IEEE Transactions on Electrical Insulation Vol. EI-20 No.5, October 1985 891

THE 1985 J. B. WHITEHEAD MEMORIAL LECTURE

CRITICAL ASSESSMENT OF THE ELECTRICAL BREAKDOWN

PROCESS IN DIELECTRIC FLUIDS

E. 0. ForsterExxon Research and Engineering Co,

Corporate Research Science LaboratoriesAnnandale, New Jersey

ABSTRACT

The phenomenon of electrical breakdown in insulatingliquids has been investigated extensively for decadesby many researchers all over the world. Severaltheories have been advanced, based on more or lesslimited experimental evidence. Also attempts weremade to apply to liquids concepts derived for thebreakdown of solids and gases. In the last decadethe advent of very high speed electrooptical toolshas made it possible to elucidate this process in muchmore detail than was previously possible. It hasbecome possible to show that at least three stagesare involved in the development of the ultimate break-down. In this paper a brief review of the earlywork will be presented. Then the more recent experi-mental results are summarized, and finally a compre-hensive model of the breakdown process is proposed.

EA5RLY WORKLewis' theory appeared to explain the experimental data

The early literature on electrical breakdown in the he had obtained under uniform field conditions usingcondensed phase invokes three primary processes to ex- electrode separations of 10 to 80 vim and dc stresses.plain the mechanism of breakdown. These are electron It was not clear from this paper how the hydrocarbonemission from the cathode, possibly via a Schottky or molecules would acquire sufficient energy for dissoci-Fowler-Nordheim process, interaction of the electrons ation, since the proposed vibrational processes hadwith the bulk of the dielectric resulting in energy associated with them less than 0.5 eV. Also there wastransfer to the latter as proposed by von Hippel, and great uncertainty of the mean free path over which anelectron multiplication by some ionization process electron could travel in a liquid to gain sufficientleading to Townsend avalanches and eventually to com- energy from the applied field. The use of relativelyplete breakdown. All these processes had been pro- narrow gaps, the assumption that the applied field wasposed earlier for gases and crystalline solids. Their the same as the local field, and the lack of informa-application to amorphous condensed matter represented, tion on the electron injection mechanisms appeared tohowever, certain problems. For example Lewis [1] be responsible for leading Lewis to incorrect conclu-tried to explain breakdown in saturated hydrocarbons sions. To overcome some of these difficulties, Watsonin this manner. He assumed that near breakdown elec- and Sharbaugh considered the formation of bubbles attron emission occurred from the cathode via a Schottky the cathode surface. These authors reasoned that atprocess. The injected electrons, drifting in the selected sites of the cathode surface such as asperi-liquid under the influence of the applied field, were ties, electron emission would occur, leading to highvisualized to lose their energy to the solvent mole- current flow. The Joule heating resulting from thiscules and thereby exciting vibrational modes of the current flow could provide the energy necessary tolatter. Collisions of such excited molecules were then vaporize the liquid in the immediate vicinity of theseable to cause ionization processes to take place. The injection sites in times as short as 1X10-6 s. Thepositive ions so produced were to lead to an insta- authors reasoned that once such a bubble was formed itbility such as an avalanche and hence to breakdown, could grow across the gap and form a spark channel.

