ieee transactions on kerr-effect studies insulating ......ieee transactions on electrical...

IEEE TRANSACTIONS ON ELECTRICAL INSULATION, VOL. EI-9, NO. 2, JUNE 1974

Kerr-Effect Studies of an Insulating Liquid Under

Varied High-Voltage ConditionsESTHER CHRISTMAS CASSIDY, SENIOR MEMBER, IEEE, ROBERT E. HEBNER, JR., MEMBER, IEEE,

MARKUS ZAHN, MEMBER, IEEE, AND RICHARD J. SOJKA, STUDENT MEMBER, IEEE

Abstract-Refined Kerr electrooptical fringe-pattern methods areused to study time and space variations in the electric field betweenthe electrodes of parallel-plate capacitors filled with liquid nitro-benzene. Photographs of fringe-pattern data recorded during applica-tion of high direct (both positive and negative) and sinusoidalvoltages, ranging in frequency from 40 to 200 Hz, are compiled toenable computation of space-charge distortions of the field in bulkof the liquid during the stress of high-field (up to 85-kV/cm) opera-tion. The measurements reveal significant differences between thefield and charge behavior under short pulse (microsecond) voltageconditions, during prolonged dc operation, after sudden changes inthe dc voltage level and polarity, and, for the first time, at variousintervals over the course of entire cycles of sinusoidal voltage. Theresults show that space-charge distortion in the interelectrode fieldis influenced by the level, frequency, and duration of applied voltage.Discussions of effects believed due to particulate charge carriers,to electrohydrodynamic motion of the liquid, and to the electrodematerials are also included.

I. INTRODUCTION

R ECENT WORK has experimentally demonstratedthe usefulness of the electrooptic Kerr effect for the

determination of various electrical parameters under theinfluence of high voltage. Among these are the measure-ment of pulsed high voltage [1]-[4], the determinationof space-charge effects in nitrobenzene [5]-[6] andchlorinated biphenyls [7], the measurement of theelectric-field distortion caused by the insertion of a soliddielectric in a liquid insulant [8], the measurement of60-Hz alternating voltages [9], and the observationof the spatial and temporal variation of the electric-fielddistribution caused by pulsed and direct voltage [10]-[12]. In addition, theoretical advances in the understand-ing of space-charge phenomena in insulating liquids havebeen made in such areas as the prediction and descriptionof bulk space charge [13] and polarization waves [14]in charged-liquid systems and the role of particulatecharge carriers in electrical conductance [15].

Realizing that further advances are predicated on aknowledge of electric-field and space-charge behavior in

Maniuscript received September 21, 1973. This work was sup-ported in part by the U. S. Atomic Energy Commission through theSandia Corporation, Albuquerque, N. Mex. One of the authors(M. Zahn) received partial support from the National ScienceFounidation under Grants GK-27803 anid GK-37594.

E. C. Cassidy, R. E. Hebner, Jr., and R. J. Sojka are with theElectricity Division, National Bureau of Standards, Washington,D. C. 20234.M. Zahn is with the Department of Electrical Engineering, Uni-

versity of Florida, Gainesville, Fla. 32601.

liquid dielectrics, we have used refined Kerr electro-optical fringe-pattern techniques [10]-[11] to measureand compare electric-field and charge distributions innitrobenzene-filled parallel-plate capacitors during pulsed,direct, and low-frequency alternating high-voltage oper-ation. The purpose of this work, which supplements thesubstantial quantity of data published from earlier nitro-benzene studies, is the determination of, and ultimatelycontrol of, the complex mechanisms that lead to electricalbreakdown in insulating liquids. The present work wasalso undertaken partly to increase the precision of 60-Hzvoltage measurements. The results demonstrate both thefeasibility and the effectiveness of using electroopticalfringe-pattern techniques to visualize electric-field distri-butions and space-charge behavior under steady-statealternating high voltages. Field strengths as high as 85kV/cm are applied. The effects of voltage level and fre-quency on space-charge density and distribution are in-vestigated between 0 and +50 kV; frequencies range from40 to 200 Hz.

Nitrobenzene is the liquid investigated, primarily be-cause of its high electrooptical coefficient (of order 10-12m/V2) and relatively high electrical breakdown strength( > 100 kV/cm). Though its powerful solvent, hygro-scopic, and toxic properties make this liquid impracticalfor many insulating applications, nitrobenzene is uniquelysuited to serve in this capacity for a multiplicity of diversespecialized applications, ranging from its use as thedielectric in prototype "high-energy" electrostatic gen-erators [16] to its widespread use in high-speed photo-graphic shutters [17] and laser "Q-switching" devices[18], to its use for precision measurements of pulsed highvoltages. In connection with the latter application, ex-tensive studies, now underway for more than five years,of the performance of nitrobenzene-filled Kerr cellsimmersed in oil under both transient (microsecondpulses) and steady-state de and ac operation have demon-strated that the physical behavior of nitrobenzene underhigh-field conditions is essentially the same as that of mostinsulating liquids. Its conductivity and breakdownstrength are, for example, affected by space-chargeeffects [19], whether they result simply from moisture,impurities, and particulate contaminants in the liquid,or from more complex mechanisms such as charge forma-tion by electrochemical reaction or field emission at theelectrodes. Its performance is also affected by other widelyreported high-field phenomena [20] such as electrohydro-

43

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IEEE TRANSACTIONS ON ELECTRICAL INSULATION, JUNE 1974

LENSAh4PLATE AT 90'

i \CIRCULAR APERTURE

PULSEDVOLTAGESYSTEM

END VIEWON FILM

DiRECTVOLTAGESYSTEM

ALTERNATiNGVOLTA6ESYSTEM

Fig. 1. Schematic diagram of the apparatus used to measure the electric-field distributions in nitrobenzene. TypicalKerr-effect fringe pattern is shown on the right of the drawing. The systems used to supply and measurethe applied voltage are also shown.

dynamic motion, bubble formation, electrophoretic anddielectrophoretic forces, the Sumoto effect, etc. All ofthese phenomena are known to be more or less pronounceddepending upon the condition of the liquid, the electrodedesign, the nature and level of applied voltage, etc. Yet our

understanding of electric-field and charge behavior, andthus of breakdown, in the bulk of insulating liquids re-

mains admittedly poor.

It has been our philosophy in the preparation of thispaper that much is to be gained from careful analysis ofelectrical stress and space-charge behavior which can beso easily observed experimentally in nitrobenzene by use

of Kerr-effect fringe-pattern techniques. Although thechemical composition of nitrobenzene differs significantlyfrom that of more commonly used insulating fluids, it isreasonable to expect that behavior attributable to physicalphenomena, such as conduction by particulate impuritiesor electrohydrodynamic motion of the liquid, is similar inboth cases. Accordingly, more than 600 individual ex-

periments investigating such behavior were performedwith sealed nitrobenzene-filled cells under a variety of

carefully monitored high-voltage conditions. The resultsdemonstrate significant differences between the pulsedand steady-state ac or do field and charge behavior in thebulk of the liquid. Whereas expected conditions were ob-served during short pulse operation, several significantunforeseen trends were observed in the steady-state experi-ments, thereby pointing up the fact that extreme care

should be exercised in inferring behavior at power-linefrequency from characteristics observed under dc or

pulsed-voltage operation. If, for example, it is suspectedthat the degradation of an insulating liquid is due to thepresence of space charge, whether formed chemically or

carried by particulate impurities, the data presented heresuggest questioning of the common practice of inferringthe liquid's performance at power frequency from ac-celerated aging (for shorter times at frequencies muchhigher than 60 Hz) testing or dc testing results.The experimental techniques and methods of data

analysis employed in the present work are described inSections II and III, respectively. Section IV presents anddiscusses the data obtained under direct voltage andunder alternating voltages. The final section, Section V,presents the summary and conclusions.

