US1952158A - Electrical apparatus and dielectric material therefor - Google Patents

Description

March 27, 1934.

F. M. CLARK ET AL ELECTRICAL APPARATUS AND DIELECTRIC MATERIAL THEREFOR Filed Dec. 23, 1932 TRANSFORMER GOA/TA INS DIELECTRIC OIL 60A! PAS TE Inventors: F'Pank M.Clar-k, Ralph A. Ruscetta,

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Patented Mar. 27, 1934 UNITED STATES ELECTRICAL APPARATUS AND DIELECTRIC MATERIAL THEREFOR Frank M. Clark and Ralph A. Ruscetta, Pittsfield, Masa, assignors to General Electric Company,

a corporation of New York Application December 23, 1932, Serial No. 648,630

5 Claims. ((31. 175-361) The present invention relates broadly to electrical apparatus and more especially to dielectric material for use in connection with electrical apparatus, such for example, as junction boxes, pot heads, transformers, bushings, metal-clad switchgear, lightning arresters and the like.

The principal object of the present invention is to provide a semi-solid dielectric material which is soft and plastic over a wide range of temperatures above and below room temperature, and which possesses dielectric properties superior in a number of fields of use to both the solid and the liquid dielectric materials heretofore used in such devices as will be described more fully hereinafter.

In accordance with the present invention there is employed in electrical apparatus a plastic dielectric material which consists principally of mineraloil and a relatively small proportion of metallic soap. The proportions of oil and soap are so chosen that the composition forms a colloidal gel. In some cases a few per cent of rosin may be added with accompanying advantage as will be explained later.

The accompanying drawing shows in front elevation a transformer and a junction box as illustrative of apparatus in which a paste of oil and zgiap may be used in accordance with my inven- It has been heretofore generally believed that metallic soaps are objectionable constituents in dielectric materials. They were believed to lower their dielectric value. Hence, the presence of metallic soaps has been always avoided in mineral oils used for dielectric purposes. Sludge formed in mineral oils in electrical apparatus, such as transformers, usually contain metallic soaps and sludge formation has been carefully guarded against in the past.

We have discovered that if metallic soaps employed with mineral oils and the ranges of ingredients carefully chosen we are able to obtain a plastic product which has the' character of a colloidal gel and which is stable throughout a wide range of temperature. We have, moreover, found that such a composition instead of being lower in dielectric strength than the oil itself is materially better in dielectric strength, an increase of about 25% being obtained. Hence, we are able to obtain the desiralble properties of a good dielectric in an electrical apparatus without its attendant disadvantages by simply using a small percentage of metallic soap in an oil, the proportions being so chosen that the resulting product is a stable plastic composition.

In order to prepare the dielectric composition to be employed in conjunction with the electrical apparatus a required quantity of a parafilne or asphaltic oil or a mixture ,of parafline or asphaltic oil, is heated in a suitable container and are the metallic soap is introduced in any one of a number of methods to be described below. A suitable oil for use consists of a heavy mineral oil with a viscosity of approximately 150 seconds Saybolt at 100 C. The oil either may be unrefined, or commercially refined in the ordinary sense of transformer oil, or may be a highly refined paraffine oil type of material. The type of oil used will depend upon the application to which it is subjected. For instance, if a product is desired that is highly resistant to sludge formation a highly refined mineral oil would be used as a base.

To the oil at room temperature is slowly added the requisite quantity of metallic soap. Any suitable metallic soap may be employed including metallic salts of the fatty acids of high molecular weight the metal part of the soap including such metals as magnesium, aluminum. lead, calcium, sodium and zinc, and, in general, metals of the alkali and alkaline earth groups which are combined with fatty acids of high molecular weight including such acids as stearic acid, palmitic acid, oleic acid and the like. The proportions of soap to oil may be varied within a definite range. The range which we have found necessary for carrying out the object of our invention is 88 to 98% oil and 2 to 12% metallic soap, the preferred proportions being 6 to 10% soap and 90 to 94% oil. A suitable composition is one wherein the proportions are 90% oil and 10% metallic soap. such as aluminum stearate.

