IEEE TRANSACTIONS ON ELECTRICAL INSULATION, VOL. EI-7, NO. 4, DECEMBER 1972

The Flashover Voltage of Polymethylmethacrylatein Vacuum Under Direct, Alternating, and Surge

Voltages of Various Front DurationsS. GRZYBOWSKI, E. KUFFEL, AND J. P. C. AcMATH

Abstract-This paper presents results on flashover voltagesacross insulation surfaces and on the rate of surface deteriorationin a vacuum of 10-5 torr under direct, alternating, and surge

voltages of front duration in the range from 1-600 /.s. The insulationmaterial used was polymethylmethacrylate (PMMA) of cylindricalshape fitted between uniform field electrodes. The measurementswere carried out on insulator specimens 25 mm in diameter and5, 10, and 20 mm in length.

I. INTRODUCTION

N high-vacuum high-voltage devices, the insulationis provided by the vacuum space, and/or by a

dielectric material, or a combination of both. Ingeneral, a flashover across the dielectric surface takesplace at a lower voltage than in vacuum gap, and thesurface flashover usually becomes the limiting factorin the operation of high-voltage high-va.cuum devices.While a considerable amount of information may be

found in the literature on breakdown of vacuum gaps,much less is known about the flashover mechanismacross dielectrics in vacuum. The results of earlier inves-tigators on the latter subject published prior to 1968[1]-[4] have been summarized by Hawley [5]. In addi-tion, several papers have appeared in the literaturewithin the last three years [5]-[9] dealing mainly withthe factors that effect the surface flashover voltage. Thevarious investigators have used direct voltages, powerfrequency voltages, and standard impulses. High-vacuum devices are now finding their application in high-voltage power systems and as such will frequentlyexperience switching surge voltages. Feasibility studies[14] indicate a good chance for the application ofvacuum-insulated cryogenic cables in the transmissionof electrical power in the distant future. The design ofsuch cables will include the use of spacers, which, forpractical reasons, are likely to be of organic nature.Consequently, it was felt that a systematic study of themore common organic insulation materials is justified.The present series of investigation included surface

Manuscript received August 24, 1971; revised June 16, 1972.This work was supported by the National Research Council ofCanada.

S. Grzybowski is with the University of Manitoba, Winnipeg,Man., Canada. He is currently on leave from Politechnika,Poznanska, Poland.E. Kuffel was with the University of Manitoba, Winnipeg,

Man., Canada. He is now with the University of Windsor, Wind-sor, Ont., Canada.

J. P. C. MeMath is with the University of Manitoba, Winnipeg,Man., C'anada.

flashover studies across polyethylene, tetrafluorethylene,and polymethylmethacrylate. The results obtained on theformer two materials have been reported elsewhere [15].The experiments were designed to investigate specificallythe effect of the wave shape of surge voltages upon thesurface flashover values. For comparison purposes mea-

surements were also carried out under direct and power-

frequency voltages and standard impulses of both polari-ties. The studies also included the progressive surfa-cedegradation caused by consecutive flashovers.

II TEST EQUIPMENTThe tests were carried out in a vacuum system con-

tinuously evacuated by a rotary pump of a pumpingrate of 190 1/min and a 4-in oil-diffusion pump, thelatter equipped with a liquid-nitrogen trap capable ofgiving an ultimate vacuum of about 10-7 torr. The pres-

sure was indicated by a Penning-type gauge mountedin the base of the chamber. Earlier experiments [10]showed that within the range of 10-61_0-4 torr the surfaceflashover values remained nearly independent of thepressure; hence, the results presented in this paper were

obtained at a pressure of 10-5 torr.

