OIL-PAPER DIELECTRICS Effect of Asphalt as a Contaminant in Oil and in Impregnated Paper JOHN D. PIPER, PORTIA TREEND', AND KATHRYN S. BEVIS2 The Detroit Edison Company, Detroit, Mich.
Addition of a n asphalt to a n insulating oil causes the oil to have a high power factor and conductivity. This conductivity is nearly independent of frequency. The dielectric properties of the asphalt solutions are not markedly different from those of solutions of electrolytically dissociable materials, such as lauryl sulfonic acid, in oil. Although asphalt, like the lauryl sulfonic acid, is strongly sorbed from oil by paper, the dielectric properties of papers impregnated with solutions of these contaminants are strikingly different. Whereas the 60-cycle conductivities of impregnated paper containing lauryl sulfonic acid are markedly greater than those of the impregnating soleution, the conductivities of impregnated paper containing asphalt are markedly less than those of the impregnating solution. In the systems containing lauryl sulfonic acid the increased conductivity, over t h a t of the paper impregnated with the solvent oil alone, is
caused by dielectric absorption; in the systems containing asphalt most of the increase is independent of the frequency and is caused by conduction. The data concerning the asphalt-containing systems are used to further the previously proposed classification of contaminants of insulating oils and papers. A means is suggested for determining whether high conductivity in a n insulating oil is caused primarily by electrolytically dissociable materials, such as acids, or by colloidally dispersed materials, such as asphalt. This means consists in impregnating filter paper with the oil under examination and comparing the 60-cycle conductivity of the impregnated paper with t h a t of the impregnant. Increase in conductivity upon impregnation means t h a t the substances primarily responsible for the high conductivity are of the electrolytically dissociable types; decrease means t h a t they are primarily of the colloidal types.
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completely soluble in oil. This constituent appears to be stabilized in suspension either by other constituents or by its own soluble portion. Usually, Class A additives are essentially neutral in character or faintly acidic. The characterization is not yet complete, as is evidenced by the fact that not all substances having the properties described cause high power factors when added to oils. However, most of the substances that do cause high power factors in oils appear to be of Class A. The effect of Class A additives on the power factors of impregnated papers has not heretofore been examined systematically. Class B additives belong to the type of substances, notably acids, that in water or other solvents dissociate electrolytically. If those substances are truly soluble in insulating oil, when they are added to oils the resulting power factors are generally low. Only the strongest of those substances, such as sulfonic acids and salts of organic bases, cause an increase in the power factor of the oil to an extent that is of practical significance. Often, however, electrolytically dissociable substances affect the power factors of impregnated paper more than they do the power factors of the oil. Those that are so strong as to cause significant increases in the conductivity of oil cause markedly increased conductivities in paper. Of those having a negligible effect on the power factor of oil, many that have a strong polar radical and only a small hydrocarbon radical become strongly sorbed on paper, and these cause high power factors. In brief, all substances that in water solution tend to dissociate electrolytically are termed Class B substances, provided that, when present in reasonable concentrations as contaminants, they cause the power factors of either oil or oil-impregnated paper to be significantly increased. One purpose of the present investigation was to gain further knowledge concerning Class A substances, and particularly to contrast their dielectric effects in oil and in impregnated paper, A further purpose was to evaluate, for a substance of Class A, the technique of Balsbaugh, Assaf, and co-workers (11 for differentiating substances that cause high conductivities in impregnated papers by conduction from those that cause high alternating current conductivities by dielectric absorption.
