INVESTIGATION OF INSULATING OIL DETERIORATION A Critical Study of Tests’ In these investigations of the deterioration, usually by oxidation, of electrical insulating oils, a major portion of the research has been spent in studying and improving existing tests, and in developing additional tests which can be applied to oils of this type. These tests may be grouped into three classes : chemical tests, electrical tests, and physical tests. They involve the determination of hydrocarbon type, oxygenated components, both volatile and nonvolatile, metallic components, and other components (e. g., sulfur and nitrogen); the measurement of direct current conductivity and of power factor and dielectric constant over the audio-frequency range; and the measurement of light absorption over the visible frequency range, viscosity, and state of subdivision in the case of colloidal components. The purpose of this paper is to point out, in the light of experience obtained in this research, the significance and limitations of some of these tests as applied to insulating oils.
J. C. BALSBAUGH J. L. ONCLEY
AND
Massachusetts Institute of Technology, Cambridge, Mass.
ject are being carried out by most of the cable companies and public utility laboratories and by a few university laboratories and oil companies.
The Problem Rossini (24) recently discussed the problems involved in fundamental research on lubricating oils. I n a similar manner we can approach the problem of increasing the economic utility of insulating oils by asking the following questions: 1. What are the desirable properties of an insulating oil? 2. How are the desirable physical, chemical, and electrical properties affected by the chemical constitution of an oil? 3. What refining methods may be used for obtaining the de-
sirable constituents in an insulating oil from an original crude?
1. The desirable properties of an insulating oil were discussed by Halperin and Betzer (8), Riley and Scott (%??), Shanklin and McKay Sommennan (28), and others. For the particular application of insulating oil to impregnated paper insulated cables some of the properties of importance are: a. Physical properties such as viscosity, set point, coefficient of expansion, and surface tension. These physical properties are determined by both manufacturing and operating conditions. For example, with solid-type cables the physical roperties are important in determining oil migration and void grmation due to temperature variations under load cycles. b. High stability t o deterioration, such as might be caused by oxidation or the influence of heat in the presence of materials in contact with the oil under service conditions or manufacture. An important criterion of this stability is the resultant increase in power factor of the oil and of the impregnated paper. The economic importance of dielectric losses was evaluated and discussed by Roper (IS). c, High stability to gas evolution and wax formation under electrical discharge, such as might be caused by ionization in voids formed in the insulation under service conditions. d. High stability to electric stress.
(n),
N IMPORTANT problem in insulation research is the measurement of physical and chemical properties of oils used for various purposes. This is especially true for the oils used in the impregnation of high-voltage oil-filled cables. In the early days of electrical distribution, the oils were subjected to low electrical stresses and temperatures, and necessary improvements of the impregnating materials were made by the trial-and-error method of introducing various materials with the oil. Impregnated paper cables (10,000 volts) were first made about 1890, the impregnating material being hot wax, rosin oil, rosin, and castor oil. Improvements by the introduction of petrolatum, etc., were made about 1918. A still later improvement was the introduction of light oil-filled cables by Pirelli first used in Italy in 1924. These two types of cable, known as solid and oilfilled, are in common use today. In the solid type we have a copper conductor lapped with layers of paper “tapes” and sheathed with lead, the paper layers being impregnated with a viscous mineral oil or a compound of mineral oil and rosin. The oil-filled type is similar, except that the impregnating material is a low-viscosity mineral oil which may flow through longitudinal channels incorporated in the cable and into pressure reservoirs placed a t various intervals. The introduction of higher stresses and temperature gradients in these cables for practical and economic reasons and the prospect of even more severe working conditions in the future have provoked considerable interest in the more fundamental aspects of the problem; long-range research problems on this sub-
A
The relative importance of a, b, c, and d depend upon whether the cable is of the solid or oil-filled type. For example, b is more important in the oil-filled than in the solid type, and c is more important in the solid than in the oil-filled type. 2. The question as to how the desirable physical, chemical, and electrical properties are affected by the chemical constitution of an oil is the particular problem which is being studied in a joint research project of the Utilities Coordinated Research Ino. (Association of Edison Illuminating Companies) and the Massachusetts Institute of Technology. The purpose of this joint research project is the investigation of the mechanism of, and the method of determining the de318
MARCH, 1939
INDUSTRIAL AND ENGINEERING CHEMISTRY
terioration of electrical insulating oils by processes related to oxidation with special reference to the influence on electrical characteristics; the development of tests for quantitatively determining the extent of such deterioration; and the study of the influence on deterioration of normal and added constituents of commercially refined insulating oils and of materials frequently in contact with the oil during service. More specifically, the work may be discussed with respect to oil samples, initial chemical and physical data to be obtained on the samples, deterioration studies, and tests made during deterioration: a. The oil samples which are being studied in this program include: commercial insulating oils, a survey group of samples obtained from different crude sources and refined by different commercial methods, a surve group of solvent-refined samples obtained from Fenske and d n n o n , of the Pennsylvania State College, special complimentary groups of solvent-refined samples obtained from the Gulf Research and Development Company and the Shell Petroleum