BACKGROUND ART In several kinds of high-voltage electric devices, such as switchgear, conductors are electrically insulated from other conductors or other conducting objects, such as metallic enclosures or structural elements, by an intervening solid, liquid and/or gaseous electrical insulation. In the case of electric devices in which at least part of the insulating capability is provided by a liquid or a gas, a spacer or other form of support member is employed to ensure that the conductor is held sufficiently spaced apart from other conductors or electrically con- ductive components.

There are two basic types of electric devices using gaseous insulation; air-insulated devices with an open structure and devices enclosed in a sealed enclosure in which a gas, such as SF6 gas or air is enclosed to serve as insulation. In such devices, unsuitable design or surface con- ditions of the insulating support members can considerably reduce the voltage across the support member at which partial discharge starts and the voltage at which breakdown is ini- tiated. It therefore is important to consider at least two aspects: -the design of the triple point, i. e. the point where the conductor, the insulating gas and the support member are all in contact with each other, because an unsuitably designed triple point amplifies the electric field forces if, as is normally the case, the support member has a relative dielectric constant higher than the relative dielectric constant of the insulating gas or air, and -the surface conditions of the support member, i. e. its topology and the surface energy, because surface defects and moist or other electrically conductive depositions on the surface also amplify the electric field forces and under unfavorable conditions can lead to partial dis- charge inception along the surface and eventually dielectric breakdown.

When discussing the electrical strength of an insulation system or individual component making up the insulation certain terms are used which need to be defined: -"breakdown voltage", or"puncture voltage"as used in this application, is the voltage across the insulation at which the insulation loses its electrical insulation function, -"partial discharge inception voltage", as used in this application, is the voltage across the insulation for which a partial breakdown occurs or, more accurately, starts in a part of the insulation system, which part may be a complete component or a portion of a component in the insulation system.

According to an unpublished proposal aiming at providing more compact designs of both open and closed gas or air insulated electrical devices the conductor is at least partly covered or coated with an electrically insulating layer. The insulating layer shall have sufficient di- electric strength and in combination with the basic gaseous insulating medium provide suffi- cient insulation for the given spacing and voltage difference, the dielectric strength for a given material being determined by the thickness of the insulating layer. The electrically in- sulating coating provided on the conductor shall have a thickness sufficient to withstand electrical puncture at a voltage which is at least 30 %, and preferably at least 50%, of the breakdown voltage of the insulation system of which the insulating coating forms part.

For an electric device according to the above-mentioned proposal the triple point of impor- tance will be the point where the conductor coating, the insulating gas and the support member are all in contact with each other.

According to the same proposal propagation of creeping discharges along a coated or covered conductor is prevented by means of a barrier arranged to increase the creep distance, e. g. a barrier in the form of one or more discoid protrusions. Such a barrier may be combined with an electric field smoothing termination such that the starting and propagation of any creeping discharge are avoided by a combination of a barrier effect and an electric field smoothing.

However, where a support member is provided, it is also extremely important to prevent any creeping discharges from passing over from the coating to the support member. The compact designs made possible by the insulating coating provided on a so-called covered conductor or electrode increase the demands with respect to the partial discharge inception voltage of any spacer or other support member used. In the case of a conventional gas or air insulated electric device, a spacer with a partial discharge inception voltage well below the breakdown voltage of the gas can be used, but in the case of a device in which part of the insulation is provided by an insulating coating on a covered conductor, the operating voltage can be in- creased to a level fairly close to the breakdown voltage of the gas. For that reason, the partial discharge inception voltage of any spacer or other support member used has to be close to the breakdown voltage of the gas to ensure that the device will be essentially free from partial discharge and dielectric breakdowndo not easily occur.

As will be appreciated from the foregoing, great care should be taken to ensure that a spacer or other support member is carefully designed and that its surface is subjected to a treatment that reduces the risks for partial discharge inception and dielectric breakdown. Consequently, for spacers or support members in electric devices a material having a low relative dielectric constant should be chosen, and the spacer or support member should be specially designed at the triple point and have a carefully treated surface. This is in all essential respects true also for electric devices comprising coated or covered conductors. However, if the spacer could be made from a material with a relative dielectric constant close to the relative dielectric con- stant of the insulating gas or air, that is, close to 1, then only the conductor or electrode de- sign and the insulating strength of the gas would determine the dielectric strength of the in- sulation system. The influence and importance of the triple point and the surface conditions would be substantially reduced.