OR a number of years the authors and their colleagues have been systematically investigating the physicochemical nature of substances that can cause significant increases in the power factors and conductivities of insulating oil? and oil-impregnated papers. The kinds of substances studied have been limited to those that conceivably could be formed by the deterioration of oil and.papcr in contact with materials with which they are used in service or that might enter the oil and paper as contaminants. The maximum concentrations have been limited to those that. could be found after severe service conditions. The method of study has consisted simply in adding, to an oil with a very low conductivity, various concentrations of highly purified substances of the types selected to represent constituents, deterioration products, or contaminants of commercial insulating oils; and in determining the resultant changes in dielectric properties. For the studies involving insulating paper, the dielectric studies have been made upon cable and filter papers after they were impregnated with the oil alone and with the oil containing the additives. All the studies have been designed to show the effect of a given type of material while it exists as such, and not to show any effect it may have on the ultimate stability of a dielectric as the resdt of any chemical reaction it may enter or affect cstalytically. The rcsults of the investigations to date may be dmmarized as follows: Oil-soluble substances of a surprisingly large variety, including most medium molrcular weight acids, alcohols, aldehydes, esters, ketonrs, and peroxides, have little effect on power factor and conductii7ity of oil or oil-impregnated paper. All additives thus far found to cause high power factors and conductivitirs in either insulating oils or oil-impregnated papers belong in Class A or Class B (4), as summarized in the nest two paragraphs. Class A additives contain a t least one constituent that is in1
Present address, Michigan State College. Lansing, Mich.
* Present address, E. I. du Pont de Xemours & Company, Inc., ton, De!.
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INDUSTRIAL AND ENGINEERING CHEMISTRY CHOICE OF ADDITIVE
In selecting an additive for use in studying substances of Class A, it was necessary to depart temporarily from the desirable policy of effecting vigorous purification of the additive before use. The one consistent characteristic of Class A substances so far observed is that of incomplete solubility in oil of one or more of their constituents. Incomplete solubilit,y of one of the constituents !dl not, in itself, however, cause the substance to produce high conductivity in oil. This x a s shonn (9) by esperimenu with suspensions of linear hydrocarbon polymers. Observations to date indicate that in order to cause high conductivity, the dispersed phase must have a dielectric constant that is materially greater ( 7 ) than that of the oil. Preliminary experiincnts in which efforts were made to disperse oxygenated polymers rcsulted in unstable suspensions of low conductivity. Experiments n-ith copper soaps (6) indicated that the degree of dispersion is an important factor. I n those experiments copper soaps were made progressively less soluble by adding, t o an oil solution of a soluble soap, successive portions of an acid whose copper salt was insoluble. High conductivities resulted at an intermediate stage during addition of stoichiometric portions before the particles had agglomerated to the extent that the mixture could be judged visually to be a suspension rather than a solution. After the particles agglomerated the conductivities dropped sharply, Other less productive experiments emphasized the observation that, although stable conducting oil sols are prevalent in nature, they are difficult t,o prepare in the laboratory from highly purified materials. It, was for this reason that the practice of rigorous purification mas abandoned for the experiments described in this paper and a natural substance was chosen as the additive.
TABLE I. PROPERTIES OF GILSOSITE;~DDITIVE Kame, gilsonite brilliant black selects Source, Utah Gilsonite Co., courtesy George E. 3Ioser and Son, Detroit Melting pointa, 170-175° C. Solubility in benzenea, 99.8% Solubility in carbon tetrachloriden, 90.4Y0 Total sulfur, 0.20% Acidity, 1.1 mg. of KOH per gram of gilsonite a Manufacturer's data.
