US2997528A - Electrical apparatus having insulation for eliminating creepage tracking - Google Patents

Description

Aug. 22, 1961 ss ET AL 2,997,528

ELECTRICAL APPARATUS HAVING INSULATION FOR ELIMINATING CREEPAGE TRACKING Original Filed Jan. 9, 1957 2 Sheets-Sheet 1 lnveniors: Alvcn A.KeSSel ROberT S. N rman Thelr Airorney Aug. 22, 1961 A. A. KESSEL ET AL 2,997,528

ELECTRICAL APPARATUS HAVING INSULATION FOR ELIMINATING CREEPAGE TRACKING Original Filed Jan. 9, 1957 2 Sheets-Sheet 2 lnventors Aivcm A. Kessel Roberi S. Norman by wu'dg/ Their AHorney Patented Aug. 22, 1961 2,997,528 ELECTRICAL APPARATUS HAVING INSULATION FOR ELIMINATING (CREEPAGE TRACKING alvan A. Kessel, Wakefield, and Robert S. Norman,

Marblehead, Mass assignors to General Electric Company, a corporation of New York )riginal application Jan. 9, 1957, Ser. No. 633,356. Divided and this application Feb. 14, 1958, Ser. No.

6 Claims. 01. 1'74 137 The present invention relates to improvements in the nsulation of electrical apparatus and, more particularly, 0 electrical devices and improved insulations therefor vhich are subject to contaminated conditions which 'romote creepage electrical discharge conditions. As s well known, certain types of electrical equipment subacted to contaminating atmospheric conditions such as moisture, dust, fog and salt frequently fail due to reepage between points of different potentials on the quipment. While the insulating components of elecrical equipment desirably include organic materials vhich are inexpensive and readily molded or otherwise abricated, it has been a particular disadvantage that he organic constituents of such components tend to orm carbonaceous deposits upon exposure to conditions vhich promote such creepage. These carbonaceous devosits ultimately provide paths of sufficiently low reistance to occasion breakdown of the equipment. This ,pplication is a division of our copending application,

ierial No. 633,356, filed January 9, 1957 which latter .pplication is in turn a continuation-in-part of our coending application, Serial No. 523,445, filed July 21, 955 now abandoned.

By way of example, in the instrument transformer ield, it has heretofore been a distinct limitation to the lsefulness of dry molded instrument transformers that he molded organic insulation components thereof form ow-resistance carbonaceous deposits upon exposure to reepage electrical discharges. High-voltage transformrs of this dry type may include molded organic inulating compounds not only as the insulation between lements but as the outer protective casing as well, and re thus distinguished from those which are filled with il or other insulating liquids. In outdoor installations, '1' others where there may be accumulations of dust, rain 11d other environmental contaminants, random surface lischarges or arcing known as surface creepage are |romoted between elements. These discharge conditions ICCQSIOI]. the formation of carbonaceous deposits in the usulation, ultimately yielding low-resistance paths or racks which destroy further utility of the apparatus. )ischarges of the creepage type are to be distinguished rom those caused by the establishment of an are through r directly between two parts of the apparatus having lilferent potentials. Under arcing conditions, while the vrganic material adjacent to the arc is carbonized, the re track so formed is not random in character but forms direct path along the line of the arc. On the other and, tracks due to creepage are random in effect and roduce a tree-like path. The difference between trackng due to arcing and tracking due to creepage is further lointed out in ASTM Test D495-48T in which it is tated specifically that the test directed to determining he resistance of insulating material to arcs does not in ;enera1 permit conclusions to be drawn as to the reistance of the material to other types of arc such as hose promoted by conducting contaminates with which atter the present invention is concerned. It is further vointed out that in the creepage type of electrical failure, ailure of the material can occur not only due to surace failure but to subsurface failure. In other words,

even if the surface of the organic material is devoid of carbonaceous or conducting material, tracking due to surface creepage may nevertheless occur between two points of different potential beneath the surface of the material itself. It is evident from the above and it has been found that materials which are effective in protecting against the effects of direct arcing are not necessarily effective in protecting against creepage breakdown.

