EP0020405A1

EP0020405A1 - Mit schwefel-hexafluorid gefüllte kompakte isolatorbuchse mit reduziertem gasgefülltem volumen

Info

Publication number: EP0020405A1
Application number: EP19790901135
Authority: EP
Inventors: Lorne D. Mcconnel; Allen E. Stringfellow
Original assignee: BBC Brown Boveri AG Switzerland
Current assignee: BBC Brown Boveri AG Switzerland
Priority date: 1978-10-10
Filing date: 1980-04-22
Publication date: 1981-01-07
Also published as: WO1980000762A1; BE879277A

Description

COMPACT SULFUR HEXAFLUORIDE-FILLED INSULATOR BUSHING WITH REDUCED GAS-FILLED VOLUME

BACKGROUND OF THE INVENTION

This invention relates to electrical insulation bushings and more specifically relates to a novel sulfur hexafluoride gas-filled electrical bushing which is at least partially filled with a solid or relatively immobile insulation material to reduce the free volume of gas within the porcelain weather jacket or casing of the bushing and to reduce the potential explosion hazard of the bushing if the casing should fracture. Sulfur hexafluoride gas-filled electrical insulation bushings are well known and a bushing of this type is typically shown in U.S. Patent 3,566,001, in the name of J.R. McCloud, dated February 23, 1971 and assigned to the assignee of the present invention. The disclosure of that patent is incorporated herein by reference.

A similar insulation bushing which is filled with sulfur hexafluoride gas is also shown in copending application Serial No. 763,422, filed January 28, 1977, entitled REINFORCED SHATTERPROOF GAS-FILLED HIGH VOLTAGE BUSHING, which is assigned to the assignee of the present invention and which discloses the use of an elastomeric layer on the interior surface of a bushing to reduce its explosion hazard by adhering to particles of porcelain which are created during the fracture of the casing and thus preventing their scattering. The present invention provides a novel means for limiting the explosion hazard of gas-filled bushings of this type by reducing the volume of the free gas within the bushing.

BRIEF DESCRIPTION OF THE PRESENT INVENTION

In accordance with the present invention, a sulfur hexafluoride gas-filled bushing of the-prior art type has the gas volume therein. reduced without affecting the dielectric integrity of the bushing by replacing at least a portion of the gas volume with a solid modular body or relatively immobile body.

In one embodiment of the invention, a sulfur hexafluoride foam, which is made in accordance with the disclosure of Canadian Patent No. 880,377, dated Sept. 1971, in the name of David H. Reighter and assigned to the assignee of the present invention, displaces a substantial part (more than about 50%) of the free gas volume within the bushing interior. Other foam formulations can also, be used where the solid fόam material will displace the free dielectric gas such as sulfur hexafluoride within the bushing.

As a further feature of the present invention and since a solid module of foam or other material may introduced into the bushing interior, it also becomes possible to support grading capacitance layers within the module. Thus, a plurality of concentric conductive tubes can be formed within the module, thereby to serve as the grading capacitor for grading the dielectric stress across the outside surface of the bushing. This in turn, enables a reduction in.the size of the bushing The interior of a given bushing can receive different kinds of modules: one, adjacent the grounded mounting flange containing the capacitance layers, and another, disposed in the upper region of the insulator where the dielectric stress is lower, not containing the grading layer and simply being a mass of sulfur hexafluoride-filled solid material.

In a second embodiment of the invention, the solid material which displaces the gas can be formed of a wound cloth or macerated fibrous material such as wound dacron, orlon, alumina, mullite, cotton or other suitable material which is impregnated with sulfur hexafluoride gas. A wound unit of this type, which may incorporate conductive capacitance layers concentrically spaced within the wound material, will entrap sulfur hexafluoride gas within the fibrous material, thereby preventing the rapid discharge of the sulfur hexafluoride gas in the event of a fracture of the porcelain weather jacket. Thus, this module again will reduce the explosion hazard of the bushing and further permits a significant reduction in the height and diameter of the bushing.