00183-93567/85/1000-089l$0l .00 @3 19835 IEEE

892 IEEE Transactions on Electrical Insulation Vol. EI-20 No-5, October 1985

They attributed the high currents to field-enhanced Table 1thermionic emission. They calculated a field enhance-ment factor of 5 and postulated a metal work function Typical Cathode Initiated Streamer Growthof between 0.25 and 5 eV. The bubble model thus Rates in Liquid Hydrocarbonsavoided the need for an electron multiplication process.These conclusions were based on highly purified n-hexane as the test liquid, two parallel plate elec- Type Growth Rate in cm/strodes with rounded edges ranging in separation from60 to over 600 jm, using rectangular voltage pulses Primary (bush or tree) (<2.5)x104as well as dc conditions. The attractive features of Secondary (l_S>x05this bubble model caused Krasucki [3] to investigate Slx106the influence of viscosity on this mechanism. He ex- Fast Event >1x106tended the bubble concept to include initiation ofthese bubbles by submicroscopic particle impurities inthe liquid. The author was able to actually photographthe bubble development in very viscous hexachlorobi- There was no noticeable dependence of the growth ratesphenyl under uniform field conditions using parallel on the molecular composition of the liquids. Theplate electrodes 100 pm apart and an applied voltage of actual electrode separation and the externally applied22 kV. The attractive feature of Krasucki's develop- voltage had some effect on these values. At appliedment was its ability to predict correctly both temper- voltages below the breakdown voltage only primaryature and pressure effects. streamers formed and their growth rate slowed with

time. At or above the breakdown voltage, all threephenomena were observed. In the rush for new knowledge

EXPERIMENTAL RESULTS to be gained by these new electro-optical techniques,the metal/liquid interface was temporarily neglected.

Apparent support for the bubble theory came from ex-perimental studies using optical techniques such as During the last few years it became evident that anythose of Hakim and Higham [4], Farazmand [5], Chadband explanation of the propagation of these disturbancesand Wright [6], and Morikawa [7]. These investigators required a more detailed knowledge of their initiationphotographed prebreakdown phenomena in n-hexane and at the electrode surface and of the events occurringsimilar dielectric fluids using shadowgraph and *Schlieren techniques. Both techniques revealed the wihntercnie. Te rbe ftecag nSchlieren techniques. Both techniques revealed the jection process was addressed recently by Forster et al.existence of a region of refractive index differing [13-15]. The authors drew on the experience gained byfrom that of the bulk. Since the fluids studied by Cox and Williams [16] in studying the charge injectionthese authors were much less viscous than those usedthese authorsweremuhlessiscoushanthprocess in vacuum or in gases. The latter authors hadby Krasucki, these regions of different refractive in- determined that there were only a few injection sitesdex lasted for much shorter times. It should be men-. . ~~~~~~~~~~onthe cathode surface, each being rather small and nottioned here that unlike the earlier work by Lewis [1], 2 .TWatson and Sharbaugh [2], and to some extent Krasucki geiter rain bOundaries, moleular spities, orr3] th nee stde.47 sdpitpaegoe either grain boundaries, molecular asperities, or small[3], the newer studies [4-7] used point-plane geome- dust (dielectric) particles. They reported that thetries. Under non-uniform field conditions it was dut(ilcrc atce.Te eotdta htries. Uner non-unform fied conditins it wa local field enhancement factors were of the order offound that breakdown occurred at lower applied voltages 100at 300 and emwor fucton at the sites waantatth foemntondprbrakow oud e b 100 to 300 and the work function at these sites wasand that the aforementioned prebreakdown could be ob-..

served readily at the point electrode. In view of the less than 1 eV. These findings explain the problemlower voltage requirements, it was possible to increase confronting earlier investigators who assumed that the

' ~~~~~~~wholecathode surface was injecting charges and thusthe electrode gap by at least an order of magnitude wovertatedtes actual areatinv org eample,thufaiittn opia obevain. Asteopia overestimated the actual area involved. For example,thusfacilitating optical observations. As theoptical

