Electrical Insulating Oil Deterioration
Chemical and Electrical Tests
J. C. BALSBAUGH, A. H. HOWELL, AND A. G. ASSAF Massachusetts Institute of Technology, Cambridge, Mass.
The electrical and oxidation stability of two series of related samples of electrical insulating oils have been studied. One of these series represents different solventrefining extracts and finishing treatments, from an appropriate distillation cut. The other series represents varying amounts of aromaticities, one of the samples being aromatic-free. The samples were deteriorated by oxidation in the presence of paper and copper in an improved deteriorating system. The results show: The electrical and oxidation stabilities of an oil are differently affected by the initial chemical constituency and the refining of an oil; and the electrical loss as a function of oxidation time or oxygen absorbed may start with a rapid increase to a relatively high value to be followed by an equally rapid decrease to a relatively low value with further oxygen absorbed, after which the loss again increases with continued oxidation.
H E selection of electrical insulating oils for various service applications represents a difficult and important problem. Such selection must be based on the original physical, chemical, and electrical properties of the oil in terms of the resulting initial and deterioration characteristics imparted to the oil-insulated equipment. The information necessary for such selection must be obtained from appropriate deterioration tests and from results obtained from service. Thus the development of adequate tests and measurements are necessary in the application of electrical insulating oils.
T
Refining and Properties of Oil Samples The work reported in this paper covers studies which have been made on two series of specially prepared oils. One of these series consisted of forty-one samples; studies on eleven of them are included in this paper. This series was prepared by the Gulf Research & Development Company. The other series consisted of a total of fifteen samples, and studies on five of them are included here. These samples were prepared by the Shell Petroleum Corporation. The base stock from which the complete Gulf series was refined was selected so that it would be as nearly identical as
possible to the stock which the Nationtll Bureau of Standards was studying, as A. P. I. Project No. 6 (19), by successive fractionation. The stock used by the Bureau of Standards was a blend of 150 and 450 mid-continent distillates, which when fractionated into nine distillates yielded a ninth cut of 730 seconds Saybolt Universal viscosity at 100' F. (37.8' C.). However, for this study it was desirable to have a fraction with a viscosity of about 1600 seconds, which would be in the range of cable saturants. To produce this heavier fraction, a raw dewaxed cylinder stock was blended with the 150 and 450 distillates. The stocks used for this blend were supplied by the Continental Oil Company from a crude charge which was a mixture of Wilcox sand crudes from the Oklahoma CityLucien-Crescent pool. Three stocks having viscosities of 159, 676, and 14,640 seconds were blended in the respective proportions of 42.4, 28.6, and 29.0 per cent. They represented 9.4, 6.4, and 6.5 per cent, respectively, of the original crude. This blend was vacuum-flash-distilled, and bottom, side, and overhead streams were obtained. The overhead stream was distilled and again bottom, side, and overhead streams were taken off. The overhead stream from this second distillation was then distilled to give three more cuts, making a total of seven distillation cuts of approximately equal volume. The Gulf series of samples reported here was obtained by solvent refining the side stream from the third distillation above, and represented 2.7 per cent of the original crude. This cut, called D6, had a viscosity of 109 seconds which is in the oil-filled cable oil range. This cut was furfural-solvent-extracted to give seven cuts; the first extract was labeled D6E1 and the last D6E6. The raffinate from the last extraction was called DOE7. These extracts were treated for 20 minutes a t 350" F. (176.7' C.) with 20 per cent by weight of Filtrol clay to give samples, D6ElF, D6EZF, . . D6E7F. Portions of the second, fourth, and sixth extracts (D6E2, D6E4, D6E6) were treated with 55 pounds (24.9 kg.) of 98 per cent sulfuric acid per barrel, neutralized with 50 per cent excess of caustic, and then clay-contacted as above. The acid-treated samples were labeled DBEZAF, D6E4AF, and D6E6AF. Sample D6F2 is a portion of distillation cut D6, which was given the same clay-contacting treatment as the other samples. These eleven samples composed the Gulf series studied in this paper. Data presented in Table I show that they have a rather wide range of properties. With continued refining the aromaticity drops to approximately zero (as shown by specific dispersion) and the character changes from highly naphthenic to highly paraffinic (shown by kinematic viscosity index), while the viscosity drops from 900 to 100 seconds. The Shell series was prepared from a plant cut of a special West Texas crude of low pour point. The cut had a viscosity of around 100 seconds. The first sample, 100, was not solvent-extracted; the other samples were extracted to give decreasing degrees of aromaticity, 100-4 being completely
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TABLEI. PHYSICAL PROPERTIES OF GULFSAMPLES Sample h-0. Gravity O A. P I. Sp. gr.,.bO/po C. Refractive index 20° C. Refractive disperkon, 20° C. Sp. dispersion Kinematic visc (4) centistokes l o O D F. (37.8" C.) 210° F. (98.9' C.) Saybolt Univ. visc. (18), 880.
