Paper insulation for transformers

April 12, 1966

J· G. FORD paper insulation for transformers

Original Filed Sept. 25, 1951

3,246,271

Sheets-Sheet 1

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Fig. I

WITNESSES

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INVENTOR

James G. Ford

ATTORNEY

April 12, 1966

Original Filed Sept.

j. g. ford 3,246271 paper insulation for transformers

25, 1961

Sheets-Sheet s

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3,246,271

Patented Apr. 12, 1966

United States Patent Office trical conductors or coils the adhesive on the cellulosic insulation bonds the conductors and applied cellulosic insulation into a unitary member, and when thereafter immersed in oil or other liquid dielectric, and subjected to 5 vacuum drying treatment the air, moisture, and other gases in the cellulose interstices are removed and are replaced rapidly and substantially completely by the liquid dielectric so that there are no gas bubbles or pockets which will result in undesirable radio interference.

It has been discovered that the resin, which bonds the wound electrical conductors into an integral unit, must be applied to the insulation in a particular critical configuration. Briefly, the resin must be applied to one or both surfaces of the sheet insulation as a thin coating 15 in the form of essentially discrete areas of limited size.

The shape of the resin areas is, with some limitations, a matter of choice and/or convenience. For example, the resin areas may be in the shape of diamonds as indicated by the numeral 12 of FIG. 1 or they may be circular 20 (numeral 28, FIG. 3), rectangular, or other selected shapes such as the lightly connected squares 30 of FIG. 2. The dimensions of the selected resinous areas, however, are extremely. critical. This is particularly true where the insulation is to be employed in a liquid dielectric en25 vironment such as in a transformer employing such dielectric.

It has been found that, regardless of the shape of the applied resinous film, or area, the distance from any point in the area to the nearest edge thereof must not exceed o0 about % inch nor should the areas be so small that their least dimension is less than about inch. Where the above smallest area dimension is less than %₀ inch, the final bond between the treated insulation and conductors or between layers of treated insulation is insufficiently 35 strong and, under stress, the bonded unit may readily fail mechanically.

. The upper limitation of resin area is related to absorption, by the treated insulation, of liquid dielectric. This is particularly, true when the resinous areas overlie one another in adjacent layers or where resin is applied to both surfaces of the sheet insulation. Obviously, where the resin areas overlie one another, the liquid dielectric can only penetrate the insulation laterally beneath the resin. In such instances air and other gases present in 45 the. interstices of the insulation become trapped and remain therein, in the form of pockets or bubbles. It has been determined that the entrapped gaseous bubbles are the source of radio frequency emission from the transformers under relatively high voltage stress due to ioniza50 tion of the gases. Where, therefore, the liquid dielectric must penetrate laterally into the resin treated insulation for a distance exceeding about -¾ inch, the reduced radio interference benefits of the present invention are materially reduced.

.- In addition to the size of the individual resin areas, it is requisite that the total area of insulation covered thereby shall lie within the range of about 25% to about 75% of the total surface area. This range of surface area coverage insures that sufficient bond is obtained and that 60 concurrently, radio frequency emission is virtually eliminated.

It has been stated hereinbefore that certain configurations of resin areas are undesirable. Obviously, a continuous resin coating is intolerable. Continuous rela65 tively wide strips of resin are also undesirable inasmuch as they impart an excessive degree of stiffness to the treated sheet, insulation. It is therefore clear that the above limitations must be adhered to in order to derive the benefits of this invention.

In the application of discrete resinous areas to sheet cellulosic insulation according to the invention, any suitable resin may be used such as shellac, epoxy, phenolic,

3,246,271

PAPER INSULATION FOR TRANSFORMERS Janies G. Ford, Sharon, Pa., assignor to Westinghouse

Electric Corporation, Pittsburgh, Pa., a corporation of Pennsylvania

Continuation of application Ser. No. 140,567, Sept. 25, 1961. This application Apr. 16,1965, Ser. No. 450,252

Claims. (CI. 336—94)

This is a continuation of application Serial No. 140,567, filed September 25, 1961, now abandoned, in the name of James G. Ford and assigned to the same assignee as this application.

The present invention relates to cellulosic electrical insulation and to electrical apparatus embodying such cellulosic electrical insulation. More particularly the invention relates to resin treated sheet electrical insulation characterized by greatly improved electrical properties.