As a result of such disadvantages, organic insulating components have been avoided in the construction of electrical equipment wherein such components would be subjected to the influences of creepage electrical discharges, even though these components would otherwise have been attractive because of different considerations. Organic insulating materials which experience this limitation include polyester resins.

Accordingly, it is one object of this invention to provide improved electrical apparatus having polyester resin organic insulation which eliminates tracking due to creepage electrical discharge conditions.

It is another object to provide improved electrical apparatus including certain polyester resin compositions which have insulating characteristics and in which formation of carbonaceous deposits upon exposure to creepage type electrical discharges is eliminated.

By way of a summary account of one practice of the teachings of this invention, an improved high-voltage outdoor instrument transformer includes a magnetic core member with linked primary and secondary windings each connected to terminals, and a polyester resin composition having hydrated alumina dispersed therein in proportions defined hereinafter. The polyester composition with the hydrated alumina is formed about the transformer elements to electrically insulate them to provide an outer covering which protects these elements against influences of the ambient environment. Uniquely, the creepage discharges occurring across the surface of the transformer even under the most severe contaminating conditions do not occasion tracking and breakdown, inasmuch as low-resistance carbonaceous deposits are not permitted to form.

Although the features of this invention which are novel are set forth in the appended claims, greater detail of the invention in its preferred embodiments and the further objects and advantages thereof may be readily comprehended through references to the following description taken in connection with the accompanying drawings, wherein:

FIGURE 1 provides a perspective view of a typical molded casing type transformer embodying our invention;

FIGURE 2 is a cross-sectional view of the transformer of FIGURE 1; and

FIGURE 3 is a pictorial view of electrical test equipment utilized to procure operating life data.

Referring to FIGURES 1 and 2 of the drawings, the electrical apparatus there shown is a current transformer having primary terminals 19 and 11 and secondary terminals 12 and 13. The positioning of the primary windings l4 and the secondary windings 15 will be apparent from the view of FIGURE 2. These windings are electromagnetically linked with a core 16 of magnetic material which is provided with a pair of tubular separators 17 and 18 which serve to insulate the core from the windings.

The transformer of FIGURES l and 2 includes composition 19 which encapsulates the various components as shown and thereby provides a completed physical enclosure as Well as electrical insulation. A support plate 20 facilitates mounting. As has been pointed out above, organic insulating materials heretofore employed as such components of transformers of this type, and in other applications wherein similar conditions may occur, have been subject to the basic problem of carbonaceous residue formation and accumulation occasioned by random electrical creepage discharges which are likely to occur under adverseclimatic conditions in outdoor installations. This difficulty is attributable to the inherent characteristics of organic materials of this type which cause them to break down to form low-resistance carbonaceous deposits when exposed to creepage type electrical discharges. Composition.19 includes polyester resin and hydrated alumina dispersed therein in sufficient quantity to prevent accumulation of carbonaceous deposits upon exposure to creepage electrical discharges such as those which may occur between the conducting members 10, ll, 12, 13 and 20.

With apparatus not embodying our teachings, it is believed that the breakdown process with respect to creepage failures occurs as follows. The localized high-temperature discharges produced across the surface of the polyester resin breaks down the polyester resin components to form carbon deposits. The process is cumulative, as has been previously described, and low-resistance tracking failure occurs rapidly.

Electrical apparatus wherein exposure to contaminating conditions is likely to occur and in which our teachings may be embodied to great advantage includes also apparatus other than current transformers which are unprotected from such contaminating conditions. In accordance with one practice of our invention, we provide highvoltage electrical apparatus in which conductors having a large potential difference are found with an insulating polyester resin material having interspersed therein a hydrated alumina in proportions such that material breakdowns ordinarily experienced under creepage electrical discharge conditions do not occur. The concentration of the hydrated alumina employed is critical since it has been found that only a sufficient concentration eliminates carbon accumulation and electrical breakdown. As will be demonstrated, this effect can be achieved by employing a hydrated alumina only in the hereinafter defined critical proportions, A very marked increase in operating life of the apparatus under creepage electrical discharge conditions is observed as the critical levels of concentration are reached.

