Electrical Apparatus

Abstract

Electrical apparatus having an electrical conductor insulated with solid insulating means, which is impregnated with a liquid dielectric. The solid insulating means includes paper having a fibrous web formed of wholly aromatic polyamide fibers.

Description

BACKGROUND OF THE INVENTION

1. Field of The Invention

The invention relates in general to electrical apparatus, such as transformers, and more specifically to liquid cooled electrical apparatus.

2. Description of The Prior Art

Cellulosic paper is used in power transformers to insulate the electrical conductors from one another and from ground. The cellulosic insulation surrounding the electrical conductors is impregnated with liquid insulating means, such as mineral oil, and the magnetic core-winding assembly is immersed in the insulating liquid, with the liquid also serving as a coolant. While cellulosic paper impregnated with oil provides a good electrical insulating system, the use of cellulosic paper has some disadvantages. For example, cellulosic paper may limit the average operating temperature of the transformer, because of its limited thermal stability. Hot spots in the windings, which may be substantially higher than the average operating temperature of the transformer, make the thermal limitations imposed by cellulosic paper an even more severe design limitation.

Cellulosic insulation must be dried to remove absorbed moisture, before it is impregnated with the liquid insulating means. Further, when cellulosic insulation deteriorates during usage, one of its byproducts is moisture, which contaminates the insulation system of the transformer.

In certain locations within a transformer, such as the turn insulation on the electrical windings, and the insulation on tap leads, electrical insulation having a higher electrical impulse strength then cellulosic insulation would be desirable. While higher electrical strength cellulosic papers are available, they are generally characterized by being considerably more brittle than conventional papers. Thus, in many instances, mechanical considerations dictate the choice of conventional cellulosic paper, with the resulting lower allowable voltage stress, which places restrictions on the transformer design.

Also, high electrical strength cellulosic papers have a higher specific gravity and dielectric constant than conventional cellulosic papers. While solid insulation having a higher dielectric constant may be desirable in specific locations within a power transformer, it is, in general, undesirable as it transfers voltage stress from the paper to the liquid dielectric. In most instances it is preferable to closely match the dielectric constants of the solid insulation and the liquid dielectric insulation which surrounds the solid insulation.

Clothlike mats and/or papers formed of noncellulosic fibers have, in general, been found to be unsuitable as solid insulation in liquid filled transformers, as their electrical impulse strengths when impregnated with liquid dielectric is no greater than that of the liquid alone, resulting in an insulation system which has a lower electrical strength than cellulosic insulation. Further, the synthetic solid insulations are often limited in chemical resistance and physical properties, when subjected to the elevated transformer operating temperatures.

Impervious dielectric films, such as a polyester film, are also unsuitable in most applications for solid insulation in liquid filled power transformers, as their electrical impulse strength in volts per mil drops rapidly when used in thicknesses above about five mils.

SUMMARY OF THE INVENTION

Briefly, the present invention is new and improved electrical apparatus, such as transformers, wherein at least certain of its electrical conductors are immersed in a liquid dielectric, such as mineral oil, with these conductors being insulated with solid insulation impregnated with the liquid dielectric. The solid insulation is paper, i.e., a felted sheet of fibers, formed of wholly aromatic polyamide fibers. It has been found that unlike most insulation formed of noncellulosic fibers, that the combination of paper formed of wholly aromatic polyamide fibers impregnated with conventional transformer liquid dielectric and cooling means, such as mineral oil, provides insulating qualities which exceeds by a substantial margin the insulating qualities of either the noncellulosic paper or the liquid dielectric taken alone. Further, the specific combination of noncellulosic paper containing the wholly aromatic polyamide fibers and insulating liquid dielectric has an impulse fail strength which is much higher than that of cellulosic paper impregnated with the same insulating liquid dielectric, it has greater thermal stability than cellulosic paper and a greater mechanical strength, it does not absorb moisture, and it does not form moisture as a byproduct when it deteriorates

BRIEF DESCRIPTION OF THE DRAWINGS

Further advantages and uses of the invention will become more apparent when considered in view of the following detailed description and drawings, in which:

FIG. 1 is a graph which compares impulse fail strengths of insulating systems constructed according to the teachings of the invention, compared with insulating systems constructed according to the teachings of the prior art;

FIG. 2 is a perspective view, partially cut away, of a liquid filled transformer which may utilize the teachings of the invention; and

FIG. 3 is a diagrammatic view, in section, of a portion of the transformer shown in FIG. 2, which illustrates a specific application of the teachings of the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

