US2953629A

US2953629A - Porcelain condenser bushing

Info

Publication number: US2953629A
Application number: US695021A
Authority: US
Inventors: Grover W Lapp
Original assignee: Individual
Current assignee: Individual
Priority date: 1957-11-07
Filing date: 1957-11-07
Publication date: 1960-09-20
Anticipated expiration: 1977-09-20

Description

Sept- 20, 1960 G. w. LAPP PoRcELAm coNDENsER BusHrNG Filed Nov. 7, 1957 s Sheets-sheet 1 Sept. 20, 1960 G. w. LAPP PORCELAIN coNDENsER BUsHING 3 Sheets-Sheet 2 Filed Nov. 7, 1957 con/bac r/Vf L 4 vifs w .NNN

COA/006 7'/ VE L VERS tlllllillllllillaailill lll: l ggllilllllil'lllllllll!! lllln .il

INVENTOR. GROVER W LAPF Sept. 20, 1960 G, w. LAPP 2,953,629

PORCELAIN CONDENSER BUSHING Filed NOV. 7, 1957 5 Sheets-Sheet 3 INVENTOR. GROVER Ik LAPP A TTORNEY United States Patent O PORCELAIN CONDENSER BUSHING Grover W. Lapp, 68 W. Main St., Le Roy, N.Y.

Filed Nov. 7, 19-57, Ser. No. 695,021

Claims. (Cl. 174-143) This invention relates to electrical insulator bushings for high voltage use, and more particularly to procelain condenser bushings such as are used in leading high voltage terminals from oil-immersed switches or transformer windings out through the tank into the air high enough to prevent flashover.

The usual insulating bushing comprises a center conducting member, an insulating structure concentric of the conductor, and a metal grounding sleeve by means of which the bushing is maintained in position with respect to the tank. Where used with oil-immersed electrical equipment, the portion of the bushing, which projects in side the tank or enclosure, is usually either wholly or partially immersed in oil.

In bushings of this type, the internal dielectric stress between the central conductor and the grounded tank is distributed by means of equal capacitances in series. Usually, the capacitances are in the form of concentric conductive tubes which are of decreasing length from the central conductor outwardly, and the outermost tube is grounded. The conductor tubes are spaced apart by dielectric layers whose thicknesses vary to obtain a desired constant dielectric relationship. The lengths of the conductive tubes can be graded in a series of equal steps to correspond with the potential gradients of the external path along the upper and lower surfaces of the bushing. The purpose of this construction is to produce condenser layers of equal capacitance to distribute the voltage in approximately equal steps from the center conducting member carrying the high potential current to the outside grounding sleeve at ground potential that supports the bushing on the transformer cover.

Two basic problems have to be met to achieve a satisfactory condenser bushing. The first is to produce stable dielectric layers between the conductive layers. The second is to make the flashover path resistant to llashover under Wet and dusty atmospheric conditions as well as in dry, clear Weather.

A conventional way of making an insulating bushing has been by winding up paper sheet interlaid with thin conductive foil. In such a conventional bushing, electrical discharges from the sharp edges at the ends of the conductive layers of foil at the ends of the foil tend to creep along the joints between adjacent paper sheets. Because of the type of construction used there is no opportunity to modify the shape to reduce concentration at the edges of the foil. only fa very low voltage per mil of thickness of the dielectric layer. Moreover, because bushings of this laminar type are usually immersed in oil, the life of these bushings is limited since oil vand paper are notoriously susceptible to progressive deterioration. Other organic insulating materials contain carbon and, when exposed to leakage currents or flashover arcs, tend to trac or establish conductive carbonized paths, and finally to reduce the tiashover voltage.

One object of the invention is to provide a sturdy,

Accordingly, this type of construction can carry simplied condenser bushing having excellent and stable distribution of internal dielectric stress, free from progressive deterioration, and capable of carrying high voltage per mil of thickness of the dielectric material.

Another object of the invention is to provide a condenser bushing of high quality, capable of utilization at high eiciency with both low and high voltages and without the requirement for oil immersion.

Another object of the present invention is to provide an improved condenser bushing of simplified, substantially monolithic, ceramic structure.

Still another object of the invention is to provide an improved porcelain condenser bushing having a. self-cooling feature.