Watson and Sharbaugh [2] erred when they assumed thattechniques became more refined, it was noted that in * 2. . . ' ~~~~~~~theinjecting area was of the order of 10 jim with athe low viscosity liquids studied, these "bubbles" conventional work function of 5.2 eV and a field en-were actually irregularly shaped regions resembling hancement factor of 100. The obvious difficulty inbushes or trees. Further improvements of the optical defining this area in their calculations of the currenttechniques came with the introduction of powerfulpulsed lasers by Morikawa [7] and Thomas [8]. At the dest ldtohsecnuin. Tisosntma,plsmedtime,rhigh speedforikam camerTasbecame Avaiabe however, that their suggestion of vaporization was in-same time, highspeedframingcamerasbecameavailable correct. On the contrary, as Forster pointed out [13],allowing framing rates of up to 2X0 ' frames per second there is now general agreement that the charge injec-[7,9-11]. The stage was now set for extensive research tion at favorable sites on the cathode surface leads toof both the prebreakdown and breakdown processes. the formation of low-density regions that grow until anWhere earlier on only one or two picutres were obtained instability leads to the actual breakdown of the dielec-per experiment, it became possible now to get 10 to 12 tric fluid. These low-density regions are essentiallyframes of the same event and thus measure the growth what Watson and Sharbaugh referred to as bubbles. Theyrate of these disturbances in O.S to 0.6 us. This re- appear to be hemispherical at low magnification but atmoved the ever lingering doubt as to the validity of high magnification they are found to be irregular inearlier procedures in which photographs were taken in shape, mostly cylindrical in cross section, and more orconsecutive experiments at different times during the less branched. In the more recent literature they areapplication of the voltage or by chopping the voltage referred to as streamers and their propagation has beenat different times [12]. From these studies it was studied in great detail in pure and contaminated fluidsconcluded that cathode generated streamer growth pro- [11,13,15].ceeded in essentially three steps . The first involvedthe formation of a bush or tree-like structure which Information on the processes occurring within thesewas followed by a much thinner secondary streamer and structures came from spectroscopic studies of thea final, very fast, event leading to breakdown. Typi- light emitted during the prebreakdown period [16].cal growth rates of these three phases are summarized The spectral quality of the light emitted during thebelow, growth of these streamers was shown by Wong and

Forster: Understanding electrical breakdown in liquid dielectrics 893

Forster [17] to be similar to that emitted during the negative ones. They were found to produce a patternactual breakdown event although much weaker than the reminiscent of that of a river delta. In the presencelatter. This suggests that within the streamers the of certain additives having low ionization potentials,vapor phase breaks down and a plasma condition is the number of these tributaries to the mainstreamcreated, the chemistry of which is essentially the seemed to increase and the overall pattern formed bysame as that of the overall breakdown process. It them approached a more or less semicircular structure.appears logical, therefore to associate the events The secondary streamer leading to breakdown was foundoccurring within the streamers with the concept of to move much faster than the primary, similar to thepartial discharge or corona phenomena. Hanna et al. observations made with cathode-initiated streamers and[18] actually measured the coincidence of these light under uniform field conditions. In very pure liquidspulses with current pulses in liquid hydrocarbons and in gaps of 1 cm or larger Hebner's group could ob--under prebreakdown stresses. serve these secondary streamers in great detail. They

estimated their propagation to be in excess of 106Up to this point the discussion has foucsed on cm/s. The streamer formations produced in the pre-