After the module described above has been formed, it can be vacuum-dried until the moisture Level of the bushing is reduced to a suitable point. The bushing is thereafter pressurized with sulfur hexafluoride to form the necessary dielectric.

In the above, the invention has been described in connection with the use of sulfur hexafluoride gas under pressure such as 100 p.s.i.g. However, other electronegative gases at other pressures can be used as desired.

In carrying out the invention and where the bushing is associated with circuit breakers of the type shown in U.S. Patent 3,602,669, in the name of H.G. Meier et al, dated August 8, 1971 and entitled PURGING AND DRYING SYSTEM FOR GAS BLAST CIRCUIT INTERRUPTERS and assigned to the assignee of the present invention, the bushing interior communicates with the interiox of the circuit breaker housing. In accordance with the invention, the communication can be through a small pressure regulator which limits the rate of flow of gas from the gasfilled housing to the interior of the bushing. In addition, when a macerated fibrous material is used to form the module in the interior of the bushing which reduces the free gas volume, the fibrous material can serve as a heat pipe using a Freon of suitable boiling point. Thus the Freon gas can rise when the boiling point is attained by passing through holes in the conductor, rising to the top of the bushing and then refluxing from a heat sink on the top of the bushing back through the cloth to a reservoir at the bottom of the bushing, thus acting as a temperature Controller.

BRIEF DESCRIPTTON OF THE DRAWINGS

Figure 1 is a cross-sectional view taken through an ultra-high voltage insulator bushing having a precast sulfur hexafluoride epoxy foam module with capacitance grading layers to replace sulfur hexafluoride gas volume. Figure 2 is a cross-sectional view of a frame for a foam capacitor module of the type used in connection with Figure 1 before the sulfur hexafluoride. foam is fixed in place.

Figure 3 is a cross-sectional view of Figure 2 taken across the section line 3-3 in Figure 2. Figure 4 is a cross-sectional view of a second embodiment of the invention in which the bushing capacitance grading module uses SF₆ impregnated cloth with split condenser layers. DETAILED DESCRIPTION OF THE DRAWINGS

Referring first to Figure 1, the ultrahigh voltage bushing shown therein is typically a bushing which can be used for voltages of 145 kV through 550 kV and greater The bushing is Schematically illustrated and typically employs a porcelain weather casing 10 which is pressed against a metallic mounting flange 11 which is fixed to the apparatus with which the bushing operates. A second insulation portion 12 cooperates with the porcelain weather casing 10 and is clamped against. the opposite side of the metallic support flange 11. A central bushing conductor 13 then passes through the cylindrical aligned insulation members 10 and 12 to permit electrical connection to the interior of the apparatus to which the flange 11 is fixed. Flange 11 is usually connected to ground potential.

A conductive cap 14 is sealed to the top of weather casing 10 and is electrically connected to the conductor 13. Any suitable construction- can be used for fixing or sealing the top and bottom of weather casing 10, which is of porcelain, to the cap 14 and to the flange 11, respectively, in order to form a sealed interior volume within the weather casing 10. Note that the interior volume within porcelain weather casing 10 may be connected to the interior of the apparatus to which flange 11 is secured. The structure described to this point is shown in detail in U.S. Patent 3,566,001, referred to above and which is incorporated herein by reference. The invention to be described hereinafter can directly apply to the bushing shown in Patent 3,566,001.

The prior art bushing described above is normally filled with sulfur hexafluoride gas or some other suitable electronegative gas at a pressure of up to 100 p.s.i.g. Because of this high pressure, it is possible that if the porcelain weather casing should become fractured due to any cause such as impact by a rifle bullet, there would be a high-speed release of gas through the fractured casing causing an explosion or scattering of porcelain fragments which could be dangerous to personnel in the surrounding area.

