US2792871A - Electrical insulating structure and method of making the same - Google Patents

Description

May 21, 1957 P. DolGAN 2,792,871 ELECTRICAL TMSULATTNC STRUCTURE AMD METHOD A 0E MAKING THE SAME Filed May 2T, 1955 'fm/627251 Paal Dogan, gy iwf M His frqg;

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ELECTRICAL INSULATING STRUCTURE AND METHOD OF MAKING THE SAME Paul Doigan, Pittsfield, Mass., assignor to General Electric Company, a corporation of New York Application May 27, 1955, Serial No. 511,601

3 Claims. (Cl. 1542.6)

The present invention relates to electrical insulating material, and more particularly to resin-impregnated insulation structures for electrical apparatus such as transformers, and particularly for dry-type transformers.

The invention is especially concerned with resin impregnated asbestos insulating sheet material which is wound in the form of cylinders for use as dielectric barriers on transformer cores, e. g., for insulating the wound transformer coils from the core and from each other. Such dielectric barriers especially when used in transformers and other electrical apparatus of high capacity must withstand high voltage and high temperature conditions without breaking down. Prior types of insulating cylinders made of inorganic base material and particularly of asbestos have not proved satisfactory for many applications due to their low dielectric strength and poor physical and electrical properties on aging tests. Other types of known insulating sheet materials which have improved electrical properties such as higher dielectric strength have proved unsatisfactory in `that they are diflicult to handle and wind into the desired form, due to their stiff and brittle quality, and more over when wound into cylinders tend to lose the good electrical characteristics which they have in other insulation forms.

It is, therefore, an object of the present invention to provide electrical insulation products which are suitable for high temperature, high voltage applications, and which overcome the disadvantages of known types of such insulation.

It is another object of the invention to provide an improved insulating barrier of the above type which has superior electrical and mechanical strength properties even under prolonged high temperature and high voltage conditions, and which can be readily wound into cylindrical form with conventional winding devices.

It is still another object of the invention to provide a method of making wound insulating barriers of the above type composed of asbestos base material impregnated with a polyester resin material.

It has been found in accordance with `the present invention that wound insulation products of superior electrical and physical characteristics may be made by impregnating asbestos base material containing organic compounds (hereinafter also referred to as organic type asbestos material) with a polyester resin composed -of a mixture of diallyl phthalate and diethylene glycol maleate, the resin components being preferably present in 4approximately equal parts.

The invention will be better understood from the following description taken in conjunction withl the accompanying drawing, in which:

Fig. l schematically shows in partly sectional view a transformer core having arranged thereon cylindrical coil insulation barriers to which the present invention vis applicable; and

Fig. 2 is a fragmentary view of la lpartially 'nrolled insulating cylinder of the type shown iii Fig. 1.

United States Patent O rice Referring now to the drawing, there is shown a transformer core member 1 having an inner insulating cylinder 2 arranged thereon, around which one set of coil windings 3 is wound. Surrounding the inner cylinder 2 and windings 3 is an outer insulating cylinder 4 similar in structure to that of cylinder 2 and having wound thereon another set of windings 5. In practice, the sets of coil windings are often placed on their respective cylinders before the cylinders are assembled on the transformer core, and then the coilwound cylinders are slipped over the end of core leg 1 before the latter is joined with the upper yoke 6. The illustrated insulating arrangement is adapted for use, for example, in a drytype ventilated distribution transformer, although it is to be understood that the invention is not limited to this type of transformer or to the arrangement illustrated.

The cylinders, one of which is shown in detail in Fig. 2. are normaly formed in known manner on conventional cylinder winding machines by Winding the resin-impregnated asbestos sheet, which may have a thickness of about 0.010, into a multi-ply cylinder of tubular form of the desired diameter and of a wall thickness of about one-quarter inch. A typical process of fabricating such cylinders is set forth in detail hereinafter.