streamers initiated at the cathode surface under uni- sence of certain impurities lead, as mentioned above,form or non-uniform field conditions. These streamers to a more uniform field condtion (sphere-plane), whichare quite different in appearance and behavior from required obviously a higher voltage for the initiationthose initiated at the point anode. The latter of the secondary streamers thus explaining why understreamer-like structures appear to grow from the point these conditions breakdown was observed to occur atanode into the liquid dielectric. They are thinner higher voltages [21]. These structural differencesand less branched. Their growth can no longer be asso- are illustrated in Fig. 1. The actual breakdown vol-ciated with electron injection and a different mechan- tages and conductivities of these fluids are given inism has to be involved. Devins et al. [12a] pointed Table 3.out that the growth rate of these "positive" dischargesor streamers was remarkably constant during their Forster [13] proposed a somewhat different mechanismpropagation across the gap. These authors suggested from that of Devins [12] and Chadband [20] to explainthat the positive streamers grow by a field ionization the initiation and propagation of these positiveprocess. Support for this concept came from Chadband streamers. He postulated that thermalized electronsand Calderwood [19] and Chadband [20]. However, most present in the liquid (due to its contact with theof these conclusions were based on single-shot photo- metallic electrode surfaces) were focused near thegraphs taken at different intervals after application anode surface. At that anode surface, like that of aof the voltage pulse. A more detailed study of the cathode, there exist favorable sites for electronprocesses involved in the positive streamer growth was transmission that are characterized by a large en-presented by Hebner et al. [21] who used a high-speed hancement factor (100 to 300) and a low work function.framing camera to determine the effect of purity and Such a site then enhances the local field and causesvarious additives on the growth rate of both negative field-induced ionization of the liquid column existingand positive streamers in gaps ranging from 0.3 to between the focused charge and the anode spot. Kelley2.5 cm. Based on their observations these authors and Hebner [22] have shown that the resulting streamerconcluded that breakdown, when the needle point is an structures are conductive and thus they can be con-anode, involved two types of streamers similar to the sidered to represent an extension of the electrode in-observations made when the point is a cathode and un- to the liquid. The point of the streamer replaces theder uniform field conditions. But unlike the latter surface spot on the anode and the field ionizationtwo, the growth rate of the primary positive streamer process can repeat itself in any direction in whichwas rather constant and at least one order of magni- focused charges can accumulate near the tip. Overall,tude faster than that of the primary negative streamer. the streamer appears to grow steadily into the liquid,There was also noted some dependence of their growth while actually it grows in spurts too short to be re-rates on the molecular makeup of the liquids. This solved by the available photographic equipment. Sinceis illustrated in the Table below. the proposed process depends entirely on the local

field conditions existing between electronic chargesin the liquid and the streamer tip, the growth ratewill most likely be determined by the time it takes

Table 2 for the charges to focus near a given tip. This focus-ing process would depend to some extent on the state

Effect of Molecular Structure on Anode of these charges. The data shown in Table 2 illustrateInitiated Streamer in Liquid Hydrocarbons this point. Toluene has a considerable electron affin-

ity which n-hexane has none at all. Thus solvatedelectrons may be more readily available for focusing

Type of Fluid Growth Rate, cm/s x 10-5 in n-hexane than in toluene. In Marcol 70, the pre-sence of saturated cyclic structures and the higher

n-hexane 3.2 ± 0.2 viscosity may explain the intermediate behavior oftoluene 1.2 ± 0.1 this liquid. Subsequent recombination of the ionized

molecules may well account for the observed weak lightMarcol 70, a paraffinic 1.8 ± 0.2 emission associated with this streamer growth process.(white) oil This model would explain also why branching is not as

predominant as it is in negative streamers and why thepositive streamers are much thinner than their nega-tive counterparts.These primary streamer growth rates were within exper-

imental error independent of the presence of impuri-ties or conductivity reducing additive [21] as shownin Table 3. The distance these primary positivestreamers grew, depended on the level and type of im-purity. As mentioned earlier, the structure of theseprimary streamers was also different from that of the

894 IEEE Transactions on Electrical Insulation Vol.. EI-20 No.59 October 1985

Table 3

A. Effect of Additives on Electrical Breakdown Voltage of Base Fluids

(250C, 0.3 cm Gap, Trapezoidal Pulse)

Breakdown Voltage, kV( )Needle Cathode Needle Anode

Fluid n-hexane toluene n-hexane toluene

Purified >203 >203 39.6 ± 2.6 58.9 ± 1.1

Chemical Grade 148 ± 7 121.4 ± 1 36.5 ± 7 57.1 ± 18

10 ppm ASA3(2) - 84.5 ± 0.8 36.5 ± 0.4 57.1 ± 2

0.10M DMA 3) >200 >200 40.7 ± 3 57.2 ± 2

C Particles 120.7 ± 5.5 95.9 ± 2.5 38.8 ± 1.1 57 ± 2

(1) Average of seven consecutive determinations.

(2) The antistatic additive ASA3 is a mixture of 1/3 chromiumsalicilate, 1/3 calcium sulfosuccinate, and 1/3 vinyl copolymerin kerosend (50/50 mixture by volume.