looo F.
210" F,. Flash point (I). Pour point,
O
O
F. (" C.)
F. (" C.)
Color ( 5 ) Carbon residue (Conradson), % Neutralization No. Sulfur ( 8 ) . % Aniline point, a F. (" C.) Kinematic viscosity index 14) Viscosity- ravity constant (16) Mol. weiggt (16) Grignard ( B ) , cc. Oz/kg. Evolved Added Consumed Olefinic unsatn. (8) % Aromatic unsatn. (6) % Power factor (60cycl&, 85' C.),
%
..... .....
D6E2F 15.5 0.9589 1.5450 0.0124 129
D6E2AF 16.9 0.9498 1.5368 0.0156 164
D6E3F 23.1 0.9123 1.5091 0.0106 116
D6E4F 27.9 0.8840 1.4900 0,0104 118
22.65 4.00
197.1 8.60
72.80 6.21
58.33 5.72
33.29 4.71
24.84 4.17
108.8 38.9 385 (196.1) 1 5 (-15) 1 0.07 0.06 0.24 187 (86.1) 88 0.843 315
915 53.5 390 (198.9) +15 (-9.4)
336 45.7 380 (193.3) 0 ( - 17.8) 8+ 0.60 0.18 0.60 100 (37.8) 35 0,927 335
270 44.2 395 (201 7) 20 (- 28.9) 4 0.21 0.07 0.57 112 (44.4) 13 0.915 315
155.6 41.1 370 (187.8) -5 ( - 20.6) 8+ 0.35 0.11 0.35 151 (66.1) 54 0.873 325
118.4 39.4 375 (190.6) 10 ( - 12.2) 4'/2 0.10 0.04 0.19 181 (82.8) 81 0.845 320
.....
2300 750 3050
600 500 1100 18 33
220 90 310 8 13
380 310 690 10 7
140 120 260 4 12
2.79
.....
D6F2 28.1 0.8829 1,4920 0,0107 120
D6ElF 6.3 1.0230
..... ..... ..... .....
.....
-R +,
2.31 0.19 1.09 34 (1.1) 170 1.004 340
-
.....
.....
.....
-
-
-
60.5
freed of aromatics by treatment with fuming sulfuric acid. of the samples received a finishing treatment with sulfuric acid, followed by clay contacting. This resulted in aromaticities as measured by specific dispersion of from 142 (highly aromatic, benzene = 189) for sample 100, to 96 (aromaticfree) for sample 100-4. Data on the physical properties of these samples are given in Table 11.