The cross section of the test chamber is shown inFig. 1. It consisted of a glass cylinder about 40 cm highand 30 cm in diameter fitted with stainless-steel endplates. Neoprene gaskets sealed the system. The elec-trodes consisted of smooth nickel-plated brass of 6-inchdiameter and were designed to give uniform field forspacings up to 30 mm. The direct high voltage was

obtained from a 200-kV source and was mneasured witha resistance divider.The surge voltages were derived from a two-stage

impulse generator shown in Fig. 2. By varying the cir-cuit resistances R1 and R2 and the capacitance C1, itwas possible to obtain surges with front duration extend-ing from 1-600 I,s and with the corresponding wave-tailduration extending from 50-3000 js. Capacitances C3and C4 constituted the capacitor divider that was usedin conjunction with a Tektronix oscilloscope type 585Afor measuring the impulse voltages.The test specimen is shown placed between the elec-

trodes. In order to monitor the discharge visually a tele-scope was set up outside the chamber, which enabledviewing discharges in the vicinity of the electrodes as

well as on the insulation surface. In this way it was

possible to distinguish between prebreakdown discharges

180

GRZYBOWSKI et al.: FLASHOVER VOLTAGE OF POLYMETHYLMETHACRYLATE

Fig. 1. Vacuum chamber. 1-nickel plated electrodes, 2-testspecimen, 3-insulators supporting the grounded electrode,4-Glass cylinder, 5-metal end plates.

Ro G R2 C3

Fig. 2. Circuit arrangement of surge generator. DC-chargingunit, TG-trigation, G-sphere gap, S-test specimen, C-im-pulse generator capacitance 0.25 sF, Ro charging resist-ance 3.5 ko, Ri-tail resistance 520 2 to 100 kQ, Z0 =- K-cable impedance 75 Q, C1-load capacitance 0.005 ,uF (notused for standard impulse), C2/C3-capacitance divider0.001/0.1 AF.

originating from the electrodes and those originating atthe dielectric surface.

III. EXPERIMENTAL PROCEDURE

It is well known that in a simple test arrangementsuch as is shown in Fig. 1, the cathode-insulator junctionplays an important role in the initiation of the dischargeby supplying the initiatory electrons. To secure a goodcontact between the specimen and the electrodes, theends of the cylinders were ground and polished andwere then silver plated by a vacuum-deposition process.

The contact pressure at specimen-electrode junction was

maintained constant in all tests by allowing the topelectrode to rest on the specimen, giving a pressure ofabout 2 kg/cm2 on the contact surface. Before mountingthe specimens between the electrodes, they were washedin trichloroethylene and dried with a clean cloth. Inseveral experiments an acetone was used as an alterna-tive washing liquid giving no significantly differentresults.

Preliminary measurements showed that the amount ofprebreakdown discharge and the rate of the specimens'deterioration depended largely upon the time of de-gassing before voltage application. As the material inves-tigated polymethylmethacrylate (PMMA) has a rela-tively low softening temperature, it was impossible toapply the baking-out technique. Consequently, beforetaking readings, the specimen was left inside the chamberand the system was pumped for 24 h at 10-6 torr.Under direct or alternating voltages prebreakdown

discharges were observed both near the electrode junc-tions and on the insulator surface on all new uncondi-tioned specimens at voltages in excess of about 50 per-

cent of the flashover value. In the former case theintensity of discharges was more severe near the anode.

181

The discharges were luminous and self-extinguishing butwere too weak to cause any appreciable drop in the gapvoltage. Their frequency of occurrence decreased withthe time of voltage application, and usually after severalminutes under applied voltage the predischarges ceasedcompletely.To obtain consistent and reproducible flashover values

each new specimen was conditioned by applying a directvoltage slightly above the discharge onset value untildischarges ceased. Then the voltage was increased insteps of 1 kV every 2 min until flashover occurred.Most previous investigators have stated that their

measured flashover values were strongly dependent uponthe conditioning of the samples but no standard condi-tioning technique has yet been suggested. Our measure-ments with both dc and impulse voltages showed thatthe flashover values not only depended upon the degass-ing period but also on the intervals between successivevoltage applications, e.g., when the waiting period was1-2 min, the second and subsequent flashover valueswere erratic and in some cases as much as 50 percentbelow the first flashover. To obtain consistent results(within +5 percent) under direct or 60-Hz voltages, itwas necessary to allow about 30 min between successiveflashovers. Such long waiting periods suggest the possi-bility that space charge, partially trapped in some wayat the surface of the specimen, may have been respon-sible for the low flashover values.With surge voltages, preliminary measurements