(3) N,N'-dimethylaniline

B. Effect of Various Additives on the Conductivity of Base Fluid

Conductivity, Scm-1 (25°C, 1000 Hz)(1)

Fluid n-hexane toluene

Purified <1x10-16 6.25xl0-14

Chemical Grade 1.6x10-5 1.lxlO13

10 ppm ASA3 l.9x101-0 5x100-l

0.10M DMA l.5X10-15 6.3xlO-13

(1) Experimental error is 10%

It has to be pointed out here that under these con- Experimental evidence for the existence of such layers

ditions electron injection will still take place at comes from the work of Wong and Forster [17] who ex-

the cathode as soon as an external potential is appli- amined electrode surfaces under the scanning electron

ed. Thus the supply of thermalized electrons could microscope after they were subjected to electricalbe maintained in the bulk of the liquid during the breakdown. The second and somewhat similar conceptgrowth process. This mechanistic picture appears to was developed by Latham [24] who proposed a micro-

explain quite satisfactorily all the experimental inclusion model. In this model, a very small dielec-

observations including the lower breakdown voltage. tric particle is postulated to be involved in the emis-

Obviously it should be easier to remove electrons from sion process. This particle, in direct contact with

a liquid than to push them into it. the emission site, changes its insulating propertiesunder the influence of emitted electrons to becomeconductive and eventually allowing a very small metal-

A MODEL FOR BREAKDOWN lic whisker to grow through it to its surface. Thiswhisker of atomic dimensions will then serve as a

Recently three contributions appeared in the litera- powerful electron emitter. This model could be appliedture that deal with theoretical aspects of the electri- equally well to cathode and anode surfaces. These two

cal breakdown model and they need to be discussed here. contributions are very helpful in supporting the

The first one by Lewis [23] reviews the various steps cathodic electron emission process as well as the re-

leading to breakdown. Of particular interest here are moval of electrons at the anode.his ideas on the charge injection process. He pointsout that practical electrode surfaces are covered by The third contribution by Kao [25], deals with a

thin oxide layers (as was suggested also by Watson and novel model to account for the formation of negative

Sharbaugh [2]) and perhaps some organic layers gener- and positive streamers. The author proposes that the

ated by electric field-induced polymerization. Their injected electrons, after being trapped, form a homo-

presence would greatly facilitate electron injection, space charge which tends to modify the local electric

Forster: Understanding electrical breakdown in liquid dielectrics 895

10 9 10 9

8 _ 7 8 7

6 5 6 5

4 3 4 3

2 _ 1 2

Pure n-hexane 0.1M Dimethylaniline inn-hexane

Fig. 1: Effect of low ionization potentia2 additiveon positive streamer structureTrapezoidal pulse, 130 kV crest voltage,1.25 cm gap, 2x106 frames/s

field and thus eventually help create low density re- namic instability occurring at the surface of the pri-gions in whcih subsequent electrons can enjoy a large mary streamers. Experimental evidence has shown thatmean free path. There, they can gain energy from the the presence of impurities may enhance the onset offield and thus cause impact ionization. The problem this instability in the case of negative streamershe addresses is how these low-density regions are [11,21]. The exact mechanism involved in the develop-formed. For this purpose he proposes an Auger type ment of this instability remains unkown, however, andprocess in which an injected electron during trapping further investigations will have to be made to eluci-releases energy to another electron to form a hot date this process.electron, capable of causing impact ionization. Thisprocess is visualized by the author to go on as a SUMMARY AND CONCLUSIONSchain reaction to produce more and more molecularfragments due to dissociation of C-H and C-C bonds. The experimental data and the theoretical studiesIn this manner the associated energy release and the published in the last decade have helped greatly tocoulombic repulsion term will help create these low- understand more thoroughly the elusive electric break-density regions. While this model appears to work down process. As was surmised correctly by the earliervery well to explain negative streamer growth, it is investigators, electric breakdown represents an insta-less satisfactory in accounting for positive streamer bility. It is caused by three sequences of events.development in amorphous insulators such as liquids. In the case of uniform fields or point cathode theClearly, once the low-density region is formed near first one is the unavoidable electron injection pro-the cathode surface, the stage is set for impact cess, leading to the formation of low-density regionsionization to occur and the associated energy release in which processes similar to those in gases can taketogether with coulombic repulsion can account for the place. The resulting streamers eventually develop ansubsequent growth. From the aforementioned structural instability which leads to the extremely rapid growthdifference of positive streamers and associated growth of a secondary streamer that crosses the gap and estab-patterns it appears that Kao's model does not describe lishes a conducting path. When the point is an anode,the latter in proper terms. At this stage Forster's the first step involves the focusing of thermalizedmodel [13] seems to provide a more adequate descrip- electrons near the anode surface, resulting in thetion of the positive streamer process. build-up of an intense local field which causes local