II. EXPERIMENTAL APPARATUS ANDTECHNIQUES

A schematic diagram of the experimental apparatus isgiven in Fig. 1. The system consists of a nitrobenzene-filled parallel-plate capacitor within a glass vessel (Kerrcell), a triggerable pulsed argon laser, crossed polarizersoriented at + and -45° to the interelectrode-field direc-tion, retardation plates oriented as indicated for elimin-ation of field-directional fringes [21] not pertinent inthe present work, and appropriate photographic equip-ment for recording of the transmitted fringe patterns ob-served when the laser is flashed during high-voltageoperation. The laser-beam diameter is expanded andcollimated as indicated to illuminate the area between andaround the test-call electrodes. The beam is focused andpassed through the circular aperture to exclude extraneousreflections and, on occasion, to allow Schlieren observationsof the liquid motion invariably produced by steady-statehigh-voltage operation. For purposes of illustration, thediagram includes a typical fringe pattern photographed

44


CASSIDY et al.: KERR-EFFECT STUDIES

by flashing the laser with - 20-kV dc applied to the testcell. During such direct voltage studies, the high voltage(0 - +50 kV) was connected to the parallel combinationof the test cell and a calibrated 100-MQ resistive divider[22] as shown. The divider was used to measure the highvoltage across the cell. For current measurements a micro-ammeter was placed in series with the Kerr cell, betweenthe cell and ground.

For the ac studies, alternating high voltage was obtainedby amplifying the output of an audio oscillator. Thispower-amplified signal supplied a step-up transformer, andthe latter's output voltage, as high as 40 kV rms withfrequencies over the range from 40 to 200 Hz, was appliedto one electrode of the cell, the other electrode beinggrounded. The rms value of the applied voltage wasmeasured using a 1000:1 metering tap on the transformer.

In order to control the timing (during a cycle) of theac fringe-pattern observations, triggering of the laserpulse (flash duration 6 Mts) was synchronized with thehigh alternating voltage applied to the cell. This wasaccomplished by using the audio-oscillator output signalto trigger an oscilloscope equipped with a manually ad-justable delayed-output voltage pulse for triggering of thelaser pulse. The delay and synchronization of this delayedtrigger signal were adjusted, Fig. 2, so that the laserflashed in a strobelike manner to allow discrete observa-tions of the fringe pattern at a selected time(s) during acycle. The time interval At between the triggering of theoscilloscope and the triggering of the laser was monitoredusing a conventional counter which counted its internal

wILI010 0.

,0U'

0U)

I

I

I

Fig. 2. Timing diagram of system used to observe the electric-fielddistribution at various instants of time during a cycle of alternatingvoltage. The upper trace shows one cycle of the applied voltage.The center trace represents the time for one sweep of the horizontaltime base of the oscilloscope. The oscilloscope is externallytriggered by the applied-voltage waveform. The bottom traceshows the laser trigger pulse. The delay between the start of theoscilloscope sweep and the laser trigger pulse is manually adjust-able and the duration of the laser ]ight pulse is of order 6 ,us.

clock pulses during At. In this way At could be very ac-curately measured. Two methods were used to correlatea value for the interval At with a specific point during acycle of the applied voltage. The first was to detect aportion of the laser light pulse with a photomultiplierand display the time relationship between the laser lightpulse and the applied voltage on the oscilloscope screen.The time interval At was then adjusted until the light pulsecoincided with the peak of the positive half-cycle. Thesecond and somewhat more accurate method was toadjust At until the fringe pattern resulting from the elec-trooptic Kerr effect indicated that the laser was pulsed atthe peak of the positive half-cycle, i.e., until the numberof fringes produced was maximum. All attempts to cor-relate a specific point on the applied-voltage waveformwith a value of At were reproducible to within +20 ,us,i.e., to within +t0.8 percent of the shortest half-cycle used.The present work was conducted using two specially

prepared cells (see Table I), one with electropolishedstainless-steel electrodes of dimensions 4 cm wide by 12cm long, and the other with glass-blasted nickel electrodesof dimensions 2 cm wide by 12 cm long. The electropolish-ing and glass-blasting procedures were adopted to min-imize the effects of electrode-surface contamination. Theelectrode edges and corners are rounded to avoid pre-mature electrical breakdown. The nominal electrodespacing in both cases is 0.5 cm. Glass-to-metal seals areused for inserting tungsten rods which provide electricalconnection and support for the electrodes. An expansionvolume is included to prevent failure of the glass vesselwhen the test liquid expands due to heating. A therm-ometer is sealed into the nitrobenzene for monitoring of itstemperature. (Temperature increases as large as 200Cwere encountered during prolonged high-voltage acoperation.) External surface and air flashovers are avoidedby immersing the test vessel in transformer oil. Morecomplete details of our cell-construction technique werepresented elsewhere [11].With regard to the test-cell insulant, nitrobenzene was,

as previously mentioned, selected primarily because of itshigh electrooptical coefficient and relatively high electricalbreakdown strength. Though the electrical resistivity ofcommercially pure grades is inadequate (of order 103 Qi.m)for steady-state high-voltage operation, increased resistiv-ity is readily obtained by passing reagent-grade liquidthrough a chromatographic adsorption column of activatedalumina. In the present work, this procedure, which haslong been used to remove moisture and other impuritiesfrom insulating oils [23], increases the nitrobenzene'sresistivity to the order of 108-109 U m. At this level, pro-longed steady-state high-voltage dc and ac operation couldbe maintained without excessive liquid heating or turb-ulence due to current leakage. Gas chromatographicanalysis of the liquid before and after processing showedthat, as is often the case with insulating oils, wvater is theprincipal contaminant. Thus, because of the hygroscopicand toxic properties of nitrobenzene, our entire cell-filling and sealing procedure is performed under vacuum.

45



TABLE IPARAMETERS FOR THE TEST VESSELS DESCRIBED IN THE TEXT

ELECTRODE ELECTRODE STRUCTURE CELL CONSTANTCELL MATERIAL & PREPARATION (E d)m

A Nickel Parallel-plate 4900 VGlass-blasted

B Stainless Parallel-plate 4600 VSteel Electro-polished

Note: The cell constants listed are valid for 514.5-nm argon laserradiation at a liquid temperature of approximately 298 K.

