ELECTRIC HIGH VOLTAGE DEVICE WITH A FIBER COMPOSITE CASING AND A METHOD TO MANUFACTURE SUCH DEVICE

Abstract: An electric high voltage device with a fiber composite casing (15) and a method for manufacture of such an object comprising a mechanically load carrying structure of associated and connected fibers impregnated with and essentially coated by the resin composition which at least in the surface areas of the object comprises a particulate with a resistivity dependent of the electrical field it is subjected to.

Description: Electric high voltage device with a fiber composite casing and a method to manufacture such device. TECHNICAL FIELD The present invention relates to an electric high voltage device with a fiber composite casing and in particular a device with a tubular fiber composite casing. More specific, the present invention relates to an electric high voltage device with a fiber composite casing for use in an electrical installation where the casing is subjected to an electrical field. In particular the present invention relates to an electric high voltage device with a fiber composite insulant or an electrically insulating casing in an insulated electrical high voltage installation with an insulation system comprising liquid or gaseous electrically insulating media. The casing can also be used in a device comprising a solid or a gelled electrically insulating media. The present invention also relates to a method for manufacturing such an electric high voltage device with a fiber composite casing. BACKGROUND ART A conductor in an electric device is electrically insulated from other conductors or any grounded casing or structural element made from an electrically conductive material and which are at a different electric potential. The electrical insulation is provided by an electrically insulating media and can comprise solid, liquid and/or gaseous media. Electric devices comprising insulants or casings of the hollow core type such as breakers, bushings, instrument transformers, etc and support insulations often have an insulation system comprising liquid or gaseous media, in some cases also solid and gelled media. Devices with an open structure such as high voltage switchgear using air as insulating media are shielded by a casing and gas insulated devices using a gas, e.g. SF 6 -gas or air are enclosed in a casing. To ensure the mechanical integrity of a resin based casing they often comprises a mechanically reinforcing self-supporting coherent struαure. This supporting, load carrying structure, which can be achieved in various ways, are typically impregnated with and covered by an electrically insulating resin composition. The structure typically exhibit an open reinforcing, sup- porting and load carrying net like structure of fibers, bars or wires surrounded and essentially covered by the electrically insulating resin composition. The structure can be an ordered net but is often a weave of the non- woven type, such a fiber mat that has been formed to the desired shape. A typical shape is tubular. The casing can also comprise reinforcing fibers dispersed in a matrix of the electrically insulating resin composition. Although any body, structure or fiber used will be essentially covered by a layer of the electrically insulating resin composition only electrically insulating materials are suitable for use and an other criteria for choosing reinforcement will be the mechanical properties required of the casing. Of course should also any interactions between the reinforcement and any electric or electromagnetic field created at the conductor be considered when designing the casing. Suitable materials for the casing are electrically insulating polymeric materials, such as a polyolefin, a polyamide, a phenolic resin or the like. The reinforcement can comprise electrically insulating fiber materials or yarns, wires or nets made from as glass, polymeric materials, e.g. polyamide based materials, polyolefin or the like. However, in a casing essentially made from electrically insulating materials, space charge accumulation has been noted. Especially when the dielectric is subjected to a DC-field. Such space charge accumulations distorts the electrical stress distribution and persist for long periods because of the high resistivity of the polymers. Space charges in an insulation body do when subjected to the forces of an electric DC-field accumulate in a way that a polarized pattern similar to a capacitor is formed. There are two basic types of space charge accumulation patterns, differing in the polarity of the space charge accumulation in relation to the polarity. The space charge accumulation results in a local increase at certain points of the actual electric field in relation to the field, which would be contemplated when considering the geometrical dimensions and dielectric characteristics of an insulation. The increase noted in the actual field might be 5 or even 10 times the contemplated field. Thus the design field for an insulation must include a safety factor taking account for this considerably higher field resulting in the use of thicker and/or more expensive materials in the insulation. The build up of the space charge accumulation is a slow process, therefore this problem is accentuated when the polarity of the field acting on the insulation after being operated for a long period of time at the same polarity is reversed. As a result of the reversal a capacity field is superimposed on the field resulting from the space charge accumulation and the point of maximal field stress is moved from the interface and into the insulation. When discussing the electrical strength of an insulation system or individual components making up the insulator some expressions are used that needs to be defined; - "breakdown voltage" means in this application the voltage across the insulation at which the insulation loses its electrical insulation function, -"partial discharge inception voltage" means in this application the voltage across the insulation for which a partial breakdown occurs or more accurately starts in a part of the insulation system, the