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The additiw selected, asphalt, !vas chosen for its practical sigriificarice as well as to typify Class .4 materials. For many years asphalt iws used in the electrical industry in cablc terminals and joints because of its physical properties and its excellent dielectric strength. Practical experience sholred, hen.ever, that the asphalt dissolves in insulating oils, thus causing the contaminated oils and also paper cables to acquire very high p o w r factors and making necessary the development of an oilresistant filling niat,erial (9). The asphalt selected for the tests was not the type that n'as normally used for filling potheads and joints (penetration approxiinately 0.5 a t 25' C. according to A.S.T.M. D5-25, and of random origin) but one whose origin m-as known-namely, gilsonite brilliant black selects-the properties of Lyhich are listed in Table I. Inasmuch as even small concentrations of this material Ivere not completely soluble in t,he base oil upon being heated a t temperatures up to 100' C. for 24 hours, it was unnecessary t o resort t o any fractional solvation t,o prove that an incompletcly soluble constituent existed. BASE MATERIALS AND PROCEDURE
The base oil used was the same as that described in the previous studies-namely, a liquid paraffin having a viscosity of 285 Saybolt Universal seconds at 40" C. and 50.5 seconds at 100" C. The cable paper was a highly calendered kraft paper of
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American manufacture, made from Snedish y o o d pulp. The average thickness was 5.4 mils, the Gurley air resistance (1 square inch orifice) was 243 seconds per mil, and the specific gravity was 0.968. The filter paper used was Whatman 41H. The cells and the bridge used for the electrical measurcmcnts, the technique for making the meaanrements, and the relationships used for expressing the resulk in terms of the specific conductance have been described (4). The apparatus and technique uscd for the sorption experiments were patterned after those usctf with nonvolatile acids (6). The technique involved t,he removal of part of the oil from the impregnated paper by means of a centrifuge and a comparison of the asphalt concentration in the oil removed with that of the impregnating oil. The conccntrations of asphalt in t,he oil removed were determined colorimetrically by means of empirical standards. The oil-paper rat,io for both the dielectric and sorption experiments was 0.6 for citble paper and 1.0 for filt,er paper. As was shown in the original doscriptions (4,5 ) , t,he impregnant for t'he dielectric tests entcrcd t,he stack of eight disks from the top, whereas that for the sorption tests entered from one end of a tightly wound cylindrical roll. For both the dielectric and the sorption test's the gilsonite was dissolved in C.P. benzene and the resulting solutions were added to the liquid paraffin and well mixed. The benzene mas removed during the heating and evacuation treatment, to n-hich all samples were subjected in order i o effect drying of the paper immediately before impregnation. ELECTRICAL PROPERTIES OF SOLUTIONS
The conductivities a t 80°, 60", and 40" C. of the solutions of gilsonite in liquid paraffin are shown in Figure 1, which also shows the approximate 60-cycle power factors that correspond mho per em. I n addition, Figto these conductivities up t o ure 1 shows the conductivities at 80" C. of lauryl sulfonic acid solutions as reproduced from the work previously reported (4, 8). Of all the electrolytically dissociable (Class B) materials investigated, lauryl sulfonic acid gave solutions having the highest conductivity for given weight concentration of added material, Nevertheless, Figure 1 shows that gilsonite in equal concentration gave solutions having conductivities from seven to fifteen times greater over the range of concentrations studied. The greater effect of the gilsonite is particularly noticeable at the lower concentrations, as shown. Except for the difference in the conductivities of the solutions
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Figure 2. Conductivity 8t 60 Cycles and 80' C. of Gilsonite in Oil and Impregnated Paper A . Liquid paraffin-gilsonite solutions B . Cable paper impregnated with A C . Filter paper impregnated with A
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having a given weight concentration of added material, there was little in the dielectric behavior of the solutions to disA . Li uid paraffin-lauryl sulfonic acid solutions B . Ca%le paper impregnated wjth A tinguish between the Class A additive, C. Filter paper impregnated with A gilsonite, and the Class B additive, lauryl sulfonic acid. In both cases the conductivities were essentially independent of frequency between 20 and 20,000 cycles except that a t 40" C. the conductivities of the gilsonite solutions increased slightly with increase of frequency. Further, the slopes of the conductivity-concentration curves shown in Figure 1 do not differ greatly, that for the gilsonite being 0.8 and that for the lauryl sulfonic acid 1.1. The viscosity of the liquid paraffin at 80' C. was almost identical with t h a t of the transformer oil used by Gemant (8) at 25' C., and the dielectric constants of the two oils were also within the same range. The slope of 1.1 for lauryl sulfonic acid in mineral oil is i n better agreement with Gemant's data for solutions having dielectric constants greater than those of oil than is the slope of 0.8 determined by him in the transformer oil. The slope of 0.8 for the curves for gilsonite agrees fairly well with Gemant's curves for two asphalts which ranged from 0.5 a t low concentrations to 0.7 a t the higher.