III. THEORYComprehensive theoretical discussions of the electro-

optic Kerr effect, including derivations of the Kerrconstant [241-[26], the application of the Kerr effect tohigh-voltage measurement, and methods for eliminatingthe effects of fringing fields [27], are available in theliterature. The purpose of this section is to make explicitthe assumptions used in the analysis of the data presented.The governing equation for the Kerr effect is

ni-n = XBE2 (1)

where nll(L) is the index of refraction for light polarizedparallel (perpendicular) to the direction of the appliedfield E. The Kerr constant of the liquid is B and X is thewavelength of the monochromatic light traversing theKerr cell. Equation (1) can be solved for the phase shift,sp, between the components of the light beam polarizedparallel to and perpendicular to the direction of theapplied field

L

=2-rf E2B dl. (2)

In (2), L is the geometrical length of the optical paththrough the applied field. To reduce (2) to a more con-venient form it is necessary to make a number of observa-tions concerning the electric-field distributions in thecells used. To clarify this discussion, define a set of co-ordinate axes so that the z direction is the direction ofpropagation of the light beam, the x direction is per-pendicular to, and the y direction is parallel to the surfaceof the electrodes. In this notation (2) can be rewritten

L

ep(x,y,t) = 2r 1 BE2(x,y,z,t) dz0 (2a)

which also explicitly includes time dependence. The firstassumption is that B is a constant, so that

sP(x,y,t) = 2-rB f E2(x,y,z,t) dz. (2b)

In the present work, all field mapping is done in the xdirection. We have therefore analyzed data only at aspecific value of y, the geometric center of the electrodes.It is shown in our data and has been shown in previous

studies in cells with similar geometry that so(y) variesonly slightly near the center of the plates so that errorin locating the exact center of the plates are negligible incomparison to other errors in the analysis. We can there-fore rewrite (2b) as

L

<p(x,t) = 2rB f E2(x,z,t) dz.0

(2c)

Because the experimental setup is such that light polarizedat an angle of 450 to the direction of the applied field passesthrough the Kerr cell and then impinges upon an analyzeroriented perpendicular to the direction of polarization, thelight transmitted by the analyzer obeys the relation

I(x,t) = sin2 [so(x,t) /2].Im (3)

In this equation we have again suppressed the y de-pendence and I(x,t)/Im is the relative irradiance of thetransmitted light at a position x at any given time t, Imbeing the maximum transmitted irradiance. We haveassumed that temporal and spatial variations in Im, whichare dependent solely on the light source and the attenu-ation in the media through which the light beam propa-gates, are negligible; especially so in the present work, aswe measured only the values of x for which I (x,t) /Im, = 0,i.e., the positions of the dark fringes.

It has been shown [27] that (3) can be rewritten

t(I(t) ) 2 7r (E (-t))2](N0) = sing[2 ( )

where the overbar (e.g., in A) denotes the following:

A = [L AA2dz]

and [I(t)/Im] denotes I(x,t)/Im at a specifie value of x.In (4), Em is the smallest effective field strength that

produces a maximum I,,, in the amount of transmittedlight. Methods have been developed to correct the ob-served values of (I(t) /Im) x for variations in the x directiondue to the fringing fields at the electrode ends [27].However, these corrections were not applied to the presentdata because it has been shown previously that theymodify values of (E (t) /Em)x measured with parallel-

(4)

46



plate electrodes greater than 10 cm in length and spacedless than 1 cm apart by less than 1 percent.

In this study information was obtained concerning[E'(t)]z, and thus the space-charge density, at certaintimes during the cycle of an applied steady-state alternat-ing high-voltage operation. These data were obtained byilluminating the cell with a light pulse which was shortcompared to the observed temporal variation of the field.(Details were given in Section II.) To make explicit thissituation (4) can be written

(i§) in2 [1r ( E )2 ]

(5)

where t denotes the instant at which the observation ismade.

Electric-field and space-charge distribution informationwere obtained from photographs of the transmitted fringepattern at time t (see, for example, Figs. 3 and 4). Therelative positions (x/d) of the centers of the dark bandsalong a centrally located path (of total length d) betweenand perpendicular to the inner electrode surfaces weremeasured and recorded. The center of each dark band orfringe was assumed to indicate a relative intensity of(III-),, = 0, and thus, from (5), a point where therelative electric-field intensity (R/IP) x,, was equal to thesquare root of an even integer. In keeping with the con-vention established earlier [10], [11], numerical values nwere assigned to each dark fringe by counting (n =0,2,4,6, ..) successively from the outermost dark regionwhere n = 0 inward to the desired fringe located on thescanning path (from x/d = 0 to 1) between the electrodes(see Figs. 3 and 4). The experimental values of (x/d) and(P/Pm) l, obtained from each photograph were fitted toa first-degree polynominal

= a + b(x/d)

or to a second-degree polynominal

(G.)1= A + B(x/d) + C(x/d)2

(6)

(7)

Fig. 3. Fringe pattern photographed (exposure time 0.3 ys) at thepeak of a 65.4-kV total-duration 10-ps pulse. The fact that thefield is uniform between the plates is demonstrated by the ab-sence of interelectrode fringes. The side fringes are due to thefringing field along the length of the electrodes.

STAY-TT VOLATAGDES AP ' ,

-22.62 kv PEAK

Fig. 4. Typical observations of the fringe patterns resulting fromthe application of direct and 60-Hz alternating voltages in cell A.The interelectrode fringes show that the interelectrode field is notuniform under these conditions. The fringe numbering procedureis described in Section III.

not needed, and, neglecting the effects of fringing endfields, (9) was reduced to

d(R/Im) ta (x/d)

pd(10)

where a, b, A, B, and C were coefficients determined by aleast squares best fit method [28]. The appropriatepolynominal was judged to be sufficiently accurate when-ever

IF" fe(E.)fitted = IA;)ep

4 0.05().Gim eP

Computations of the coefficients in (6) and (7) were usedto plot the electric-field distribution, i.e., (E/Em), as afunction of (x/d) and to allow convenient examination ofspace-charge effects in terms of the reduced space-chargedensity Pr

(8)

In order to obtain the space-charge density p from theelectric-field distribution, the differential form of Gauss'law was used, i.e.,

aE p (9)ax e (9)

where e is the dielectric constant of the liquid medium.For the purposes of this paper numerical values of p were

(11)a(R/Eld)tPI a(9(xd)

Whenever a linear electric-field distribution, of which thedc pattern in Fig. 4 is typical, was observed, (6) wasfitted to the data and the reduced space-charge densitywas given simply by

p, = b. (12)

Whenever a parabolic-field distribution was computed

47



(see, for example, the ac pattern in Fig. 4), the reducedspace-charge distribution was determined as follows:

Pr = B + 2C(x/d). (13)

IT. EXPERITMENTAL RESULTS

It was reported earlier [11], [27] that electric fieldsproduced by applying short (5 -20-,us duration) pulses ashigh as 300 kV show no evidence of space-charge distor-tion. A typical Kerr-effect fringe pattern demonstratingthis conclusion is given in Fig. 3. This photograph, whichis a 0.3-,As exposure taken at the peak of a 10-,us-long65.4-kV nearly rectangular pulse, shows the irradiancetransmitted through a parallel-plate cell similar to thoseused in the present work. With this geometry, the fieldbetween the electrodes should be uniform. The pulsedfield's compliance with this condition is verified experi-mentally by the uniformity in the irradiance (the absenceof fringes) transmitted between the electrodes.