Other objects of the invention will become `apparent to those skilled in the art from the following detailed descn'ption of the invention.

To meet the external problem or making the outside ashover path resistant to ashover regardless of weather conditions I make my bushings with an outer shell of fired ceramic material and use corrugations on this outer shell. Preferably helical corrugations of a drainage type are used to cause rain water to flow in and out over the surface of the bushing while rain IWater is coursing around the bushing in continuous helical paths. This type and form of corrugations make it possible to use small corrugations having long creepage distance.

To meet the internal problem of obtaining a stable dielectric between the conductive layers of the bushing I embed the conductive layers between successive layers of ceramic dielectric.

Two separate embodiments of the invention are illustrated in the drawings. In one embodiment of the invention, the condenser bushing has a center conducting member, a novel vitried, substantially monolithic conducting and insulating structure concentrically and symmetrically arranged around the conductor, and a grounding sleeve adapted to secure the bushing in position on a grounded tank casing. The novel insulating structure comprises a plurality of unied layers of clay concentric with the center conducting member, each layer of clay being separated, for the major portion of its length, yfrom the overlying layer of clay by a conductive glaze. The concentric cylindrical or tubular layers formed by the conductive glaze are of different lengths, arranged in a graded series of decreasing lengths radially outward from the center conducting member. Each end of each conductive layer is flared or bulged slightly outwardly, then inwardly, to

avoid a high concentration of ilux at the ends of the con-v ductive layer. The inwardly turned tips ofthe ared ends provide a less pointed emission of grading flux through the vitried clay or porcelain to the exterior of the bushing and the outside ashover path. The outer bushing layer is of clay forming a sheath around the bushing, except for one small spot inside the ilange to bring out the current through a meter, as is usual, for the purpose of measuring capacitance or charging current and power Ifactor.

At the ends of the bushing, without intervening conductive layers, so that the vitrified clay with the intervening conductive layers is unified and integrally united, so that an extremely strong structure is obtained in which the conductive layers are permanently enclosed and are completely weather-proofed.

To car-ry away lthe heat from the central conductor and to minimize ltemperature differences through the bushing, the central conductor is made self-cooling. For this purpose it is made tubular, is completely plugged at both ends, and contains water or other suitable liquid under reduced pressure. Radially-nned heat-dissipating tubes are connected to -this central tubular conductor at its the clay layers overlap upper end4 to radiate therefrom; and these nned tubes are sealed lat their outer ends, and an arc guard ring is attached around their outer ends. A drainage channel is rattached below each finned -tube to conduct rain water collected by this topassembly `and'carry it-down over the bushing; The centralftubular conductor and connected radial nned tubes `form a-tightly; sealedunitwhieh -func-A tions to cool the central conductor. When'` thecentral conductor tube cools to ambient temperature, thevapor pressure inside the tube-falls to correspondto that temperature. If the temperature-rises the water in the tube boils; vapor rises up in the tube and into the-radial heatdissipating tubes where it is condensed; and-the condensate drips back. to the warmer bottom-of the centralconductor `tube to be vaporized again. In-this-way, heat isconveyed from the conductor and dissipated into air by the condenser. The larger the condenser the more nearly will the temperature of the tubular central conductor inside the bushing approach the ambient temperature of the yair above the bushing. With thisstructure the central conductor can get rid of its heat, andserve to cool the inside layers. of dielectric instead of heating them.

When extremely high voltages are to be carried, the bushing is made in two concentric sections, each having a group of layers of dielectric alternating with conducting glaze. Bach section corresponds in structure generally to thenfor-m of bushing already described. The central conductor isinserted in the bore of the innerl section and is provided with heat exchangerneans such as described. Between the two sections there is an annularl clearance, and ia concentric double walled tubeis located in the annular clearance. Waterl or other liquid is` sealed in this double-walled tube under reduced pressure, andthis tube isl interconnected with external heat exchange means, such as, for example, radially iinned tubes and iagua-rd ring, so that heat will be dissipated from the double-walled tube `and the outer section of the bushing in the same general manner as from the central conducting tube of the inner section of the bushing.