Corporation, and synthetic hydrocarbons of known structure. Some of the results obtained from electrical and chemical studies of oil oxidation, using oil samples from the first three of the foregoing groups, were given by Bdsbaugh, Larsen, and Oncley (4). b. The initial chemical and physical data t o be obtained on the samples are: viscosity, specific gravity, refractive index, specific dispersion, molecular weight, flash point, pour point, aniline point, distillation test, sulfur content, nitrogen content, carbon-hydrogen ratio, and the chemical, physical, and optical data under d. c. The deterioration studies include oxidation and the influence of heat with an inert gas; they are carried on individually or in the presence of materials in contact with oil under service conditions, the volatile deterioration products being in some cases removed and in other cases retained. The foregoing are made for the determination of the kinetics of the oxidation and for obtaining oil samples at various stages of deterioration for obtaining the test data under d. d. The test data t o be obtained as a function of the deterioration under e are: acids, esters, peroxides, Grignard evolved, Grignard consumed, unsaturation, volatile products, oxygen absorbed, power factor over the audio-frequency range and as a function of temperature, conductivity at low gradients as a function of temperature, dielectric constant and its change during deterioration, copper (for those tests in which copper is used as a catalyst, either in bulk, colloidal, or soap form), and light transmission as a function of wave length (400-700 millimicrons),
3. It is believed that there are now available commercial refining methods which permit an effective separation of the chemical constituents of a crude oil (11). The more important problem a t present is the determination of the desirable constituents of an insulating oil. The purpose of this paper is to point out the significance and limitations of some of these tests in the light of these investigations. They may be classified into three groups: chemical tests, electrical tests, and physical tests. A review by Ambrose (1) states that “recent developments in research on insulating oils are largely concerned with methods of testing and evaluation. Without adequate test methods (other than long time service tests), no criteria are available for distinguishing between good and bad liquid dielectrics.” Because of the importance of this phase of the problem, some of these developments will be discussed, using results obtained in the investigation as illustrations. This is a factor of the greatest significance in the conduct and analysis of any investigations of oil deterioration, since the success of such investigati0n.s depends entirely upon the significance and importance of the tests used.
Hydrocarbon Constituents Insulating oils, like all petroleum oils of a similar viscosity. are a complex mixture of hydrocarbons of many types. These types are generally classified as paraffinic, naphthenic, aromatic, and olefinic. The paraffinic type consists of straight-
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chain and branched-chain hydrocarbons. The naphthenic type consists of the various cycloparaffins. The aromatics are the hydrocarbons with condensed ring structures of the benzene type, such as benzene, naphthalene, anthracene, pyrene, etc. The olefinic type contains double bonds not in aromatic ring structures. I n general, almost all materials of the latter type have been removed in the refining process. A large majority of the hydrocarbons present are not examples of these four distinct types but are combinations. Thus, in a molecule such as butylcyclohexane we have six carbon atoms of naphthenic and four of paraffinic character. Such a hydrocarbon contributes roughly 60 per cent to the naphthenic and 40 per cent to the p a r a f i i c nature of the oil. Besides these hydrocarbon components, there may be certain sulfur-, nitrogen-, and oxygen-containing substances, the nature of which is not yet clearly understood (86). These substances are nearly always almost completely removed in the refining process, although certain oxygen- and some nitrogen-containing materials are occasionally added as inhibitors. Acidic oxygen compounds are usually found as oxidation products and are considered detrimental in nature. It is very desirable to have tests that will give us some idea as to the relative proportions of the materials, mentioned in the last two paragraphs, which are present in a given oil Sample. A considerable number of tests have been developed for determining the percentage compositions of hydrocarbon atoms of the types mentioned. Because of the complexity of the mixtures involved, these tests are of necessity rather empirical in nature. This problem was recently discussed by Rossini (94) who concluded that “once a complete set of data is obtained on the physical properties of representative compounds of the different types of hydrocarbons of high molecular weight, more certain and effective methods of deducing the type or types of hydrocarbons present in a given lubricating-oil fraction can be established.” This problem is being attacked a t the National Bureau of Standards (American Petroleum Institute Project No. 6) by Rossini and his co-workers (24, 25‘), by Mikeska (16) of the Standard Oil Development Company, and by others. The following properties are useful in characterizing an oil: viscosity, change of viscosity with temperature, density, molecular weight, refractive index, change of refractive index with frequency (dispersion), boiling point, surface tension, aniline point, empirical formula, etc. Various combinations of these properties give us such characterization values as the viscosity index, viscosity-gravity constant, specific dispersion (dispersivity), U. 0. P. characterization number, etc. The most precise method available for the complete specification of the oil type is the Waterman method (29)which depends upon measurements of aniline point, specific refraction, and molecular weight of the original oil and of the hydrogenated oil; the hydrogenation is conducted under high pressure with a nickel catalyst a t temperatures under 350” C. This hydrogenation is long and difficult, since the nickel catalyst is easily poisoned by most oil samples. Waterman gives an empirical method for eliminating this step which seems fairly satisfactory. Further investigations of this method, especially in the use of other catalysts which are less readily poisoned by these oils, should prove profitable.