In contrast, Fig. 4 shows typical records taken duringsteady-state dc and 60-Hz ac experiments with the cellhaving parallel-plate glass-blasted nickel electrodes. Thepresence of space charge and its distortion of the inter-electrode field is evident from the fringes recorded betweenthe electrodes. Further, plotting (see Fig. 5) of the dis-torted field distributions from the fringe numbers andpositions, as described in Section III, shows a profounddifference between the dc and 60-Hz field and chargedistributions. Observations and discussions of these andother anomalies in steady-state dc and ac field distributionsare discussed separately in the following sections. Dis-cussions of the reproducibility of the electrical andelectrooptical measurements are included, and severalalternative explanations are presented and discussed.Because the data presented typify results from over 600individual photographs, the majority of the data are

ALTERNATING VO_-At_ lT

DIRECT VOLTAGE

3 4 3~-

- --------

I3 -3_

2 4 6 8 2 4 6 8x d X /d

E T*r 308'/

4Q~~~~~~~-.. 4~~~~~~~~~~~~c

2 4 .6 ~8x/d

1t._ .l _ I

2 4 6 8

x/Jd

Fig. 5. The relative electric-field strength and the associatedspace-charge density as a function of relative position between theplates (x/d). These data were obtained from the two photographsin Fig. 4. Although the average field strengths are approximatelythe same in the two cases, the electric-field distribution, andconsequently the space-charge densities, are radically different.

presented in graphical form. A number of photographsof raw data are included to indicate to readers unfamiliarwith this technique the types of observations that can bemade.

A. Mleasurements of Current Under Direct Voltage

When the test cell is first filled with the purified nitro-benzene the current passed by the cell upon application ofsteady-state high voltage (alternating or direct) is veryhigh, being as much as a thousand times greater than thesteady-state current. For several minutes the conductionand electric-field properties of the cell are not stable andmeasurements, either electrical or optical, made duringthis period are nonreproducible. After high voltage hasbeen applied for about half an hour, the current throughthe cell approaches a steady-state value, and furtherelectrical and electrooptical measurements are repro-ducible. However, when there is a significant timeinterval between voltage applications (e.g., overnight),or when the type of voltage excitation is changed (e.g.,from direct to alternating or even if the polarity of directvoltage is changed), the current initially increases abovethe steady-state value but within a few minutes returnsto the same steady-state value. During this period theelectrooptic fringe pattern also changes slowly indicatingchanges in electric-field and space-charge distribution.Thus, whenever changes in voltage excitation are made,reproducible measurements reoccur only after severalminutes of high-voltage conditioning. The cells in thisstudy were used over a three-month period with the re-producibility of all measurements occurring after thebrief conditioning periods. Other sealed cells have been inuse for over three years wxith similar results. (Becausenitrobenzene is hygroscopic, unsealed cells do not maintainthis reproducibility.)

Other authors [29], [30] have noted both the decreasein current with time during application of a constant highvoltage and the dramatic increase in current with thedecay back to a steady-state when the polarity is reversed.In their work, however, the electrodes were coated with anelectrodialytic varnish. In that case, the following explan-ation for current behavior was postulated [30]:

... the oncoming residual ions are trapped withinthe polymer matrix while electrical neutrality is pre-served thanks to the stationary counterions; chargedisplacement in the reverse direction cannot occurunless the matrix has sufficient electronic conduc-tivity. Of course, if voltage is reversed, the mobileions are injected into the liquid and the arrange-ment works as a very efficient injector.

However, as previously noted, the same time- andpolarity-dependent behavior of the current was observedin our cells in which the electrodes were not varnished.A different, equally qualitative explanation for thisbehavior has been postulated [31]. This hypothesis, whichseems a plausible and perhaps a more accurate explanation

48

a

cr

v

J IJ

Er


49CASSIDY et al.: KERR-EFFECT STUDIES

for our observations, contends that electrical conductionis primarily the result of the motion of micrometer-sizeparticles carrying charges between the electrodes. If thisis the case, two processes involving trapping of particleson the electrodes can account for the current behavior.

1) Conducting micrometer- and submicrometer-sizedparticles become trapped on insulating areas of the metallicelectrodes because the insulating areas prevent them fromreadily giving up their charge to the metal electrode. Thisexplanation accounts for both the decrease of current withtime (the trapping sites slowly fill, thereby reducing thedensity of charge carriers and thus the current) and forthe increase of current with polarity reversal [31](coulombic repulsion on reversal may be expected torestore the density of charge carriers temporarily to itsoriginal high value).

2) Fluid motion can bring small insulating particlesin contact with one of the electrodes [15]. The particleseventually accept enough charge from the electrode to berepelled to the other electrode [32], where the chargingand repulsive action are repeated. Under a constantapplied voltage this process continues until the chargedistribution in the liquid bulk eventually reaches a con-dition of steady-state current leakage and charge-induceddistortion in the interelectrode field. Again, polarityreversal, whether under dc or periodic ac conditions,interferes with these slower processes, causing increasedrepulsion and/or attraction of the particles from (or to)the electrode to which they are attached, and accordingly,an increase in current.

It should perhaps be noted that in addition to the workpresented and referenced here concerning the effects ofparticles on the space-charge distribution in liquid insu-lators, there is a substantial body of literature concernedwith the fact that the insulation strength of compressedgases is greatly reduced by free particles [33].

B. Electric-Field Measurements and Charge Behavior withHigh Direct Voltage

Typical field distribution data under direct voltage areshown in Fig. 6. This figure contains information obtainedfrom fringe-pattern measurements by applying 0- to 430-kV direct voltage to one electrode of cell A (see Table I).Several minutes of conditioning were allowed at eachvoltage level to insure steady-state operation. Schlierenobservations during these experiments indicated that thedielectric fluid was in motion. The fringe patterns were,however, stable and reproducible after the conditioningperiod. This indicates that the equilibrium space-chargedistribution in the liquid was stable in the presence of andin spite of steady-state fluid motion.

Fig. 7 shows explicitly the effects of high-voltageconditioning described earlier. The dashed lines show theelectric-field distribution immediately after polarity re-versal (the power-supply polarity was reversed so that thenongrounded electrode was changed from positive tonegative). During 1/2 h of negative high-voltage oper-ation, with sequence as shown in the figure, the field dis-

Fig. 6. Typical field distribution data under direct voltage in cellA. Photographs from which some of the data were taken arepresented in the center of the figure. The presence of fringes be-hind the negative electrode is discussed in the text.

-E1w

I

cc2-10 kV

9 3 _ _ __

4i

L)4

, 6

00 0.2 0.4 0.6 0.8 1.0Et RELATIVE DISTANCE FROM HV ELECTRODE (x/d)

Fig. 7. This demonstrates the effects of high-voltage conditioningon the data presented in Fig. 6. The dashed lines represent theelectric-field distribution immediately after the application ofnegative high voltage while the distribution obtained after highvoltage was applied for about a half hour is shown by the solid line.

tribution shifted to those shown by the solid lines. Theseresults are in agreement with those reported by previousworkers [20], [34], insofar as all observations show thatthe field distribution shifts with time when high voltage isfirst applied to the plates. It is noted, however, that theshape of the field distributions reported by earlier workersdiffers from those shown in Figs. 6 and 7. This differenceis probably attributable to differences in our electrodes;



they used varnished brass electrodes, while the presentresults were obtained using a cell equipped with barenickel electrodes. Further evidence suggesting that thefield shape is affected by the electrode material is pre-sented in discussion of Figs. 11 and 14 which show thedifference between ac field-distribution results when usingstainless-steel and nickel electrodes, respectively.