The `features of the invention will be best understood by consideration of the following description, with reference to the drawings, in which:

Figs. 1 and 1A, taken together, show an axial section of an improved condenser bushing made accordingto one embodiment of this invention;

Fig. 2 is a section of apparatus for manufacturing the condenser bushing shown inFig. 1; and

Fig. 3 is an axial section of-a modified form of con-v denser bushing made accordingtothe invention for carrying high Voltage loads.

Referring now in detail to the drawings, and first to the embodiment shown in Figs. 1 and lA, the condenser bushing comprises 'a centraltubular conductor 11, an insulating assembly 12 arranged concentrically around the central conductor but slightly spaced radially-therefrom, `and a short metal grounding sleeve.14 mounted around the `insulating assembly about midway the length thereof.

The central conductor 11 is an elongated tube having a bore 16 closed in liquid-tight fashion adjacent each end thereof by plugs 18 and 18', respectively. A body of water or Vother liquid 20 is conned within the bore 16 under reduced pressure. The insulating. assembly 12 lis shorter in length than Ithe tubular conductor 11; land the conductor 11 protrudes both -at top andV bottom beyond the. insulating assembly. The insulating assembly may be spaced slightly radially from the tubular conductor; and

a snug, leak-proof, iianged collar 22 is mounted on topv of lthe insulating assembly 12 around the central conductor 11', to keep rain `water out of the annular space between.

ring 23 is generally conical in shape and seats on the collar 22, extending at its bottom around the ange of the collar. It surrounds the conductor 11 near the upper end of the conductor and may be made of a conductive material. Each tube 24 has its bore 25 connected at its inner end with the bore of the central conductor and is closed at its other end. Each tube 24 is provided along its length with radial fins 26. A trough 2S is mounted -below each tube 24 to provide a drainage channel to conduct rain water which may accumulate on and drip from each finned tube toward the central conductor 11. An arc guard ring 27 is attached around the outer ends of the tubes 24.

The insulating core or assembly 12 comprises a plurality of concentrically disposed layers 3), 32, 34, 36 and 38, respectively, of clay, with which 4are bonded, as a Wholly monolithic s-tructure, interposed concentric

conductive layers

40, 42, 44, 46 and 43, formed as coatings of conductive glaze on the underlying clay layers. Thus, a conductive glaze layer 40 is interposed between the innermosty clay layer 30, `and the adjacent, concentric clay layer 32. A second, shorter tubular conductive glaze layer 42 is disposed between the second clay layer 32 and the third concentric clay layer 34, and so forth. Each successive conductive glaze layer, progressing from the central conductor 11 outwardly, is of less length than the next inner conductive layer, so that the lengths are graded in a seriesof preferably equal steps.

The. clay layers 30, 32, 35, 36 and 38 `are formed to extend, over the entire length of the insulating assembly, `and for purposes'of illustration only, their boundaries 142 are indicated in Fig. 2. In the vitrified insulating assembly, the several clay layers are fused into a -monolithic whole, s0 that the boundary lines 142 are indistinguishable.

The whole is enclosed in a thick, outer coating of clay 56 having preferably helical drainage corrugations 58 on the .outer periphery thereof. The whole insulating assembly is, of course, baked to vitrify it. The drain channels 28 are disposed to discharge any rain water onto the upper end or ends of the helical corrugations.

The portion of each clay layer underlying the extremity of each tubular conductive glaze layer is `formed with an outward flare 5), so that the conductive glaze layer is correspondingly contoured, providing Va belled or flared end 52. Each conductive glaze layer terminates at a part of the iiared end 52 that is directed inwardly and is formed at its end with a beadv ring 54 formed in a corresponding groove-in the clay layer.

The metal grounding sleeve 14 is disposed centrally of the bushing 10 around the outer layer of vitriiied clay 56. The grounding sleeve 14 is secured by cement 60 or otherl means` on the bushing. An aperture 62 is provided inthe clay layer 56 inside the ilange of the grounding sleeve 14 to permit a plug or probe 145 to be mounted on the flangel to bring out current from the outermost conductive layer 48 through the line 146 to a meter 147, and; back to the sleeve 14 through the line 148, as is usual, to measure capacitance or charging current and power factor.

In a typical installation,the ground sleeve 14 is adapted to be supported on and in electrical Contact with a grounded tank casing 64, in which case the lower portion ofv the bushing is adaptedv to be submerged in oil.