breakdown of the liquid. Growth of the positiveIt has been pointed out earlier that these primary streamer thus most likely involves a sequence of field-

streamers do not bridge the gap. While early studies enhanced ionizations which eventually leads to the[1-3] with electrode separations of less than 0.5 cm formation of the fast secondary streamer and subsequentsuggested that this might be the case, analysis of breakdown.photographs obtained with cameras having framing ratesof 2x107 frames/second have shown that in all cases a The above sequences appear to be equally applicablefast event occurs just prior to breakdown. This fast to amorphous solids. Thus the understanding gainedevent has been observed under all field conditions from gaseous and liquid breakdown may well prove to be[9,11,15]. The onset of these secondary streamers has helpful in elucidating the failure mechanism occurringbeen attributed by Watson [26] to an electrohydrody- in highly stressed amorphous solids such as polymers.

896 IEEE Transactions on Electrical Insulation Vol. EI-20 No.5, October 1985

This reveiw has attempted to show that the solution [12] (a) J. C. Devins, S. J. Rzad and R. J. Schwabe,

of an age-old puzzle depended on many contributions. "Prebreakdown Phenomena in Liquids: Electronic

It illustrates some of the pitfalls of research when Processes", J. Phys. D 9, L87-L91 (1976)

inadequate tools or inappropriate experimental condi- (b) J. C. Devins, S. J. Rzad and R. J. Schwabe,

tions are used. It also shows that the pioneers in "Prebreakdown Phenomena in Sphere-Sphere Elec-

this area of electric breakdown exhibited excellent trode Configurations in Dielectric Liquids",

intuition. Today's understanding could not have been Appl. Phys. Lett. 31, 313-314 (1977).

achieved without their contributions.[13] E. 0. Forster, "Electrical Breakdown in Liquid

ACKNOWLEDGEMENTS Hydrocarbons", J. Electrostatics 12, 1-12 (1982).

The author is greatly indebed to his colleagues, [14] E. 0. Forster, "The Metal/Liquid Interface:

Drs. R. E. Hebner and E. F. Kelley of the National The Charge Injection Process", IEEE Trans. EI-19,

Bureau of Standards for many helpful suggestions in 524-528 (1984).

the development of the overall picture of the pre-breakdown and breakdown processes. Thanks are also [15] G. J. FitzPatrick, E. 0. Forster, R. E. Hebner

due to his collaborators, Messrs. P. P. Wong and and E. F. Kelley, "Analysis of Prebreakdown

G. J. Fitzpatrick who helped carry out some of the Events in Liquid Hydrocarbons", submitteG" for

experimental work. publication in IEEE Trans. on Electrical Insu-lation.

REFERENCES [16] B. M. Cox and W. T. Williams, "Field EmissionSites on Unpolished Stainless Steel", J. Phys. D.

[1] T. J. Lewis, "Mechanism of Electrical Breakdown 10, LS-L9 (1977).in Saturated Hydrocarbon Liquids", J. Appl. Phys.27, 645-650 (1956). [17] P. P. Wong and E. 0. Forster, "The Dynamics of

Electrical Breakdown in Liquid Hydrocarbons",[2] P. K. Watson and A. H. Sharbaugh, "High Field IEEE Trans. EI-17, 203-220 (1982).

