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What is Corona Effect

MARCH 23, 2011 22 COMMENTS(HTTP://ELECTRICALNOTES.WORDPRESS.COM/2011/03/23/WHAT-IS-CORONA-EFFECT/#COMMENTS)

Introduction:

One of the phenomena associated with all energized electrical devices, including high-voltage

transmission lines, is corona. The localized electric field near a conductor can be sufficientlyconcentrated to ionize air close to the conductors. This can result in a partial discharge of electrical

energy called a corona discharge, or corona.

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, thelocalized electric field near energized components and conductors can produce a tiny electricdischarge 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 noisethat 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 andrarely noticed.

During wet and humid conditions, water drops collect on the conductors and increase coronaactivity. Under these conditions, a crackling or humming sound may be heard in the immediate

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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 thereduction of losses.

Source of Corona:

The amount of corona produced by a transmission line is a function of the voltage of the line, thediameter of the conductors, the locations of the conductors in relation to each other, the elevation ofthe 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 conductorshave 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 wereselected 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 suspensionhardware) concentrate the electric field at these locations and thus increase the electric field gradientand the resulting corona at these spots. Similarly, foreign objects on the conductor surface, such asdust 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 anincrease 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 toComanche 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 alsosources of surface irregularities that can increase corona. During fair weather, the number of thesecondensed 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 dropsstanding on the conductor) and corona effects are therefore greater.During wet or foul weather conditions, the conductor will produce the greatest amount of coronanoise. However, during heavy rain the noise generated by the falling rain drops hitting the groundwill typically be greater than the noise generated by corona and thus will mask the audible noisefrom the transmission line.

Corona produced on a transmission line can be reduced by the design of the transmission line andthe selection of hardware and conductors used for the construction of the line. For instance the useof conductor hangers that have rounded rather than sharp edges and no protruding bolts withsharp edges will reduce corona. The conductors themselves can be made with larger diameters andhandled so that they have smooth surfaces without nicks or burrs or scrapes in the conductorstrands. 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 theappearance is similar to the round ends of a bottle brush. The audible noise associated with brushcorona 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 namedbecause of its general resemblance to a plume. When viewed in the dark it has a concentrated stemthat 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 significantRIV/TVI.These observations are based on fair weather conditions. Under wet conditions virtually allenergized 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 theline.

Physical Parameters of Corona:

Corona is caused by the ionization of the media (air) surrounding the electrode (conductor)Corona onset is a function of voltageCorona onset is a function of relative air densityCorona 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 onlines 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 surfaceIrregularities 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-serviceCorona is significantly increased in foul weather.

What’s The Fuss?

Corona from conductors and hardware may cause audible noise and radio noiseAudible noise from conductors may violate noise standards

Radio noise from conductors may interfere with communications or navigationCorona loss may be significant when compared with resistive loss of conductors

Corona can cause possible damage to polymeric insulators

Methods to reduce Corona Discharge Effect:

Corona can be avoided

1. By minimizing the voltage stress and electric field gradient.: This is accomplished byusing utilizing good high voltage design practices, i.e., maximizing the distance between

conductors that have large voltage differentials, using conductors with large radii, and avoidingparts that have sharp points or sharp edges.

2. Surface Treatments: Corona inception voltage can sometimes be increased by using a surfacetreatment, 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 inturn results in less resistance, which in turn reduces losses.

6. Elimination of sharp points: electric charges tend to form on sharp points; therefore whenpracticable 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 widerarea, thereby reducing the electric field and the resulting corona discharges.

8. Whether: Corona phenomena much worse in foul weather, high altitude9. 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 bereduced 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 beimproved.

Sources of Corona and Arcing in PolymerInsulators:

Loose hardwareContamination and surface tracking

Missing corona rings

Damaged or incorrectly installed corona ringDamaged end fittings or end fitting seal

Exposed internal rod due to: Carbonized internal rod by internal discharges Split sheath due toweathering

Electro Magnetic Inductions:

EM1 field or radio noise field from high-voltage transmission lines are caused by corona, which isessentially 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 electricalbreakdown 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 ofsurface gradient locally leads to the corona discharge.

The streamer generated during corona discharge, transports electric charge into the surrounding airduring 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 linepasses 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, thenthe 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 resultsavailable in the literature. In the computational work, earth is considered as an infinitely conducting

ground.

Physical description of corona and ElectroMagnetic Induction:

When alternating supply voltage energizes the conductor, the conductor surface electric fieldexceeds 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 coronadepending 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 heavypositive ions are, moving towards the higher electric field, their motion is very rapid which gives riseto 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 thanin the case of positive polarity. Thus the negative corona pulses are lower in amplitude and lower inrise and fall times as compared to positive corona pulses. They have also higher repetition rates thanthe positive pulse



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 thecorona.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 arcingThe 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 detectionequipment 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 toobscure the corona. In its simplest form the pulse detection network is a resistor monitored by anoscilloscope. Don’t dismiss this simple technique as crude, as we once used this method to observethe 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 (asabove) 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 sphereRI = Radius of inner concentric sphereR = Sphere radiusr = 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) cangenerate a small amount of sound energy. This audible noise can increase during foul weatherconditions. 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 lineaudible noise is measured in decibels using a special weighting scale, the “A” scale that responds todifferent 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 rainyweather 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 andrain.