A bottom terminal 66, of metallic conducting material, is mounted on the lower end of the central conductor 11 in electrical contact with it. This terminal may be connected. in conventional manner with the oil-immersed switch or other electrical equipment with which the bushingis to. be used. The portion of the bushing above the grounding sleeve 14 is adapted to be exposed to the atmosphere, and the upper exposed end of the central conductor 11 is adapted. to beconnected with an externall electrical circuit.

One method of manufacturing the condenser bushing of the presentinvention is described in detail in my copending application, Serial No. 695,058, filed November 7, 1957. For convenience, a brief description of a method of manufacture of the bushing is described here.

A typical apparatus for use in the manufacture of the bushing is illustrated in Fig. 2. 4

This apparatus includes a cylinder 110 formed by an elongated tubular pipe 112 interposed between a pair of end plates 114, with a sealing gasket 116 interposed and compressed between each end of the pipe and each end plate 1214. The end plates are drawn together by any convenient means, such as, for example, a plurality of nuts and bolts indicated generally by the numeral 118.

An elongated, completely threaded bolt 120 passes through apertures 122 in the end plates and centrally through the cylinder 110. A mandrel 124 is mounted around the elongated bolt 120 Within the cylinder 110. The mandrel 124 is preferably sectional and is made from a porous or Water-permeable mixture of Portland cement and sand or `other filler. The mandrel surface 126 is preferably slightly tapered from one end to the other to facilitate its removal from the completed blank.

The mandrel has an open bore 128 through which the threaded bolt 120 passes freely. At each end of the mandrel a metallic end bushing 130 is mounted. Each end bushing has an inner end face 132 coinciding in size with the diameter of the mandrel at that end `and engaging the adjacent end `of the mandrel, from which the end bushing tapers gradually outward to coincide approximately with the outer diameter of the condenser bushing at that end. A gasket 136 is interposed between the opposed surfaces of each end bushing 130 and each end plate 114.

In manufacturing a condenser bushing with this apparatus, the mandrel 126 is mounted on the elongated bolt 120, together with the end bushings 130. Wet plastic clay is built up in a single layer around the mandrel in excess thickness to form the first dielectric layer 30. This may be done by casting from clay slip, or by plaster" ing the clay on, or by laying it on by an injection-plugging method in my copending application referred to above.

It is preferred to lay a perfectly air-free layer, but not necessary since any inclusions of air in the clay will be removed in the next step in the process. A rubber sheath, similar to sheath 138, is placed over, or a rubber ribbon is wound around the layer of wet plastic clay. Care is taken that the sheath or ribbon extends around the end bushings 130. The sheathed cast is then placed in the cylinder 110. Hydraulic pressure on the order of 1000 p.s.i. is then applied to the sheath. This pressure forces the sheath tightly against the layer of clay, to cause the clay to be dewatered, compacted, and densied. Water forced out of the clay, passes through the porous mandrel 126, and escapes in the void space between the threads of the bolt 120 and the bore 128 of the mandrel. The pressure is maintained until water stops dripping, or

until the clay is considered sufficiently dewatered andV densiied. If desired, the pressure may be progressively increased during this step. Y

When dewatering is completed, the pressure is released and the mandrel and its clay coating are removed from the cylinder 110. The rubber sheath or rubber ribbon is then removed and the layer of clay is turned in a lathe, preferably in a vertical position to avoid any tendency to sag. The consolidated or compacted clay is -then in excel lent condition to be precisely turned to size, leaving the portion that is to be given a conductive coating slightly smaller than the ends. The ends of the precisely sized section are ilared slightly to give a gradually thickened wall, and the outer tips of the flarred ends can be turned inwardly-slightly. This is for the purpose of shaping the clay so that when the conductive layer is deposited on the compacted clty layer, concentrated ilux at the ends of the conductive layer will be avoided, and there will be less pointed emission ofthe grading ux through the porcelain to the outside ashover path of the nished bushing. The

clay is turned precisely to the particular length, diameter, and ared end shape desired.