Certain types of liquid cooled electrical apparatus, such as transformers, utilize paper formed of cellulosic fibers, to electrically insulate its winding turns and leads. Cellulosic insulation is used not only because it is economical, but because when it is impregnated with a liquid insulating dielectric, such as transformer oil, it has excellent electrical insulating characteristics. These factors, plus the lack of acceptable substitutes, makes its use as an insulating system in transformers practically universal, even though certain disadvantages of cellulosic insulation place restrictions on transformer design. For example, the electrical insulating qualities and mechanical strength of cellulosic insulation deteriorate rapidly, even when impregnated with transformer oil, at temperatures above about 100.degree. C. when cellulosic insulation deteriorates, it liberates water as a byproduct, contaminating the remaining portion of the insulation system. Thus, the use of cellulosic insulation limits the average operating temperature of a transformer because of the limited thermal stability of the cellulosic insulation. Hot spots which occur at certain points in the transformer, which exceed the average operating temperature of the transformer, make the thermal limitations of cellulosic papers an even more severe limitation on the design of the transformer, reducing the allowable average operating temperature of the transformer below that which cellulosic insulation would ordinarily withstand. Cellulosic papers of greater electrical strength are available, but these papers are generally characterized by being more brittle than conventional cellulosic papers, and hence mechanical considerations often dictate the choice of the specific cellulosic paper used, and the allowable voltage stress on the selected paper dictates transformer design.

While cellulosic papers become brittle with age when subjected to elevated operating temperatures, their mechanical strengths are limited even at the time of manufacturing the electrical apparatus, making its use extremely difficult in those applications where tearing of the paper may be experienced.

Another character of cellulosic papers which is a disadvantage in certain applications is its relatively high dielectric constant. Cellulosic paper impregnated with transformer oil has a dielectric constant in the range of 3.2 to 4.2, while the oil has a dielectric constant of 2.2 Since electrical stress distributes itself across a media in inverse proportion to the dielectric constants of the strata of elements which make up the media, the oil impregnated cellulosic insulation transfers electrical stress to oil filled ducts and other oil filled voids adjacent the solid insulation. To prevent this nonuniform stress distribution, it would be preferable to more closely match the dielectric constant of the oil impregnated solid insulation with that of the transformer oil.

The impervious synthetic insulating films, and the papers and clothlike mats formed of synthetic fibers are, in general, unsuitable for use as solid insulation in liquid filled transformers, even if cost is disregarded. The impulse fail strength of the impervious films in volts per mil drops rapidly with thickness, making their use in the thickness range required in power transformers unacceptable. The papers and mats formed of noncellulosic fibers, in general, have a relatively low impulse fail strength in volts per mil, and the combination of solid noncellulosic insulation and liquid insulation fails to exhibit the synergistic effect that the combination of cellulosic insulation and liquid dielectric does. Further the synthetic fibers, in general, are limited in their chemical resistance and physical properties at elevated temperatures, in a manner similar to that of natural organic fibers. Thus, the use of synthetic films, cloths and paper has been confined to dry-type transformers, with the emphasis in liquid cooled transformers being on a thermally stabilizing cellulosic insulation, by using certain additives, such as disclosed in U.S. Pat. No. 2,722,561, which is assigned to the same assignee as the present application.

The present invention is new and improved liquid cooled electrical apparatus which utilizes the discovery that paper formed of wholly aromatic polyamide fibers, unlike other noncellulosic papers, exhibits a synergistic effect when impregnated with liquid dielectric, such as transformer oil, increasing its 60 cycle puncture strength and impulse strength by a factor of at least two. Further, the impulse fail strength for this specific synthetic paper when impregnated with transformer oil, is 30 to 50 percent higher than conventional transformer solid insulation of the same thickness, i.e., rope-kraft paper, when it is impregnated with transformer oil, and its 60 cycle puncture strength is 25 to 35 percent higher than impregnated cellulosic paper. Also, paper formed of wholly aromatic polyamide fibers has excellent mechanical strength, it is thermally stable, it has a dielectric constant when impregnated with transformer oil which more closely matches that of the oil, than does cellulosic insulation impregnated with transformer oil, it does not absorb moisture and it does not liberate water as a byproduct when it deteriorates.

To illustrate the unexpected superiority of synthetic paper formed of wholly aromatic polyamide fibers when impregnated with transformer oil, compared with cellulosic paper impregnated with transformer oil, for use as electrical insulation, tests were made to determine the impulse fail strength, the 60 cycle puncture strength, and 60 cycle dissipation factors of various samples of wholly aromatic polyamide paper and samples of cellulosic paper.