Calculations of the interelectrode field distribution,i.e., of (E/Im)x as a function of relative position (x/d)between the electrodes, and of the reduced space-chargedensity Pr were performed as described in Section III.In the dc cases the field distributions were computed andplotted by fitting the data to a first-degree polynominal,(6). In this case Pr was independent of the relative position(x/d) between the electrodes and its value was determinedfrom (12).

Results calculated from the measurements of Figs. 4and 5 with cell A and from similar typical experimentswith cell B are listed in Table II. Inspection of the resultsshows several trends over the range studied: 1) the netcharge in the liquid was always positive and uniformlydistributed in the bulk; 2) the space-charge densitv in thebulk decreased as the applied voltage (average field)increased; 3) the net charge in the liquid bulk was slightlygreater when negative high voltages were applied; 4) thenet charge in the liquid was reduced by extended high-voltage operation (conditioning); and 5) the net densityof space charge was greater in the cell with nickel electrodes(cell A).These trends lend evidence to the postulate that leakage

conduction is dominated by the motion of small particlesbetween the plates. Previous work [29] has shown thatif the particles were metallic with a work function smallerthan the work function of the electrodes, the particleswould receive a greater positive charge at the positiveelectrode than negative charge at the negative electrode.Because nickel has a very large work function it is likelythat in our case such conducting particles would have asmaller work function and therefore that the net space-charge density would indeed be positive.

Similarly, these trends are also compatible with thepostulate that the current conduction is governed by theaction of insulating particles, in this case perhaps of sub-micrometer-size alumina particles which were imper-ceptibly carried with the liquid in passing from thechromatographic column during the process described inSection II. If this be the case, the insulating particles maybe expected to be attracted to and form a resistive layer ordielectric coating on the electrode surface. This layer wouldbe expected to increase in thickness with voltage and time,thereby slowly reducing the net charge circulating in thebulk and thus the leakage current through the liquid. Onremoval of the voltage, the attached particles woulddetach from the electrode and drift slowly back into thebulk. Upon reapplication of voltage they would repeatthe same procedure, except that the initial current andrate of decay may be expected to be smaller due to thefact that some particles will undoubtedly remain at-

TABLE IIREDUCED SPACE-CHARGE DENSITY IN BULK OF LIQUID DURING

HIGH DIRECT VOLTAGE OPERATION

CELL A

Reduced Charge DensityAppliedVoltage Before Conditioning After Conditioning

(kV) Pr Pr+10 _ 0.87+20 _ 0.88

+25 _ 0.77

+30 _ 0.62

-10 2.35 2.35

-20 2.64 1.54

-25 1.43 0.90

CELL B

tached to the electrodes. On polarity reversal, similarbehavior would be expected except that the electrode ofattraction would change with the polarity.

Schlieren observations of the liquid behavior on re-moval of the applied high voltage also tend to substantiatethe particle postulate. Schlieren movies beginning im-mediately before and continuing after sudden removal ofhigh direct voltages show development of localizedchanges in the index of refraction of the liquid near theelectrodes, with gradual development (over a period ofseveral minutes) of differing refractive indices in theupper and lower regions of the test cell in a manner sug-gestive of gradual settling of the particles under gravityto the lower regions of the cell.One additional discussion of the dc data is in order.

The typical photographs in Fig. 6 show significant lighttransmission behind the negative electrode. Some lighttransmission is, of course, expected due to fringing fieldsbehind the electrodes, but these fringing fields would besymmetric about both electrodes and hardly evident atlower voltages. Yet Fig. 6 indicates that a field of sig-nificant intensity exists behind the negative electrode. Thefringes behind the cathode in the +20-kV pattern, forexample, indicate that (4)1/2 < (EjEm) < (6)1/2 in theregion adjacent to the back of the cathode surface. In thiscell, for which Em = 9.8 kV/cm, this indicates thath' 22 kV/cm immediately adjacent to the back surface,that E = 0 at a point located about 0.5 cm from the backsurface of the negative electrode. This enhancement of thefield behind the negative electrode probably results fromthe presence of positively charged particles in the liquidbulk. Under the influence of the imposed electrostaticforces the particles would move to and collect about the

50


51CASSIDY et al.: KERR-EFFECT STUDIES

cathode, including its back side, and cause the observedfield enhancement. The irradiance around the cathode-support rod in Fig. 6 supports the postulate that chargedparticles also collect around this portion of the electrode.The fact that E goes to zero at a point on the electrodesupport could indicate that the accumulation of chargedparticles in this region of the liquid is limited by theshielding of charged particles already attached to theelectrode or by fluid motions which cause the particles tobe concentrated in certain regions of the liquid.Such observations, indicating the existence of space

charge in the bulk of the liquid behind an electrode, havenot, to the authors' knowledge, been previously reported.Accordingly, this unforseen behavior and its contributionto electrical stress in liquids are areas for further theoreticaland experimental work. In the meantime, several trendsare noted from the present work as characteristic of thiseffect. 1) The field enhancement behind the electrodealways appears behind the cathode, whether or not it isgrounded, thereby ruling out the possibility that it mayresult simply from stray capacitance between the high-voltage electrode and ground. 2) The field enhancement ismost pronounced under steady-state direct-voltage oper-

ation; it is barely noticeable under steady-state ac con-

ditions (see Figs. 4 and 8). Under ac conditions the rapidvariation of the electric field will not allow charged particlesto collect behind an electrode; the particle does not haveenough time to travel to the back surface of an electrodebefore the electrode polarity is reversed. Furthermore,Schlieren observations of the liquid during ac steady-state operation show that the fluid motion is much more.

turbulent than under dc conditions. This greater turbu-lence would also tend to disperse any particles tending tocollect near a given electrode. 3) The magnitude of theenhancement is affected both by the level of appliedvoltage and by the duration of exposure to high-voltageoperation: in a continuous experiment the enhancementwas observed to increase with voltage level, as is evidentfrom the + 10- and +20-kV fringe patterns in Fig. 6 (onefringe behind the electrode at 10 kV and nearly three at

20 kV) which were taken after days of intermittent ex-

posure to voltage of positive polarity. In widely spacedexperiments and those performed without extensiveconditioning, the behavior of this enhancement is as yetless predictable. For example, the -20- and - 10-kVpatterns were taken in that order after about 50 min ofexposure to voltage of negative polarity; the patterns showgreater enhancement at -10 kV (after longer condition-ing) than at -20 kV. Another fringe pattern, that re-

cording the data for the' - 10-kV curve in Fig. 6 (im-mediately after reversal of the voltage polarity), showsgreater enhancement (3-4 fringes) behind the electrodebefore conditioning. The cells were oriented with theplates horizontal and it was consistently observed thatthere was greater enhancement when the negative elec-trode was the lower rather than the upper electrode. It istherefore conceivable that the gravitational force mightnot be negligible in this case. It is evident, however, that

further specific experimental observations are needed fora detailed and accurate explanation of the causes of fielddistortion and current leakage in the liquid bulk.