After the clay layer has been machined to the precise shape desired, the ends of the clay layer, which are beyond the contour portion can be taped to mask them; and a conductive glaze 40 of metallic oxides is then applied, having ared ends 50 and small beads 54 in the end grooves of the clay layer 30. The glaze must be refractory so that it will not fuse to a pre-vitried layer. It must remain permeable to the exit of gases and must be free of constituents such as copper, that tend to migrate into the clay during ring.

After the conductive glaze coating has set so that it will not Wash, a second clay layer 32 is applied. The clay layer is then wrapped again in the rubber sheath 138 or in a rubber ribbon, which is extended around the end bushings 130, and the assembly is placed in the cylinder and pressure is again applied. The cast is thus dewatered by hydraulic pressure. Then the whole is removed from the pressure tank 110, and unsheathed,

and the newly-applied clay layer is turned to the thick-A ness and contour desired, and another conductive coating 42 is applied, which is shorter and of larger diameter than coating 40.

This sequence of operations is repeated until the nal conductive glaze coating 48 has been applied. In this way, successive layers of dielectric and conductive coat` ings are built up. The iinal or outside cover layer of porcelain body is then applied as a final thick coating of clay around the conductive coating 48, one small spot being left inside the flange 23 to bring out the current `from the ground sleeve 11 through a meter for the purpose of measuring capacitance, or charging current and power factor. The outermost layer may be wound in a porous layer of glass Wool or tape 137, which in turn is covered by the rubber sheath 138. A pipe or metallic tubing 140, that extends through the end plate 114, is provided to drain oif water from the pressure tank.

During successive dewatering steps, the pressure applied to the sheath 138 is programmed so that dewatering of the iinal thick outer layer 56 is accomplished at the highest pressure. This has the desirable result that the successive clay layers 30, 32, 34, etc. and the intermediate

glazed coatings

40, 42 44, etc. are compacted about the mandrel, and formed as a substantially monolithic whole. At lthe ends of the successive clay layers, where conductive glaze is not interposed between them, their boundaries, indicated in the drawing by the dotted lines 142, are not distinguishable.

When the dewatering and densication steps on the outer layer 56 have been completed, the rubber sheath and the porous layer are removed; and the insulating assembly blank is placed in a lathe and turned to form the helicoidal corrugations 58 in the outermost clay layer 56. If convenient, the blank may be left on the porous mandrel 126 while turning on the lathe.

After the mandrel is removed, the clay blank is reamed to a diameter approximately 5% larger than the desired diameter. The bore of the blank is then coated with a conductive glaze, and the blank is tired. During ring, plastic ilow completely fuses adjacent clay layers so that a homogeneous, unitary, and monolithic bushing is thus obtained. After tiring, the remaining components of the bushing are assembled on the tired core.

This construction provides conductive contoured layers embedded in solid vitriiied porcelain, hermetically sealed away from the weather and atmospheric moisture. Such a bushing may be housed in a porcelain, oil-filled housing as usual and will give excellent performance. However, because of the presence of the outer thick corrugated layer of porcelain, the bushing may also be employed as a dry type of bushing without oil. Because the bushing is suitable for use as a dry bushing, even at high voltages, troublesome design problems, which would otherwise be encountered, are avoided.

Simplicity of structure is a markedV characteristic of this type of bushing. Because of the stability ofY fused ceramic asl compared with organicvr dielectric material, the structure will have long life without increase in dielectric power factor. service can, therefore, be avoided.

The flared end contour` of each conductivel layer eliminates hot spots, or points of critical dielectric stress.

As a result ofthe described method of manufacturing my bushing, when each layer` of clay is. dewateredunder known pressure, and then a conductive layer is applied, followed by another pressure-dewatered` layer of clay, there is thorough consolidation ofthe ceramic granules, nodrying shrinkage, andthe-final firing shrinkage isvery much reduced. The residual firing shrinkage can be controlled by programming the successive` pressures and the finalconsolidating pressure to achieve a final solid structure when fired. By eliminating: all drying shrinkage andav large part of the firingv shrinkage, the residual final shrinkage occurs in the range of early vitrication when the body is hot enough to be safer against cracking because it is somewhat plastic. Since the successive layers are thoroughly compacted, differential shrinkages are largely eliminated and warping is minimized. thisway, roundness and straightnessof` the tubular forms are improved, and dimensional control is made more exactly predictable.

