What is Corona

5/25/2018 What is Corona

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What is Corona Effect?

March 23, 2011

One of the phenomena associated with all energized electrical devices, including high-voltagetransmission lines, is corona. The localized electric field near a conductor can e sufficientl!

concentrated to ionize air close to the conductors. This can result in a partial discharge ofelectrical energ! called a corona discharge, or corona.

What is Corona?

"lectric transmission lines can generate a small amount of sound energ! as a result of

corona.

#orona is a phenomenon associated with all transmission lines. $nder certain conditions,

the localized electric field near energized components and conductors can produce a tin!

electric discharge or corona that causes the surrounding air molecules to ionize, orundergo a slight localized change of electric charge.

$tilit! companies tr! to reduce the amount of corona ecause in addition to the low

levels of noise that result, corona is a power loss, and in e%treme cases, it can damage

s!stem components over time.

#orona occurs on all t!pes of transmission lines, ut it ecomes more noticeale at higher

voltages &3'( )* and higher+. $nder fair weather conditions, the audile noise from

corona is minor and rarel! noticed.

uring wet and humid conditions, water drops collect on the conductors and increase

corona activit!. $nder these conditions, a crac)ling or humming sound ma! e heard inthe immediate vicinit! of the line.

#orona results in a power loss. ower losses li)e corona result in operating inefficiencies

and increase the cost of service for all ratepa!ers a ma/or concern in transmission linedesign is the reduction of losses.

Source of Corona:

The amount of corona produced ! 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 aove sea level, the condition of the conductors andhardware, and the local weather conditions. ower flow does not affect the amount of

corona produced ! a transmission line.

The electric field gradient is greatest at the surface of the conductor. arge-diameter

conductors have lower electric field gradients at the conductor surface and, hence, lower

corona than smaller conductors, ever!thing else eing eual. The conductors chosen for

the #alumet to the line were selected to have large diameters and to utilize a twoconductor undle. This reduces the potential to create audile noise.


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rregularities &such as nic)s 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. imilarl!, foreigno/ects on the conductor surface, such as dust or insects, can cause irregularities on the

surface that are a source for corona.

#orona also increases at higher elevations where the densit! of the atmosphere is less

than at sea level. 4udile noise will var! with elevation. 4n increase in 1000 feet of

elevation will result in an increase in audile noise of appro%imatel! 1 d5 &4+. 4udile

noise at (000 feet in elevation will ( d5 &4+ higher than the same audile noise at sealevel, all other things eing eual. The new #alumet to #omanche 3'( )* doule circuit

line was modeled with an elevation of 6000 feet.

7aindrops, snow, fog, hoarfrost, and condensation accumulated on the conductor surface

are also sources of surface irregularities that can increase corona. uring fair weather, the

numer of these condensed water droplets or ice cr!stals is usuall! small and the coronaeffect is also small.

8owever, during wet weather, the numer of these sources increases &for instance due to

rain drops standing on the conductor+ and corona effects are therefore greater.

uring wet or foul weather conditions, the conductor will produce the greatest amount of

corona noise. 8owever, during heav! rain the noise generated ! the falling rain dropshitting the ground will t!picall! e greater than the noise generated ! corona and thus

will mas) the audile noise from the transmission line.

#orona produced on a transmission line can e reduced ! the design of the transmission

line and the selection of hardware and conductors used for the construction of the line.

9or instance the use of conductor hangers that have rounded rather than sharp edges and

no protruding olts with sharp edges will reduce corona. The conductors themselves can

e made with larger diameters and handled so that the! have smooth surfaces withoutnic)s or urrs or scrapes in the conductor strands. The transmission lines proposed here

are designed to reduce corona generation.

TYPES OF CORONA:

There are three t!pes of corona.

4 glow discharge occurs at a gradient of appro%imatel! 20 )* rms:cm. ;low discharge is

a light glow off sharp points that does not generate o/ectionale 7*:T* or cause an!

audile noise.

4t aout 2( )* rms:cm, negative polarit! hen viewed in the dar) it has a

concentrated stem that ranches and merges into a violet-colored, tree-li)e halo. The


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audile noise associated with plume corona is a rather intense snapping and hissing

sound. lume corona generates significant 7*:T*.