In the electrical industry, it has been commonplace for many years to insulate conductors and the like with cellulosic insulation. Materials such as paper, cotton cloth, cotton tape, and similar materials have been wound around the conductors, positioned between them, and in other manners employed to effect insulation thereof. As the electrical art has progressed it has been increasingly necessary to provide improved electrical insulation. It has been desirable that the insulation be thinner, while providing improved insulating properties, in order to permit reduction in the size of equipment. To some extent this aim has been achieved, frequently with the employment of resinous materials in conjunction with the cellulosic insulation. For example, polyvinyl alcohol has been used as a coating resin for the sheet insulation for transformer coils. Other resins, more recently the phenolics, epoxies, phenolic-epoxies and the like resinous materials have been employed.

. However, there has been an increasing problem, particularly in the case of liquid dielectric transformers, of the emission of radio frequency radiation. This, is particularly true in the case of high voltage equipment since the undesirable radiation increases with voltage. Consequently, there have been many complaints about radio reception in the vicinity of transformers. This objectionable feature has been a matter of great concern in the industry.

It is therefore a primary object of this invention to provide electrical equipment embodying cellulosic insulation with greatly lessened radio interference, characteristics.

It is a further object of the invention to provide cellulosic electrical insulation . which is characterized by greatly improved electrical properties, chief among which is the virtual elimination from liquid dielectric filled electrical equipment insulated therewith of radio frequency emission originating in the coil structure thereof.

Other objects will, in part, appear obvious and will, in part, become apparent from the following detailed description of the invention.

The description will be made in conjunction with the accompanying drawings, in which:

FIGURE 1 is a side elevation, partly in section, of a transformer coil;

FIG. 2 is a top elevation illustrating a particular resin pattern applied to cellulosic insulation;

FIG. 3 is a similar elevation of another resin pattern; and,

FIG. 4 is a perspective view, partially in section, of the transformer coil illustrated as a side elevation in FIG. 1.

In accordance with the present invention, cellulosic insulation, and particularly paper and pressboard sheets, are coated with a resin adhesive composition applied in a predetermined pattern so that when applied to elec3,346,271

Ο polyvinyl alcohol, phenolic-epoxy, or any resinous material which is either thermoplastic with reasonably high melting point or capable of being applied to the insulation in the “B” stage and remain fusible for further processing. The resin should also be one which does not penetrate into the paper to any substantial degree. It is highly desirable, of course, that the resin is dried to tack-free condition subsequent to its application to the insulation so as to prevent sticking or blocking during storage prior to its ultimate use. The resin may be applied by conventional means although it has been found that roll application is very convenient and economical. A further limitation resides in the thickness of the resin films. The minimum thickness oridinarily acceptable while still providing adequate adhesion is about 0.4 mil while the maximum is about 2.5 mils. Within these limits, a preferred range of film thickness is about 1 mil±0.25 mil.

An additional feature of the invention resides in the application of the discrete resin areas to thermally stabilized sheet cellulosic insulation. Insulation having present within its interstices from about 0.02% by weight thereof to about 5% or more by weight of a stabilizing agent is particularly adapted for use. A group of stabilizing agents suitable in the invention includes melamine, triethylmelamine, triphenylmelamine, diallylmelamine, tris-tertiary butylmelamine, N-tertiary butylmelamine, dicyandiamide, polyacrylamide, succinonitrile and related nitriles. The stabilizing agents may be used separately or in synergistic conjunction with one another.

Ordinarily, the stabilizing agents are incorporated in the sheet during manufacture of the cellulosic insulation. For example, in the manufacture of paper there is usually a sizing tank between sections of the drying rolls. Conveniently, the stabilizing agent, in the form of aqueous solution, may be applied to the paper here as an impregnant. Subsequent to drying, the agent is to be found uniformly distributed throughout the interstices of the paper. Paper, or other sheet cellulosic insulation so treated, is characterized by greatly enhanced thermal stability in air or in liquid dielectrics. Further, improved dielectric strength is obtained, particularly at elevated temperatures. It is therefore a primary feature of the invention to employ resin coatings, within the hereinbefore prescribed limits, in combination with thermally stabilized sheet insulation.

In order to describe the invention more fully the following illustrative example is given. It is to be remembered, however, that the example is given for illustrative purpose only.

Example

Using kraft paper about 14 mils thick and having a density of about 1, an epoxy resin solution is applied to one or both surfaces thereof in a diamond shape pattern. This is shown in FIG. 1 wherein paper sheet 10 has diamond pattern adhesive areas 12 applied thereto. The resin application is carried out by means of application rolls having the diamond pattern in their surfaces. The kraft paper contains about 3% by weight of equal amounts each of melamine, dicyandiamide, and polyacrylamide. The diamond shape resin areas are approximately % inch on a side and are spaced about %g inch apart. The resin, which covers about 37% of the surface of the paper, is dried to tack-free condition and the paper is rolled for storage.