When the condenser bushing becomes heated in operation, the liquid, which is sealed in the bore of` the central conductor, boils under the reduced pressure obtaining. Vapor rises up and condenses in the radially iinnedtubes 24, and the condensate returns to the bottom of the conductor 11, where it, again takes on heat. in this way, heat is transferred from the interior of the bushing through the heat exchange tubes 24 to the atmosphere. Moreover, the radially finned heat exchange tubes 24 serve to dissipate electrostatic flux to help grade the flashover path.

For simplicity of illustration and description, the fins 26 of the tubes 24 have been illustrated and described as of uniform size. In order to increase the clearance distance between the surrounding arc guard rings, the finned tubes can be made shorter on the near sides. and longerV on the outside. Also, other sets of finned tubes can be placed above the rst set of tubes.

For bushings of higher voltage ratings, the thickness of wall may exceed the practical thickness. for firing ceramic bodies. In this case, the bushing may be manufactured in two or more sections, as illustratedin Fig. 3. This practice is advisable also because of differences in temperature that may exist between the outside and inside layers due-to heating and cooling by atmospheric changes on the outside, or due to the suns heat, or duetto heating by dielectric losses and by resistance losses in the central conductor which carries the load current. Differential heating and cooling cause expansion and contraction, resulting in strains in the structure.

The high capacity bushing illustrated in Fig. 3 includes a central tubular conducting member 111 and a first or inner insulating assembly, 119 and a second, or outer insulating assembly 1349. In each assembly, layers of conductive glaze are interposed between layers of clay, as in the first describedy embodiment of my invention. The central tubular conducting member is cooledy as in thel first-described embodiment of my invention by radially finned heat exchange tubes 123. In addition, in the bushing of Fig. 3, a sealed, double walled tube 132 is locatedy in the annular space between the two sections of the bushing, The tube is closed and contains water or other liquid underV reduced pressure. A plurality of heat exchange tubes 125 are secured around the bushing to the double walled tube 132, and the bore of each interconnects at its inner end with the bore of the tube, while the outer end ofT eachV isy closed. The karray of heat dissipating tubes serves, not only for Much testing, and replacement inv InA coolingthe bushing but also to dissipate electrostatic flux to help grade-the flashover path. Additional sets of finned tubes can obviously be mounted above the sets illustrated, if desired, to increase the dissipating capacity.

The bore 121 in the inner assembly 119 of the insulating assembly is coated with a conductive glaze (not shown) to avoid corona from the central conductor 111 to the inside` wall. Similarly, the exterior surface 133 of the inner section 119, which is juxtaposed with the double walled tube 132, and the opposed inner surface 135 of the outer section 130, are coated with conductive glaze (not shown).

A collar 134 is mounted on top of the inner section 119, toV prevent seepage of rain water into the space 121. Appropriate gaskets, not shown, are provided between the top of the porcelain of the bushing and the top conductor flange, and also between the inner assembly 119 and the outer assembly where exposed to the weather.

A grounding sleeve 114 is mounted on a smooth portion of the outer section 130 of the insulating assembly.

The operation of this multi-sectional condenser bushing is similar to that of the bushing previously described. The heat exchange means provided between the two sections eliminates any difference in temperature that may exist, between the two sections and thus preventsdifferential expansion or contraction which would result in strains in the-structure.

Eachsection is separately made by the general process previously described, and the sections and other cornponents of the bushing are 'assembled to complete the final structure shown.

While the invention has been described in connection with specific embodiments thereof, then, it will be understood that itis capable of further modification, and this application is intended to cover any variations, uses, or adaptations of the invention following, in general, the principles of the invention and including such departures from the present disclosure as come within known or customary practice in the art4 to which the invention pertains and as may be applied to the essential features hereinbefore set forth, and as fall within the scope of the invention or the limits of the appended claims.