These oservations are ased on fair weather conditions. $nder wet conditions virtuall!

all energized electrodes will e in corona of one form or another.

Man! are under the impression that the dielectric strength of air is greater under dr!conditions. That is not true. n fact, the dielectric strength of air increases with increased

moisture up to the dew point when moisture egins to condense on the surface of

insulators and other components of the line.

Physical Parameters of Corona:

#orona is caused ! the ionization of the media &air+ surrounding the electrode

&conductor+

#orona onset is a function of voltage

#orona onset is a function of relative air densit!

#orona onset is a function of relative humidit!

1 Corona an! the Electric Fiel!

#orona is ?OT solel! a function of the "lectric 9ield

#orona is a function of the electric field on the surface of the electrode &conductor+

#orona is also a function of the radius of curvature of the electrode &conductor+

#orona is also a function of the rate of deca! of the electric field awa! from the electrode

&conductor+

9or the preceding reasons, selecting the conductor with the smallest electric field at its

surface is not correct.

" Corona an! the Relati#e Air $ensity

#orona has an inverse relationship with air densit!

tandard line designs that perform well at sea level, ma! have significant corona issues if

used on lines that are installed over mountainous areas

% Corona an! the &umi!ity

#orona has an inverse relationship with humidit! at power freuencies

9air weather corona is more prevalent in low humidit! environments

' Corona is $e(en!ent Surface Con!ition of the Con!uctors

#orona is enhanced ! irregularities on the conductor surface


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rregularities include@ dust, insects, urrs and scratches and water drops present on new

conductors

#orona will generall! e greater on new conductors and will decrease to a stead!-state

value over a period of appro%imatel! one !ear in-service

#orona is significantl! increased in foul weather.

What)s The Fuss?

#orona from conductors and hardware ma! cause audile noise and radio noise

4udile noise from conductors ma! violate noise standards

7adio noise from conductors ma! interfere with communications or navigation

#orona loss ma! e significant when compared with resistive loss of conductors

#orona can cause possile damage to pol!meric insulators

*etho!s to re!uce Corona $ischar+e Effect:

#orona can e avoided

1. ,y minimi-in+ the #olta+e stress an! electric fiel! +ra!ient: This is accomplished !

using utilizing good high voltage design practices, i.e., ma%imizing the distance etween

conductors that have large voltage differentials, using conductors with large radii, andavoiding parts that have sharp points or sharp edges.

2. Surface Treatments: #orona inception voltage can sometimes e increased ! using a

surface treatment, such as a semiconductor la!er, high voltage putt! or corona dope.3. &omo+enous .nsulators: $se a good, homogeneous insulator. *oid free solids, such as

properl! prepared silicone and epo%! potting materials wor) well.

'. .f you are limite! to usin+ air as your insulator/then !ou are left with geometr! as the

critical parameter. 9inall!, ensure that steps are ta)en to reduce or eliminate unwanted

voltage transients, which can cause corona to start.

(. 0sin+ ,un!le! Con!uctors: on our 3'( )* lines, we have installed multiple conductors

per phase. This is a common wa! of increasing the effective diameter of the conductor,which in turn results in less resistance, which in turn reduces losses.

6. Elimination of shar( (oints:electric charges tend to form on sharp points thereforewhen practicale we strive to eliminate sharp points on transmission line components.

A. 0sin+ Corona rin+s:On certain new 3'( )* structures, we are now installing coronarings. These rings have smooth round surfaces which are designed to distriute charge

across a wider area, there! reducing the electric field and the resulting corona

discharges.


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B. Whether:#orona phenomena much worse in foul weather, high altitude

C. Ne Con!uctor:?ew conductors can lead to poor corona performance for a while.

10. ,y increasin+ the s(acin+ 2eteen the con!uctors: #orona ischarge "ffect can e

reduced ! increasing the clearance spacing etween the phases of the transmission lines.

8owever increase in the phaseDs results in heavier metal supports. #ost and pacereuirement increases.