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which the paper was impregnated, and the conductivities of the impregnated filter paper are even lower, especially for the s m a l l e r concentrations. The sharp contrast betn-een Class .A and Class B additives is shown by a comparison between Figures 2 and 3. Figure 3 shows the behavior of systems containing the Class B additive, lauryl sulfonic acid, as reproduced from the previous work (4). Whereas in Figure 2 the progression of conductivity from oil to cable paper to filter paper is in descending order, in Figure 3 the progression is in ascending order. This marked difference in behavior supports the authors' belief that a fundamental difference exists between Class A and Class B additives despite the similarity of their dielectric behavior in oil solution. SORPTION
In previous work ( 6 ) it Ras shown that one of the properties that characterizes additives of Class B and serves to differentiate the class from additives that, although they may belong to the
ELECTRICAL PROPERTIES OF IMPREGNATED PAPER
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The similarity in dielectric behavior between systems containing Class A and Class B additives disappears when consideration passes from oil solutions to paper impregnated with these solutions. Figure 2 shows the conductivity a t 60 cycles and 80" C. of liquid paraffin-gilsonite solutions and of paper impregnated with these solutions. Curve A , which is reproduced from Figure 1, is for the liquid paraffin solutions. Below A is curve B for cable paper impregnated with the solution and still lower is curve C for the impregnated filter paper. Neither curve B nor curve C approaches being a straight line. The significant fact is that the conductivities of the impregnated cable paper are approximately only a fifth of those of the gilsonite solutions with
Figure 4.
Degree of Sorption of Gilsonite from Liquid Para5n by Cable and Filter Papers
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Figure 5 . Condurtivity at Various Temperatures and Frequencies of Cable Paper Impregnated with Gilsonite Liquid Parafin Solutions
Figure 6. Condiicti\ ity a t Various Temperatures and Frequencies o!'Filter Paper Impregnated with Gilsonite Liquid Paraffin Solutions
Concentration of Gilsoiiitr in Impregnant, o/o by K e i g h t 0.00 A 0.02 B 0.06 C
Concentrntion of Cilsonite in Impregnant, % by Weight o.no A 0.02 B 0,06 C
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same homologous series, have only a very small effect on the dielectric properties of impregnated paper, is the property of bcing strongly soybed from oil by paper. In order to dctermino whether this property of being strongly sorbed nould serve t o diffcrcntiate additives of Class B from those of Class A, sorption experiments
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taining gihonite are, however, perceptibly flatter a t the lower frequencies. The lower frequency data at 80” C. are plotted on a linear scale in Figure 7 for analysis by the “intercept” method of Balsbaugh and Assaf
(11. According t o Balsbaugh and Assaf’s interpretation, materials that are selectively sorbed by the paper increase the dielectric absorption loss of the paper but decrease the conduction loss of the impregnant, the conduction of which is obtained by extrapolating the conductivities to zero frequency. Analysis of the data for the systems containing lauryl sulfonic acid supported ( 4 ) Balsbaugh and Assaf’s interpretations in that it showed that the lauryl sulfonic acid, which was nearly entirely sorbed, gave rise to high alternating-current conductivities. When these Conductivities were plotted against frequency on linear scales, no intercept could be obtained on the conductivity axis. Instead, the curves, two of which are reproduced as the dotted lines on Figure 7 , were concave downward and pointed toward the origin as shown. This was in keeping with the results FREQUENCY, CYCLES PER SECOND of the sorption and conductivity exFigure 7. Intercept Method of Balsbaugh and Assaf for Distinguishing between periments that showed that the conDielectric Absorption and Conduction