Using polyester resins the hydrated almina is used in the proportion of from to 70% by weight of the insulation and preferably to 70% The preferred specific hydrated alumina content of each insulating composition is 60% by weight.

While we do not wish to be bound by any particular theory, it being sufiicient that our invention accomplishes the desired end, we believe that the combined water in the hydrated alumina serves to oxidize the carbonaceous particles formed under creepage conditions and that the aluminum oxide component itself acts as a catalytic agent to indirectly promote this oxidation. It has been found that while unhydrated aluminum oxide is useful in delaying tracking under surface creepage conditions, it is not efficacious in oxidizing carbonaceous materials or in preventing the eventual failure of insulation due to creepage tracking. It has also been found that the water must be chemically bound to the aluminum oxide as in the hydrated compound. It will be obvious that in practical commercial applications, failure due to surface creepage tracking must be not merely delayed but wholly eliminated as by the practice of our invention.

In order that those skilled in the art may better understand how the present invention may be practiced, the following examples are given to illustrate organic insulation components containing polyester resin materials coming within the scope of this invention. These examples are given by way of illustration and not by way of limitation.

Example I Tests were conducted with apparatus mock-ups including various insulating compositions of polyester resin and selected .concentrationsby.weight of hydrated alumina of F trodes extending parallel to each other.

the chemical composition Al O .3H 0. These samples of insulation components were prepared as molded sheets about 0.076" thick and about 6" square and were mold cured at 150 for about 20 minutes. For testing purposes these samples identified by numeral 21 in FIG. 3 were mounted in a high voltage test cage resting flat against a conducting metal plate 22 tilted at an angle of about 15 from the horizontal.

Two electrodes, 23 and 24, each about 1" by 2" in cross section were placed perpendicularly against the surface of each sample 21 under test, that is, against the upper side of the sample opposite that in contact with the metal plate 22. The electrodes '23 and 24 were positioned about 1" apart with the 2" dimensions of the elec- One of the two electrodes, identified by numeral 23, was connected to the metal plate 22 by clips 25 and 26 and conductor 27. The two electrodes were electrically connected across the alternating voltage output of an adjustable high-voltage transformer through clips 28 and 29. It will be appreciated that, with the circuit just described, creepage breakdown of the sample may occur between the two electrodes 23 and 24 or between electrode 24 and the metal'plate 22 by means of a surface failure, or between electrode 24 and the metal plate 22 through the thickness of the sample by means of an erosion type failure.

After the test samples were placed in the apparatus as just described, they were dusted with a synthetic dust representative of atmospheric dust accumulations, the dust particles being designated by reference character 30. A fine water spray 31 was directed against the samples for the duration of the test, a nozzle 32 being coupled with water and air lines 33 and 34 for this purpose. A 60-cycle voltage of about 1500 volts was applied to the electrodes 23 and 24 to set up surface discharge conditions of the intensity capable of causing decomposition of organic materials. The samples were removed from the apparatus for redusting purposes at the end of each hours of testing, or sooner for inspection purposes if failure occurred.

It will be observed by referring to the dust-spray test data presented, that a large increase in operating life of the apparatus under electrical discharge conditions is obtained as the concentration of hydrated alumina is increased beyond a certain level. A more significant consideration is, however, the concentration level at which the type of failure encountered changes from a surface type failure to an erosion type failure.

The surface failure is one which occurs by reason of formation of a random carbon path along the surface of the material indicating that the life of the material is being limited by carbonization, while an erosion failure is one which occurs because the insulation component 21 is reduced in thickness by an eroding effect not involving carbonization. That is, it is determined by the inherent qualities of the insulation component itself rather than by some external cause such as carbonization.