In accordance with the present invention, the explosive scattering, which could be caused in casje of a fracture of the porcelain weather casing which is relatively brittle, is limited by limiting the free volume of dielectric gas wϊthin the interior of the weather casing 10. Thus, a solid rigid or resilient material or a fibrous material which would limit the expansion rate of the gas in case of a fracture of thesurrounding weather casing 10 is inserted into the interior of the casing.

In Figure 1, the upper portion of the interior of weather casing 10 is filled with a cylindrical foam module 15 which is formed of a sulfur hexafluoride foam body of the formulation disclosed in Canadian Patent 795,446. Other formulations can be used as will be later described. The foam body 15 can, if desired, extend the füll length of the interioX of the bushing and thus substantially limit the amount of free gas in the interior of the weather casing 10.

In accordance with a further feature of the invention, the use of the solid module permits the application of condenser layers to the bushing in order to grade dielectric stress across the outside surface of the bushing. Thus, in Figure 1, the lower portion of the bushing contains a second module 16 which contains a plurality of conductive cylinders embedded therein including conductive cylinders 17, 18, 19, 20, 21 and 22 which function as conventional capacitance grading cylinders which will help reduce the diameter and length of the bushing by grading the dielectric stress along the bushing. Cylinder 17 may be connected to grounded flange 11 in any desired manner. Each of cylinders 17 to 22 have their ends outwardly flared to assist in the control of the dielectric stress. Note that the module 16 can be directly adhered to the lower portion of conductor 13 while the module 15 is adhered directly to the upper portion of the conductor 13.

Figures 2 and 3 illustrate the general frame construction which can be used to form the module 16. Thus, in Figure 3, the module has the central conductive tube 30 suitably supported in a jig (not shown). relative to the concentric thin conductive tubes 17 to 22. Each of the tubes 17 to 22 are held in place by two split foam disks at their either end, shown as. split foam disks 23a-23b to 28a-28b for the tubes 17 to 22, respectively. Each of the disks have circular openings therethrough which permit the circulation of a foamed SF₆ through the openings during the foaming or filling Operation.

The entire assembly is then placed in a suitable mold which is filled with the foamed SF₆ material as disclosed in Canadian Patent 795,446 which fills out the entire assembly to the solid line outline shown in Figures 1 and 2, with the capacitance shields or screens being fixed in the foam material. Moreover, the module is fixed to the central conductor 50 which becomes an integral part of the conductor 13 in Figure 1.

Note in Figure 3 that the openings through the disks are typically shown by the series of openings 40 around the periphery of the support disks 23a.

In the arrangement of Figure 1, the use of the two modules 15 and 16 reduces the free gas volume for a typical bushing which might be rated at 550 kV from 1,225,000 cubic centimeters to 78,000 cubic centimeters. Thus, the explosion hazard of the bushing is substantially reduced. Figure 4 shows a second embodiment of the invention in which the module is formed of a wound cloth or macerated fibrous material. Thus, in Figure 4, the module 16 of Figure 1 is formed on a central conductor 30 (which can become continuous with the central conductor 13 in Figure 1) and can contain the illustrated five concentric tubular conductive cylinders 17 to 21. Note that the condensor layer 22 has been eliminated in the; arrangement of Figure 4. The conductive layers 17 to 21 may be aluminum foil capacitance layers which are respectively formed on top of wound layers 50 to 54, respectively, of the cloth or macerated fibrous material after each layer is wound. way of example, layers 50 to 54 can be formed of ribbon of dacron, orlon, alumina, mullite, cotton or other similar material which is fibrous and which will tend to be filled with a gas which is loaded into the bushing.

The module of Figure 4 can be formed by using the central conductive tube 30 as a mandrel and then winding a sufficient number of layers of the above material to form the layer 54 which might have a thickness of 80 to 140 mils.