It is known, e. g. from a presentation paper given on August 27,1997 at the 10"'International Symposium on High Voltage Engineering, Aug. 25-29,1997 Montreal, Canada, and entitled "Behavior of Surface Discharges Along a Polymer Foam Insulator in Air, U. Fromm, Li Ming, M. Leijon and D. Windmar., that electrically insulating components with a low relative dielectric constant can be created by connecting solid volumes and gaseous volumes in series.

The example given in the presentation is a spacer made from a foamed thermoplastic poly- meric material with a low solids content, in the example a solids content of less than 5 % by volume. For this spacer a relative dielectric constant of 1.1 was measured. The material had a porous structure having closed gas-filled essentially spherical voids with a mean diameter of about 50 m and a density of 0.05 g/crri 3, corresponding to a gas volume exceeding 95 % of the total volume of the material.

OBJECT OF THE INVENTION The primary object of the present invention therefore is to provide a compact high-voltage electric device that possesses a high capability to withstand partial discharge and dielectric breakdown resulting from it. The high-voltage electric device should be suited for use in high-voltage switchgear applications or in other applications where at least one conductor operates at high voltage and a substantial part of the electrical conductor insulation is pro- vided by a gaseous insulting medium.

A further object of the present invention is to provide a compact high-voltage electric device comprising a support member for a high-voltage conductor and having a low relative di- electric constant and a sufficient mechanical strength.

It is also an object of the present invention to provide a suitable support member for a com- pact, gas or air insulated high-voltage electric device comprising a conductor that is at least partly covered by an electrically insulating coating. It is also an object of the present inven- tion to provide a method of manufacturing such a support member.

SUMMARY OF THE INVENTION In accordance with the invention there is provided a gas or air insulated electric device com- prising at least one conductor, an electrically insulating support member surrounded by a gaseous insulating medium and keeping the conductor spaced apart from other electrically conducting bodies of the device, such as structural or enclosure members, the support member comprising a porous insulating material, characterized in that the conductor is at least partly covered by an electrically insulating coating, the porous insulating material is a polymeric material having a solids content of 30 % by volume or less, and the relative di- electric constant of the support member is 1.5 or less.

Preferably, the support member contacts the conductor where the conductor, which may be a single solid body or comprise a plurality of strands, is covered by the electrically insulating coating.

In one embodiment especially suitable for a gas insulated electric device comprising a sealed enclosure filled with a gas having a high dielectric strength, such as SF6 or the like, the pores of the porous polymeric insulating material are open. However, in the case of other gases with a lower dielectric strength, and especially in the case of an open air insulated device, support members with closed pores are preferably used. Preferably the pores, whether open or closed, have a mean equivalent diameter of 250 Fm or less.

The electrically insulating coating on the conductor should have sufficient dielectric strength to provide in combination with the gaseous insulating medium an insulation that is adequate for the given spacing and voltage. For any given insulating coating material, the dielectric strength is determined by the thickness of the coating. It is preferred, therefore, for the coating to have a thickness sufficient for it to withstand electrical puncture at a voltage up to 30 %, and more preferred, 50 %, of the breakdown voltage of the insulation system between the conductor and a neighbouring conducting body. In a preferred embodiment, this is achieved with a polymeric coating material having a low dielectric constant, such as silicone rubber, if the thickness of the electrically insulating coating on the conductor is at least 5 % of the radius of the conductor in the case of a conductor of circular cross-section. Generally, the thickness preferably is at least lmm, preferably 3 mm, in the case of a conventional con- ductor or conducting bar in a high-voltage installation.

Other preferred embodiments are characterized by the features of additional independent claims and will be described by way of examples.

Preferably, the relative dielectric constant of the support member is close to the relative di- electric constant of the gaseous insulating medium, i. e. close to 1. To achieve a relative di- electric constant in the range of 1.05-1.15, the porous insulating material of the support member, at least wherever it is contacted by the gaseous insulating medium, has a mean equivalent pore diameter of 100 llm or less, preferably about 50 m, i. e. ranging from 25 to 75 Fm, and a solids content of 20 % by volume or less, preferably about 5 % by volume, i. e. ranging from 3 to 10 % by volume."Mean equivalent pore diameter"is defined as the dia- meter of a sphere the volume of which is equal to the mean pore volume.