part being one whole component or a part of a component in the insulation system. As understood from the foregoing, great care should thus be taken to ensure that the conductor, any spacers and the casing are carefully designed such that all parts of the electrical insulation system exhibits the required electrical properties. It is known to increase the capability to withstand partial discharge and thus reduce the risks for electric breakdown, i.e. increase the breakdown strength for solid electrically insulating materials which are subjected to electrical fields, by applying a coating comprising a sufficient amount of a particulate filler with a non-linear resistivity, and in particular a field dependent resistivity. Such a coating is described in US-A-4 666 142. However it has been experienced in field tests that such a coating is not capable to withstand the combined thermal and electrical conditions prevailing on the inside of a casing of a gas or air insulated high voltage direct current device. It has also been suggested, in EP-A-0 440 865, to incorporate such a particulate filler with a non-linear, field dependent resistivity in the resin used for the bulk of a spacer in a gas or air insulated high voltage electric device. SUMMARY OF THE INVENTION It is the object/aim of the present invention to provide a gas or air insulated electric device comprising a mechanically load carrying and electrically insulating casing with a decreased tendency for space charge accumulation when subjected to an electric field, thereby providing an increased capability to withstand partial discharge and a reduced risk for electric breakdown. Further it is an object of the present invention to provide a method for manufacturing such a fiber composite object. To achieve this the invention suggest a gas or air insulated high voltage direct current device comprising a conductor and a casing spaced apart from each other according to the preamble of claim 1, which is characterized by the features of the characterizing part of claim 1. Further developments of the invention are characterized by the features of the additional claims 2 to 10. The invention also suggest a method for manufacture of such an electric device with a fibber composite casing comprising a self supporting mechanically load carrying structure of associated and connected fibers that is impregnated with and essentially coated by a resin composition according to claim 11 which is characterized by the measures of the characterizing part of claim 10. Further developments of the invention are characterized by the features of the additional claims 12 and 14. For an electric high voltage device according to the present invention the impregnating resin composition comprises at least in the internal surface areas of the casing a particulate filler with a resistivity dependent of the electrical field its subjected to. Preferably the particulate filler exhibits a non-linear resistivity dependent of the electrical field its subjected to. According to one embodiment the resin based composition comprises a particulate filler with an intrinsic resistivity at from 10 4 to 10 8 ohm m at 25 °C and low electrical field strength. Suitable fillers have shown to be particulate chromium oxide, Cr 2 O 3 , iron oxide, Fe 2 O 3 , silicon carbide and doped zinc oxide or an metal phtalocyanine or a mixture of at least two of these substances. As example of usable metal phtalocyanines may be mentioned phtalocyanines of copper, iron, nickel, cobalt, magnesium, aluminum, manganese, tin, chromium and zinc, separately or as a mixture. Particularly preferred among others for economical reasons is copper phtalocyanine and then in particular α-copper phtalocyanine. Preferably the particulate filler is added at a level of 10 % by volume or more. The filler containing fiber composite casing is according to one embodiment intended for use as a part in the electrical insulation system for a high voltage electrical alternating current, AC device. According to an alternative embodiment, the fiber composite casing is intended for use as a part in the electrical insulation system for a high voltage electrical direct current, DC device. According to one preferred embodiment the fiber composite casing has a tubular form. The tubular fiber composite casing comprises a wound or pultruded fiber structure impregnated with a resin based composition and that at least the resin contained in the wound or pultruded layer adjacent to the internal face of the tube comprises the particulate filler with a non-linear resistivity. Alternatively the wound or pultruded fiber structure is impregnated in full with a resin based composition comprising the particulate filler with a non-linear resistivity. Thus a tubular fiber composite casing with a field dependent resistivity in the surface and with a surface finish as good as for conventional fiber composite tube is provided. Such a tubular fiber composite casing is in particular suited for use as an insulant for a gas or air insulated electric high voltage device, such as a bushing, a reactor or switch-gear a support insulant. An electrical device comprising such fiber composite casing according to the present invention with a particulate filler with a field dependent resistivity exhibit a substantially reduced tendency for space charge accumulation and thus an improved electrical strength. The present invention also provides a method for manufacture of an electric device with a composite casing comprising a self supporting, load carrying structure of associated and connected fibers that are impregnated with and essentially coated by a resin composition, wherein a particulate filler with a field dependent resistivity according to the present invention is added to at least the resin composition that is used for impregnation of outmost areas of the tubular casing, such that at least the inner surface of the tube comprises said particulate filler with field dependent resistivity. According to one embodiment also the outer surface of the tube also comprises said particulate filler with field dependent resistivity. According