11. ,y increasin+ the !iameter of the con!uctor: iameter of the conductor can e

increased to reduce the corona discharge effect. 5! using hollow conductors corona

discharge effect can e improved.

Sources of Corona an! Arcin+ in Polymer .nsulators:

oose hardware #ontamination and surface trac)ing

Missing corona rings

amaged or incorrectl! installed corona ring

amaged end fittings or end fitting seal

"%posed internal rod due to@ #aronized internal rod ! internal discharges plit sheath

due to weathering

Electro *a+netic .n!uctions:

"M field or radio noise field from high-voltage transmission lines are caused ! corona,

which is essentiall! due to the electrical rea)down of the air surrounding the conductors

at higher voltage.

>hen the conductor surface electric field e%ceeds the corona onset electric field, a

partial electrical rea)down occurs in the surrounding air medium near the conductorsurface and is called the corona discharge. The increase of conductor surface gradient

ta)es place with increase of suppl! voltage. n addition, organic contamination or

attachment -of water droplets also ma! contriute to localized field enhancement.

>hen organic particles or water droplets are attached to the conductor surface, the chargeaccumulation at that point increases which enhances the local electric field. Theintensification of surface gradient locall! leads to the corona discharge.

The streamer generated during corona discharge, transports electric charge into the

surrounding air during the discharge c!cle. These moving charges contriute directl! tothe noise fields. EThe! also cause currents to e induced on the transmission line

conductors. ince the charge is moved ! a time var!ing electric field, it is euivalent to

a current pulse and this >hen a communication line passes near the corridor of a 8* or


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"8* transmission line, if the freuenc! of the radiated "M signal due to corona matches

with that of the transmitted signal on the communication line, then the communication

signal ma! get distorted. To mitigate this effect, the communication line should pass at asafe distance awa! from the transmission line.

8ence there is a need to estimate the radiated "M1 signal in d5 at a given distance from

the 8* or "8* transmission line. n this paper, radiated "M1 in d5 is computed for a

single conductor high voltage over headline. This theoretical result is compared with the

pulished e%perimental results availale in the literature. n the computational wor),earth is considered as an infinitel! conducting ground.

Physical !escri(tion of corona an! Electro *a+netic .n!uction:

>hen alternating suppl! voltage energizes the conductor, the conductor surface electric

field e%ceeds the corona on set electric field of the conductor. The corona discharge

occurs in oth positive and negative half c!cle. o the corona is divided into positive andnegative corona depending upon the polarit! of the suppl! voltage.

>hen the conductor ispositive with respect to ground, an electron avalanche moves

rapidl! into the conductor leaving the heav! positive-ion charge cloud close to conductor,which drifts awa!.

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.

>hen conductor is negative with respect to the ground, an electron avalanche moves

awa! from the energized conductor and the positive heav! ions move towards theconductor. ince the heav! positive ions are, moving towards the higher electric field,

their motion is ver! rapid which gives rise to a much sharper pulse than the positive

pulse. ue to rapid moment of the electrons from the conductor surface, the electric fieldregains its original value at conductor surface ver! uic)l! than in the case of positivepolarit!. Thus the negative corona pulses are lower in amplitude and lower in rise and fall

times as compared to positive corona pulses. The! have also higher repetition rates than

the positive pulse

Corona $etection:

ight $ltraviolet radiation@ #orona can e visile in the form of light, t!picall! a purple

glow, as corona generall! consists of micro arcs. ar)ening the environment can help to

visualize the corona.

ound &hissing, or crac)ing as caused ! e%plosive gas e%pansions+@ Fou can often hearcorona hissing or crac)ing ound.

n addition, !ou can sometimes smell the presence of ozone that was produced ! the

corona.

alts, sometimes seen as white powder deposits on #onductor.