The support member may have to carry a substantial load, e. g. because it has to support the weight of the conductor and absorb bending and traction forces and also attraction or re- pulsion forces applied to it as a result of a high-amperage current flowing in it or a neigh- bouring conductor. To ensure the required mechanical strength of the support member, especially when the solids content is low, embodiments of the present invention comprise an internal mechanical reinforcement which is embedded in and essentially completely covered by the porous insulating material of the support member.

The mechanical reinforcement can take different forms: -one or more solid bodies arranged as a supporting, load carrying structure embedded in and completely or essentially completely covered by the porous polymeric insulating material of the support member; -an open reinforcing, supporting and load carrying net-like structure of bars or wires em- bedded in the porous polymeric insulating material of the support member; -an open rigid foam structure the open porosity of which is at least partly impregnated by the porous polymeric insulating material and completely or essentially completely covered by that material; -reinforcing fibers dispersed in a matrix of the porous polymeric insulating material, the re- sulting fiber-reinforced porous body being embedded in and completely or essentially com- pletely covered by a layer of the porous polymeric insulating material essentially free from reinforcing fibers.

Although any body, structure or fiber used for the reinforcement should be completely or essentially completely covered by a layer of the porous polymeric insulating material, only electrically insulating materials are suitable for use and another criterion for choosing the reinforcement will be the required mechanical properties of the support member. Of course, any interactions between the reinforcement and electric or electromagnetic fields created at the conductor should be taken into account when designing the reinforced support member.

Suitable materials for a solid reinforcing body in the interior of the support member are: electrically insulating polymeric materials, such as a polyolefin, a polyamide, a phenolic resin or the like, and electrically insulating ceramics, such as porcelain, an oxide, a silicate, glass or the like. The open net-like structures and the reinforcing fibers can comprise electrically in- sulating materials such as fibers, wires or nets made from glass or polymeric materials, e. g. polyamide based materials, polyolefins or the like.

In one embodiment of the invention, the support member comprises, at least at the surfaces thereof exposed to the gaseous insulation, a porous polymeric foam structure produced from gas filled polymeric spheres with a diameter ranging from a few pm to a few hundred pm, so called microspheres. In one preferred embodiment, the polymeric foam was produced from a mixture of expanded microspheres with a diameter of about 50 llm and unexpanded micro- spheres with a diameter of about 15 m and no extra binder or resin additions. The mixture of expanded and unexpanded microspheres was expanded under such conditions that an essen- tially rigid porous body with a foamed structure of microspheres bonded to each other was obtained. The resulting body had a mean equivalent pore diameter of about 50 m, a density of about 0.05 g/cm3, a gas content of about 95 percent by volume and a relative dielectric constant of about 1.1. Suitable microspheres have been found to be butane filled poly- vinylidene chloride spheres, but of course other microspheres made from an electrically insulating thermoplastic resin and filled with a suitable gas can also be used. Examples of suitable microspheres include those sold under the trade mark EXPANCEL by Akzo Nobel.

The electrically insulating coating covering at least a part of the conductor may suitably be combined with means for preventing propagation of creeping discharges, so-called streamers, along the covered conductor and especially to prevent such discharges from passing over to the support member. Preferably, the conductor coating is combined with a barrier extending the creep distance, e. g. a barrier in the form of one or more discoid protrusions and provided on opposite sides of the area of contact between the coating and the support member. In one preferred embodiment the barrier is associated with an electric field smoothing termination whereby the propagation of any creeping discharge from the covered conductor to the support member is avoided by a combination of a barrier effect and electric field smoothing.

In a further preferred embodiment the electrically insulating coating on the conductor com- prises, in addition to an electrically insulating base layer, at least one inner semi-conducting layer arranged as a shield to eliminate essentially all electric field concentrations at or adja- cent to surface defects or other irregularities on the conductor surface. The semi-conducting shield is formed around the conductor, preferably in the form of a layer within the coating.

The invention will be described in more detail with reference to preferred embodiments shown in the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 a simple schematic cross-sectional illustration of an electric device according to the invention comprising a coated conductor and support member; Figure 2 is a simple schematic cross-sectional illustration of an electric device according to the invention with a reinforced support member; Figure 3 is a simple schematic cross-sectional illustration of an electric device according to the invention comprising a support member with a modified internal support structure; Figure 4 is a schematic longitudinal sectional view of an electric device according to the invention comprising a coated conductor and two support members; Figure 5 is a simple schematic cross-sectional illustration of an electric device according to the invention comprising a coated conductor, three support members and a casing; Figure 6 is a simple schematic cross-sectional illustration of an electric device according to the invention comprising a three-phase assembly of conductors and support members.

