US2326324A - Dielectric or insulating oil - Google Patents

Description

Patented Aug. 10, 1943 DIELECTRIC R INSULATING OIL Henry G. Berger,'Woodbury, Lyle A. Hamilton,

Wenonah, and Everett W: Fuller, Woodbury, N. .L, assignors to Socony-Vacuum Oil- Company, Incorporated, New York, N. Y., a corporation of New York No Drawing. Application May 24, 1940,

Serial No. 336,960

integration of the materials, particularly of the Claims.

This invention is directed to the maintenance of the dielectric stability 'of insulating oils such as are used in high voltage power cables and similar situations.

In certain types of high voltage power cables insulating oils form a part of the dielectric medium. Such cables, in general, consist of one or more copper conductors, each main conductor Wrapped with many overlapping layers of paper tape and the whole encased in a lead sheath. All space in the cable, not occupied by the paper or the conductors, is filled with an insulating oil. In some types of cables 3. low viscosity oil is used to impregnate the cable after applying the lead sheath and this oil is then maintained under pressure from a reservoir at the end of the cable. In other types a high viscosity oil is used to impregnate the paper and, after impregnation, the cable is sheathed and sealed. In both cases the object is to completely fill all space between the conductor and the lead sheath with an insulating material.

In practice it has been found that, under conditions of 'use, such oils are prone to decomposition of some form with the resultant formation of gas and a wax-like substance known as cable wax," This decomposition may lead to loss of dielectric properties of the combination of oil with paper or other structural insulating material used and eventual cable failure. It is known that one of the major causes of failure has been traced to "void formation, 1. e., to the presence of small pockets in the structure, actually filled with gas, which may have been caused by the attendant expansion and contraction of the insulating structure when in use under a normal load-temperature cycle, or by the incomplete impregnation of paper, or other insulating structural material with oil, or voids" may be formed by handling, as by bending of a cable, causing parting of insulation. These pockets or voids are filled with gases, some possibly being residualair, and some originating from the dielectric material. This invention is directed to a method of protection of a composite insulating structure such as is used in a cable or capacitor against de terioratlon resulting from voids 0f the type discussed above.

It has been suggested that the deterioration referred to above most probably has its origin as the result of the electrical bombardment of the surrounding oil, and structural material by high speed electrons formed within the void under oils. This manifests itself by the splitting out of hydrogen from the hydrocarbon molecule and the formation of the insoluble waxy material called X-wax or cable wax of unknown com tion will. become progressively worse as voids" are enlarged and new ones formed by the formation of hydrogen. This results in ultimate electrical breakdown.

It has been suggested that the tendency towards gas formation in cables may be lessened by adding to a cable oil certain compounds. Thus Nederbragt, U. 5. Patent 2,087,578 discloses that adding a small amount of a relatively volatile aromatic hydrocarbon having a compact molecule and a molecular weight less than 225 serves to inhibit gas evolution for a limited time. Berberich, U. S. Patent 2,176,952 discloses a number of aromatic compounds which serve the same purpose. The formation of gases in a cable oil is lessened to some extent by the substances disclosed by these patents but, is not whollyprevented; thus oils containing these materials, while they do not deteriotate at such a rapid rate as untreated oils, nevertheless eventually fail because of excessive gas evolution and void" formation.

This inventionhas for its object the maintenance of the dielectric stability of systems containing insulating oils under conditions of use wherein they are subjected to the deteriorating influence of electric bombardment such as occurs in a cable in service. It furthermore has as its object the prevention of void" formation in the dielectric medium used in high voltage cables.

A further object is the provision of an oil composition for such use.

This invention depends upon the introduction into the dielectric medium, in such a manner that it will be in intimate contact with the oil, of a hydrogenation catalyst, with or without added hydrogen acceptors in addition to the oil. The hydrogenation catalyst may be a solid in or on the surface of the paper or other insulating structural material, or it may be a material dispersed in the oil, or it may be an oil-solubl material dissolved in the oil. It operates by promoting the absorption of such hydrogen as is formed as a result of electric discharge, in hydrogenation processes, and by this continuous absorption eliminates voids as they first begin to form. This continuous healing of incipient.

voids, or prevention of such voids. keeps at a minimum the dielectric losses of a cable and enables the dielectric to give satisfactory service for an indefinite period of time.

It is well known that many petroleum oils under the influence of the proper catalysts will absorb hydrogen; likewise under the influence of proper catalysts aromatic compounds and unsaturated non-aromatic compounds in general will absorb hydrogen.

Hydrogenation catalysts in great variety have been disclosed in the chemical literature. Thus, various metals and metal oxides among other materials will serve under the proper conditions to cause hydrogen to be absorbed by petroleum oils, and by all manner of aromatic and olefinic compounds, and also by other reducible compounds such as nitro compounds.

It should be noted that the conditions such as temperature, pressure, etc., required to hydrogenate a certain material in the absence of a high voltage electric field, may be no criterion of the conditions under which the same reaction will occur when under the influence of such a field; thus a catalyst which requires high temperature and high pressure to be effective in causing a petroleum oil to absorb hydrogen may efiectively cause the same reaction to occur at normal temperature and pressure under the influence of a high voltage electric field such as occurs in the cables of the type with which we are concerned.