Conduction Currents in Liquid n-hexane UnderMicrosecond Pulse Conditions", J. Electrochem. [18] M. C. Hanna, J. E. Thompson and T. S. Sudarshan,

Soc. 107, 516-521 (1960). "The Simultaneous Measurement of Light and Cur-rent Pulses in Liquid Dielectrics", 1983 Annual

[3] Z. Krasucki, "Breakdown of Liquid Dielectrics", Report, Conference on Electrical Insulation and

Proc. Roy. Soc. A 294, 393-404 (1966). Dielectric Phenomena, IEEE #83CH1902-6, 245-250(1983).

[4] S. S. Hakim and J. B. Higham, "A Phenomenon inn-hexane Prior to its Electric Breakdown", Nature [19] W. G. Chadband and J. H. Calderwood, "The Propa-

189, 996 (1961). gation of Discharges in Dielectric Liquids", J.Electrostatics 7, 75-91 (1979).

[5] B. Farazmand, "Study of Electric Breakdown ofLiquid Dielectrics Using Schlieren Optical Tech- [20] W. G. Chadband, "The Propagation of Positive

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[6] W. G. Chadband and G. T. Wright, "A Prebreakdown [21] R. E. Hebner, E. F. Kelley, E. 0. Forster and

Phenomenon in the Liquid Dielectric Hexane", G. J. FitzPatrick, "Observations of PrebreakdownBrit. J. Appl. Phys. 16, 305-313 (1965). and Breakdown Phenomena in Liquid Hydrocarbons

Under Non-uniform Field Conditions", IEEE Trans.

[7] E. Morikawa, "Optical Observation of Prebreak- EI-20, 000 (1985).down Phenomena in Dielectric Oil", ElectricalEngineering in Japan 92, 11-17 (1972). [22] E. F. Kelley and R. E. Hebner, "The Electric

Field Distribution Associated with Prebreakdown[8] W. R. L. Thomas, "An Ultra-high Speed Laser Phenomena in Nitrobenzene", J. Appl. Phys. 52,

Schlieren Technique for Studying Electrical 191-195 (1981)Breakdown in Dielectric Liquids", 1973 AnnualReport Conf. Electrical Insulation and Dielec- [23] T. J. Lewis, "Electronic Processes in Dielectrictric Phenomena, NAS/NRC Washington, DC, 130-136 Liquids Under Incipient Breakdown Stress", IEEE

(1974). Trans. EI-20, 000 (1985).

[9] H. Yamashita, H. Amano and T. Mori, "Optical [24] R. V. Lantham, "The Origin of Prebreakdown Elec-

Observation of Prebreakdown and Breakdown Phen- tron Emission from Vacuum-Insulated High Voltage

omena in Transformer Oil", J. Phys. D. 10, Electrodes", Vacuum 32, 137-140 (1982).1753-1760 (1977).

[25] K. C. Kao, "New Theory of Electrical Discharge

[10] E. F. Kelley and R. E. Hebner, Jr., "Prebreak- and Breakdown in Low Mobility Condensed Insula-

down Phenomena Between Sphere-Sphere Electrodes tors", J. Appl. Phys. 5S, 752-7SS (1984).in Transformer Oil", Appl. Phys. Lett. 38, 231-233 (1980). -[26] P. K. Watson, "EHD Instability in the Breakdown

of Print-Plane Gaps in Insulating Liquids",

[11] R. E. Hebner, E. F. Kelley, E. 0. Forster and 1981 Annual Report Conf. on Electrical Insula-

G. J. FitzPatrick, "Observation of Prebreakdown tion and Dielectric Phenomena, IEEE #81CH1668-3,

and Breakdown Phenomena in Liquid Hydrocarbons", 370-376 (1981).J. Electrostatics 12, 26S-283 (1982).

Manuscript w:as receiv)ed 1 April 1985.