showed a large scatter in the breakdown results obtainedfor new unconditioned specimens. Prebreakdown dis-charges were also observed. Their presence could benoted by a transient increase in pressure and by theirfaint luminosity. To obtain consistent results, beforeapplying surges the specimens were conditioned underdirect voltages in the same way as above. Then 10 surgeswere applied at each voltage level at intervals of 1 min.The voltage was increased in steps of 5 percent of theflashover value starting from about 70 percent of theanticipated breakdown value. Five samples were testedfor each spacing and for each voltage waveshape, andthe average value was taken as the flashover value.

IV. RESULTS AND DISCUSSION

A. GeneralNumerous investigators have shown that the condition

of electrodes is the most important factor effecting thebreakdown voltage of a straight vacuum gap. Less infor-mation is available on the effect of the electrodes andtheir condition upon the flashover voltage across insula-tion surface in vacuum. The present experiments indi-cated that the surface condition of the insulation had aprevailing influence upon the flashover voltage whilethe influence of the electrodes' surface condition wasrelatively insignificant as long as the electrode surfaceremained smooth and clean. The condition of the insula-tion surface also had a strong effect upon the intensity

182

of prebreakdown discharges that increased with thelength of the insulator.

Specimens subjected to alternating voltages showedmore intensive prebreakdown discharges than identicalspecimens subjected to direct voltages. In the formercase discharges could be observed near both electrodes,and for specimens of 10-mm length and longer, the dis-charges continued as the voltage approached breakdown.

It also should be emphasized that numerous dischargeswere observed that originated from the dielectric surfaceand terminated at another point on the surface withoutbridging the gap. Such discharges are probably asso-ciated with space charges on the surface as suggested byFryszman et al. [11]. Both their intensity and theirfrequency of occurrence increased with the roughness ofthe dielectric surface.

IEEE TRANSACTIONS ON ELECTRICAL INSULATION, DECEMBER 1972

Fig. 3. Flashover voltage-Insulator length relationship for dif-ferent types of voltages (p = 10-5 torr). The ac curve representsthe rms values.

B. Relation Between Breakdown Voltage andLength of Insulator

The flashover voltage-insulator length relationship isshown in Fig. 3 in which the results obtained underdirect, alternating (60-Hz rms), and surge voltages ofvarious front duration are included. The range of scatterof the results for dc and standard impulse voltages isindicated by arrows. The range remained approximatelythe same for other type of voltages for conditionedspecimens (within 45 percent). The flashover voltagesfor each type of testing voltage increased with lengthof the specimen. However, the rate of increase of theflashover voltages with specimen lengths is different foreach type of voltage.

It is seen that the rate of increase in flashover voltagewith increasing specimen length is lowest for 60-Hz volt-ages. Under surge voltages the rate increases as thefront duration is decreased and the standard impulsevoltages giving the highest values. However, the patterndid not remain progressively uniform. For example,surges of 80-,ts front duration gave lower flashover valuesthan 600-as front surges. This problem is discussedfurther in the next section where a, histogram includingall data is presented.

Fig. 3 showis that the flashover values under directvoltages were highest for all specimens studied. It wasmiientioned earlier that the high and consistent directflashover values are most likely the result of the pro-longed conditioning technique adopted in the presentwork, in which the specimens were subjected to highvoltage gradually increasing in value until flashovertook place.

C. Effect of Voltage Waveshape on Flashover VoltageTo demonstrate the effect of the surge waveshape upon

the flashover values, the results obtained under switch-ing surge voltage of various waveshapes (shown inFig. 3) are reproduced in the form of a histogram inFig. 4, where for comparison purposes the correspondingvalues for direct, alternating, and standard impulse

Fig. 4. Influence of voltage waveshape on flashover voltage.p = 10l-5 torr. Specimens thickness 5, 10, 15, 20 mm.