+
0.60
D6E4AF 29.5 0.8752 1.4835 0.0092 107
D6E5F 30.9 0.8676 1.4795 0.0084 97
D6E6F 32.4 0.8596 1.4735 0.0083 97
D6EBAF 32.9 0.8570 1.4720 0.0075 88
D6E7F 36.1 0.8406 1.4645 0.0078 93
22.89 4.08
19.83 3.80
30.08
19.20 3.80
16.52 3.56
109.8 39.1 365 (18;)
96.7 38.2 365
94.0 38.2 380 (193.3) 10
82.9 37.5 37a (190 6 , 10 n
40 40 80 0 0
10 50 60 1 0
0.01
0.01
3.90
1+ 0.02 0.04 0.07 190 (87.8) 95 0.835 325
0.01 0.03 0.11 197 (91.7) 104 0.822 315
97.7 38.6 370 (187.8) 10 (-12.2) 1 0.00 0.03 0.10 209 (98.3) 113 0.817 325
30 200 200 0 15
100 80 180 0 13
30 300 300 1 5
- 17.8)
0.11
-5
(-20.65 11/2
0.10
+
0.09
+
10.00 0.03 0.08 211 (99.4) 114 0.815 320
0.00
0.03 0.03 226 (107.2) 131 0.798 325
portions of distilled water, methanol, benzene, and finally methanol; and a drying Procedure using clean hot air.
Oxidation Procedure The procedure used in the deterioration of these mineral oils by oxidation has resulted in satisfactory check data. Since previous work in this and other laboratories has shown the procedure to be of importance, it is described in detail.
Deterioration Apparatus Experience has shown that deterioration studies on oils, involving electrical measurements, rate of reaction measurements, and chemical analyses, require certain precautions and carefully designed apparatus in order t o obtain reproducible data. The deterioration system briefly described here was developed in a research program on the deterioration of ~ be completely described in a insulating oils. T E system subsequent paper.
oxidation a t 760 mm. ~ All of the ~samples were~deteriorated~gp'bys&8n5d. r 2:~ of the Shell oils was 75" C. Whatman No. 41-H ashless filter paper was used because of its similarity to pure cellulose. Fifty grams of paper were employed in each test. It was desired to use the maximum amount of paper consistent with adequate agitation of the oil and dispersion of the gas through the oil. Twenty grams of copper were employed, previous work having shown that this amount brought about the deterioration within a satisfactory period (usually 2 weeks). This amount gives 1.6 sq. cm. of cop er surface area per gram of oil. Dornte and Ferguson (IO)founcfthat when the copper oil ratio was greater than about 0.4 sq. cm. of copper per ram of oil, it did not have a significant effect on the oxidation. borresponding ratios in oil-impregnated paper-insulated cables are of the order of magnitude of ten to one, whereas in other oil-insulated equipment it is much less. If electrical tests alone were to be made, the system was charged with 468 grams of oil; if the deterioration was being made for
EQUIPMENT,An all-glass system (total volume 1000 cc.) has an internal glass pump for mechanical agitation of the oil and for dispersion of gas contained in the system through the sample, and provision for introducing copper and paper in the oil sample. The electrical measuring equipment includes a three-terminal platinum measuring cell [guard supporting and measuring section made from 0.25-inch (6.35-mm.) 0 . d., 7-mil wall tubing, 50-mil spacing, measuring section 1 inch (2.5 cm.) long, air capacitance micromicrofarad 4.21 sealed to a No. 15 male ground joint, for making electrical measurements on the oil in the system during the deterioration (7). An additional cell of similar construction is rovided in case electrical measurements are desireJon an oil-impregPROPERTIES OF SHELL SAMPLES TABLE11. PHYSICAL nated paper sample in the system during the deSample No. 100 100-1 100-2 100-3 terioration. 30.4 31.9 An automatic oxygen feed apparatus maintains Gravity A P I 23.1 28.1 the oxygen pressure constant at 760 mm. and allows Saybolt'Uni;. (loo'' F,)* 110 102 97 94 the quantity of oxygen supplied to be read on a F 1 ~ ~ ~ p o i nF. t b o c.) 345 (173.9) 345 350 (176.7) 350 390 390 390 (198.9)a 390 buret (500 cc.); the apparatus is actuated by elecFire point, F. (" C.) -1s -10 ia 1'/¶ + Color 108
