IEEE TRANSACTIONS ON ELECTRICAL INSULATION, VOL. Ei-5, NO. 2, JUNE 1970

[51 N. G. MeCrum, B. E. Read, and G. Williams, Anelastic andDielectric Effects in Polymeric Solids. New York: Wiley, 1967.

[6] P. J. Burkhardt, "Dielectric relaxation in thermally grownSiO2 films," IEEE Trans. Electron Devices, vol. ED-13, pp.268-275, February 1966.

[7] R. Kono, G. E. MeDuffie, and T. A. Litovitz, "Viscoelasticrelaxation and non-Arrhenius behaviour in diols," J. Chern.Phys., vol. 44, pp. 965-970, 1966.

[8] C. P. Smyth, Dielectric Behaviour and Structure. New York:McGraw-Hill, 1955.

[9] J. E. Anderson and R. Ullman, "Molecular relaxation in afluctuating environment," J. Chem. Phys., vol. 47, pp. 2178-2184, 1967.

[10] G. P. Johari and W. Dannhauser, "Effect of pressure ondielectric relaxation in isomeric octanols," J. Chem. Phys..,vol. 50, pp. 1862-1876, 1969.

Stability of Electrical-Insulating OilsB. P. KANG, SENIOR MEMBER, IEEE

Abstract Knowing that oil is a weak link in a composite or im-pregnated dielectric system with reference to both dielectricstrength and ease of contamination, many investigators have devotedgreat efforts to try to ascertain the best types of oil for variousoperating conditions. However, different investigators seem to havedifferent opinions as to the best-suited type of oil for this purpose.It is the object of this study to involve a wide variety of oils and testconditions to shed some light on this controversy.Ten original oils and three blended samples selected from an

array of over 50 original specimens tested have been chosen toillustrate the tenacity of this problem. The samples include bothmineral and synthetic oils, coming from domestic sources as wellas from abroad. Their sources or ethnic groupings seem to indicateno preference, but the care in refining these samples seems to be afactor.

In this investigation, two criteria have been chosen to indicatethe service quality of the oils: gas-absorption and liberation charac-teristics and electrical deterioration under moderate electricalstress. In the presence of carbon dioxide, no oil has been found toliberate gases. In the presence of air, nitrogen, or oxygen, althoughgas liberation has been observed in some of the European oils atsome stages of aging, there has been no sharp rise in dissipationfactor or indication of internal discharge or failure.The most stable oils found in this study are those with fairly low

viscosity of not over 45 Saybolt universal seconds (sus) at 850C andthe purity of which also satisfies the National Formulary require-ments for pharmaceutical uses. It indicates the importance of thepurity and exactness of these compositions.

INTRODUCTION

K~,NOWLEDGE of the stability or aging charac-teristics of electrical insulating oils under variousoperating conditions is of utmost importance to

both electrical-equipment designers and operating engi-neers. In composite oil-impregnated dielectrics for high-voltage insulation, oil is usually the weaker componentof the system, both in dielectric strength and in environ-mental contamination. Earlier, Whitehead and Mauritz[1] dealt with the effects of oxidation at length, and

Manuscript received February 20, 1969; revised January 5, 1970.The author is with the Industrial Condenser Corporation, Chicago,

Ill. 60618.

recently, Melchiore and Mills [2] presented a briefcomparison of stability between mineral and syntheticoils, with mineral oils as their favorite, while Pilpel [3]rendered a comparison of four synthetic fluids for electricalinsulation, and Pelagati [4] advocated the extremedesirability of dodecylbenzene. These discussions providea stimulant for further continuation of this study.To substantiate a part of their findings and to point

out the diversions and the statistical nature of the stabilitytest data, no matter where, when, or by whom the testsare conducted, condensed results from a rather extensiveinvestigation, involving practically all electrical-insulatingoils available, commercial and experimental, mineraland synthetic, tested under a wide variety of environ-ments and extended through a period of nearly twodecades will be revealed for examination.Many engineers in both the extra-high-voltage cable

and capacitor fields are deeply concerned with the stabilityof dielectric loss and the gassing phenomena underelectrical stress. The general belief heretofore is thatlow-viscosity white oils are more easily subjected togassing under electrical stress than the less refined vari-eties. It is an objective of this study to see whether amoderate electrical stress of 200 V/mil initiates gassing.Whitehead [5] in his life-long dedication to the study

of electrical-insulating oils came out with a low-viscositywhite oil as most promising, but unfortunately no one inthe industry so far has accepted his proposition. Nearlyall negative responses to his proposal stem from the fearof probably excessive gassing of white oils in operation.Though with careful selection of typical or repre-

sentative data from the voluminous experimental resultsobserved, one can only point to the general trend butnot exclusively or conclusively enough to postulate ageneral aging behavior or to suggest the definite superi-ority of a particular type of oils. However, the extremelyhigh viscosity of some oils seems to have obscured theirpoor stability to some extent and three low-viscosityfluids appear somewhat superior.