Our invention thus comprises a method of preventing deterioration of dielectric compositions such as insulating oils when these are used under conditions wher they are subjected to high voltage stresses such as occurs in high voltage power cables. It is carried out by placing in or on the surface of the-structural dielectric a solid hydrogenation catalyst and/or dissolving or dispersing in the insulating oil a hydrogenation catalyst which is effective in causing the liquid dielectric to absorb such hydrogen as may be formed, substantially as it is formed, under the conditions existing in the dielectric. The catalyst and the liquid dielectric shall be so chosen as to complement one another; thus certain catalysts will serve eifectively when no addition is made to the insulating oil, while others will be eflective only when a suitable hydrogen acceptor, such as an olefinic material or certain types of aromatic substances or other reducible materials, have been added to the liquid dielectric. We desire to claim the process without limitation as to the hydrogenation catalyst or as to any hydrogen acceptor which we may wish to use, other than that they shall be efiective to accomplish the desired result.

As will be obvious to one skilled in the art, and as stated in our information, only those petroleum oils which contain aromatic materials or olefinic materials or to which reducible compounds of some type have been added, may be expected to benefit by our invention. A petroleum oil from which all aromatic and olefinic constituents have been removed, as by treating with large amounts of sulfuric acid and oleum would not b expected to adsorb hydrogen under the influence of a hydrogenation catalyst, and therefore such a cable oil would not be improved by our invention, except by the concurrent addition of an acceptor.

As an indication of the practical performance of this invention, the followingtest data are presented.

While actual operating conditions would indicate that tests of oil to high tension electric discharges should be made at or near atmospheric pressure, or in actual cable tests, such tests are not only diflicult to perform, but are considerably less rigorous than the following test.

A discharge chamber is arranged, consisting of two co-axial glass cylinders, the annular space between which is sealed from the atmosphere, the outside of the outer cylinder being'coated with a conducting paint, and the inner cylinder filled with mercury, thus providing two conductors with an-insulating space between. The annular space is evacuated to less than 0.05 mm. mercury pressure, absolute, then partially filled with oil while maintaining the vacuum to insure vacuum, the action of the hydrogenation catalyst is quite difilcult, and it is obviously far less effective than at normal cable'oil operating conditions, for the vacuum encourages escape of the hydrogen. from the catalyst and the liquid phase wherein hydrogenation occurs. Even in the face of this adverse condition, highly significant results were obtained, as appears below.

Using this procedure two oils were tested alone and with a palladium on carbon catalyst suspended in the oil.

Oil A was a regular commercial cable oil prepared from a naphthene base crude distillate which contains a substantial'amount of aromatic constituents by refining with a moderate amount of sulfuric acid. It was to be expected that our invention would improve the service characteristics of this oil in a cable thru lessening or preventing gassing" with void formation.

Oil B was a white oil of the viscosity used for 5 cable oils, prepared by refining a parailinic crude distillate with large amounts of sulfuric acid and oleum. It was to be expected that our invention would not improve the service characteristics of such an oil.

The following results were obtained; noting pressure in the system after various time intervals of exposure to discharge.

Oil A+ Oil A hours on test Oil A hydrogenation catalyst Mm Mm. 0 i 01 of a hydrogenation catalyst.

Oil B+ Oil 13 hours on test Oil B hydrogenation cataly Mm Mm.

With this oil where no hydrogen acceptor was present, the hydrogenationcatalyst actually increased gas formation.

Th results indicate that the use of a hydrogenation catalyst to decrease or prevent void formation in cables due to the gassing of the cable oil is of practical value and, properly applied, should result in a greatly increased service life for oil filled cables, so treated.

Obviously hydrogenation could not be accomplished in the absence of a hydrogen acceptor. Such hydrogen acceptors, as pointed out previously, are most conveniently aromatic compounds or unsaturated non-aromatic compounds. may be present in the oil, or be added thereto, and our invention embraces both phases.

While palladium on carbon was utilized many other suitable hydrogenation catalysts will be suggested to those skilled in the art, such a for example, finely divided platinum, nickel, cobalt, copper, nickel oxide, ,or nickel salts of organic acids.

The most usual manner of use of the catalyst will be as a suspension in the oil prior to its application to the use. It is also within the scope of our invention to incorporate the catalyst within or upon some solid portion of the dielectric structure, as for example, the paper wrapping or paper tube previously mentioned, or to use an oil soluble catalytic material, dissolved in the oil.

We claim:

1. An oil for use in dielectric structures com- They prising mineral oil, aromatic hydrocarbon material capable of accepting added hydrogen, and catalyst material capable of promoting hydrogenation.

2. An oil for use in dielectric structures comprising mineral oil containing aromatic hydrocarbon compounds capabl of accepting added hydrogen and in suspension therein a small amount of a hydrogenation catalyst of the nature of palladium on carbon.

3. An oil for use in dielectric structures comprising mineral oil, unsaturated hydrocarbon material capable of accepting added hydrocarbon, and catalyst material capable of promoting hydrogenation.

4. An oil for use in dielectric structures comprising mineral oil containing unsaturated hydrocarbon compounds and in suspension therein a small amount of a hydrogenation catalyst of the nature of palladium on carbon.

5. An oil for use in dielectric structures comprising a mineral oil containing an oil miscible material capable of accepting added hydrogen, and, selected from the group consisting of arcmatic and unsaturated hydrocarbons and in admixture therewith catalytic material capable of promoting hydrogenation. HENRY G. BERGER.

' LYLE A. HAMILTON.

EVERETT W. FULLER.