Mechanical erosion of surfaces ! ion omardment


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8eat &although generall! ver! little, and primaril! in the insulator+

#aron deposits, there! creating a path for severe arcing

The corona discharges in insulation s!stems result in voltage transients. These pulses are

superimposed on the applied voltage and ma! e detected, which is precisel! what corona

detection euipment loo)s for. n its most asic form, the following diagram is a corona&or partial discharge+ measuring s!stem@

t is important that the voltage source and the coupling capacitor e%hiit low noise so as

not to oscure the corona. n its simplest form the pulse detection networ) is a resistor

monitored ! an oscilloscope. onDt dismiss this simple techniue as crude, as we once

used this method to oserve the presence of corona in an improperl! terminated high

voltage connector, even after a dedicated corona tester failed to find an!. #ommerciall!availale corona detectors include electronic t!pes &as aove+ as well as ultrasonic t!pes.

Corona Calculations

The following corona calculations are from ielectric henomena in 8igh *oltage

"ngineering

1 For Concentric Cylin!ers in Air:

#orona will not form when 7O : 7 G 2.A1B. &4rcing will occur instead when the voltage

is too high.+

" For Parallel Wires in Air:

#orona will not form when H : r G (.B(. &4rcing will occur instead when the voltage istoo high.+

% For E3ual S(heres in Air:

#orona will not form when H : 7 G 2.0'. &4rcing will occur instead when the voltage is

too high.+

4rcing difficult to avoid when H : 7 G B

>here

7O I 7adius of outer concentric sphere 7 I 7adius of inner concentric sphere

7 I phere radius

r I wire radius

H I istance etween wires or etween spheres


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Effects of Corona:

415 Au!i2le Noise

uring corona activit!, transmission lines &primaril! those rated at 3'( )* and aove+

can generate a small amount of sound energ!. This audile noise can increase during foulweather conditions. >ater drops ma! collect on the surface of the conductors and

increase corona activit! so that a crac)ling or humming sound ma! e heard near a

transmission line. Transmission line audile noise is measured in deciels using a specialweighting scale, the


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8ardware is designed to e prolem-free, ut corrosion, wind motion, gunshot damage,

and insufficient maintenance contriute to gap formation. ;enerall!, interference due to

gap discharges is less freuent for high-voltage transmission lines than lower-voltagelines. The reasons that transmission lines have fewer prolems include@ predominate use

of steel structures, fewer structures, greater mechanical load on hardware, and different

design and maintenance standards.

;ap discharge interference can e avoided or minimized ! proper design of the

transmission line hardware parts, use of electrical onding where necessar!, and !

careful tightening of fastenings during construction. ndividual sources of gap dischargenoise can e readil! located and corrected. 4rcing on contaminated insulators can e

prevented ! increasing the insulation in high contamination areas and with periodic

washing of insulator strings.

4%5 6aseous Effluents

#orona activit! in the air can produce ver! tin! amounts of gaseous effluents@ ozone and

?OH. Ozone is a naturall! occurring part of the air, with t!pical rural amient levels

ranging from aout 10 to 30 parts per illion &pp+ at night and pea)s at appro%imatel!100 pp. n uran areas, concentrations e%ceeding 100 pp are common. 4fter a

thunderstorm, the air ma! contain (0 to 1(0 pp of ozone, and levels of several hundred

pp have een recorded in large cities and in commercial airliners.

Ozone is also given off ! welding euipment, cop! machines, air fresheners, and man!

household appliances. The ?ational 4mient 4ir Jualit! tandard for O%idants &ozone is

usuall! C0 to C( percent of the o%idants in the air+ is 120 pp, not to e e%ceeded as a

pea) concentration on more than one da! a !ear.

n general, the most sensitive ozone measurement instrumentation can measure aout 1

pp. T!pical calculated ma%imum concentrations of ozone at ground level for 230 )*transmission lines during heav! rain are far elow levels that the most sensitiveinstruments can measure and thousands of times less than amient levels. Therefore, the

proposed transmission lines would not create an! significant adverse effects in the

amient air ualit! of the pro/ect area.

4'5 .n!uce! Currents

mall electric currents can e induced ! electric fields in metallic o/ects close to

transmission lines. Metallic roofs, vehicles, vine!ard trellises, and fences are e%amples of

o/ects that can develop a small electric charge in pro%imit! to high voltage transmission

lines. O/ect characteristics, degree of grounding, and electric field strength affect theamount of induced charge.