There after,, the low and high voltage coils of a transformer are wound and insulated with the treated paper as shown in FIGS. 1 and 4. Referring now to FIGS. 1 and 4, it will be seen that the transformer coil comprises low-voltage coils 14 and 16, as well as high voltage coils 18, 20 and 22, insulated by layer-to-layer application of the treated paper. In addition the low voltage coil 14 is insulated from the high voltage coils by the treated winding-to-winding insulation 24. In the finished transformer, a liquid dielectric will fill the channels 26 and will, as well, completely permeate the paper insulation.

Subsequent to being wound and assembled the entire assembly is vacuum treated to remove air and moisture from the paper and is baked to eliminate fully any moisture and to cure the resin to its final insoluble thermoset state.

Referring to FIGURE 4, which is a perspective view, partially in section, of the transformer coil illustrated in FIG. 1, like reference numerals in FIG. 4 refer to like elements of FIG. 1. In the coil illustrated in FIG. 4 a sheet of stabilized electrical insulation paper 10 having a thermosetting resinous coating applied thereto in the form of diamond shaped spots 12 is first wound upon itself to provide a coil form 13. The diamond shape spots of resin are non-tacky at the winding temperature. After the first turns of paper have been wound to provide the coil form 13, then a low voltage electrical winding 16 is wound on the coil form 13. This low voltage winding 16 comprises two layers of electrical conductors separated by a layer of electrical sheet insulation 10. After the low voltage winding 16 has been wound on the coil form 13, other layers of electrical sheet insulation 10 having resinous spots 12 thereon are then wound on the low voltage winding 16. Spacer means 17 are then provided to provide oil ducts or channels 26. At least one sheet of electrical insulation material 10 having resinous spots 12 thereon is then wound over the spacer means 17 to provide the oil ducts 26, then high-voltage windings 22 are wound over this insulation material 10. Adjacent the high-voltage winding 22 other spacer means in the form of a corrugated sheet 30 are provided to provide another oil duct 26, then another high-voltage winding 20 is wound over this corrugated spacer sheet 30. Another corrugated spacer sheet 31 is then provided around the high-voltage winding 20 to provide another oil duct 26, then a third high-voltage winding 18 is wound over the corrugated sheet 31. Additional spacer means 33 is provided against the high-voltage winding 18 to provide another oil duct 26. A plurality of layers of electrical insulation material 24 having resinous spots 12 thereon is then wound over the last oil duct 26. Finally a second low-voltage winding 14 is wound over the layers of sheet electrical insulation 24 and a final layer of electrical insulation material 36 having resinous spots 12 thereon is wound over the final layer of the low-voltage winding 14. The resinous spots 12 on all of the turns of paper used in construction of the coil shown in FIG. 4 are illustrated as being diamond shaped; however, it is understood that they may be square, circular or another suitable shape. The resinous spots are non-tacky at winding temperatures; however, after the complete coil is wound the coil is placed in a furnace and heated to an elevated temperature. This elevated temperature causes the resinous spots 12 to soften and rigidly stick or bond all of the turns of the sheet electrical insulation material 10 and all of the turns of the high-voltage and low-voltage windings together in a rigid structure, then the resin thermosets.

When the transformer is tested according to NEMA Standard TR 1-0.05 for Radio Influence Voltage and Levels, it is found that the transformer has average transmission values of about 5-20 microvolts. The standard adapted by the industry specifies that the maximum transmission shall be 250 microvolts. The test itself is to be found in Standards Publication #TR-1-196O, National Electrical Manufacturers Association. In contrast to the good radio interference properties of the product of the present invention, when a transformer built in the same manner, but wherein the resin is applied to the sheet insulation as parallel strips about 2.5 inches in width and spaced 2 inches apart, as known in the art, is tested by this method the average radio transmission values will be about 2000-2500 microvolts. Inasmuch as the transmission of radio frequency signals by transformers is due, largely, to the ionization of the entrapped air in the insulation, it Will be appreciated that the present invention pro3,346,271 vides an outstanding improvement over the prior art means.

While the invention is disclosed as being applied to transformers, the treated paper can be employed in cables, regulators, and other electrical apparatus. The treated 5 paper may be wrapped on an electrical conductor, bare, or provided with enamel or other insulation, or wrapped about a coil or winding composed of one or more layers or turns of an electrical conductor.

The adhesive resin on the cellulosic insulation may be jq rendered active by applying a solvent for the resin, water being suitable for polyvinyl alcohol, or by heating the adhesive to a temperature where it becomes tacky and will bond.

It will be appreciated from the foregoing description 15 that the invention represents a distinct advance in the art. The novel electrical insulation is simply manufactured, economical, and provides those engaged in the art with insulation characterized by greatly improved physical and electrical properties. 20