Hence, at the point where erosion failure begins to occur instead of surface failure, the concentration of hydrated alumina has reached a level where carbonization is no longer the limiting factor with respect to ability of the high-voltage apparatus to withstand creepage electrical discharges. The samples used to obtain the data presented were inspected upon failure to determine the type of failure involved in each case; that is, whether breakdown occurred across the surface through carbonization or whether it occurred through the thickness thereof by erosion between the ungrounded electrode 24 and the metal plate 22.

Test pieces as described above were made of commercially available polyester resin known as Laminac No. 4116 manufactured by American Cyanamid Company to which was added 1% by weight of tertiary Butyl perbenzoate, as a catalyst, based on the weight of the resin.

When this material with added hydrated alumina was tested as above, the results were as follows:

Example I was repeated except that the polyester used was H. H. Robertson Companys Stypol 107E catalyzed with 0.5 part of benzoyl peroxide per each 100 parts by weight of resin. When hydrated alumina was mixed with the resin in varying amounts and the material tested as in Example I, the results were as follows:

TABLE II Material Percent Hours to Failure Type of A12Oa-3H2O Failure 0 1 Surface. 50 SuEriace and H. H Robertson T951011 20 180 c. EIOSlOll. stypol 107E 30 Greater than 200---. Do. 40 .do Do. 50 .do D0.

From Examples I and II it will be seen that polyesters mixed with hydrated alumina exhibit desirable creepage resistance. Under dust-spray condition creep tracking is prevented when from 20 to 70% by Weight of the composite insulation is hydrated alumina. Under salt fog conditions a minimum of about 30% hydrated alumina is preferred to eliminate creep tracking. The upper limit of hydrated alumina is imposed only by the physical re quirements of the insulation.

The term polyster resin refers to a material comprising the reaction product of a polyhydric saturated or unsaturated alcohol and a saturated or unsaturated polybasic acid either with or without a modifying unsaturated monomer such as styrene, etc. Specific examples of the basic material are, for instance, diethylene glycol maleatc, dipropylene glycol maleate, diethylene glycol fumarate and so forth. Such materials are readily polymerized by peroxy catalysts such as benzoyl peroxide, tertiary Butyl perbenzoate, etc.

It will also be apparent that the particular examples set forth above by way of illustration are not limiting in nature and that various forms of electrical apparatus having greatly improved resistance to breakdowns caused by creepage electrical discharges may include insulation components having the organic insulating materials set forth above and a hydrated alumina in concentration defined by the foregoing disclosure, and that such arrangements deriving the advantages of our invention, will fall within the true scope and spirit of our invention as defined in the appended claims.

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

1. Electrical apparatus comprising at least two spaced electrically conducting members between which electrical potentials may be developed, an organic insulating material disposed and completely filling the space between said members and having an outer surface intermediate said members exposed to ambient contaminating atmospheric conditions, said surface material comprising polyester resin which tends to form low resistance carbonaceous deposits under the influence of creepage type electrical discharges occurring under contaminating conditions and means for preventing the formation of said carbonaceous deposits due to creepage, said means comprising hydrated alumina interspersed in said surface material in the amount of from 20 to 70 percent based on the weight of said surface material and hydrated alumina.

2. Molded electrical apparatus comprising at least two electrically conducting members between which electric potentials may be developed and an insulating composition disposed between said members and having an outer surface intermediate said members exposed to ambient contaminating atmospheric conditions and fixing said members in spaced relationship such that electrical discharges of the creepage type may occur therebetween and across said surface of said insulating composition, said composition including polyester resin which leaves a low resistance carbonaceous residue under influence of creepage type electrical discharges and further including hydrated alumina interspersed in said composition in the amount of from 20 to 70 percent based on the weight of said composition and hydrated alumina, said hydrated alumina preventing accumulation of carbonaceous material upon exposure of said surface of said insulating composition and hydrated alumina to creepage type electrical discharges occurring across said surface under contaminating conditions.