in Impregnated Paper at 80’ C. ductivity of the impregnant with its Additive in Impregnant low equilibrium concentration of lauryl Concentration, sulfonic acid was so low as to be Kind % by weight Kind of Paper Designation nearly indistinguishable from zero in Lauryl sulfonic acid 0.14 A Cable paper 0.022 Lauryl, sulfonic acid B Filter paper Figure 7. Gilsonite 0.6 Cable paper C 0.2 Gilsonite D Cable paper The contrast between the behavior of 0.06 Gilsonite E Cable paper 0.02 Gilsonite F Cable paper the systems containing gilsonite and None Cable paper G those containing lauryl sulfonic acid, 0.6 Gilsonito Filter paper H 0.2 Gilsonite I Filter paper when analyzed according to the “inter0.06 Gilsonite Filter paper J 0.02 Gilsonite Filter paper K cept” technique, is clearly shown in None L Filter paper Figure 7 . The curves for the gilsonite systems are essentially straight lines having intercepts on the conductivity axis. Particularly is did the cable paper. Thus the behavior of the Class A and Class this true for the impregnated cable papers, for which all corrcenB materials was qualitatively the same. The behavior differed, trations of gilsonite produced definite intercepts although the however, in that the percentage sorption of the Class B additive curve for the paper impregnated with the solvent oil alone passed increased as the concentration increased, whereas it decreased through the origin. for the ClasB A additive. This decrease was fairly regular, from The data for the gilsonite systems provide an opportunity to 30 to 20%, for the cable paper but was irregular for the filter test the Balsbaugh-Assaf hypothesis further. According to paper. For the latter, approximately 90% was sorbed for conthis hypothesis the unsorbed gilsonite in the impregnant should centrations of gilsonite in the impregnant up to 0.2% by weight. contribute to the conductivity a t all frequencies to the extent At the higher concentrations the percentage sorption was markshown by Figure 1 for the equilibrium concentrations obtainable edly less, as shown. These differences in sorption behavior befrom Figure 4. The sorbed gilsonite, on the other hand, should tween the Class A and Class B additives are not, however, contribute to the dielectric absorption. The effect of the sorbed sufficient for characterization purposes. gilsonite should, therefore, be a function of frequency. VARIATION O F CONDUCTIVITY WITH FREQUENCY k ? D In order to determine whether the conductivities corresponding TEMPERATURE to the intercepts on Figure 7 are equal to the conductivities of gilsonite solutions containing the equilibrium concentrations deFigures 5 and 6 show, respectively, the variation in conductermined in the sorption experiments, these conductivities were tivity of the impregnated cable and filter papers with change plotted against the concentrations of gilsonite left in the imin frequency, temperature, and concentration of gilsonite in the pregnant, and the results were compared with the data for the impregnant. Except that the conductivities for a given percentimpregnating solutions as shown in Figure 8. The points for the age concentration of additive are so much lower than the conducimpregnant in cable paper fell along a fairly straight line. Intivities previously shown (Figures 4 and 5 of 4 ) for the same imstead of coinciding with the line for the impregnant as the hypregnated papers containing lauryl sulfonic acid, the shapes of the pothesis requires, however, it fell markedly below, as shown, and curves are similar. The curves for the impregnated papers con-
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CONCENTRATION OF GllSONlTE IN IMPREGNANT, PER CENT BY WEIGHT
Figure 8. Comparison at Equal Concentrations between Conductivity of Gilsoni te in Impregnant Alone and Apparent Conductivity in Impregnant in Contact with Paper Apparent conductivities a:e intercept yalues of Figure 7 : concentrations used as coordinates with them are derived from sorption d a t n of Figure 4