Thereafter the alumina foil layer 21 is deposited on the outer surface of layer 54 and the outer ends of the conductive layer 21 are outwardly flared as illustrated. Thereafter, a second layer 53 is wound on the outer surface of conductive layer 21 and is wound to the desired thickness and this process is then repeated until the outer conductive layer 17 is formed on the device. Thereafter a few additional turns of fibrous material is wound to hold the conductive layer 17 in place.

After the module has been completed, it is assembled within the bushing in Figure 1, and the entire interior of the bushing is vacuum-dried until the moisture level of the bushing is reduced to a suitable point and all moisture and gas have been xemoved from the fibrous material layers 50 to 54 in Figure 4. Thereafter the bushing is pressurized wi h sulfur hexafluoride gas or any other sufficient gas to a pressure of up to 100 p.s.i.g. in order to form the necessary dielectric.

Note that other types of electronegative gases and other pressurεs could be used. Significantly, however, the free σas space which remains after the module or modules are inserted in place is very limited and can, for example, have a radial thickness surrounding the module.16 of no more than about 1/2 inch. This ensures ease of assembly of the module within the weather casing 10 but permits a substantial reduction in free gas space so that any explosive action would be greatly limited if the weather casing 10 should fracture. Thus, if there should be a fracture of the weather casing, the gas which is trapped within the fibrous material of layers 50 to 54 cannot be easily expelled to contribute to the explosive effect of a fractured bushing.

The free. gas volume remaining within weather casing 10 can, if desired, be filled by high dielectric gas which is relatively immobile. By way of example, hollow microspheres made of a suitable plastic, such as polyethylene, can be loaded with sulfur hexafluoride under pressure and these microspheres can then fill the free Space remaining within the bushing. The interstices between the microspheres can then be filled with free sulfur hexafluoride gas. These microscopic spheres which are loaded with sulfur hexafluoride gas are relatively immobile. Thus, for the free gas between the interstices of the spheres to escape through a fracture in weather casing 10, the gas would have to flow around or between the millions of sulfur hexafluoride filled spheres, thus impeding the escape of gas and limiting the explosion potential of the bushing. Note that the entire interior of the bushing could be fiiled with such microscopic spheres which are filled with sulfur hexafluoride under pressure.

The available free volume within the bushing interior can also be filled, if desired, with molecular sieves loaded with sulfur hexafluoride or some other electronegative gas. These gas-loaded molecular sieves are again relatively immobile in comparison to free sulfur hexafluoride gas and the interstices between the molecular sieves would be filled with free gas such as sulfur hexafluoride.

A typical sieve which could be used could be 4-8 mesh type 13X molecular sieve beads loaded with sulfur hexafluoride. For example, Grace Company molecular sieve beads can be loaded to up to about 20% by weight of sulfur hexafluoride. These sulfur hexafluoride loaded beads may then be poured into the remaining cavity within the bushing (pr can completely fill the bushing interior) and displace the free gas within the bushing volume.

The dielectric strength of the system remains high because of the pressurized sulfur hexafluoride.

Moreover, the molecular sieve alumina silicate structure will not carbonize due to its inorganic composition.

The sulfur hexafluoride loaded molecular sieves, like the sulfur hexafluoride filled microscopic spheres described above, can be simply poured into the cavities of the existing bushing and are relatively immobile so that there will be a substantially reduced explosive force in the event of a fracture of the weather casing 10.

The sulfur hexafluoride foam used for forming the modules previously described used a polymeric material for the foam body. It is also possible to use an epoxy foam which is pressurized or foamed with sulfur hexafluoride or some other suitable electronegative gas. By way of example, an epoxy foam can be made by whipping sulfur hexafluoride directly into a suitable liquid epoxy. Violent agitation is preferably used in order to produce a light-weight frothy material. This mixture is then injected into a mold or injected directly into the porcelain bushing with the injection continuing until the pressure equals about 2 to 3 atmospieres. After the epoxy eures, the injection-molding pressure is removed and the pressurized sulfur hexafluoride will be contained by the light-weight cellular epoxy foam.