4n electric current can flow when an o/ect has an induced charge and a path to ground

is presented. The amount of current flow is determined ! the impedance of the o/ect toground and the voltage induced etween the o/ect and ground.


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The amount of induced current that can flow is important to evaluate ecause of the

potential for nuisance shoc)s to people and the possiilit! of other effects such as fuel

ignition.

The amount of induced current can e used to evaluate the potential for harmful or other

effects. 4s an e%ample, when an average woman or man grips an energized conductor,

the threshold for perception of an electric current is 0.A3 milli ampere &m4+ and 1.1 m4,respectivel!. f the current is graduall! increased e!ond a personDs perception threshold,

it ecomes othersome and possil! startling.

8owever, efore the current flows in a shoc) situation, contact must e made, and in the

process of estalishing contact a small arc occurs. This causes a withdrawal reaction that,

in some cases, ma! e a hazard if the involuntar! nature of the reaction causes a fall orother accident.

The proposed 230 )* transmission lines will have the highest electric field within the

right-of-wa!, appro%imatel! 0.2 to 1.( )* per meter &)*:m+, and appro%imatel! 0.1 to

0.C )*:m at the edge of the right-of-wa!. These fields are less than man! other 230 )*transmission lines due to the use of cross-phasing on the doule-circuit lines and higher

clearance aove ground. nduced currents have een calculated for common o/ects for aset of worst-case theoretical assumptions@ the o/ect is perfectl! insulated from ground,

located in the highest field, and touched ! a perfectl! grounded person. "ven though the

ma%imum electric field onl! occurs on a small portion of the right-of-wa!, and perfectinsulation and grounding states are not alwa!s common, the calculated induced current

values are ver! low therefore, in most situations, even in the highest field location,

induced currents are elow the threshold of perception and are far elow hazardous

levels.

4gricultural operations can occur on or near a transmission line right-of-wa!. rrigations!stems often incorporate long runs of metallic pipes that can e su/ect to magnetic fieldinduction when located parallel and close to transmission lines. 5ecause the irrigation

pipes contact moist soil, electric field induction is generall! negligile, ut anno!ing

currents could still e e%perienced from magnetic field coupling to the pipe. ipe runslaid at right angles to the transmission line will minimize magneticall! induced currents,

although such a la!out ma! not alwa!s e feasile. f there are induction prolems, the!

can e mitigated ! grounding and:or insulating the pipe runs. Operation of irrigations!stems eneath transmission lines presents another safet! concern. f the s!stem uses a

high-pressure nozzle to pro/ect a stream of water, the water ma! ma)e contact with the

energized transmission line conductor. ;enerall!, the water stream consists of solid and

ro)en portions. f the solid stream contacts an energized conductor, an electric currentcould flow down the water stream to someone contacting the high-pressure nozzle.

Transmission line contact ! the ro)en-up part of the water stream is unli)el! to present

an! hazard.

475 Fuel .+nition


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f a vehicle were to e refueled under a high-voltage transmission line, a possile safet!

concern could e the potential for accidental fuel ignition. The source of fuel ignition

could e a spar) discharge into fuel vapors collected in the filling tue near the top of thegas tan).

The spar) discharge would e due to current induced in a vehicle &insulated from ground+

! the electric field of the transmission line and discharged to ground through a metallicrefueling container held ! a well-grounded person. Theoretical calculations show that ifa numer of unli)el! conditions e%ist simultaneousl!, a spar) could release enough

energ! to ignite gasoline vapors. This could not occur if a vehicle were simpl! driven or

par)ed under a transmission line. 7ather, several specific conditions would need to esatisfied@ 4 large gasoline-powered vehicle would have to e par)ed in an electric field of

aout ( )*:m or greater. 4 person would have to e refueling the vehicle while standing

on damp earth and while the vehicle is on dr! asphalt or gravel. The fuel vapors and airwould have to mi% in an optimum proportion. 9inall!, the pouring spout must e metallic.

The chances of having all the conditions necessar! for fuel ignition present at the same

time are e%tremel! small.

*er! large vehicles &necessar! 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 )* transmission line electric field levels are too low &aout 0.2-1.( )*:m on theright-of-wa!+ for the minimum energ! necessar! for fuel ignition under an! practical

circumstances.