Oils and Hydrocarbons Studied I n order to profit as much as possible from the work done a t the Bureau of Standards on A. P. I. Project No. 6, in establishing the types of hydrocarbons present, a crude oil was chosen for a major part of the work which was as similar as possible to that used in the bureau’s investigations. The stock chosen represented commercial productions from a mixed charge of Wilcox sand crude from the Oklahoma-
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1 OXIOIZEO
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TIME FIGURE
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600
TIME IN HOURS AND PROIDUCTS FORMED AS A
Lucien-Crescent pool and was a mixture of 42.4 per cent raw 150 pale oil, 28.6 per cent raw 400 pale oil, and 29.0 per cent raw overhead cylinder stock. Each of these materials was dewaxed, and the resulting stock represented 22.3 per cent of the original crude oil charge. The breaking up of this stock will be described in a later paper, and this information is given here simply to indicate how i t is hoped to connect this work with that being carried out in the A. P. I. Project No. 6. One of the oils upon which some tests are reported in this paper (G-DGF) was a distillation cut from this stock. Some properties of this oil, as well as those on a commercial insulating oil (S-1), are given in Table I. They are both oils in the low-viscosity range (approximately 100 Saybolt Universal seconds a t 100' F.). Along with these two oils, one a typical well-refined insulating oil and the other an oil of similar characteristics which had been refined only by a clay treatment, results are reported on three synthetic hydrocarbons; one of them was prepared and all were purified in this laboratory. They represent a typical p a r a f i (cetane), a typical naphthene (Decalin), and a hydrocarbon which is half naphthenic and half aromatic (Tetralin) , The cetane was prepared (16) from spermaceti (cetyl palmitate). The ester was cracked in an iron still, and the liberated cetene was distilled at 230-250" C. and a pressure of 300 mm. This crude yellow cetene was then shaken with 10 per cent sodium carbonate until acid determinations showed it to be acid-free. The cetene was then vacuum-distilled from a Claissen flask a t 100' C. and a pressure of 2 mm. After distillation, the straw-colored cetene was hydrogenated at
FUNCTION OF TIME
180' C. and a pressure of 35 atmospheres of hydrogen, using Raney nickel catalyst. The colorless cetane (hexadecane) obtained gave only a 0.3 per cent unsaturation test, and was subsequently vacuum-distilled under carbon dioxide (boiling point, 112' C. a t 2 mm.), and stored in a brown bottle flushed with carbon dioxide. TABLE I. PROPERTIES OF OILS a-D6F
F f i o eravity (25/4O C.) ash point, F. Pour oint F.
SayboTt Udiversal viscosity at looo F., 680. Refraotive index at 25" C. Neutralization Sulfur Aniline point F. Kinematio vikooaity index Dielectric constant at 25' C. Power factor at 25' C BO cycles D . 0 . conductivit at ib' C y X 101' Grignard evolvedl(N. T. P.j: 00. On/kg. oil
0.8808 380 -15 109.4 1,4909 0.02 0.25
186 77
2.40 0.00015
0.8 310
8-1 0,8960 320 -55 104.0
....
Neutral
.... .... ....
2.20