41

IEEE TRANSACTIONS ON ELECTRICAL INSULATION, JUNE 1970

TABLE IFUNDAMENTAL PROPERTIES OF SOME HIGH-GRADE ELECTRICAL INSULATING OILS

Viscosity (sus)

Oil 100OF 85 °C 100 0C

6 Specific Pour Aliphatic Naphthenicpercent Gravity Point Content Content85°C at 60°F 0C (percent) (percent)

Aromatic Average AnilineContent Molecular Point(percent) Weight 0C

0.897 -40.8 700.905 7.0 100

2

0

0.882 -57.0 12 640.868 -43.0 9 90.8

0.850.8550.820.872

- 6.7-40.0-51.0-62.0

60100100

400

0

0

22 3140 1190

24 2960.2 337

0

0

0

355450327243

79 3.2513.4

78 1.297.9 0.07

102 037.20.2

50 0.03

Because of the wide variety of environments andeffects, it seems rather difficult to tailor the results intoa complex but complete story; hence this exposition isonly to state the objective, to enumerate the specimens,to describe the equipment and procedures, and to presentthe results without attempting to go into any depthof discussion.

PURPOSES OF THIS INVESTIGATION

The original purpose of this study was strictly com-

merical in nature. It was to look for an oil, or a group

of oils, or some combinations of oils, that mnight indicatethe longest useful life or the best electrical stability forimpregnation of extra-high-voltage oil-impregnated paper

cables and capacitors and for impregnation of syntheticfilm insulated cables or capacitors. However, becauseof the importance of this knowledge to high-voltageinsulation engineers in general, an added purpose is torender these findings to a larger technical segment forfuller utilization.

SPECIMENS

When the original purpose was disclosed to the variouselectrical insulating oil suppliers, 12 sources, domesticand overseas, began to submit specimens for evaluation.Some sources had submitted more than half a dozenspecimens either of different crudes or of different methodsof production. Six domestic petroleum refineries suppliedmineral oils and three synthetic oil plants suppliedseveral varieties of polybutene, while two Europeansources supplied mineral oils from foreign crudes andone source supplied several grades of polyisobutylene.The fundamental properties of the oils selected forillustration are shown in Table I.

A simple way to group these oils by their viscositiesand their probable end uses may be to separate theminto five categories. At 1000C, those having viscositiesup to 35 sus are generally used for transformers, between35 and 40 sus for oil-filled high-voltage cables and X-rayequipment insulations and coolants, around 100 sus forsolid cables, between 500 and 1000 sus for high-stresscapacitors, and over 1000 sus for high-pressure pipe-typehigh-voltage cables. This is only a very general classifi-

cation and their actual uses may be overlapped to some

extent. This study deals with one or more specimens ofeach group.

TEST EQUIPMENT AND PROCEDURES

To avoid unnecessary copper contamination, test unitswere of four concentric nickel alloy tubular electrodes,with a nominal separation of 25 mils between any twoneighboring tubings. These electrodes were mounted on

two I2-inch nickel rods that extended through the ground-glass hermetically sealed stopper and that also servedas external connectors. The electrode system was housedin a glass container with a ground-glass mouth to fitthe stopper and a per anently cemented and groundedcylindrical nickel shiele attached- to the outer surfaceof the container wall directly opposite to the activeelectrodes inside. A vent in the-shape of a glass tubewas provided to expose each specimen to the chosenatmosphere. The cell had a capacity of 50 ml to maintainan upper oil surface level of approximately 2 inch abovethe upper edges of the active electrodes.

Engineering of this compact electrode system mustbe accredited to the late C. J. Balsbaugh. His primarypurpose was to observe the day-to-day changes of dis-sipation factor and gassing characteristics of a specimen.

Due to the lack of absolute uniformity and exactnessof the electrode separations, they are only satisfactoryfor this type of indicative investigation but not forprecision measurements.The vent was connected to a closed system, containing

approximately 170 ml of the gas under study, in thiscase, air, carbon dioxide, nitrogen, and oxygen. Thesystem, in turn, consisted of a mercury-sealed gas reser-

voir, which was a calibrated burette, the necessary con-

necting tubes, and a U-tube mercury barometer. A merc-

ury bottle that supplied mercury to the calibrated burettecould be raised or lowered to adjust the gas pressure

in the system to one atmosphere once a day. The changein the volume of gas in the system each time was ob-served and recorded. The system was always adjustedto atmospheric pressure before electrical measurementswere taken.

