguish the arc during breaker opening, the arc chutes may be of
a sand-crystal cast material (like space shuttle heat tiles), asbestos layers,
and electrical insulating board to protect the works during an explosive event
when temperatures get hotter than the sun. There is specific NFPA training for
arc flash exposure.
§ Arcing horns are also commonly used to protect insulation
from impulse and other overvoltages. The horn gap (distance between arcing
horns) is set to ensure that flashover occurs across the gap rather than along
the insulation surface thereby protecting the insulation surface and preventing
arc termination and associated damage to the end terminals or line and ground
end hardware. They may also be used to connect a surge arrester to protect
transformers and other equipment from overvoltage surges (gapped arrester). A
gapped connection is one method of preventing line lockout in the event of
arrester failure
§ Corona rings are meant to distribute the electrical field
and neither the hardware protected or the corona ring should have corona, the
typical line voltage that corona rings are applied is 150KV and higher,
altitude or high temperatures can reduce the voltage to 138KV lines. Properly
designed corona rings do not have corona.
§ Corona can appear to start and stop at essentially the same
voltage, there are other variables. Corona produces light (from UV thru visible
and into the infrared), sound (thru all wavelengths), ozone, and nitric acid
(in the presence of moisture).
§ Arcing arrestors were used long ago, some of the old-old
transmission lines. They were opposing arcing fingers mounted in parallel with
the insulators; the gap determined the flash-over voltage. The intent was to protect
insulators from lightening surges. I don’t know if those old lines are
energized anymore. You don’t see arcing fingers on modern (post WWII war)
transmission lines.
§ To break an arc the voltage must be decreased below about
60% of the voltage an arc starts at, thus if a transmission line insulator
arcing arrestor flashes over and maintains an arc the line is going to be
shutdown. Thus arcing arrestors (without an arc extinguishing capability)
decrease the reliability of a transmission line.
What’s The Fuss?
§ Corona from conductors and hardware may cause audible noise
and radio noise
§ Audible noise from conductors may violate noise standards
§ Radio noise from conductors may interfere with
communications or navigation
§ Corona loss may be significant when compared with resistive
loss of conductors
§ Corona can cause possible damage to polymeric insulators
Electro Magnetic Inductions:
§ EM1
field or radio noise field from high-voltage transmission lines are caused by
corona, which is essentially due to the electrical breakdown of the air
surrounding the conductors at higher voltage.
§ When the conductor surface electric field exceeds the
corona onset electric field, a partial electrical breakdown occurs in the
surrounding air medium near the conductor surface and is called the corona
discharge. The increase of conductor surface gradient takes place with increase
of supply voltage. In addition, organic contamination or attachment -of water
droplets also may contribute to localized field enhancement.
§ When organic particles or water droplets are attached to the
conductor surface, the charge accumulation at that point increases which
enhances the local electric field. The intensification of surface gradient
locally leads to the corona discharge.
§ The streamer generated during corona discharge, transports
electric charge into the surrounding air during the discharge cycle. These
moving charges contribute directly to the noise fields. ‘They also cause
currents to be induced on the transmission line conductors. Since the charge is
moved by a time varying electric field, it is equivalent to a current pulse and
this When a communication line passes near the corridor of a HV or EHV
transmission line, if the frequency of the radiated EM signal due to corona
matches with that of the transmitted signal on the communication line, then the
communication signal may get distorted. To mitigate this effect, the
communication line should pass at a safe distance away from the transmission
line.
§ Hence there is a need to estimate the radiated EM1 signal in
dB at a given distance from the HV or EHV transmission line. In this paper,
radiated EM1 in dB is computed for a single conductor high voltage over
headline. This theoretical result is compared with the published experimental
results available in the literature. In the computational work, earth is
considered as an infinitely conducting ground.
Physical description of corona and
Electro Magnetic Induction:
§ When alternating supply voltage energizes the conductor, the
conductor surface electric field exceeds the corona on set electric field of
the conductor. The corona discharge occurs in both positive and negative half
cycle. So the corona is divided into positive and negative corona depending
upon the polarity of the supply voltage.
§ When the conductor is positive with respect
to ground, an electron avalanche moves rapidly into the conductor leaving the
heavy positive-ion charge cloud close to conductor, which drifts away.
§ The rapid movements of electrons and motion of positive ions
gives the steep front of the pulse, while the further drift of positive ions
will give slow tail of the corona pulse.
§ When conductor is negative with respect to the ground, an
electron avalanche moves away from the energized conductor and the positive
heavy ions move towards the conductor. Since the heavy positive ions are,
moving towards the higher electric field, their motion is very rapid which
gives rise to a much sharper pulse than the positive pulse. Due to rapid moment
of the electrons from the conductor surface, the electric field regains its
original value at conductor surface very quickly than in the case of positive
polarity. Thus the negative corona pulses are lower in amplitude and lower in
rise and fall times as compared to positive corona pulses. They have also
higher repetition rates than the positive pulse
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