Since the sulfur hexafluoride gas trapped within the epoxy foam is at; about 2 to 3 atmospheres, the dielectric strength of the composite is somewhat increased. Moreover, because 2 to 3 atmospheres positive molding pressure is used, the advantage of pressure gelation is realized. Note that the epoxy foam modules made aecording to the above process can also encapsulate condenser layers. If the module fills only a portion of the interior volume of the bushing, the remaining portion of the bushing can be filled with relatively immobilized gas-filled bodies such as sulfur hexafluoride filled molecular sieves or sulfur hexafluoride filled microspheres. In the event that it is desired to keep the weight of the foam modules as low as possible, it is possible to produce an extremely light-weight foam having a speeifie gravity of 0.1 or less which is pressurized with sulfur hexafluoride gas (or some other suitable gas) to a pressure of 2 to 3 atmospheres. Thus, an open celled foam of any base resin can be formed where the open-celled foam is of light weight. The foam is then placed in a vacuum and all air and other gases are removed from the open cells. Thereafter, the vacuum Container is pressurized with sulfur hexafluoride under pressure to force-fill of all open cells with sulfur hexafluoride gas at a pressure of 2 to 3 atmospheres.

While in the pressurized State, the open-celled foam outer body surfaces are coated with a relatively gas-impervious plastic such as an SHF-14 type res in wh ich is then cured . Thus , the module wil l be an open-celled material which is sealed on all exposed surfaces to retain the pressurized sulfur hexafluoride within the open-cell material. The module can then be assembled the bushing as described previously.

In the above, the open-celled foam was sealed on all its surfaces before being inserted into the bushing. If desired, an unsealed open-celled foam body can be inserted into the bushing. The bushing can then be filled with sulfur hexafluoride to the desired pressure, with the gas being forced into the öpen cells of the foam body. After filling, the bushing itself may be capped and sealed. Sulfur hexafluoride gas will then fill and be retained in the open-celled foam. The free space outside of the open-celled body can be filled with other relatively immobile bodies such as SF₆ loaded molecular sieves and the like although these should be loaded only after the open-celled foam has been filled to its desired pressure. The resulting device will exhibit reduced explosion hazard since the bulk of the gas in the open celled foam will be replaced slowly if the casing is fractured.

Note that when an open-celled foam is used, condenser layers can be supported within the foamed body . Preferably, these condenser layers will be gas-pervious so that the open-celled body can be filled with gas which can penetrate the body from any direction.

The sulfur hexafluoride foam modules described above can be made by a process which uses microwave energy to cause the foaming of a resilient cellular foam This process can be used to create a module within its own mold or can foam the module directly within the porcelain weather casing 10 with the interior of the weather casing being used as the mold. Since the foam which is formed is relatively flexible, it will adhere snugly to the inner porcelain surface and will not crack the porcelain surface by exerting undue force on the porcelain.

In carrying out this process, sulfur hexafluor-ide gas (or some suitable other electronegative gas) is conveyed by molecular sieves loaded with the gas and dispersed throughout a suitably polymer. The entire unloading of the molecular sieves and gelling of the foam is done under a positive pressure to obtain maximum insulation effect from the gaseous component. While a wide ränge of polymeric materials can be selected, good results have been obtained with urethane. One formulation which has given good cellular structure and which has been blown with sulfur hexafluoride gas which adsorbed onto a molecular sieve (äs described in Canadian Patent 880,377) was as follows: Vibrathane B-601 (available from Uniroyal Corporation) 100 parts by weight; Poly "G" 40Q (available from Olin Chemical). 22 parts by weight; and 4 parts by weight of a sulfur hexafluoride loaded .sieve which was a 13X molecular sieve powder (availabe from Union Carbide Corporation) with up to 35% of sulfur hexafluoride gas adsorbed. This formulation is effectively blown through the use of microwave heating and under a pressure ranging from slightly above ambieiit pressure to as high as 100 p.s.i. The formulation is loaded into the porcelain jacket which is to be filled with the module (or into any other suitably mold). The porcelain jacket 10 and central conductor 13 are kept at a temperature slightly less than the gel temperature of the resin formulation to prevent the cold components from restricting blowing and gelling at their interfaces.