Twenty-four cells were connected in parallel, each

BC

B

GJ

M

NQRx

100100 000

71609410085

7804346

42

40374443431003433

383000

3435383938703132

0.160.020.110.0850.0450.130.0240.060.020.103

BromineNumber

-42

KANG: STABILITY OF ELECTRICAL-INSULATING OILS

AGING T0M4.-HOURSSPECIMEN TESTEDDNAJR Wri1OUT COPPER ORSTRE$S

Fig. 1. Gas-absorption or liberation and dissipation factor charac-teristics of electrical insulating oils aged under nolrmal atmos-phere.

fused separately with a 0.1-ampere little fuse. The powerwas supplied by a 5.0-kV, 1.0-kVA transformer, whichcould supply 0.2 ampere at 60 Hz continuously. Withonly approximately 200 V/mil, no failure was experi-enced, in a few cases up to 1200 hours. Had a failureoccurred, whether the transformer could carry thetransient overload could not be ascertained. The inputof the transformer was from the commercial 115-volt60-Hz power source. A timer system was set up so thatthe down time due to power interruption of any kind,including the time taken for measurements, was auto-matically excluded from the aging time.

All the oil-filled portion of each cell was placed in aconstant temperature oven maintained at 85°C. Thepower for the heating system was not from the samesource that supplied the test transformer but directlyfrom the main line. It should have no interruption unlessa general power failure should take place in the entirecommunity.

Electrical measurements were made with a standardhigh-voltage Schering bridge at 1.0 kV and 60 Hz.Although the day-to-day observations were made everyday, they were not exactly at the 24-hour intervals,but rather each measurement was taken at a time thatwas as close to the 50-hour intervals as possible so thatall measurements could be taken during the day-time

et

F:-

z

0

Pjj

UCJ

CC

0

to

U

2

Via

A61NG TI EME-HOUR:SSPFC)MENTNTTED IN AIR WITH COPPER ANDO3TRE55

Fig. 2. Gas-absorption or liberation and dissipation factor charac-teristics of electrical insulating oils aged under normal atmosphereand a 60-Hz electrical stress of 200 V/mil.

working hours. The actual times of measurements were 0,50, 100, 150, 200, 240, 296, 344, 392, 440, and 504 hours.Therefore, for the neat appearance of the graphs, thevalues have been rounded up to the nearest 50-hourintervals.

EXPERIMENTAL RESULTS

Gas-absorption or liberation and dissipation factorcharacteristics under four different atmospheres of thespecimens chosen for illustration are shown in Figs. 1-8.Oil X is shown only in Figs. 1 and 2.The gas-absorption curves actually give the net results

of both gas absorption and gas liberation combined.Each oil may have absorbed some of the gas in theatmosphere of the system and at the same time mayhave liberated some hydrogen or gaseous hydrocarbonsof its own. It seems in most cases, liberation of gasesis offset by the absorption of the gas in the atmosphere.For instance, Fig. 4 shows that at 500 hours, oil J absorbed160 ml of the 170 ml of CO2 originally in the system.Actually it might have absorbed the entire 170 ml of CO2and liberated 10 ml of hydrogen or gaseous hydrocarbons.Unfortunately the differentiation between gas absorptionand gas liberation was not attempted. However, in allbut two cases in 96, the oils have not liberated moregases than they have absorbed at the end of 500 hours

43

44

100)

0.

LEGEND-OIL a v OIL M OIL R

2 0.8 o E 9E 9 Y CX

Q:A G a7",C * ¢QX _/ _CLJ

U_

IL A J4Q 5

O L .00 °oo 200 .3010 400 0AGING TIME-HOURS

SPECIMEN TE5TE D IN CO2WITtioUT COPP ER OR STRESS

Fig. 3. Gas-absorption or liberation and dissipation factor charac-teristics of electrical insulating oils aged under carbon dioxideatmosphere.

of aging. In other words, absorption has never gonethrough a maximum.The largest quantity of gas absorbed by each oil is

carbon dioxide and the second place goes to oxygen.This is quite reasonable.The dissipation factor in the absence of copper and

electrical stress is generally low under all types of at-mosphere and it is also low under CO2 even in the presenceof copper and electrical stress. This indicates that gasabsorption is a function of gas solubility of the oil. Carbondioxide is physically more soluble in hydrocarbons butchemically most inert so that although very large quantityof the gas is absorbed, apparently no chemical reactiontakes place and only very little deterioration is noted.The results, further, show that absorption of air,

carbon dioxide, nitrogen, or oxygen bears no definiterelationship with dielectric loss of the oil. It does notcause any ionization or electrical instability at the overallapplied voltage of about 200 V/mil, which represents agas pocket electrical stress of about 400 V/mil, muchabove the ionization stress of any of the gases presentif they remain in the gaseous state in the oils. However,some less refined oils were observed to have liberatedmore gas than they had absorbed in some stages of thetest, but no sharp rise of dissipation factor or internaldischarge appeared.