voltages are also included. Under surge voltages theaverage flashover values depended upon the wavefrontduration. The standard impulse voltage gave the highestvalue. As the front duration wias increased, the flashovervalues decreased and reached a minimuin for a surgeof front duration near 80 jAs. For longer front waves theflashover increased gradually with the front duration.Increasing the length of test samples shifted the min-imum value of flashover towards longer front duration,e.g., for a 5-mm specimen the minimum value occurredwith a switching surge 80/700 as. The relative decreasein flashover voltage with increasing front duration washighest for the longest samples. The U-shape flashovervoltage surge front-duration relationship exhibited inFig. 4 is no news to insulation engineers. Numerousinvestigators have studied breakdown in gases, liquids,and solids under switching surge voltages and have estab-lished a minimum breakdown strength at some particularwavefront duration voltage. Kalb [13] has studied thecorresponding relation for long insulators in air andfound that the most severe surges are those of frontduration between 50 and 150 js. The effect is generallyattributed to presence of space charges in some formor other, but no clear understanding of the precisemechanism is yet on hand.The effect of wave-tail duration on the flashover volt-

age was not studied systematically since flashover usu-ally occurred at or near the peak voltage. Occasionally

183OItZYBOWSKI et al.: FLASHOVER VOLTAGE OF POLYMETHYLMETHACRYLATE

Fig. 5. Oscillograms of consecutive surges 600/3000 ,s. Specimens thickness-5 mm; pressure-10- torr; voltage-28.0kV. Surges from 1 to 5.

flashovers were observed several microseconds beyondthe crest value.

D. Surface Deterioration With the Numberof Applied Surges

The rate of specimen deterioration depended on factorssuch as conditioning, type of voltage applied, and energyof surge. With direct and alternating voltages the limit-ing resistance used was 3 Mu2 and deterioration causedby consecutive breakdowns was slow. After the iniiialconditioning process the flashover values remained con-stant for some 30-50 voltage applications. Then thevalues started to decrease gradually. With surge voltagesthe rate of deterioration was much more rapid.Not much is known about the mechanism of deteriora-

tion of the dielectric material during a flashover invacuum. The information presented in this paper givesonly an indication of the rate of deterioration of PMMAduring flashover in vacuum. Fig. 5 shows oscillogramsof a sequence of 600/3000-ps surges. The voltage in allcases was of the same magnitude and of a sufficient valueto cause flashover. It is seen that at the beginning ofthe tests, flashover may or may not take place. Sub-sequently, the surges cause a rapid degradation of thesurface and multiple flashovers with a single surgefollow, leading ultimately to the formation of a completetrack on the surface. The shorter front-wave surges,obtained with the lower values of front resistors, whichalso acted as limiting resistors, tended to cause a morerapid deterioration.

It was stated earlier that some flashovers occurreda few microseconds after crest value. Fig. 6 shows sucha case, where the first flashover (surge 2) is followedby three reignitions, but surge 3 is followed by only onereignition, which occurred on the tail of wave; this doesnot imply that the dielectric withstand voltage is in-creased. The deterioration of the dielectric surface insome cases is gradual but in others it proceeds veryrapidly. Fig. 7 illustrates a gradual surface damagecaused by breakdowns under 400/2000-,ts surge voltages.

Fig. 6. Oscillograms of consecutive surges 80/700 Ms. Specimenthickness-5 mm; pressure-10-5 torr; voltage-21.5 kV. Surgesfrom I to 5.

The second surge caused a flashover at a lower voltageand was followed by several reignitions. Not all samplesdeteriorated as systematically as is shown in Fig. 7.Fig. 8 shows a case where the first flashover damagedthe specimen surface so badly that the next flashoverled to the formation of a complete conducting track.On inspecting the test samples after flashover it was

observed that a tree-type conducting path had devel-oped on the surfaces, indicating a similarity between theflashover mechanisms in vacuum and in gases. Themultiple tree-type channels tended to be longer andmore pronounced when alternating voltages were used.Their length also increased with increasing the lengthof the test specimens.