(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 thehigher frequencies associated with TV signals).Corona interference to radio and television reception is usually not a design problem fortransmission 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 transmissionline.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 betweenmechanically 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 andquality 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 complaintsare 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, orbrackets. 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, andinsufficient maintenance contribute to gap formation. Generally, interference due to gap dischargesis 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, fewerstructures, 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 fasteningsduring 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 highcontamination 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 about10 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 incommercial airliners.Ozone is also given off by welding equipment, copy machines, air fresheners, and many householdappliances. 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 morethan 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 linesduring heavy rain are far below levels that the most sensitive instruments can measure andthousands of times less than ambient levels. Therefore, the proposed transmission lines would notcreate 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 transmissionlines. Metallic roofs, vehicles, vineyard trellises, and fences are examples of objects that can developa small electric charge in proximity to high voltage transmission lines. Object characteristics, degreeof 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 ispresented. The amount of current flow is determined by the impedance of the object to ground andthe voltage induced between the object and ground.

The amount of induced current that can flow is important to evaluate because of the potential fornuisance 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. Asan 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 isgradually increased beyond a person’s perception threshold, it becomes bothersome and possiblystartling.

However, before the current flows in a shock situation, contact must be made, and in the process ofestablishing contact a small arc occurs. This causes a withdrawal reaction that, in some cases, maybe 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 ofthe right-of-way. These fields are less than many other 230 kV transmission lines due to the use ofcross-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 isperfectly insulated from ground, located in the highest field, and touched by a perfectly groundedperson. 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 currentsare 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 whenlocated 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 frommagnetic 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. Ifthere 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 withthe energized transmission line conductor. Generally, the water stream consists of solid and brokenportions. If the solid stream contacts an energized conductor, an electric current could flow downthe 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 concerncould be the potential for accidental fuel ignition. The source of fuel ignition could be a sparkdischarge 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 theelectric field of the transmission line and discharged to ground through a metallic refuelingcontainer held by a well-grounded person. Theoretical calculations show that if a number ofunlikely 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 vehiclewould 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 spoutmust 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 lineelectric 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 thepossibility of interference with cardiac pacemakers. There are two general types of pacemakers:asynchronous and synchronous. The asynchronous pacemaker pulses at a predetermined rate. It ispractically 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 circuitrydetermines 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 spurioussignal, such as a 60 hertz (Hz) signal, they are programmed to revert to an asynchronous or fixedpacing 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 to12,000 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, onlythe lower bound electric field value of 1.5 kV/m is reached. Therefore, the potential impact wouldnot 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 screendistortion, “jitter,” or other visual defects. In most cases it is annoying, and at its worst, it canprevent use of the monitor. Magnetic fields occur in the normal operation of the electric powersystem.This type of interference is a recognized problem by the video monitor industry. As a result, thereare manufacturers who specialize in monitor interference solutions and shielding equipment.

Possible solutions to this problem include: relocation of the monitor, use of magnetic shieldenclosures, software programs, and replacement of cathode ray tube monitors with liquid crystaldisplays that are not susceptible to 60 Hz magnetic field interference. Because these solutions arewidely 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 doesso 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 lineto ground (at the bushing flange). The bushing is likely a condenser bushing, which contains acapacitance-graded core to grade the voltage radically from 100% at the central conductor toground 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 aregrounding 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 usuallymade from 6061 thin wall tubing or pipe that is formed and welded; with appropriate brackets andother mounting provisions.Corona grading ring should be designed to reduce the critical dielectric voltage gradient (typ. 20 to30 kVrms/cm) to prevent corona effect, internal discharge and reduce E-field in live parts and fittingthat 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 preventingconcentration of over stresses.For porcelain post insulators, some manufacturer recommends one corona ring and for 500 kV andabove 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 CoronaRing:

At transmission line voltage the arcing horns, when the breaker is closed normally have nothingexcept 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. Theplan is to keep the sliding contacts from getting arc metal removal so the contacts maintain lowresistance, arcing horns are sacrificial.At switchgear voltage, there are arc chutes and usually puffers to extinguish the arc during breakeropening, the arc chutes may be of a sand-crystal cast material (like space shuttle heat tiles), asbestoslayers, and electrical insulating board to protect the works during an explosive event whentemperatures 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 gaprather than along the insulation surface thereby protecting the insulation surface and preventingarc 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 equipmentfrom overvoltage surges (gapped arrester). A gapped connection is one method of preventing linelockout in the event of arrester failure

Corona rings are meant to distribute the electrical field and neither the hardware protected or thecorona ring should have corona, the typical line voltage that corona rings are applied is 150KV andhigher, altitude or high temperatures can reduce the voltage to 138KV lines. Properly designedcorona 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 opposingarcing 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 areenergized 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, thusif a transmission line insulator arcing arrestor flashes over and maintains an arc the line is going tobe shutdown. Thus arcing arrestors (without an arc extinguishing capability) decrease the reliability

of a transmission line.