0a-

0

U,

U,2

et

z

0:

In

IEEE TRANSACTIONS ON ELECTRICAL INSULATION, JUNE 1970

14000 30 40 0

12A E

SPEC.MENTESTED IN CO WIT LEGENDR*OIL B OIL M A OIL R

. _ ~~~~~oE v'. N Y GX_ * t ~~~~~G CX QX)( -IO 2 O 300 400 SOO

AGING TIME-HOURS5PEGIMP-N TESTE~D IGt4CWI-tiCOPPtr AN4D5aTfSS

Fig. 4. Gas-absorption or liberation and dissipation factor charac-teristics of electrical insulating oils aged under carbon dioxideatmosphere and a 60-Hz electrical stress of 200 V/mil.

GENERAL DISCUSSIONAmong the oils selected for this presentation, oil X

is an alkylated benzene, consisting predominately ofdodecylbenzene. Its use for blending with higher-viscositypolybutene serves a two-fold purpose, to lower theviscosity to 100 sus at 100°F and to act as a gassinginhibitor. Aging by itself in the presence of air, it takesabout its equal volume of air in about 500 hours. Thedissipation factor remains constant, around 0.15 percent,up to 500 hours. However, in the presence of copperand electrical stress, the dissipation factor reaches 3.5percent in about 150 hours. The susceptibility of thismaterial to oxidation is shown further and most clearly bymixtures CX and QX when they are subjected to con-centrated-oxygen atmosphere. This agrees on one handand yet disagrees on the other with Pelagati's findings [4],depending on the condition of its use.The three most stable oils, as received and without

additive, are oils J, N, and R. Oils J and N are highlyrefined mineral oils, suitable for pharmaceutical formulary,and oilR is a very-low-viscosity highly purified polybutene.

Oil R, a very-low-viscosity polybutene, from the stand-point of its purely aliphatic molecular structure shouldliberate either hydrogen or gaseous hydrocarbons readily.However, all tests show that it is a good gas absorber. Area 200-V/mil stress on the oil and a 400-V/mil stress on

KANG: STABILITY OF ELECTRICAL-INSULATING OILS

V)

zti-

I

U-'-a-A

:r0co

tn

I-

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z0-

InU,

LEGEND0.0. rOILM A O)L.Ro E 7 N y "CX*G G" XOX

60.-

40 -

20-_

-20 -

0.24 - Z _ _

7~_ =

01 1-I- I I-O oo 200 300 400 500AG)MN TIME-HOURS

.SPECIMEN TESTED IN NITROGENWITOJT COPPER OR 5IRE55Fig. 5. Gas-absorption or liberation and dissipation factor charac-

teristics of electrical insulating oils aged under nitrogen atmos-phere.

any gas pocket, which might be present, high enough toinitiate gassing?

Oil E, a highly recommended and widely used Europeanrefined oil, shows gassing at some stages of the stabilitytest under atmospheric aging without copper or electricalstress and shows higher values of dissipation factor thanpractically all domestic oils at the termination of 500-houraging. This tends to show that long time stability testsmay produce evidence liable to raise serious criticism fromsome reputable electrical insulation engineers.Many oils show a decline in dissipation factor during

the first 50 or 100 hours of test under all types of atmo-sphere except concentrated oxygen. This may have beendue to the evaporation of certain unstable and volatileconstituents of the oils. In the presence of concentratedoxygen, the volatile constituents may form oil-solubleoxidized products quickly and maintain the steady increasein dissipation factor.