The mixture of diallyl phthalate and diethylene glycol maleate which is used as an impregnant in the present invention and which has been found to provide insulating cylinders of outstanding properties, is of a class of resins which are 100% polymerizable. Upon curing, the present resin composition solidifies without the evolution of water or water vapor as do the so-called condensation curing plastics. The present copolymer does not evolve deleterious volatile solvents or thinners during the heat curing cycle, and the mass impregnating the organic type asbestos sheet cures to the solid state without the formation yof bubbles, cracks, or other voids which, in an insulating barrier are extremely undesirable in that they lower the dielectric strength of the insulation and raise the noise level. This property is of particular advantage in the winding of cylinder types of insulating barriers, since where the previously employed types of resin dielectric impregnants upon curing would leave voids or separations between adjacent turns of the wound impregnated sheets, the present 100% polymerizable resin forms a continuous compact void-free binder between, as well as within, the turn laminations. Moreover, the present resin composition is characterized by high dielec tric strength and is extremely resistant to high temperatures while maintaining its superior electrical properties.

The asbestos base material used in accordance with the invention is preferably one which contains a substantial proportion of organic material, which makes the asbestos more adapted than the solely inorganic types of asbestos for the handling and winding processes involved in making the wound insulating barriers described. A particularly suitable type of such base material for the present invention is a sheet material formed of chrysotile asbestos fibers and having a composition of about purified asbestos and 15% organic content. Not only does such asbestos material have a high dielectric strength, but also has the requisite pliability for the vprocess of winding it` into multi-ply cylinders. lt is furthermore especially compatible with the diallyl phthalatefdiethylene -glycol maleate resin impregnant disclosed herein and in 0.5%. The organic content of this asbestos material is about 1/3 chlorinated starch and 1% bleached sulfite pulp. This highly refined and purified asbestos has considerably higher dielectric strength than the usual commercial asbestos which has a magnetic particle content of 'about 6% and a greater concentration of other impurities which result in less satisfactory dielectric properties in the final composite insulating cylinder. The advantage of a material with a lower magnetic particle content, aside from the more uniform dielectric strength afforded throughout the material, is the considerably reduced risk of burnouts in the cylindrical type of inslulation where it is likely that conducting paths will be formed through the wall of the cylinder where magnetic particles become superimposed in the winding operation.

The following table shows a comparison of the properties of the Quinorgo type of asbestos used in the present invention and commercial asbestos of the type heretofore commonly used:

Supplementary electrical tests made using each of the above different types of asbestos sheet impregnated with the diallyl phthalate-diethylene glycol maleate mixture under factory conditions showed that the average dielectric strength of the impregnated commercial type asbestos was about 120 Volts per mil, while that of the impregnated Quinorgo type was about 168 volts per mil.

Tests conducted on cylinders made of resin impregnated purified asbestos material in accordance with the invention show that cylinders having about 250 mils wall thickness could withstand up to 55 kilovolts and that additional cure improves the dielectric strength of the resin treated asbestos material. In these tests the puriiied organic type asbestos sheet was impregnated with the following composition, in percent by weight:

Diallyl phthalate-diethylene glycol maleate The cylinders thus made were cured on the following cycle:

50 C.-2 hours 125 C.-16 hours 75 C.-2 hours 150 C.-- 2 hours 100 C.--2 hours 200 C.- 2 hours The cured cylinders were then subjected to aging tests at 200 C., in which the following results were obtained:

As shown by the above data, the dielectric properties of the resin-impregnated asbestos cylinders not only were excellent initially but increased to considerably higher values after prolonged aging at elevated temperature.

It is to be noted that while solvents such as toluol and alcohol may be used, as in the above composition, for controlling the viscosity ofthe resin to provide the most effective and uniform impregnation of the asbestos sheet, practically all of the thus added solvent is driven off before the final curing of the impregnated sheet, so that the undesirable effects of volatilization during curing, as discussed above, are avoided.

In making fiat laminated, resin-impregnated insulating structures for various electrical uses, it is relatively easy to provide effective dielectric properties in the finished product even where ordinary resins and insulating base materials are employed, due to the fact that the high molding and laminating pressures used in making such at products produce a compact, dense structure which resists arcing 'and voltage stresses, as well as a mechanically strong laminate. In the process of forming wound insulating cylinders, however, it is not feasible in normal winding operations to use such high pressures and as a result the previous wound types of resin-impregnated insulating structures have not had satisfactory dielectric properties, particularly 'at elevated temperatures. By virtue of the present invention, however, the cylinders `made of the materials disclosed may be formed on the conventional winding and tube rolling devices with the application of little or no pressure, and yet are characterized by superior properties in terms of mechanical and thermal strength and excellent stability under high voltage conditions.

The following is a typical procedure for making cylindrical insulating barriers in accordance with the invention.