Having thus described my invention, what I claim is:

1. An electricalinsulator bushing having a central bore and comprising alternating layers of ceramic insulating and of electrically conductive material, said insulating layers extending at their ends beyond the conductive layers, said conductive layers comprising conductive films dcposited on the respectively subjacent insulating layers and arranged concentrically of each other in a graded series in which the films are of progressively decreasing lengths outwardly from said bore, the corresponding ends of successive films being displaced progressively axially of said bore, each film ycomprising a layer of metallic oxide, each end of each film being convex and being directed radially inwardly of said member, said films being mutually spaced and electrically insulated from each other by said interposed layers of insulating material, and an outer layer of ceramic insulating material surrounding and encasing said alternating layers, the extended end portions of said ceramic insulating layers being integrally united together and to the outer layer-of ceramic insulating material to form a substantially monolithic structure, the exterior surface of said member comprising alternating lands and grooves extending around the periphery of said member.

2. An electrical insulator bushing comprising a central conductor rod having a bore sealed Aat each end thereof, heat exchange fluid confined within said bore, radially finned tubes mounted radially around said rod adjacent the top of said bushing and centrally bored with the bores thereof interconnecting with the bore of said rod, an insulating and condenser assembly concentrically mounted on said rod, and a grounding sleeve mounted concentrically around thel insulating and condenser assembly, said insulating and condenser assembly comprising alternating layers of ceramic insulating and of electrically conductive material, said insulating layers extending at their ends beyond said conductive layers and being integrally united at their ends to form a substantially monolithic structure, said conductive layers cornprising a series of concentric, substantially tubular conductive members and being arranged according to a decreasing progression of length outwardly from said rod, said conductive members being mutually spaced and electrically insulated from each other by the said interposed layers of ceramic insulating material.

3. An electrical insulator bushing comprising a central conductor rod, an insulating and condenser assembly concentrically mounted on said conductor rod, and a grounding sleeve mounted concentrioally around said insulating and condenser assembly, said insulating and condenser assembly comprising at least two concentric sections, each section having a central bore and comprising alternating layers of insulating and electrically conductive material, the insulating layers extending at their ends beyond the conductive layers and being integrally united at their ends to form a substantially monolithic structure, said conductive layers comprising a series of substantially tubular conductive members concentrically disposed relative to said central conductor rod and having areas which are arranged in a decreasing progression outwardly from said rod, said conductive members being mutually spaced and electrically insulated from each other by the interposed layers of insulating material.

4. An electrical bushing comprising a central con* ductor tube, an insulating and condenser assembly concentrically mounted on said tube, and a grounding sleeve concentrically mounted around said insulating and condenser assembly, said insulating and condenser assembly comprising at least two concentric sections, each section having a central bore and comprising alternating layers of insulating and electrically conductive material, a closed chamber disposed in the annular space between each pair of concentric sections, liquid under reduced pressure confined in each chamber and in said central conductor tube, finned tubes communicating at their inner ends with said central conductor tube and secured to said tube and mounted above said insulating and condenser assembly and being closed at their outer ends, and other finned tubes communicating with each closed chamber between concentric sections and also being closed at their outer ends.

5. An electrical bushing adapted for use in )a high voltage circuit and comprising a central conductor tube through which courrent is adapted to pass, an insulating and condenser assembly concentric of said tube, and a grounding sleeve mounted concentrically around said insulating `and condenser assembly and adapted to be at ground potential, said insulating and condenser assembly including at least two concentric sections, each section having a bore, said conductor tube extending through the bore of the inner section, and each section comprising alternating layers of insulating and conductive ma* terial, the insulating layers extending at their ends beyond the conductive layers, said conductive layers comprising substantially tubular electrically conductive iilms arranged concentrically of said tube in a series in which the iilms are of progressively decreasing areas outwardly from said tube, each end of each iilm being displaced from the corresponding end of the next subjacent lm axially of the bushing toward said grounding sleeve and each end of each film being flared outwardly through a convex path and terminating along a slope of said path directed radially inward toward said conductor tube, each iilm comprising a metallic oxide, said iilms being mutually spaced andelectrically insulated by the interposed layers of insulating material, an outer layer of insulating material encasing said yalternating layers, the extended ends of said alternating layers of insulating material being united together and united with the outer layer of insulating material to form a monolithic structure for each section, said outer layer of each section being helically corrugated over at least a portion of its length, heat exchange means for said conductor tube, and heat exchange means interposed between each pair of concentric sections of said insulating and conductive assembly, the outermost of said conductive ilrns in the outer section being electrically connected to said grounding sleeve.

References Cited in the le of this patent UNITED STATES PATENTS