Figure 7 is a longitudinal sectional view of an example of a covered conductor which can be included in a device according to the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS, EXAMPLES.

A compact high-voltage electric device according to the invention as shown in Figure 1 com- prises a conductor 1 having a coating, e. g. of silicone rubber, forming an electrical insulation 2, and an electrically insulating support member 3 for holding the conductor 1 and keeping it spaced apart from any other conducting body, here represented by component 5 made of an electrically conductive material and included in the electric device, such as a casing or a por- tion of a switchgear enclosure or a structural switchgear element. The electrically insulating coating 2 at least partly covers the conductor, and the support member 3 comprises an electrical insulation formed of a porous polymeric material. A gaseous insulating medium surrounds and contacts the conductor 1 with its coating 2, and the support member 3.

It is known that the solid/air interfaces in an insulation system like the present, and especially the so-called triple point, where each of the three components of the insulation system con- sisting of the insulating coating 2 of the coated conductor 1, the insulating support member 3 and the insulating gas, contacts the other two components, constitute weak parts of the system. The breakdown of the insulating gas is fairly well understood but the understanding of the surface discharges along the insulator surface is incomplete. However, it is known that the relative dielectric constant of the insulator is a factor in the field strength near the surface and can contribute to a low partial discharge inception voltage and a low flashover voltage.

See e. g. T. S Sudarshan, R. A. Dougal,"Mechanism of Surface Flashover Along Solid Di- electrics in Compressed Gases", IEEE Trans. El. Vol. 21 (1986), pp 727-746 and J. R. Laghari, "Spacer Flashover in Compressed Gases", IEEE Trans. El. Vol. 20 (1985), pp 83-92.

According to the present invention the relative dielectric constant of the support member 3 is close to that of air, i. e. close to 1, and the support member comprises a porous structure with a solids content of 30 % by volume or less. Thus the support member 3 has an internal structure and a surface structure such that its relative dielectric constant is 1.5 or less. Preferably, the support member 3 contacts the conductor 1 where the conductor 1, which may be a single solid body or comprise a plurality of wire strands or other conductor component parts, is covered by the electrically insulating coating 2.

According to one preferred embodiment the support member 3 comprises, at least at any sur- face thereof exposed to the gaseous insulation, a porous polymeric foam structure. The poly- meric foam is produced from gas filled polymeric spheres of a diameter in the range from a few um to a few hundred m, so-called microspheres. In one example the polymeric foam was produced from a mixture of expanded microspheres with a diameter of about 50 llm and unexpanded microspheres of about 15 pm diameter without any extra binder or resin addi- tions. The mixture of expanded and unexpanded microspheres was expanded under such con- ditions that a stiff porous body with a foamed structure of microspheres bonded to each other was obtained. The resulting body had a mean equivalent pore diameter of about 50 m, a density of about 0.05 g/cm3, a gas content of about 95 percent by volume and a relative di- electric constant of about 1.1. The microspheres used were butane filled polyvinylidene chloride spheres (EXPANCELO).

In the embodiment shown in Figure 2 the electric device additionally comprises an internal reinforcement 14 embedded in the support member 13. This reinforcement 14 has been added to improve the mechanical properties of the support member 13 and to provide the required mechanical strength of the support member 13. It is enclosed in the support member 13 such that essentially no part of it is contacted by the gaseous electrically insulating medium, i. e. the mechanical reinforcement 14 is embedded in and completely or substantially completely covered by the porous polymeric insulating material. The support member 13 has to possess adequate mechanical strength to withstand the pressure, bending and traction forces to which it is subjected as a consequence of the load imposed on it by the weight of the conductor and attraction or repulsion forces caused by a high-amperage current flowing through the con- ductor 11 or an adjacent conductor.

The mechanical reinforcement 14 can be a single body as shown in Figure 2 by a single body arranged as a supporting, load carrying structure disposed within the support member and embedded in and completely or substantially completely covered by the porous polymeric insulation. Suitable materials for a solid body 14 disposed within the support member 13 include electrically insulating polymeric materials, such as a polyolefin, a polyamide, a phenolic resin or the like, and an electrically insulating ceramic, such as porcelain, an oxide, a silicate, glass or the like.

