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What is Corona? A Clearly Explained and Illustrated Story About Three Types of Corona Discharge and Their Relationship to Radio Interference NOTE: Because Hubbell has a policy of continuous product improvement, we reserve the right to change design and specifications without notice. ® POWER SYSTEMS, INC. Tel 573-682-5521 ™ Fax 573-682-8714 ® http://www.hubbellpowersystems.com ® ® ® UNITED STATES ã 210 N. Allen ã Centralia, MO 65240 ã Phone: 573-682-5521 ã Fax: 573-682-8714 ã e-mail: hpsl

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  What is Corona? A Clearly Explained and Illustrated Story About Three Types ofCorona Discharge and Their Relationship to Radio Interference NOTE: Because Hubbell has a policy of continuous product improvement, we reserve the right to change design and specifications without notice.  ®  POWER SYSTEMS, INC. UNITED STATES ã210 N. Allen ãCentralia, MO 65240 ãPhone: 573-682-5521 ãFax: 573-682-8714ãe-mail: [email protected] CANADA ã870 Brock Road SouthãPickering, Ontario L1W 1Z8ãPhone: 905-839-1138ãFax: 905-831-6353ãe-mail: [email protected] MEXICO ãAv. Coyoacan No. 1051ãCol. Del Valleã03100 Mexico, D.F.ãPhone: 52-55-9151-9999ãFax: 52-55-9151-9988ãe-mail: [email protected]  ®  ®  ®  ® ™ Tel 573-682-5521 Fax 573-682-8714 http://www.hubbellpowersystems.com Bulletin EU1234-H @2004 Hubbell Power Systems, Inc.   – 2 –  What Is Corona? During the past several years, corona and radioinfluence (RI) problems associated with extra-high- voltage transmission have been studied extensivelyin the Ohio Brass Voltage Laboratory.The word “corona”, as well as the definitiveexpressions “plume discharge”, “brush discharge”and “glow discharge” have been used extensively inconnection with this work, under the tacit assump-tion that everyone is familiar with their meanings.It is the purpose of this article to explain themeanings of these expressions as used in suchstudies and to describe some of the physical andelectrical properties associated with them. Definition of Corona Perhaps the most general definition of coronapossible is as follows: “Corona is a discharge causedby electrical overstress.” While this definition is very general, and applies to practically all kinds of corona, it is still unsatisfactory since it introducestwo new expressions, “electrical overstress” and“discharge,” which, although they describe the“cause” and “effect”, must in turn be defined for various cases. Actually, corona can appear in solid, liquid orgaseous insulating materials, and its occurrencetherein is usually associated with the initial phasesof electrical failure of the insulation. In solids, theoccurrence of corona generally results in deteriora-tion of the material, while in liquids and gases,removal of the electrical overstress eliminates thedischarge, and the material generally recovers itssrcinal insulating properties.For transmission line studies, the insulating material in which the discharge occurs is the airadjacent to conductor or insulator surfaces, whenthe electrical stress at these surfaces exceeds thecritical value.Limiting corona to transmission line conductorsleads to the American Standards Association defi-nition of corona, which reads as follows:“Corona is a luminous discharge due to ionizationof the air surrounding a conductor around whichexists a voltage gradient exceeding a certain criti-cal value.”This definition obviously contains the same ba-sic idea given in the above general definition, buthas a rather limited scope. For the purpose of thisarticle the scope of this definition will be enlargedto include corona on line hardware and insulatorsat high voltages. Nature of Corona The corona discharges observed at the surface of a conductor are due to the formation of electronavalanches which occur when the intensity of theelectric field at the conductor surface exceeds acertain critical value.There are always a few free electrons in the airas a result of traces of radioactive materials in theearth’s crust and cosmic ray bombardment of theearth from outer space. As the conductor becomesenergized on each half cycle of the AC voltage wave,the electrons in the air near its surface are acceler-ated by the electrostatic field. These electrons,having an inherent negative charge, are acceler-ated toward the conductor on its positive half cycleand away from the conductor on its negative half cycle.The velocity attained by a free electron is depen-dent upon the intensity of the electric field. If theintensity of the electric field is not too great, thecollision between an electron and an air moleculesuch as oxygen (O 2 ) or nitrogen (N 2 ) is elastic; thatis, the electron bounces off the air molecule with notransfer of energy to it. On the other hand, if theintensity of the electric field exceeds a certaincritical value, any free electron in this field willacquire a sufficient velocity so that its collisionwith the air molecule is inelastic; that is, theelectron has sufficient energy to knock one of theouter orbit electrons clear out of one of the twoatoms of the air molecule. This is the phenomenonknown as ionization, and the molecule with themissing electron becomes a positive ion.The initial electron, which lost most of its veloc-ity in the collision, and the electron knocked out of the air molecule, which also has a low velocity, areboth accelerated by the electric field, and at thenext collision, each electron is capable of ionizing an air molecule. After the second collision, thereare now four electrons to proceed, and so on, thenumber of electrons doubling after each collision. All this time, the electrons are all advancing to-ward the positive electrode and after many colli-sions, their number has grown enormously. This isthe process by which the so-called electron ava-lanche is built up, each avalanche being initiatedby a single free electron which finds itself in anintense electrostatic field. COVER PICTURE  Brush and plume corona discharge on a two-conduc- tor horizontal bundle of Drake ACSR Conductors spaced 16 inches apart at a test voltage of 240 kV line-to-ground.   – 3 –  The intensity of the electrostatic field around aconductor is non-uniform. It has its maximumstrength at the surface of the conductor and itsintensity diminishes inversely as the distance fromthe center of the conductor. Hence, as the voltagelevel in the conductor is raised, the critical fieldstrength is approached and the initial dischargesoccur only at or very near to the conductor surface.For the positive half cycle, the electron avalanchesmove toward the conductor and continue to growuntil they hit the surface. For the negative half cycle, the electron avalanches move away from theconductor surfaces toward a weaker field and ceaseto advance when the field becomes too weak toaccelerate the electrons to ionizing velocity.The positive ions left in the wake of the electronavalanche move toward the negative electrode.However, they move very slowly because of theirmass, which for air molecules is in the order of 50,000 times the mass of the electron. Having apositive charge, these ions attract wandering elec-trons, and whenever one succeeds in capturing afree electron, it becomes a neutral air moleculeagain. The energy level of a neutral molecule is lessthan that of the corresponding positive ion, andhence when a free electron is captured, a quanta or“chunk” of energy is emitted from the molecule.This quanta of energy is exactly equal in magni-tude to the energy which initially was required toknock the electron out of the molecule in the firstplace. It is radiated as an electro-magnetic wave,and for air molecules such as oxygen or nitrogen,this radiation is in the visible light range. Hence,an observer can see this radiation as a soft violet-colored light, which comes principally from therecombination of nitrogen ions with free electrons. The Manifestations of Corona The discharges which are produced by electronavalanches may be observed in the laboratory inthree different ways.Perhaps the best known manifestation is “visualcorona” which appears as a violet colored lightcoming from the regions of electrical overstresswhen the test specimen is viewed in the dark. Asdescribed above, this light is produced by the re-  A 4-conductor bundle of one-inch diameter smooth tubes,spaced 20 inches apart, used for testing.Corona discharges on bundled conductors at 400 kV line-to-ground.   – 4 –  combination of positive nitrogen ions with freeelectrons.The second manifestation of this discharge is“audible corona”, which appears as a hissing orfrying sound whenever the specimen is energizedabove the corona threshold voltage. The soundwaves are produced by the disturbances set up inair in the vicinity of the discharge, possibly by themovement of the positive ions as they are suddenlycreated in an intense electric field.The third, and perhaps most serious manifesta-tion of this discharge from the point of view of thepower company, is the electrical effect which causesradio influence or RI. These avalanches, being electrons in motion, actually constitute electriccurrents, and as such, produce both magnetic andelectrostatic fields in the vicinity. Being formed very suddenly and being of short duration, thesemagnetic and electrostatic fields can induce high-frequency voltage pulses in nearby radio antennas,and hence may cause RI. These electrical distur-bances are usually measured in the laboratorywith a radio noise meter which is closely coupled tothe test specimen by means of the standard NEMA circuit. This laboratory set-up measures the gener-ated radio noise, and is usually called the “radioinfluence voltage” or RIV of the test specimen. The Aspects of Corona The three different types or degrees of coronadischarges which are recognized on EHV test speci-mens in the laboratory are called “plume discharge,”“brush discharge” and “glow discharge.”The plume discharge is the most spectacular of the three, and is so named because of its generalresemblance to a plume. When viewed in the dark,it has a concentrated stem which may be anywherefrom a fraction of an inch long to several inches inlength, depending upon the voltage level of theconductor. At its outer end, the stem branchesmany times and merges into a violet-colored tree-like halo which may range in length from a fewinches at lower voltages to a foot or more at veryhigh voltages. The audible manifestation associ-ated with plume discharges is generally a ratherintense snapping and hissing sound, readily recog-nized by the experienced corona observer.The brush discharge is a streamer projecting radially from the conductor surface. These dis-charges generally occur all around the periphery of the conductor. The length of these discharges may vary from a small fraction of an inch at low voltagesto one or two inches at higher voltages. The nameis suggested by the resemblance which the dis-charges have to the bristles of a round bottle brush.The audible manifestation associated with brushdischarges is generally a continuous backgroundtype of hissing or frying sound.The glow discharge is a very faint, weak lightwhich appears to hug the conductor surface anddoes not project there from as does a brush dis-charge. It also may appear on critical regions of insulator surfaces during high humidity condi-tions. There is generally no sound associated withglow discharges. Properties of Corona On a clean, dry, smooth conductor energized alittle above its critical voltage, only brush dis-charges occur, and these are generally limited tothe negative half cycle. Fig. 1 shows the coronawhich occurs on a smooth conductor when photo-graphed under various conditions. Fig. 1-A showsthe conductor as viewed by ordinary means. Fig. 1-B shows it on the negative half cycle only, while Fig.1-C shows how it looks on the positive half cycleonly. These last two photographs were taken witha rotating disk located in front of the camera lens.This disk, which had two 90-degree open segmentsand two 90-degree closed segments, was rotated at1800 rpm with a synchronous motor. The disk wasadjusted on the motor shaft so that the cameracould “see” the conductor only one half the time,corresponding to either pre-selected half cycle of the 60-cycle voltage wave applied to the conductor.  A. Both half cycles B. Negative half cycleC. Positive half cycle Fig. 1. Corona discharges on a one-inch diameter clean,dry, smooth conductor energized at 200 kV line-to-ground.