V. CONCLUSIONS

Flashover voltages across insulation in vacuum underdirect, alternating, and surge voltages were investigated.The measurements showed that the flashover value de-pended upon the length of the degassing period, thedegassing pressure, the conditions of the dielectric sur-

IEEE TRANSACTIONS ON ELECTRICAL INSULATION, DECEMBER 1972

Fig. 7. Oscillograms of consecutive surges 400/2000 us. Specimen thickness-5 mm; pressure-10-5 torr; volt age-34.0kV. Surges from 1 to 5.

Fig. 8. Oscillograms of consecutive surges 80/700 us. Specimen thickness-5 mm; pressure-10-5 torr; voltage-25.0kV. Surges from 1 to 7.

face, and on the intervals between successive voltageapplications. The flashover voltage across insulation invacuum also depended upon wavefront duration. Itdecreased with increasing wavefront duration andreached a minimum value at a wavefront duration thatdepended upon the length of the insulation. For therange of insulation-length studies (5-20 mm) the min-imum values were obtained with surges of front durationbetween 50 and 150 ,as.

Prolonged application of a direct or a power-frequencyvoltage to insulation specimens in vacuum at a levelbelow flashover appears to condition the surface elec-trically and improves the dielectric strength sub-stantially. In the course of conditioning, prebreakdowndischarges were to be observed near the electrodes andoccasionally on the insulation surface. The dischargesdisappeared gradually as the conditioning progressedindicating that initial minute irregularities at the cath-ode-insulator junctions may have been the cause of theearly prediseharges.

Discharge current oscillograms showed that undersurge voltages single flashovers do not generally lead tosignificant deterioration of the surface. The sample tendsto recover rapidly without discharging the total energy

of the generator. Complete discharge follows only in thecase when a full conducting track is formed on the in-sulator surface.

REFERENCES[1] E. S. Borovik and B. P. Batrakov, "Investigation of break-

down of vacuum," Sov. Phys.-Tech. Phys., vol. 3, pp. 1811-1818 1958.

[21 P. H. Gleichauf, "Electrical breakdown over insulators inhigh vacuum," J. Appl. Phys., vol. 22, no. 5, pp. 535-541,766-771, 1951.

[3] M. J. Kofoid, "Phenomena at the metal-dielectric junctionsof high-voltage insulators in vacuum and magnetic field,"AIEE Trans. (Power App. Syst.) vol. 75, pp. 991-999, 1960.

[4] R. Hayes and G. B. Walker, "Vacuum breakdown at aglazed ceramic surface," Proc. Inst. Elec. Eng. (London),vol. 3. no. 3, pp. 600-604, 1964.

[5] R. Hawley, "Solid insulators in vacuum: A review," Vacuum,vol. 18, pp. 383-390, July 1968.

[6] J. Bobo, M. Perrier, B. Fallou, and J. Galand, "Dielectricstrength of polymers at cryogenic temperatures undervac"ium." Vacuom, vol. 18, pp. 397-401, July 1968.

[7] C. C. Erven, K. D. Srivastava, and R. G. Van Heeswijk,"Prospects for vacuum installation in a.c. high voltagepower apparatus," presented at the IEEE Summer PowerMeeting, Los Angeles, Calif.. 1970, Paper 70 CP 602-PWR.

[8] P. Grancau, "Vacuum insulation for cryogenics," Vacuum,vol. 18, pp. 395-396, Juilv 1968.

[9] K. D. Srivastava and C. H. de Tourreil, "Electrical break-down across ceramic surfaces in high vacuum under d.c.nnd rin'se voltages." in Proc. 3rd Int. Symp. Discharyes andE1ectri~oa1 Insulation in Vaouem, Paris, France. Sept. 1, 1968.

[10] E. Kuffel, S. Grzybowski, and J. P. C. McMath, "Breakdownacross insulation surface in vacuum under direct, alternating

184

IEEE TRANSACTIONS ON ELECTRICAL INSULATION, VOL. EI-7, NO. 4, DECEMBER 1972

surge voltages of various waveshape," in Proc. 4th Int.Symp. Discharges and Electrical In.sulation in Vacuum,Waterloo, Ont., Canada, Sept., 1970.

[11] A. Fryszman, T. Strzya, and M. Wasinski, "On a mechanismof breakdown in high vacuum," Bull. Acad. Pol. Sci.,vol. 8, no. 7, 1960.