Alternatively, the reinforcement 14 may comprise a plurality of solid bodies, or it may be formed by a net-like open reinforcing, supporting and load carrying structure of bars or wires disposed within the support member and embedded in and completely or substantially com- pletely covered by the porous polymeric insulating material, or by an open rigid foam structure the open pores of which are at least partly impregnated with the porous polymeric insulating material and embedded in and completely or almost completely covered by the porous polymeric insulating material. Reinforcing fibers dispersed in a matrix formed by the porous polymeric insulating material can also be used. Preferably, the fiber-reinforced rein- forcement body 14 is surrounded by and completely or substantially completely covered by a layer of the porous polymeric insulating material substantially free from reinforcing fibers.

Although any reinforcement 14 used, regardless of its structure and composition, will be completely or substantially completely covered by a layer of the porous polymeric insulating material, only electrically insulating materials are suitable for use, and another factor in the choice of the reinforcement relates to the mechanical properties of the support member. Of course, any interaction between the reinforcement and the electric or electromagnetic field created at the conductor 11 should be considered when designing the reinforced support member 14. The open net-like structures and the reinforcing fibers may comprise electrically insulating materials such as fibers, wires or nets made from glass or polymeric materials, e. g. polyamide based materials, polyolefins or the like.

As shown in Figure 2, the reinforcement 14 may be fully enclosed in the support member 13, or as shown in Figure 3, the reinforcement 34 may contact the coated conductor 31 and/or a structural or enclosure member 35 with its end surfaces but may otherwise be covered by the porous polymeric insulation 33 on all surfaces exposed to the insulating gas.

In the embodiment shown in Figure 3, the electrically insulating coating 22 covering at least part of the conductor 21 is combined with means 26 for preventing initiation and propagation of creeping discharges along the coated or covered conductor 21 and especially to prevent any creeping discharge (streamer) to pass over to the support member 23. More particularly, on each side of the place where the support member 23 contacts the coating 22, the coating is combined with a barrier 26a extending the creeping distance, e. g. a barrier in the form of one or more discoid projections around the conductor, and with an electric field smoothing ter- mination (cable termination) 26b such that the propagation of any creeping discharge from the coated conductor 21 to the support member 23 is avoided by a combination of a barrier effect and electric field smoothing.

Figure 5 shows an electric device including a tubular casing or casing portion 5 in which a coated conductor 41 is kept in position by an assembly of three angularly spaced-apart support members 43a, 43b, 43c such that the insulation system comprises the insulating coating 42 on the conductor 41, the gaseous insulation around the covered conductor 41, and the porous support members 43a, 43b, 43c which may be made of a material of the kind de- scribed above. Although no support member reinforcement is shown in the figure, the support members 43a, 43b, 43c shown in Figure 5 may comprise a reinforcement as described above.

Similarly, means for preventing start and propagation of creeping discharges along the sur- face of the insulating coating 42 similar to that shown in figure 4 may also be provided.

Figure 6 shows an embodiment of the electric device according to the invention comprising three conductors 51a, 51b, 51c, each covered by an electrically insulating coating 52a, 52b, 52c and forming a three-phase assembly in which the conductors 51a, 51b, 51c are separated by support members 53a, 53b, 53c. Each such support member comprises a porous polymeric insulating material and a reinforcement 54a, 54b, 54c embedded therein. The reinforcement 54a, 54b, 54c can of course be left out, should the porous polymeric insulation provide suffi- cient mechanical properties by itself. Although not shown in this figure, means for preventing start and propagation of creeping discharges along the surface of the insulation as described above and shown in Figure 4, may advantageously be provided in this embodiment also.

Figure 7 is an exploded view of a covered conductor in the form of a T-joint which may form part of a high-voltage device according to the invention. More particularly, this figure shows a T-joint 50 for a busbar in a substation of a high-voltage energy distribution system.

T-joint 50 is made of copper or aluminium and comprises a center piece 51 and three tubular connectors 52, each receptive of busbar end sections 53, only one of which is shown. The entire exterior surface of the T-joint 50, i. e. the center piece 51 and the tubular connectors 52 as well as the interior surfaces of the connectors are covered by a coating 54 of an insulating polymer material as describe above. A similar coating 54 is applied to the circumferential exterior surfaces of the busbar end sections 53.

When the busbar is assembled, the busbar end sections 53 are slid into the associated tubular connectors 51 as indicated by an arrow in respect of the left busbar end section 53 and se- cured by means of screws 55, which can be manipulated through an opening 56 in the centre piece 51.

The illustrated T-joint 50 and the busbar end sections 53 can form a conductor supported by and in contact with a support member or spacer according to the invention so as to be kept spaced apart from, for example, a grounded portion of a metal enclosure (not shown).