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FILED UNDER UNCATEGORIZED

About Jignesh.ParmarJignesh Parmar has completed his B.E(Electrical) from Gujarat University. He has more than 11 yearsexperience in Power Transmission-Power Distribution-Electrical energy theft detection-ElectricalMaintenance-Electrical Projects(Planning-Designing-coordination-Execution). He is Presently associate

with one of the leading business group as a Assistant Manager at Ahmedabad,India. He is FreelancerProgrammer of Advance Excel and design useful Excel Sheets of Electrical Engineering as perIS,NEC,IEC,IEEE codes. He is technical Author for "Electrical Mirror" and "Electrical India"Magazines. He is Technical Blogger and Familiar with English, Hindi, Gujarati, French languages. Hewants to Share his experience & knowledge and help technical enthusiasts to find suitable solutions andupdating themselves on various Engineering Topics.

22 Responses to What is Corona Effect

Soukhito HONGVANTHONG says:April 22, 2011 at 2:46 amshoul have more picture for helf more understanding

Reply

Manuel Aguilar says:April 22, 2011 at 3:22 amwe have a query:

To reduce the RIV a stations post insulator 500 kV Corona rings are required?

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June 30, 2011 at 1:35 am

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Jignesh:

Regarding Radio-Interference values, commonly the suspension disk-type insulators are specifiedwith max RIV of 50 microvolts for test voltage of 10kV (one unit)I am facing a specification where the value to be met MUST be 1200 microvolts (near 60dB) at 30kV test voltage (one unit too)I am having a hard time to comply with such requirement. do you know any kind of radiotreatment that can be done on the bottom of tempered disk-type glass insulators, like

semiconductive coatings?

Replynaresh says:August 11, 2011 at 11:18 amhow much diameter of corona ring can be disigned for 245 isolator al.design

Reply

swaroop says:August 17, 2011 at 4:52 amsir,In already placed transmission lines if corona occurs.how can we reduce the corona at thatparticular instant.

Replyyahaya inuwa says:

October 25, 2011 at 5:24 pmThis Lesson is very helpful to me.

Replyraja says:May 30, 2012 at 2:13 pmDear Sir,

I want to know how voltage drop occurs and how to calculate voltage drop with example and howto reduce the voltage drop….

Replynandini asthana says:October 3, 2012 at 4:24 pmthe lesson is very helpful to me…….because this corona effect is easily undustood

Replywaddah says:October 21, 2012 at 4:19 pmgreat job

ReplyVASANTH rao says:

October 25, 2012 at 5:56 am

Is there any danger to health posed by living directly below or say within a radius of 100 metres










Is there any danger to health posed by living directly below or say within a radius of 100 metresfrom HV transmission line ? Similar to suspected dangers from radiations from cell phone towers .Can a lay person determine the voltage carried by the HV transmission lines approximately lookingat the number of insulators on top?

ReplyMike Marcum says:March 14, 2013 at 3:28 pm

Yes there is, but good luck proving power lines are responsible for cancer in a courtroom. Mostwould rather have cheaper electric bills than pay others’ medical/legal/funeral bills with highermonthly fees.

ReplyAfaq Ahmad says:December 2, 2012 at 5:19 pm

How corona does reduce the effects of transients produced by surges?

ReplyJ.E. says:January 11, 2013 at 9:05 amGreat article! i googled what is the corona effect and you had the perfect explanation. How wouldthis show itself in micro electronic circuitry?

ReplyOppong says:February 24, 2013 at 5:56 amWell comprehended

Reply

Bello Gafar says:February 26, 2013 at 1:16 pmSir. Please elaborate the comparison of corona effect between two countries by their weatherconditioning(Hot and cold/warm weather)

ReplyOlaniran Kayode says:

February 27, 2013 at 11:53 amComprehensive write-up

ReplyHasbi Basith Fitranda says:March 16, 2013 at 5:33 amThank You sir , sugoi desu

Replywalter says:April 12, 2013 at 8:47 pm


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this is suh a helpful information thank u very mch

Replyruel bautista says:July 22, 2013 at 4:00 pmwe are using composite insulators on our 400kv terminal ends and some are porceleine insulatorcoated with silicon paint on our terminal end is washing with water and soap when they are dirty

and have some contamination will minimized the corona discharge?

Replyrahul saini says:October 23, 2013 at 2:17 pmnice thankxxxxxxxxxxx

ReplyAsif says:January 4, 2014 at 12:05 pmThanks for this wonderfull lesson / knowledge.

ReplyPARTH says:

January 30, 2014 at 1:20 pmundergrond system corona produce ????

please detail this ans.

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What is Corona Effect _ Electrical Notes