C. Measurements with Alternating High VoltagesFringe-pattern data concerning the electric-field distri-

bution, and hence the space-charge density and distribu-tion, were taken under a variety of alternating high-voltage conditions. 1) Observations were made at thepositive peak of sinusoidal voltages of various levels (to30 kV rms) and frequencies (between 40 and 200 Hz).2) Observations were made at (1/16) T intervals over thecourse of an entire cycle of such operations. 3) As in thedc studies, these observations were conducted using bothcell A and B so as to bring out behavioral differenceswhich might be attributable to the electrode materials.Typical results from these studies are presented anddiscussed in the following paragraphs.

Fig. 8 shows the fringe patterns photographed at thepositive maximum of a 22-kV-rms sinusoidal voltage atvarious frequencies between 40-105 Hz using cell B. Thesignificant observation from these photographs is that thenumber of interelectrode fringes is frequency dependent,with a maximum at approximately 80 Hz. Typical datafrom frequency studies at 25 kV rms, obtained from twentydifferent fringe-pattern photographs, are compiled inTable III. In this table the relative position x/d of eachtransmission minimum, (E/Em) 2,t2 = n, is tabulated withthe frequency of the applied voltage as a parameter. It isimmediately obvious that the number of interelectrodefringes is frequency dependent, with a broad maximumin the vicinity of 80-105 Hz. From these data, the space-charge density as a function of the frequency of theapplied voltage can be calculated [(11) and (12)].Graphs showing the reduced space-charge density pras a function of the frequency of the applied voltage attwo different voltage levels are shown in Figs. 9 and 10.These data, like those in Fig. 8 and in Table III, are theresults of observations taken at the positive maximum ofthe applied voltage. Two conclusions can be seen from theplotted pr versus frequency curves: as the magnitude ofthe alternating voltage is increased, the frequency atwhich the maximum space charge occurs shifts towardhigher frequencies and the magnitude of the maximumspace charge increases.

During the course of this study, data were taken notonly at the positive maximum of the applied voltage butalso at selected points during the cycle of the appliedvoltage. Field distributions plotted from typical observa-tions at various times during a cycle of 15-kV-rms 60-Hzoperation using cell B are shown in Fig. 11. We note thatthe electric-field distribution is linear with positive slopethroughout the whole cycle. If we extrapolate to zero,(1/2) T, or T, the field distribution is linear, being equallypositive and negative about the center (x/d = 0.5).Fig. 12 shows some of the fringe patterns from which theseplots were obtained. Note that at time (1/2) T, the appliedvoltage is instantaneously zero, yet light appears near the



22 kV rms,Positive Maximum

dependent with a maxu a 45 75Hz.

w_fu~~~~~~~~TBEII65O5

: - 0 ~~~70 : 105

Fig. 8. Photographs showing the fringe patterns taken at the positive peak of a22-kV-rms sinusoidal voltage atvarious frequencies between 40-105. It can be seen that the number of interelectrode fringes is frequencydependent with a maximum at approximately 80 Hz.

TABLE IIIFRINGE POSITION Ir/d AS A FUNCTION OF FREQUENCY WITH 25 kV RMS APPLIED TO CELL. B

40 45 50 55 60 65 70 75 80 85 90 95 100 105

.06 .02 .02

.02 .01 .06 .07 .11 .08 .08 .05 .02

.02 .10 .09 .12 .13 .15 .13 .13 .09 .08

.10 .18 .16 .17 .18 .20 .18 .17 .13 .12

.08 .13 .11 .20 .25 .22 .23 .23 .26 .22 .22 .18 .17

.35 .31 .29 .25 .30 .32 .29 .29 .28 .32 .28 .28 .23 .22

.59 .47 .43 .39 .40 .40 .38 .35 .34 .38 .34 .34 .30 .29

.77 .62 .58 .54 .50 .49 .45 .43 .41 .45 .41 .40 .37 .35

.91 .75 .72 .71 .62 .58 .55 .51 .49 .52 .47 .47 .44 .42

.88 .85 .88 .75 .68 .65 .59 .56 .59 .54 .54 .51 .50

.97 .96 .98 .88 .79 .75 .67 .64 .65 .61 .60 .58 .57

.98 .88 .85 .75 .71 .72 .68 .68 .64 .63

.97 .91 .84 .79 .79 .74 .74 .71 .68

.91 .86 .85 .81 .81 .78 .75

.98 .93 .91 .87 .87 .85 .81

.98 .97 .93 .93 .91 .87

.98 .98 .96 .92

.97

x/d25 kv rms

110 115 120 125 130 135

.02

.07

.13

.21

.29

.37

.47

.56

.63

.70

.77

.83

.90

.97

.05

.11

.18

.27

.36

.46

.55

.64

.72

.79

.86

.92

.98

.08

.15

.23

.32

.41

.52

.60

.68

.75

.81

.97

.93

.04

.09

.16

.25

.34

.47

.57

.67

.75

.83

.89

.95

.03

.08

25

.38

.5:'

.6_

.8)

.8i

.93

.05

.11

.19

.30

.45

.59

.72

.80

.88

.94

52

90

FrequencyHz

FringeNo. n

46

48

50

52

54

56

58

60

62

64

6668

70

72

74

76

78

80



20

zLJJ0

LLICD)0rCo0

C-)n0

w

161_

12 -

04

50 70 90 110 130

FREQUENCY (Hz)Fig. 9. The reduced space-charge density p, in cell B as a function

of the frequency of an applied 15-kV-rms voltage. Each data pointwas determined from the electric-field distribution observed atthe positive peak of the applied sinusoidal voltage. The error barsindicate the standard deviation of the coefficient p, as determinedfrom a linear least squares fit of the electric field between theelectrodes. Note that the reduced space-charge density has amaximum at a frequency of approximately 60 Hz.

6

5

4

-31W

I 2

C-z

0O

g

-J

-1

-2

-J

Ld3

-4

-5

-6

0.1 0.2 0.3 0.4 0.5 0.6 07 0.8 0 9RELATIVE DISTANCE FROM HV ELECTRODE Wd)

Fig. 11. The electric field distribution in cell B at various timesduring the cycle of 60-Hz 15-kV-rms applied voltage. The periodof the applied waveform is denoted T, so, for example, the datadesignated as 1/4T were taken at the positive maximum of theapplied voltage.

2.0

I-

zw0

CD

I

w

U)

C)0wC-

LE

08-

0.4

FREQUENCY (Hz)

Fig. 10. The reduced space-charge density as a function of thefrequency of an applied 25-kV-rms voltage. The meaning of eachdata point is the same as in Fig. 9. In comparison with Fig. 9the maximum reduced space-charge density has shifted to ahigher frequency of approximately 90 Hz.

60Hz15kVrms

38T 7/16fT

1/ T 91T

5/8T 11/6TFig. 12. Photographs from which some of the data in Fig. 11 were

taken. Note that there is some light transmitted at 1/2T, i.e.,when the voltage is instantaneously zero. As explained in thetext, this indicates that the bulk space-charge density is constantthroughout the cycle.