Microwave energy is then applied to the entire assembly. Microwave heating is extremely effective since the porcelain shell 10 is very transparent to microwave energy and deep controlled heating is accomplished with substantially füll unloading of the molecular sieves.

It has been found that microwave heating is superior to oven heating since, with oven heating, the heat is most pronounced at the resin mold interface so that blowing first commences at the interface. The foam then produced at the interface tends to inhibit the flow of heat to the deeper resin regions and the unloading of the molecular sieves and subsequent gel is much slower with. the released gas rising to the surface before gelling occurs. With microwave heating, it has been found that more even gelling throughout the thickness of the resin is obtained.

It will be further noted that, with the above process, a soft cellular sulfur hexafluoride foam is produced which will accommodate itself to the shape of the porcelain shell and conductor and will not pull away from the shell or conductor. Moreover, the high pressure existing in the individual cells of the foam make available the füll advantage of the insulating properties and low- dielectric constant of the sulfur hexafluoride gas trapped within the foam.

Finally, the relatively flexible foam will not exert undue pressure on the porcelain casing if it is molded within the porcelain casing.

When using a sulfur hexafluoride loaded molecular sieve as the foaming agent for the foam to be used with the invention, it has been found useful to mix the sulfur hexafluoride loaded molecular sieves with a dielectrically insulating liquid, such as transformer oil, cabon tetrachloride, silicone oil or virtually any other non-polar liquid. The emulsion formed can then be subjected to microwave energy which is effective to release the adsorbed sulfur hexafluoride gas from the molecular sieve as described above.

The use of the emulsion of the sulfur hexafluoride loaded molecular sieve and non-polar dielectric fluid is sufficiently mobile so that it can be poured into small irregularly shaped regions where it will solidify into a semisolid highly insulating low dielectric constant insulation body. This insulation has been found resistant to moisture adsorption due to its non-polar nature and prevents the sieve from adsorbing moisture. The resulting gel is also flame-retardant due the high content of the inert gas released and, further, the oil base aids in the conduction of heat through the foam body. Another process, which can-be used to foam relatively tail modules, either directly in the bushing or within some separate mold, is a novel progessive inverse foaming process. There is a limitation to the height of a body which can be foamed since the heavier the mass to be formed, the less the foaming will be and the more variable the foam density will be. Moreover, if the gel time is incorrect, a large Variation in density will occur through the foam since the light gas bubble will tend to migrate to the top of the foam. In one process which can be used to form the relatively long module useful in connection with the present invention and wherein a sulfur hexafluoride loaded molecular sieve is used to foam a urethane base, the mold is fed from its bottom by the foamable material and a temperature gradient is placed across the mold, with the top being the hottest and the bottom being the coldest. This will then cause gelation of the foam from the top of the mold proceeding downwardly from the top in order to produce a relatively unform foam without height limitation. The module mold will preferably have a removable vent at its top and a filling orifice at the bottom, with the mold interior being quickly filled and allowed to foam downwardly toward the filling port.

The uppermost portion of the foam, as it gels, progresses downwardly and remains hot with respect. to the lower portions of the mold, due the exothermic heating. Thus, the gelling process continues smoothly from the top to the bottom of the mold cavity.

The foam module with the integral condenser layers shown in Figure 1 can be manufactured by theprocess of Figures 2 and 3. Another process which may advantageously used for the manufacture of the module employs the rotary application of the foam to build up a multilayer condenser assembly. Thus, a lathe-like apparatus can be provided for rotating a central support mandrel which may consist of or include the central conductor 30 of Figures 2 and 3.