Puried asbestos paper sheet of the Quinorgo type, .010 in thickness, is impregnated with a polyester resin solution on a vertical treating machine. The impregnating solution is prepared by mixing the following materials:

Resin mixture diallyl phthalate, 50% diethyl- The consistency of the above solution may be altered by suitable variation of the type or amount of the solvents used to allow for the use of different treating machines. Further, the particular catalyst used is not critical, and any of the known polymerization catalysts for polyester resins of the type used can be employed.

The asbestos paper is passed into the treating solution in such manner as to allow for dotation prior to dipping, and the treated paper is then conducted through a vertical infra-red drying tower. The speed of treatment iu the solution and the drying conditions are governed by the resin content and condition of the treated material. The treated sheet must be flexible and tack-free and should preferably have a resin content of labout 30-40%. The thus treated paper is then ready for fabrication into a cylinder. For this purpose, the treated paper is wound on a rotating steel mandrel of the desired diameter, until a cylinder of the desired wall thickness is obtained. For a Wall thickness of M1, about 25 layers are required. While the treated paper is being wound, a pool stock of the resin mixture is placed on the paper in the nip between the mandrel and the bed rolls, on which the mandrel rests while rotating, and is wound into the cylinder turns.

The mandrel containing the wound cylinder is then removed 'from the tube rolling machine, and placed in an oven where it is given a controlled program cure, such as described hereinbefore. After curing, the cylinder is stripped from the mandrel and cut and sanded to the proper dimensions for use.

While the `present invention has been described with reference `to particular embodiments thereof, it will be understood that numerous modifications may be made by those skilled in the art without actually departing from the scope of the invention. Therefore, the appended claims are intended to cover all such equivalent variations as come Within the true spirit and scope of the invention.

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

1. Electrical insulating structure having high dielectric and mechanical strength and high temperature stability comprising a wound multi-ply cylindrical product adapted for use as an insulating barrier for electrical apparatus, said cylindrical product being composed of insulating sheet material consisting in per cent by weight of about 80-85% asbestos bers, about 15-20% organic material, and a maximum iron content of about 0.5%, said insulating sheet material being impregnated with a copolymer of about equal parts of diallyl phthalate and diethylene glycol maleate.

2. Electrical insulating structure having high dielectric and mechanical strength and high temperature stability comprising a wound multi-ply cylindrical product adapted for use as an insulating barrier for electrical apparatus, said cylindrical product being composed of insulating sheet material consisting in per cent by weight of about 80-85% asbestos bers and about 15-20% organic material, said insulating sheet material being impregnated with a copolymer of about equal parts of diallyl phthalate and diethylene glycol maleate.

3. The method of making an electric insulating structure which comprises impregnating lasbestos sheet material composed in per cent weight of about 80-85% asbestos bers and about 15-20% organic material with a resin mixture composed of about equal parts of diallyl phthalate and diethylene glycol maleate, winding the thus treated asbestos sheet material into a multi-ply tubular structure, and curing the resin impregnant to a hardened state.

References Cited in the tile of this patent UNITED STATES PATENTS 2,443,739 Kropa June 22, 1948 2,443,741 Kropa June 22, 1948 2,530,983 Minter Nov. 21, 1950 2,542,819 Kropa Feb. 20, 1951 2,581,862 Johnson et al. Ian. 8, 1952

Claims (1)

1. ELECTRICAL INSULATING STRUCTURE HAVING HIGH DIELECTRIC AND MECHANICAL STRENGTH AND HIGH TEMPERATURE STABILITY COMPRISING A WOUND MULTI-PLY CYLINDRICAL PRODUCT ADAPTED FOR USE AS AN INSULTING BARRIER FOR ELECTRICAL APPARATUS SAID CYLINDRICAL PRODUCT BEING COMPOSED OF INSULATING SHEET MATERIAL CONSISTING IN PER CENT BY WEIGHT OF ABOUT 80-85% ASBESTOS FIBERS, ABOUT 15-20% ORGANIC MATERIAL AND A MAXIMUM IRON CONTENT OF ABOUT 0.5%, SAID INSULATING SHEET MATERIAL BEING IMPREGNATTED WITH A COPOLYMER OF ABOUT EQUAL PARTS OF DIALLYL PHTHALATE AND DIETHYLENE GLYCOL MALEATE.

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