I~ { I{Ii

08k_

I50 70 90 l10 130

I I

(1/4) T

(3/16) T

-̂' ~~~~~~~~~(1/16)T-

(9/16)T

(5/8)T -

(I1/16) T

I= I3/4)T

53



8ai 25 kV

7-_ _2fon cv

5 t15 kV

4 . X10 kV

40 Hz

.2 .4 .6 .8 /

E/Em 25 kV

; ---t ->~~~20 kVf ~~~~~~~~15 kVfi10 kV

-~~~8Q Hz

I , >- x/d.2 .4 .6 .8

25 kV

20 kV

15 kV

10 kV

60 Hz

9

8

7

6

5

4

2

I x/d

E/Em 30 kV

20 kV

15 kV

10 kV

100 Hz

.2 .4 .6 .8 1.0

Fig. 13. The electric-field distribution at selected frequencies and voltage levels in cell B. All of the data in thisfigure were taken at the positive maximum of the applied voltage.

electrodes. When the voltage is instantaneously zero, thetotal surface charges on each electrode are negative andequal in magnitude, while the total volume bulk charge isdouble in magnitude and positive, keeping the systemneutral. (The surface charge per unit area on the electrodescan be computed by the discontinuity of the normalcomponent of EE.) At other times in the cycle, the volume-charge density remains constant (independent of voltagepolarity), while the surface charge on the electrodes variessinusoidally with time.

It is also interesting to note from Fig. 12 that again somelight, although over a much smaller area than was foundwith de voltages, is also transmitted behind the cathodeduring ac operation. Further measurements over thecomplete cycle show this anomalous field enhancementfollows the sinusoidal cycle, alternating between elec-trodes, as the field enhancement always appears behindthat electrode which is instantaneously negative.Measurements of the electric field as a function of

position at the positive maximum of the applied voltageat various voltage levels and selected frequencies are

plotted in Fig. 13. From this type of observation, thefollowing results, showing relationship between the fre-quency finax at which the net space-charge density was

maximum and the applied-voltage level, were obtained:

Measurements of f at Various Voltage Levels Using Cell BVoltage fra(kV rms) (Hz)

10 45-6512 55-6515 60-8520 80-9522 80-9525 90-105

Similar measurements with cell A showed many of thesame features. The dc field was observed to be a linearfunction of position between the electrodes (see Fig. 5),again indicating a uniformly distributed net positivecharge in the liquid bulk. In addition, observations at thepeak voltages during ac operation at various frequenciesindicated that the distortion in the ac field, and thus thedensity of charge in the liquid, was again frequency de-pendent. Measurements with 20 kV rms applied showed,for example, that the field distortion and charge densitywere maximum at approximately 100 Hz, thereby demon-strating good agreement with the B-cell measurementswhich showed fmax80-95 Hz at 20 kV rms (see previoustable).

There were, however, also significant differences be-tween the A and B observations. The A-cell fringe-patternmeasurements consistently showed that the A-cell ac

electric field is not a linear function of position, and thatthe field distribution varies periodically with time duringeach cycle of ac operation, being periodically uniform andnonuniform as may be seen from the field measurementdata plotted in Fig. 14. The curves show that maximumdistortion occurs near the positive and negative peaks ofthe alternating voltage waveform. Moreover, observationsat these times showed that the interelectrode field in thiscell has an inverted "U-shaped" distribution (see the ac

results in Figs. 5 and 14). Whereas a linear field distribu-tion was found typical in cell B, this inverted U-shapeddistribution, which implies the presence of positive space

charge near the anode and negative charge near thecathode (a situation noted by Croitoru [19] as beingindicative of charge injection at the electrodes), was foundtypical of cell A at the voltage peaks during ac operation.

6

3

2

KE/EmA. E/Ern30 kV

98

7

6

5

4

3

2

.2 .4 .6 .8 1.0.

54

6

4

zo kV



-E 41W"I (3/8)Tl3r ,(i/8)T

2 -zW (7/16) Tcn

,Z o_C,, -2

H -I ~~~~~~~~~~~~~(9/16)T

-2w (5/8)T<F-3 _

-5

0.2 0.4 0.6 0.8RELATIVE DISTANCE FROM HV ELECTRODE (x/d)

Fig. 14. The electric-field distribution in cell A as a function oftime with a 60-Hz 15-kV-rms applied voltage. Note that in thiscell the field distribution is no longer linear, but varies periodicallyduring a cycle.

Finally, since the field-distributions characteristic ofeach cell were found to be reproducible for substantialperiods of time, in spite of repeated refillings of the cellswith nitrobenzene, we have felt it reasonable to attributethe differences between their ac field distributions to thedifferences in their electrode materials and/or surfacepreparations. Verification of this conclusion and distinc-tion of the relative contributions of the material from thoseof the surface conditions will, however, require furtherstudy.

V. SUMMARY AND CONCLUSIONS

Our purpose has been to document, from examinationof results from extensive Kerr-effect fringe-pattern ob-servations in nitrobenzene, several space-charge-inducedtrends in the behavior of an insulating liquid under thestress of high-voltage operation. We have demonstratedexperimentally that electric-field distributions in thebulk of the liquid are predictable only during operationunder short (microsecond) pulse conditions. Results fromobservations during dc and low-frequency ac operationhave shown that the interelectrode electric-field and space-charge distributions are dependent upon the level andfrequency of the applied voltage, the electrode materialand their surface condition, and the immediate previoushistory of the system.

In a cell with electropolished stainless-steel electrodes,the interelectrode-field distribution was approximatelylinear and the net space-charge density was positiveduring operation under high dc and low-frequency volt-ages. Further, the net charge measured during operationat a given ac voltage (i.e., at a constant rms level) was

found to be independent of the instantaneous voltage.The net charge in the liquid bulk between the electrodesremained constant throughout the entire course of theac cycle, even when the instantaneous voltage was zero,thereby suggesting that the field and charge distribu-tions in this cell are governed primarily by a steady-statearrangement of charged particles uniformlv distributedand suspended in the bulk by the steady streaming motionof the liquid between and around the electrodes.

Observations with a cell having glass-blasted nickelelectrodes showed different results. The interelectrode-field distribution was linear under direct voltage, butparabolic under alternating voltages. The observed acfields were weaker near the electrodes and stronger in theregion midway between them, thereby implying the forma-tion of positive space charge near the anode and negativecharge near the cathode. As might be expected in a chargeinjection process, the electric-field distribution and chargedensity in the bulk also varied as a function of time (andthus voltage) during each cycle of ac operation. The dif-ference between the field and charge behavior in the twocells is attributed to their differences in electrode materialsand surface conditions.

Except for these features, essentially the same behaviorwas observed in both cells. Their dc fields were bothlinear functions of position between the electrodes, andboth showed space-charge enhancement of the field behindthe negative electrode. This latter effect is maximum fordirect voltages and decreases with excitation voltagefrequency, whereas electrohydrodynamic motion of theliquid (always observed during steady-state high-voltageoperation), distortion of the interelectrode field, andcharge density in the liquid are much more pronouncedduring low-frequency ac operation.With regard to the distortions in the ac fields, the dis-

tributions observed were found to be frequency de-pendent; as the frequency of the exciting voltage was in-creased the charge measured in the liquid bulk betweenthe electrodes increased. The plot of space-charge densityversus frequency exhibited a maximum, with this max-imum shifting toward higher frequencies as the voltagelevel is increased. As the frequency of excitation wasfurther increased, the field distribution became uniformand the space-charge density in the liquid approachedzero, as would be expected when the time between polarityreversals was too short for charge collection in the liquidnear and around the electrodes.