The apparatus then includes an application head which can spray or flow on a relatively thin layer of an unfoamed resin system, such as the above-described epoxy or urethane material containing the sulfur hexafluoride loaded molecular sieves onto the rotating mandrel. These layers are applied as by causing the head to traverse along the length of the rotated body in a back and forth direction until a desired thickness is obtained. Thereafter, a suitable doctor blade or the like is applied over the mandrel surface to size and shape the outer layer which is being continuously rotated.

A suitable heating system, such as a microwave system or the like, then applies energy to the applied layer to cause foaming and solidification of the layer. A wrapping head is then brought into play to apply a conductive condenser layer in intimate (wetting) contact with the completed foamed layer. This process is then completed until the füll outer diameter desired and the total number of condenser layers desired have been obtained. Note that each layer may be only partially gelled before application of successive layers. The entire assembly may then be completely gelled after all layers have been applied. In one embodiment of the above-described forming process, the condenser layers may consist of a creped or quilted metal foil or of a vacuum-deposited conductive coating on a stretchable film such as polyethylene. This conductive layer is then intimately applied to the unexpanded and unfoamed resin layer beneath.it. Upon expansion of the layer, all corrugated or irregularities in the film would be straightened out to a more fully circular form. This action aids in controlling the thickness of the individual and final condenser layers and the roundness of the condenser form.

In the above, the individual layers are applied successively to the outer surface of the mandrel. If desired, the individual foamable layers can be applied in a centrifugal casting type system where, for example, a relatively slow rotating mold having the final outer shape desired for the module receives a foamable layer which is to be the outside layer of the module. Thereafter, a conductive layer is applied to the inner surface of the first outer foamable layer and the outer foamable layer may then be gelled. Thereafter, the successive inner layers are centrifugally applied until the final innermost layer is formed and foamed.

Although preferred embodiments of this invention have been described, many variations and modifications will now be appar-ent to those skilled in the art, and it is therefore preferred that the instant invention be limited not by the specific disclosure herein but only by the appended Claims.

Claims

WHAT IS CLAIMED IS:

1. A high voltage insulation bushing comprising, in combination: an elongated hollow cylindrical weather casing; a conductive mounting flange fixed to and sealed to one end of said cylindrical weather casing; a conductive cap fixed to and sealed to the opposite end of said cylindrical weather casing; a central conductor extending along the axis of said cylindrical weather casing and insulated from said conductive mounting flange; an electronegative gas under positive pressure filling the interior of said cylindrical weather casing; said positive pressure being high enough that, if gas fills the entire volume of said weather casing, it would form an explosion hazard if said weather casing is fractured; and a module of gas displacement material fixed to said central conductor disposed in the interior of said cylindrical weather casing to reduce the free gas volume within said weather casing.

2. The bushing of Claim 1 wherein the outer. diameter of said module is spaced from the interior diameter of said weather casing.

3. The bushing of Claim 1 wherein said module extends across the axial position within said casing which is aligned with said conductive flange.

4. The bushing of claim 1, 2 or 3 wherein said casing is made of porcelain.

5. The bushing of claim 1, 2 or 3 wherein said electronegative gas.is sulfur hexafluoride.

6. The bushing of claim 1, 2 or 3 wherein said module contains an embedded capacitive grading cylinder.

7. The bushing of claim 6 wherein said casing is made of porcelain and wherein said gas is sulfur hexafluoride.

8. The bushing of claim 5 wherein said module is formed of a foamed sulfur hexafluoride material.

9. The bushing of claim 7 wherein said modu is formed of a foamed sulfur hexafluoride material.

10. The bushing of claim 5 wherein said module is formed of a fibrous material.

11. The bushing of claim 7 wherein said module is formed of a fibrous material.

12. The bushing of claim 6 which fürthex includes a second module in axial alignment with said module; said second module being disposed adjacent said conductive cap and being fxee of capacitance gxading structure means.

13. The bushing of claim 12 wherein said casing is made of porcelain and wherein said gas is sulfur hexafluoride.