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corresponded to concentrations between 10 and 12% of thosc. determined in the sorption experiments. Lilren-ise, the points for the impregnant in filter paper, although somewhat irregular, arc,’ significantly below the curve for the impregnant. Although the degree of sorption for the stack of flat disks used for the dielectric measurements may possibly hare differed from that in the cylindrical rolls used for the sorption tests, it seems improbable that the difference could be great enough to account for the results. Two other possibilities exist. (1) The paper may sorb, t o if greater extent, the “insoluble constituent,” which according t o the authors’ hypothesis is charact,eristic of Class A additivcq, than it does the total of t,he color-producing materials by Jvhicll ld the concentration of gilsonite !vas determined. This ~ ~ - o u result in a lower conductivity for t,hc impregnant in contact Xvith paper than for the original impregnant, of equal color intensity. ( 2 ) The hypothesis of Balsbaugh and Assaf, alt,hough proved by these data to be qualitatively correct, may be too simple t o furnish quantit’ative explanation of the dielectric behavior of impregnated paper. In order to determine the effect of the gilsonite that was sorbed on t’he paper, the differences between the conduct,ivities of the papers impregnated with the gilsonite solut,ions and those iniprcgnated with t’he solvent oil alone were determined and thesc: differences mere plotted against the frequencies as shown in Figure 9. The slopes of the curves show the extent to which dielectric absorption contributes to the total conductivity. It is a t once evident that, although most of the gilsonite is sorbed from the oil by the filter paper, this sorbed material has almost no effcct upon the dielectric absorption, particularly a t the higher tcinperatures. I n the cable paper the effect, although not large, is noticeable in spite of the fact, that the degree of sorption is less in cable paper than in filter paper. From the comparison betmen the behavior of the impregnating papers containing gilsonite and those containing lauryl sulfonic acid, it is evident that the “intercept” met’hod of Balsbaugh and .4ssaf is useful for det,ermining qualitatively whetherthe high power factor of a service-degraded, impregnated-paper insulation is caused by contaminants of Class A or Class B. I t is suggested that the class of cont,aminant that is causing a high porer factor in insulating oils can be determined similarly by impregnating paper, preferably filter paper, with the oil and coinparing the dielectric properties of the oil with those of the impregnated paper as described in t’his article. For this purpose it is necessary that the oil-paper ratio be kept a t the minimum for complete impregnation. Otherwise, for contaminants of Class A the conductivity of the impregnated paper may be essentially the same as that of the impregnating oil, as was the case in the experiments of Balsbaugh and Assaf (1).
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ACKNOWLEDGMENT
The authors are grateful to Henry S. Walker, director of rcsearch, for his continued interest and support. LITERATURE CITED
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(1) Balsbaugh, J. C., Assaf, A. G., and Pendleton, W. W., IND. EKG.CHEM., 33, 1321 (1941). (2) Gemant, Andrew, J . Chem. Phus., 13 (No. 4 ) , 146 (1946). (3) Hall, H. G., and Komives, L. I. (to Hercules Powder C o . ) , U. S. (4)
Patent 2,291,961 (-4ug. 4 , 1942). Piper, J. D., and Kerst,ein, N . A . , IKD.EFG.CHEM.,36, 1104 (1944).
(5) Piper, J. D., Kerstein, N. *4., and Fleiger, 8.G., Ibid., 29, 1040. (1937).
FREQUENCY, CYCLES PER SECOND Figure 9. Conductivity of Papers Impregnated with Gilsonite Solutions Minus Conductivity of Papers Impregnated with Solvent Oil Percentages (by weight) given on each curve show original concentrstions of gilsonite in impregnant
(6) Piper, J. D., Smith, C. C., Kerstein, N . A., and Fleiger, A. G., Ibid., 31,307 (1939). (7) Ibid.,32,1510 (1940). (8) Ibid., 34,1505 (1942). (9) Sticher, Joseph, and Piper, J. D., Ibid., 33, 1567 (1941). RECEIVED February 3, 1947. Presented in abstract before the Conference OD Electrical Insulation, Kational Research Council, Ealtimore, Md., Xovember 8, 1947.