The first test performed on the samples was the impulse puncture test. Prior to testing, the samples were vacuum dried and oil impregnated using conventional transformer mineral oil. The samples were not removed from the oil after impregnation, until after the tests were completed. The impulse puncture tests were conducted with 2 inch diameter flat surface electrodes with one-fourth inch radius rounded edges. The 11/2 .times. 40 microsecond negative impulse voltage waves were applied to the top electrode, and the testa were made at room temperature. Table I lists the results obtained. ##SPC1##

Wholly aromatic polyamide papers, such as sold commercially under the trademark Nomex, is made on a conventional paper making machine, and hence is a water laid fibrous web available in different densities. The first four samples listed in Table I, represent the most common densities of wholly aromatic Polyamide Paper available, while the next three samples are of lower density, and were tested to determine if the density of the paper has a relevant influence on the electrical strength of insulation. Lower density paper is less costly, and as shown in Table I, the density of the paper is not nearly as important in determining the volts per mil electrical strength as the thickness of the paper. The next five samples of wholly aromatic polyamide papers are in the thickness range normally used for cellulosic conductor insulation in power transformers, and thus are more indicative of their value as electrical insulation than the tests on the thicker papers. The last two samples listed in Table I are conventional Cellulosic Papers, which were included to obtain an indication of the relative impulse foil strengths of the wholly aromatic Polyamide Papers and the Cellulosic Papers.

FIG. 1 is a graph which plots the impulse fail strength in volts per mil against the total thickness of the insulation in mils, for various wholly aromatic polyamide papers tested in transformer oil. Curves 10, 12, 14, 16, 20 and 22 represent typical results of tests made on different thicknesses and densities of wholly aromatic polyamide papers, while curves 24 and 26 are typical curves of tests made on 2 mil polyester films, and 3 mill rope-kraft papers, respectively. These curves illustrate that all of the wholly aromatic polyamide papers tested have substantially greatially greater impulse fail strengths than either the polyester film or three mil rope-kraft paper, in the thickness range of 1.8 to 10 mils, which is the range normally used in electrical power transformers.

Tests were also performed on certain of the samples to determine the 60 cycle puncture strength of the wholly aromatic polyamide papers. Table II lists the results of this test, with the 25 volt/second rate of rise approximating a step-by-step test, as opposed to a rapid rise test. ##SPC2##

Table III lists the results of tests made on both wholly aromatic polyamide papers, and cellulosic papers, with some of the tests being performed with 500 volts per second rise, and some step-by-step tests. Also included are average fail strength in air for some of the wholly aromatic polyamide papers. It will be noted that the wholly aromatic polyamide papers have a substantially higher 60 cycle fail voltage than the cellulosic samples. ##SPC3##

Next, it was important to determine the dissipation factor of the wholly aromatic polyamide paper, relative to the dissipation factor of cellulosic papers. The results of tests to determine the percent dissipation factor are listed in Table IV. ##SPC4##

It will be noted that the dissipation factor of the wholly aromatic polyamide papers shows only a moderate increase with increasing temperature. Further, except for the second sample listed, which has a high density (0.82), the dielectric constant of the synthetic papers is lower than the cellulosic papers.

FIG. 2 is a perspective view, partially cut away, of a transformer 30 of the type which may advantageously utilize the teachings of the invention. In almost every application for insulation in transformer 30, the wholly aromatic polyamide paper may be used to advantage, compared with cellulosic paper, with the choice in certain of the applications being influenced by the relative cost per pound of the two minerals. The higher cost of the synthetic paper may be offset by its advantages in certain applications, while not in others. Thus, while the wholly aromatic polyamide paper has greater mechanical strength, better temperature stability, and a greater electrical strength than cellulosic papers, it will not be used on a general basis until the costs per pound of the two types of paper are more competitive. Judicious use of the synthetic paper will enable design restrictions to be changed, which will enable other cost savings to be experienced.

More specifically, transformer 30 includes a magnetic core-winding assembly 32, which is disposed within a tank or enclosure 34. The tank 34 is filled to a level 36 with a liquid dielectric, such as mineral oil, or one of the synthetic liquid dielectrics commonly used with power transformers, with the core-winding assembly 32 being completely immersed in the liquid dielectric. The liquid dielectric aids in insulating the various electrical conductors from one another, and from ground, and it also serves to cool the transformer 30. Coolers 38 are connected to the tank 34, with the liquid dielectric circulating therethrough, either by thermal siphon or by forced circulation, to remove the heat from the liquid dielectric which it has picked up from the magnetic core-winding assembly 32.