485 Car!iac Pacema9ers

One area of concern related to the electric and magnetic fields of transmission lines has

een the possiilit! of interference with cardiac pacema)ers. There are two general t!pesof pacema)ers@ as!nchronous and s!nchronous. The as!nchronous pacema)er pulses at a

predetermined rate. t is practicall! immune to interference ecause it has no sensing

circuitr! and is not e%ceptionall! comple%. The s!nchronous pacema)er, on the otherhand, pulses onl! when its sensing circuitr! determines that pacing is necessar!.

nterference resulting from the transmission line electric or magnetic field can cause a

spurious signal in the pacema)erDs sensing circuitr!. 8owever, when these pacema)ersdetect a spurious signal, such as a 60 hertz &8z+ signal, the! are programmed to revert to

an as!nchronous or fi%ed pacing mode of operation and return to s!nchronous operation

within a specified time after the signal is no longer detected. The potential for pacerinterference depends on the manufacturer, model, and implantation method, among other

factors.

tudies have determined thresholds for interference of the most sensitive units to eaout 2,000 to 12,000 milli gauss &m;+ for magnetic fields and aout 1.( to 2.0 )*:m for

electric fields. The electric and magnetic fields at the right-of-wa! edge are elow these

values, and on the right-of-wa!, onl! the lower ound electric field value of 1.( )*:m is

reached. Therefore, the potential impact would not e significant.

45 Com(uter .nterference


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ersonal computer monitors can e susceptile to 60 8z magnetic field interference.

Magnetic field interference results in disturances to the image displa!ed on the monitor,

often descried as screen distortion,


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4t transmission line voltage the arcing horns, when the rea)er is closed normall! have

nothing e%cept corona from the tips and arc mar)s, the instant the rea)er egins to open

an arc is estalished across the gap etween the arc horns, when the gap is long enoughthe arc rea)s. The plan is to )eep the sliding contacts from getting arc metal removal so

the contacts maintain low resistance, arcing horns are sacrificial.

4t switchgear voltage, there are arc chutes and usuall! puffers to e%tinguish the arcduring rea)er opening, the arc chutes ma! e of a sand-cr!stal cast material &li)e spaceshuttle heat tiles+, asestos la!ers, and electrical insulating oard to protect the wor)s

during an e%plosive event when temperatures get hotter than the sun. There is specific

?94 training for arc flash e%posure.

4rcing horns are also commonl! used to protect insulation from impulse and other

overvoltages. The horn gap &distance etween arcing horns+ is set to ensure that flashoveroccurs across the gap rather than along the insulation surface there! protecting the

insulation surface and preventing arc termination and associated damage to the end

terminals or line and ground end hardware. The! ma! also e used to connect a surgearrester to protect transformers and other euipment from overvoltage surges &gapped

arrester+. 4 gapped connection is one method of preventing line loc)out in the event of

arrester failure

#orona rings are meant to distriute the electrical field and neither the hardware

protected or the corona ring should have corona, the t!pical line voltage that corona rings

are applied is 1(0L* and higher, altitude or high temperatures can reduce the voltage to

13BL* lines. roperl! designed corona rings do not have corona.

#orona can appear to start and stop at essentiall! the same voltage, there are other

variales. #orona produces light &from $* thru visile and into the infrared+, sound &thruall wavelengths+, ozone, and nitric acid &in the presence of moisture+.

4rcing arrestors were used long ago, some of the old-old transmission lines. The! were

opposing arcing fingers mounted in parallel with the insulators the gap determined theflash-over voltage. The intent was to protect insulators from lightening surges. donDt

)now if those old lines are energized an!more. Fou donDt see arcing fingers on modern

&post >> war+ transmission lines.

To rea) an arc the voltage must e decreased elow aout 60K of the voltage an arc

starts at, thus if a transmission line insulator arcing arrestor flashes over and maintains anarc the line is going to e shutdown. Thus arcing arrestors &without an arc e%tinguishing

capailit!+ decrease the reliailit! of a transmission line.

What is Corona

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