While the metallic soap, for example the aluminum stearate, is added to the oil, as set forth above, the mixture is continuously agitated. When the soap has all been added the temperature of the mixture is slowly raised, vigorous agitation being continued. As the temperature rises, the mass thickens and at about 100 (2. its consistency is that of an extremely heavy gel or paste. The temperature is slowly raised further and as the temperature increases above 100 0., the viscosity of the l quid decreases until at about 150 to 175 C. the viscosity has become comparatively low and the soap is gradually taken into solution. The temperature of the mass is raised to 200 0., continuous agitation being employed, and the mass is held at that temperature until the required characteristics are obtained. The mixture at this stage does not show any signs of stringiness. When the soap has apparently gone into complete solution the temperature of the mixture is lowered to about 175 C. When the mixture has reached approximately this temperature it is run out into relatively long and shallow pans. for example pans approximately four inches deep, ten feet long and four feet wide. In these pans the mixture is cooled to room temperature and upon cooling is a butterlike, plastic mass.

Instead of preparingthe composition as above outlined it may, for example, be prepared by mixing'a sodium soap with other oils preferably in the presence of a small amount of water; or they sodiumsoap may be mixed with the oil in the absence of. Water and an, aqueous paste or heavy solution of a-soluble aluminum salt or salt of any of the other metals desired may be added thus forming the desired soap in the oil-water mixture by double decomposition reaction.

Or, the mineral oil may be mixed with a vegetable or any other organic oil such as cotton-seed oil, corn oil or various fish oils and the vegetable oils saponified by means of sodium hydroxide in the presence of water in an autoclave. To the product is added a soluble salt of the desired metal and the desiredsoap precipitated into the mineral oil on evaporation. of the water.

- "Or, the fatty acid, such as oleic acid may be added to the mineral, oil and to the mixture a water solution of the desired metallic salt is added forming the desired soap on evaporation of the water. I v

Irrespective of the method bywhich the metallic soap is obtained the manufacturing process is substantiallythe same and follows along the general lines of that outlined in the first method of preparation.

We preferred to use aluminum stearate as the soap ingredient because of the greater stability, resistance to water and other desirable factors obtained by-the use of this material as compared to the cheaper sodium soap.

The transformer shown in the drawing comprises a metal casing containing primary and secondary windings (not shown) surrounded by a dielectric material 2 the composition of which is indicated by a legend. The high tension winding is connected to one or more bushing terminals, one of which is indicated at 3.

The low tension terminals lead to a junction box 4 mounted on the cover 5 of the transformer. It comprises a metal casing 6 containing oil-soap paste 7 and in it the conductor 8 leads from the low tension bushing 9 to the exterior. The breather 10 and the hand holes 11 and 12 are conventional.

The dielectric compositions of our invention impart superior properties to the electrical aporder as ordinary asphalt at room temperature,

namely about 1.5 x 10 ohms per centimeter cubed. Hence, the presence of the soap in the oil does not spoil its resistivity, as would be expected, but in fact gives a resistivity of the order of ordinary asphalt.

A further unexpected result obtained by the employment of the dielectric composition of our invention in electrical apparatus is that such composition presents a power factor at 25 C. of 0.27% as compared with a power factor of 1.6% at the same temperature for ordinary asphalt and as compared with a power factor of 5.4% at the same temperature for wood pitch. Moreover, where the power factor of wood pitch increases at 85 C. to 30% the power factor ob-' tained with the present composition at 85 C.

is only 10% a dielectric strength of only 5 kv. and ordinary asphalt at this low temperature is very brittle and will easily crack. The colloidal gel made in accordance with our invention will remain plastic and stable at temperatures as low as minus 20 C. and as high as 110 0.

Although the present composition is lighter than water, if water is added in small quantities to a relatively large mass of this composition, it unexpectedly prevents the water from coming through and sinking to the bottom. This property makes the composition extremely use ful in apparatus such as transformers, for example, in that it prevents water from coming through the composition and being caught in the bottom of the transformer. Moreover, the

. dielectric strength of the composition is not disfissure formation.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. Electrical apparatus containing a dielectric comprising a colloidal gel of mineral oil and metallic soap, said gel being a soft, plastic solid at ordinary temperatures.

2. A transformer containing electrical windings and a stable, plastic dielectric material therefor separating and insulating said wind ings, said dielectric material consisting of approximately mineral oil and approximately 10% aluminum stearate.

3. A junction box comprising a casing, electric terminals therein, and a semi-solid paste surrounding said terminals comprising mineral oil and a soap, the latter ingredient being. present in sufficiently high proportion to render the paste stable'at temperatures up to about 110 C.

4. A dielectric material for electrical appara-

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