3. An electrical insulator shaped to support and hold in insulating relationship a plurality of conducting members which are adapted to have electrical potentials developed thereacross and between which electrical discharges of the creepage type may occur, said insulator comprising an organic insulating material disposed between said members and having an outer surface intermediate said members exposed to ambient contaminating atmospheric conditions, said surface material comprising polyester resin which tends to form low resistance carbonaceous deposits under influence of creepage type elec trical discharges and hydrated alumina dispersed in said surface material and comprising from about 20 to 70 percent by weight of the combined surface material and hydrated alumina, said hydrated alumina serving to prevent the formation of said carbonaceous deposits under said contaminating conditions.

4. Electrical apparatus comprising at least two spaced electrically conducting members between which electrical potentials may be developed, an organic insulating material disposed and completely filling the space between said members and having an outer surface intermediate said members exposed to ambient contaminating atmospheric conditions, said surface material comprising polyester resin which tends to form low resistance carbonaceous deposits under the influence of creepage type electrical discharges occurring under contaminating conditions and means for preventing the formation of said carbonaceous deposits due to creepage, said means comprising hydrated alumina interspersed in said surface material in the amount of 60 percent based on the weight of said surface material and hydrated alumina.

5. Electrical apparatus comprising at least two spaced electrically conducting members between which electrical potentials may be developed, an organic insulating material disposed and completely filling the space between said members and having an outer surface intermediate said members exposed to ambient contaminating atmospheric conditions, said surface material comprising polyester resin which tends to form low resistance carbonaceous deposits under the influence of creepage type electrical discharges occurring under contaminating conditions and means for preventing the formation of said carbonaceous deposits due to creepage, said means comprising hydrated alumina interspersed in said surface material in the amount of from 30 to 70 percent based on the Weight of said surface material and hydrated alumina.

6. In a molded electrical transformer adapted to be energized by a source of electrical power and having spaced high voltage terminals and low voltage terminals, an insulating material molded about said transformer and 7 fixedly positioning said terminals, said material being disposed and completely filling the space between said terminals and having an outer surface intermediate said'high and low voltage terminals exposed to ambient contaminating atmospheric conditions, the improvement which comprises at least the outer surface of said material in cluding polyester resin which leaves a conducting carbonaceous deposit under the influence of creepage electrical discharges and hydrated alumina dispersed in at least said outer surface of said material to comprise from 20 to 70 percent by Weight of at least said outer surface of the combined insulating material and said hydrated alumina, said hydrated alumina obviating said carbonaceous deposits.

References Cited in the file of this patent UNITED STATES PATENTS 2,618,689 Cook Nov. 18, 1952 2,645,626 Nordlander et a1. July 14, 1953 2,679,493 Anderson May 25, 1954 2,743,308 Bardsley Apr. 24, 1956 2,768,264 Jones et a1 Oct. 23, 1956

Claims (1)

1. ELECTRICAL APPARATUS COMPRISING AT LEAST TWO SPACED ELECTRICALLY CONDUCTING MEMBERS BETWEEN WHICH ELECTRICAL POTENTIALS MAY BE DEVELOPED, AN ORGANIC INSULATING MATERIAL DISPOSED AND COMPLETELY FILLING THE SPACE BETWEEN SAID MEMBERS AND HAVING AN OUTER SURFACE INTERMEDIATE SAID MEMBERS EXPOSED TO AMBIENT CONTAMINATING ATMOSPHERIC CONDITIONS, SAID SURFACE MATERIAL COMPRISING POLYESTER RESIN WHICH TENDS TO FORM LOW RESISTANCE CARBONACEOUS DEPOSITS UNDER THE INFLUENCE OF CREEPAGE TYPE ELECTRICAL DISCHARGES OCCURRING UNDER CONTAMINATING CONDITIONS AND MEANS FOR PREVENTING THE FORMATION OF SAID CARBONACEOUS

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