Transformer 30, in this example, is a three-phase transformer of the core-form type, having a magnetic core 40 and winding assemblies 42, 44 and 46 disposed about winding legs of the magnetic core 40. Each winding assembly includes a low voltage winding and a high voltage winding concentrically disposed about a leg of the magnetic core 40. The high voltage windings are connected to the high voltage bushings, of which two bushings 48 and 50 are shown in the figure, with the third bushing being mounted in opening 52. The low voltage windings, if connected in wye, have their neutral ends connected to bushing 54, and the other ends of the low voltage windings are connected to low voltage bushings (not shown) via conductors 56, 58 and 60. A no-load tap changing mechanism 62 is shown connected to the high voltage windings via a plurality of conductors 64. A load tap changer may also be used, if desired.

The first location in transformer 30 where the wholly aromatic polyamide paper could be used to great advantage would be the high voltage cables 64 connected to the tap changing mechanism 62, and the high voltage cables which connect the high voltage windings to the high voltage bushings. The high electrical strength of the wholly aromatic polyamide paper and its lower dielectric constant than cellulosic papers, would enable the design of these leads to be simplified and would increase their reliability. The higher electrical strength of the wholly aromatic polyamide paper would enable insulating clearances to be reduced, and the lower dielectric constant of impregnated wholly aromatic polyamide paper will distribute stresses more uniformly across adjacent oil passages, which will improve the corona level of the transformer.

As the wholly aromatic polyamide papers become more competitive cost-wise with cellulosic insulation, the wholly aromatic polyamide paper may be used for the critical turn-to-turn insulation in the high voltage windings. FIG. 3 is a diagrammatic view, in section, of a portion of winding assembly 46 and magnetic core 40 of the transformer 30 when in FIG. 2, which more clearly illustrates the electrical conductors of the high voltage winding and the turn-to-turn insulation.

More specifically, winding assembly 46 includes high and low voltage winding assemblies 70 and 72, respectively, which are disposed in concentric relation about leg 74 of magnetic core 40, about a common centerline or axis 76. The low voltage winding 72 includes a plurality of conductor turns 78 which are insulated from magnetic core 40 and high voltage winding 70 by insulating means 80. The high voltage winding 70 includes a plurality of disc or pancake coils, such as pancake coils 82 and 84. The pancake coils, such as pancake coil 82, each include a plurality of radially disposed conductor turns, which are spirally wound about an opening for receiving magnetic core winding leg 74 and low voltage winding 72, with the turns forming a substantially disc shape having first and second major opposed outer surfaces and a predetermined radial build or outside diameter. The various pancake coils are stacked in spaced side-by-side relation, with their outer edges in alignment, and with their major surfaces being in spaced parallel relation to form cooling ducts between adjacent coils, such as cooling duct 86. The plurality of pancake coils are connected in electrical series, with the end coil 82 being connected to the line conductor 88, and line terminal or bushing L, and with adjacent coils being interconnected with start-start, finish-finish connections, such as the start-start connection 90 between pancake coils 82 and 84, and the finish-finish connection 92 which connects pancake coil 84 with the next adjacent pancake coil.

It is to be understood, however, that other arrangements may be used to interconnect the pancake coils. It is to be further understood that the pancake coils, instead of being of the continuous type, may be of the interleaved turn high series capacitance type.

The conductors of which the pancake coils are wound includes at least one electrically conductive strand, such as conductive strand 94, with the conductive strand or strands being wrapped with a plurality of layers 96 of insulating paper. The pancake coils are subjected to surge potentials and voltage oscillations, such as those due to lightning and switching surges, as well as other transient voltages on the electrical system, which develop high turn-to-turn stresses, high interpancake stresses across the cooling duct, and high stresses from the turns to the low voltage winding and ground. The use of the wholly aromatic polyamide paper, with its higher impulse fail strength would be excellent as turn insulation 96, and it would provide a greater factor of safety against faults due to the high transient stresses. Further, the lower dielectric constant would provide less stress in the oil filled cooling ducts than cellulosic insulation, resulting in less ionization of the liquid dielectric and an improved corona level in the transformer.

While the invention has been illustrated in combination with a three-phase transformer of the core-form type, it will be understood that it applies equally to single or polyphase electrical apparatus, and to transformers of the shell-form type, as well as to reactors, and any high voltage apparatus wherein electrical conductors are insulated with solid insulation and immersed in a liquid dielectric.