Finally, though these observations still offer no con-clusive results for reliable prediction of steady-state fieldand charge behavior (and thus of the onset of breakdown)in an insulating liquid, they do demonstrate severalimportant, as yet undocumented trends to be expected inthe bulk of the liquid when under the stress of high-voltage operation. It is hoped that the experimentaevidence presented will stimulate the further research andtheoretical investigation needed for a more thoroughunderstanding of the mechanisms of breakdown in liquid-insulated high-voltage equipment.

I I5 (-/4) T

55



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ing system based on Kerr effect," Rev. Sci. Instr., vol. 34, pp.221-224, 1963.

[2] D. C. Wunsch and A. Erteza, "Kerr cell measuring system forhigh voltage pulses," Rev. Sci. Instr., vol. 35, pp. 816-820, 1964.

[3] E. C. Cassidy, H. N. Cones, D. C. Wunsch, and S. R. Booker,"Calibration of a Kerr call system for high-voltage pulsemeasurements," IEEE Trans. Instrum. Meas. (1968 Conf.Precision Electromagnetic M11easurements), vol. IM-17, pp. 313-320, Dec. 1968.

[4] E. C. Cassidy, H. N. Cones, and S. R. Booker, "Developmentand evaluation of electrooptical high-voltage pulse measure-ment techniques," IEEE Trans. Instrum. Meas. (1970 Conf.Precision Electromagnetic Mleasurements), vol. IM-19, pp. 395-402, Nov. 1970.

[5] A. W. Bright, B. Makin, and A. J. Pearmain, "Field distribu-tion in nitrobenzene using the Kerr effect," Brit. J. Appl. Phys.(J. Phys. D), vol. 2, pp. 447-451, 1969.

[6] P. Atten and J. P. Gosse, "Regime transitoire de conductionlors d'une injection unipolaire dans les liquids isolants," inPhenomenes de Conduction dans Liquides Isolants. Paris,France: Editions du Centre National de la Researche Scien-tifique, 1970, pp. 325-343.

[71 E. A. Cherney and J. D. Cross, "Space charge effects in chloro-biphenyls," IEEE Trans. Elec. Insul., vol. EL-8, pp. 10-16,Mar. 1973.

[8] J. D. Cross and R. Tobazeon, "Electric field distortions pro-duced by solid dielectric spacers separating uniform fieldelectrodes in nitrobenzene," IEEE Trans. Elec. Insul., Vol.

El-8, pp. 25-29, Mar. 1973.[9] R. E. Hebner and E. C. Cassidy, "Measurement of 60 Hz

voltages using the Kerr effect," Rev. Sci. Instr., vol. 43, pp.1839-1841, 1972.

[10] E. C. Cassidy and H. N. Cones, "A Kerr electro-optical tech-nique for observation and analysis of high intensity electricfields," J. Res. Nat. Bur. Stand., Sec. C, vol. 73, pp. 5-13, 1969.

[11] E. C. Cassidy, "Pulsed laser Kerr system polarimeter forelectro-optical fringe pattern measurement of transient elec-trical parameters," Rev. Sci. Instr., vol. 43, pp. 886-892, 1972.

[12] C. E. Hill and H. House, "The problems in using the Kerrelectro-optic effect to measure the field distributions in non-polar liquids," in Phenomenes de Conduction dans LiquidesIsolants. Paris, France: Editions du Centre National de laResearche Scientifique, 1970, pp. 465-481.

[131 MA. Zahn and J. R. Melcher, "Space charge dynamics of liquids,"Phys. Fluids, vol. 15, pp. 1197-1205, 1972.

[14] M. Zahn, "Dynamics of stratified liquids in the presence ofspace charge," Phys. Fluids, vol. 15, pp. 1408-1417, 1972.

[15] W. R. L. Thomas, "The nature of particulate charge carriers inn-hexane and their role in electrohydrodynamic phenomena,"in 1972 Annu. Rep. Conf. Electrical Insulation and DielectricPhenomena (Nat. Acad. Sci., Washington, D. C.), 1972, pp.52-59.

[16] A. W. Bright and B. Makin, "Modern electrostatic generators,"Contemp. Phys., vol. 10, pp. 331-353, 1969.

[17] A. M. Zarem, F. R. Marshall. and F. L. Poole. "An electro-optical shutter for photographic purposes," AIEE Proc., vol.68, pp. 84-91, 1949.

[181 F. J. McCling and R. W. Hellwarth, "Giant optical pulsationsfrom ruby," J. Appl. Phys., vol. 33, pp. 828-829, 1962.

[19] Z. Croitoru, "Space charges in dielectrics," in Progress in Di-electrics, vol. 6, J. B. Birks and J. Hart, Ed. New York:Academic, 1965, pp. 103-146.

[20] W. F. Pickard, "Electrical force effects in dielectric liquids,"in Proqress in Dielectrics, vol. 6, J. B. Birks and J. Hart, Ed.New York: Academic, 1965, pp. 1-39.

[211 H. T. Jessop and F. C. Harris, Photoelasticity, Principles and,M1ethods. New York: Dover, pp. 68-70.

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[25] G. Nienhuis and J. MI. Deutch, "Structure of dielectric fluids1I. The free energy and the Kerr effect in polar fluids," J. Chem.Phys., vol. 56, pp. 235-247, 1972.

[26] P. Mazur and B. J. Postma, "On the molecular theory of theKerr effect," Physica (Utrecht), vol. 25, pp. 251-267, 1959.

[27] E. C. Cassidy, W. E. Anderson, and S. R. Booker, "Recentrefinements and developments in Kerr system electrical meas-urement techniques," IEEE Trans. Instrum. Meas. (1972Conf. Precision Electromagnetic MTeasurements), vol IM-21, pp.504-510, Nov. 1972.

[28] Details of the fitting procedure can be found in J. Hilsenrathet al., Omnitab, A Computer Program for Statistical and Nu-merical Analysis, National Bureau of Standards Handbook101, U. S. Government Printing Office, Washington, D. C., pp.124-140.

[29] R. Tobazeon, "Comportement du nitrobenzene pur souschamps intenses continus et alternatifs," in Phenomenes deConduction dans Liquides Isolants. Paris, France: Editions duCentre National de la Researche Scientifique, 1970, pp. 407-422.

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[33] For a recent discussion of these effects see A. H. Cookson anid0. Farish "Particle-initiated breakdown between coaxialelectrodes in compressed SF6," IEEE Trans. Power App. Syst.,vol. PAS-92, pp. 871-876, May/June 1973.

[34] G. Briere and J. Gosse, "Electrodialyse des solvants polaires,"J. Chim. Phys. Physicochim. Biot., vol. 68, pp. 1341-1348, 1968.

56


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