[12] I. I. Kalyatski and G. W. Kassirov, "An investigation ofpulse flashover of some solid dielectrics in vacuum," Sov.Phys.-Tech. Phys., vol. 9, no. 8, pp. 1137-1140, Feb. 1965.

[13] J. W. Kalb, "How the switching surge family affects lineinsulation," Elec. Eng., vol. 82, p. 395, 1963.

[14] P. Graneau and L. B. Thompson, "Three functions ofvacuum in cryocables," presented at the IEEE WinterPower Meeting, New York, N. Y., Jan. 1972, Paper C72 119.1.

[15] E. Kuffel, S. Grzybowski, and R. B. Ugarte, "Flashoveracross polyethylene and tetrafluorethylene surface in vacuumunder direct, alternating and surge voltages of variouswaveshapes," J. Phys D: Appl. Phys., vol. 5, p. 575, 1972.

The Effect of Oxidation on the DielectricProperties of an Insulating Oil

RAYMOND M. HAKIM

Abstract-The dielectric constant and loss, as well as the resis-tivity of a cable insulating oil, have been measured as a function oftemperature and frequency before and after successive oxidations.The dielectric loss, and its distribution of relaxation times changesslightly with the degree of oxidation, but the resistivity decreasesby more than an order of magnitude. However, the activationenergy for conduction remains unchanged. A previously unreportedresult is that the frequency of the maximum of the dielectric lossdecreases with the degree of oxidation. A tentative semiquantitativemolecular model to account for the activation energy for conductionis presented.

INTRODUCTION

r HE EFFECT of oxidation on the dielectric prop-ertics of insulating oils is important in determiningthe stability of oils during their use in power

systems. Several electrical, chemical, and other types oftests have been evolved to judge the effect of oxidationon these oils [1]. Although the correlation of these ac-

celerated tests with actual oil performance in use hasyet to be demonstrated, the mechanism of oil degrada-tion should be the same since the field conditions are

duplicated as closely as possible [2].A particularly convenient and useful comparative test

involves periodic measurement of the power factor or

dielectric loss as a function of the oxidation conditions[2]. However, these changes have not been related to thedipole or ionic processes in these oils. The present paper

is an attempt to clarify this problem.In a previous paper [3], the dielectric properties of a

commercial high-viscosity mineral oil commonly used as

an impregnant in cable insulation has been reported.

Manuscript received January 19, 1972; revised July 25, 1972.The author is with the Department of Materials Science, Insti-

tute of Research, Hydro-Quebec, Quebec, Canada.

The dispersion in the values of the dielectric constantindicated that this oil behaves dielectrically as a dilutesolution of polar compounds in a nonpolar medium. Also,the dielectric loss was characterized by an apparent dis-tribution of relaxation times, which increased in widthwith decreasing temperature, and was related to the coop-erative motion of the dipoles in the oil. In addition, nocorrelation was found between the shear viscosity or itsdependence on temperature with the intrinsic dipole re-laxation time of the oil.The same oil was used in the present study. Although

the carbon type (aromatic, naphtenic, and parafinic) wasnot specified, it was ascertained from the manufacturerthat no inhibitors were intentionnally added to the oil.

EXPERIMENTAL

The experimental details for the measurements of thedielectric loss, dielectric constant, and viscosity are sub-stantially the same as those outlined in [3], with the ex-ception of the temperature control, which was providedby an environmental temperature test chamber withforced-air circulation (INRECO, model 11901) insteadof the closed-loop methanol circulation. Temperatureuniformity and constancy were the same or better:+0.3° during the period of the frequency scan (z 20min) and a difference of 0.5° between the top and bottomof the cell.

Successive oxidations were carried out on the same oilsample, which had previously been degased and dried bysubjecting it to a vacuum of about 10 1 for a period of24 h at a temperature of 70°C. Each oxidation periodconsisted of rapidly heating the sample to 115°C in anoven and maintaining it at this temperature for 68 h.This schedule was chosen since it represents a relativelymild oxidation without the formation of sludge and other

185