In summary, there has been disclosed new and improved electrical apparatus of the liquid insulated and cooled type, wherein the electrical conductors of the apparatus to be insulated include solid insulation formed of a wholly aromatic polyamide paper, which is impregnated with a liquid dielectric such as transformer mineral oil, or the synthetic oils such as those containing chlorinated diphenyl and tricholorobenzene. The impulse strength of impregnated wholly aromatic polyamide paper, especially in thicknesses from two through 10 mils, is substantially higher than the impulse strength of impregnated rope-kraft paper. This high impulse strength, coupled with its excellent mechanical strength, its thermal stability, and its resistance to attack from the commonly used liquid dielectrics, makes its use extremely attractive in those applications where impulse strength is of the primary importance. The 60 cycle puncture strength of wholly aromatic polyamide paper is also better than rope-kraft paper.

Since density is not critical in determining the impulse strength of wholly aromatic polyamide paper, the lowest density paper which will maintain adequate turn separation in a pancake coil may be used. In other words, the density should not be so low that it will be compressed to the point of losing the desired dimensional clearances.

Impulse strength of wholly aromatic polyamide paper is controlled primarily by the thickness of the paper. Impulse strength in volts per mil reaches a peak usually at two to three layers of paper, then gradually tapers off as the thickness is increased, with the volts per mil at 50 mils thickness being about 80 percent of maximum.

The 60 cycle strengths of wholly aromatic polyamide papers in thicknesses up to about five mils are appreciably higher than cellulosic papers, and the 60 cycle strength of wholly aromatic polyamide paper holds up well in multiple layers to about 20 mils.

Further, the dissipation factor of the wholly aromatic polyamide papers shows only a moderate increase with increasing temperature, unlike the cellulosic papers and the wholly aromatic polyamide papers do not liberate water as a byproduct when they deteriorate. Finally, except for the very high density wholly aromatic polyamide papers, the dielectric constant of the wholly aromatic polyamide papers is less than that of cellulosic papers, when impregnated with liquid dielectric, which is significant in certain applications such as high voltage cables and other areas where the liquid dielectric will be directly stressed by the insulated conductor.

Since numerous changes may be made in the above described apparatus and different embodiments of the invention may be made without departing from the spirit thereof, it is intended that all matter contained in the foregoing description or shown in the accompanying drawings, shall be interpreted as illustrative, and not in a limiting sense.

Claims

We claim:

1. Electrical inductive apparatus having more inductive reactance than capacitive reactance at power frequencies, comprising:

an enclosure,

liquid dielectric means disposed in said enclosure,

at least one electrical winding, including leads, immersed in said liquid dielectric means, said at least one electrical winding being adapted for connection to an electrical potential,

and solid insulating means disposed to electrically insulate at least a portion of said electrical winding, with said liquid dielectric means impregnating said solid insulating means,

said solid insulating means including paper consisting essentially of a fibrous web formed of wholly aromatic polyamide fibers.

2. The electrical apparatus of claim 1 wherein the liquid dielectric means is mineral oil.

3. The electrical apparatus of claim 1 wherein the liquid dielectric means is a synthetic liquid.

4. The electrical apparatus of claim 1 wherein the solid insulating means includes a plurality of layers of paper, each formed of wholly aromatic polyamide fibers.

5. The electrical apparatus of claim 1 wherein the electrical apparatus is a transformer, and the solid insulating means is the turn-to-turn insulation on the at least one electrical winding.

6. The electrical apparatus of claim 1 wherein the electrical apparatus is a transformer, and the solid insulating means is disposed to electrically insulate at least one lead of at least one electrical winding.

7. The electrical apparatus of claim 1 wherein the electrical apparatus is a transformer, and including tap changing means, and wherein the solid insulating means is disposed to insulate a lead from the at least one winding of the transformer to the tap changing means.

8. Electrical transformer apparatus, comprising:

an enclosure,

liquid dielectric means disposed in said enclosure,

at least first and second electrical windings, each having a plurality of conductor turns and electrical leads, disposed in said enclosure and immersed in said liquid dielectric means,

and solid insulating means disposed to electrically insulate at least a portion of one of said electrical windings, said solid insulating means being impregnated with said liquid dielectric means,

said solid insulating means including paper consisting essentially of a fibrous web formed of wholly aromatic polyamide fibers.

9. The electrical transformer apparatus of claim 8, wherein the solid insulating means is disposed between the first and second electrical windings.

10. The electrical transformer apparatus of claim 9, wherein the first and second electrical windings are concentrically adjacent one another.

11. The electrical transformer apparatus of claim 8, wherein the solid insulating means is disposed to electrically insulate the conductor turns of at least one of the electrical windings.

12. The electrical transformer apparatus of claim 8, wherein the solid insulating means is disposed about at least one of the electrical leads of at least one of the electrical windings.

13. The electrical transformer apparatus of claim 8, wherein the liquid dielectric means is mineral oil.

14. The electrical transformer apparatus of claim 8, wherein the liquid dielectric means is a synthetic liquid.