MINERAL OILS OR SYNTHETIC OILSAs it has been stated that the original purpose of this

study was to look for an oil, mineral or synthetic, thatcould outweigh its rivals in insulation qualities or electricalstability. With the lack of unanimity among workers inthis field, literature search adds more confusion, andexperimental evidence thus far has not made the taskany easier. Excluding the alkylated benzene used only for

Q:

-A

-j

xCI

0

InCC

Q

I-zU'U

0I-U-0F

tnina

LE6GFNDOIL B v OLMM A OILR

, N CC

A*G CC' QX'a a *

I0

46 _ --_ _ -

o100I O 5O 300 400 .500

AGING TIME-HOURSSPEC)MFi4 ThSTEWINN1TRO-eN WITH COPPVER ANOSTRt5

Fig. 6. Gas-absorption or liberation and dissipation factor charac-teristics of electrical insuilating oils aged under nitrogen atmos-phere and a 60-Hz electrical stress of 200 V/mil.

the dilution of high-viscosity polybutenes, the 12 mostrepresentative specimens selected for this presentationhappen to be four, or one-third, synthetic polybutene orpolyisobutylene liquids and eight, or two-third, mineraloils. Now, the three best oils are one polybutene and twomineral oils, the exact ratio of the number of the spec-imens. Under this dilemma, advice was sought from andgiven by two most distinguished physical chemists andliquid dielectrics experts of our time, Gemant and Fuoss.Their advice may be quoted as follows [6].

The synthetic liquid polymers are, according to myinformation and experience, generally better thanmineral oils. The reason is that the former are chem-ically better defined and more uniform, while mineraloils always contain various admixtures, and if theyare excessively refined they may even lose traces ofunsaturates that are actually desirable.

And the other states, "You may recall, however," in ourjoint study some years ago, "the natural oils showed upbetter than the synthetics, confirming your results."Again the uncertainty has not been resolved.At this point, one may ask, why this report is written

and presented, if there is no contribution to the solutionof this dilemma. It must be pointed out that not alltechnical problems have unique or clear-cut solutions.The results here indicate that superiority of an electrical

45

IEEE TRANSACTIONS ON ELECTRICAL INSULATION, JUNE 1970

n

IIU-

~0

'I)

zLU

U

c:>

tU

I-

z

!R

3L

_ _- -

0.6 _ _ _ _ _

00 1C0 20 300 400 5(AGING TIME-HOURS

SPE CIMEN TESTED IN OXYGEN WITHOUT rOPPE R OR STRE SS

Fig. 7. Gas-absorption or liberation and dissipation factor charac-teristics of electrical insulating oils aged under oxygen atmos-phere.

insulating oil is not a heritage of a family of oils. It dependson the individual make-up and refinement. Either mineraloils or synthetic fluids can be carefully prepared to givetop insulating qualities.

CONCLUSIONS

1) In accordance with these results, there appears no

definite superiority on electrical insulation quality or

aging stability between mineral and synthetic electrical-insulating liquids that can be attributed to a familyheritage.

2) Conclusion on stability behavior of electrical-insulating oils from a limited scale of tests is usuallymisleading, unreliable, and dangerous, as it resembles a

blind person trying to estimate the shape of an elephantby the limited part of the body he touches.

3) Evidence has been established that under a similartreatment, low-viscosity electrical insulating fluids are

more stable than high-viscosity fluids.

00

4

00 0 0 0 11

Ul

0

VI

to0 200 300 400 500

AGJMG TlME-HOURSSPECIMEN TE5TEO IN OXYGEN WITH COPPER AND 5TRES

Fig. 8. Gas-absorption or liberation and dissipation factor charac-teristics of electrical insulating oils aged under oxygen atmos-phere and a 60-Hz electrical stress of 200 V/mil.

4) Careful purification in refining processes such as thataccepted in pharmaceutical formularies yields betterelectrical-insulating fluids as shown by oils J and N.

5) The presence of copper and electrical stress simul-taneously accelerates deterioration of all electrical-insulating oils under all atmospheres tested, with carbondioxide as the best atmosphere with reference to agingdeterioration and oil J suffers the least deterioration underall types of atmosphere.

REFERENCES[11 J. B. Whitehead and F. E. Mauritz, "Oxidation in insulating

oils," AIEE Trans., vol. 56, pp. 465-474, 1937.[2] J. J. Melchiore and I. W. Mills, "Factors affecting stability of

electrical insulating oils," IEEE Trans. Electrical Insulation,vol. EJ-2, pp. 150-155, December 1967.

[3] N. Pipel, "Four liquid insulations-Properties and potentials,"Insulation, pp. 63-69, May 1968.

[4] U. Pelagati, "A new synthetic impregnant for high tensionhollow core cables," Ann. Rept., Conf. on Electrical Insulation,publ. 1141, 1963.

[5] J. B. Whitehead, Impregnated Paper Insulation. New York:Wiley, 1935, p. 106.

[6] A. Gemant and R. M. Fuoss, private communication.

LEGEND

OILB ' O)L M 1 OIL Ro° E v N y -I CX

- - G . C g Gx_ _ _ El.J a , Q * SY

46