to one embodiment of the invented method the fiber composite structure is impregnated in full with a resin comprising a particulate filler with a field dependent resistivity. According to an embodiment of the invented method which is especially suited for production of an electric device with a tubular fiber composite casing, -a tubular fiber structure is wound or pultruded from a tape, yarn or sheet comprising a coherent fiber web, or non- woven structure, - the tape, yarn or sheet , prior to or in association with the winding or pultrud- ing is impregnated with a resin based composition, and - wherein a particulate filler with a resistivity dependent of the electrical field it is subjected to is added at least to the resin composition which is impregnated into the innermost layer of the wound or pultruded fiber composite tube, such that at least the inner surface of the tubular casing comprises said particulate filler with non-linear resistivity. According to one other embodiment a particulate filler with a resistivity dependent of the electrical field it is subjected to is added at least to the resin composition which is impregnated into the innermost layer and outermost of the wound or pultruded fiber composite tube, such that at least the inner and outer surface of the tubular casing comprises said particulate filler with non-linear resistivity. Alternatively can the tape, yarn or sheet be coated with the resin comprising the particulate filler with a non-linear field dependent resistivity be used for impregnating the fiber structure. By the adoption of any of these alternative embodiments of the process according to the present invention can a tubular fiber composite casing with a field dependent resistivity at least in a zone at the inner or internal surface and with a surface finish as good as for conventional fiber composite tube be provided. Such a tubular fiber composite casing is in particular suited for use as an insulant for a gas or air insulated electric high voltage device. And in particular for use as a hollow core type insulant for a breaker, an instrument transformer, a bushing or the like or to be used as a support insulant. A fiber composite casing comprising a particulate filler with a field dependent resistivity exhibits a substantially reduced tendency for space charge accumulation and thus an improved electrical strength. BRIEF DESCRIPTION OF THE DRAWINGS A preferred embodiment of the present invention shall be described more in detail while referring to the drawings, where; Figure 1 shows in a side view a simple schematic sketch of an electric device with a casing of the hollow core type comprising a tubular fiber composite casing according to one embodiment of the present invention. DESCRIPTION OF PREFERRED EMBODIMENTS, EXAMPLES. A compact high voltage electric device with an electrically insulated casing in the form of a resin impregnated tubular fiber composite casing according to one embodiment of the present invention and as shown in figure 1 comprises a conductor 11 spaced apart and electrically insulated from a casing 15. The casing 15 is made in an electrically insulating fiber composite body and comprises a structural hollow core from a fiber composite tube. The tube 15 is typically surrounded by a flanged exterior structure made in a polymeric material 16 to provide weather proofing and extend the creeping distances to reduce or eliminate leakage currents or flash overs along the exterior surface. This type of hollow core insulants are typically used for breakers or other switch gear, instrument transformers, bushings and as support insulant. A liquid, gelled or gaseous insulating media surrounds the conductor 11. The breakdown of the insulation is fairly well understood and the understanding of the electrical stresses in the electrically insulating casing 15 is believed to depend on the space charge profile developed in the insulation when subjected to an electric field. According to the present invention the tubular fiber composite casing 15 at least in a layer at the inner surface 150 is impregnated with a resin composition that comprises a particulate filler with a resistivity dependent of the electrical field it is subjected to . The mechanically self supporting and load carrying fiber structure is impregnated and coated with resin such that essentially no part of the fiber structure is in contact with the electrically insulating media, i.e. the fiber structure is surrounded and essentially covered by the resin. The casing 15 needs a sufficient mechanical integrity to withstand all pressure, bending and traction forces for which it is subjected, it carries all load imposed on it by the weight of the conductor and any attraction or repulsion forces caused by a high current flowing through the conductor or an adjacent conductor. Suitable materials the fiber structure is fibers, yarns, wires or nets made from as glass, polymeric materials, e.g. polyamide based materials, polyolefin or the like. Suitable particulate fillers with a non-linear field dependent resistivity typically comprises chromium oxide, Cr 2 O 3 , iron oxide, Fe 2 O 3 , is silicon carbide and doped zinc oxide or an metal phtalocyanine or a mixture of at least two of these substances. Preferably the particulate filler is added at a level of 10 % by volume or more. The tubular fiber composite casing in the embodiment shown in figure 1 comprises a wound or pultruded fiber structure impregnated and essentially coated with a resin based composition. At least the resin contained in the wound or pultruded layer adjacent to the internal face of the tube comprises the particulate filler with resistivity dependent of the electrical field it is subjected to. Alternatively the wound or pultruded fiber structure is impregnated in full with a resin based composition comprising the particulate filler with a field dependent resistivity.

Claims: CLAIMS 1. An electric high voltage device with a fiber composite casing (15) and at least one conductor (11), the fiber composite casing comprising fibers associated and connected to a mechanically load carrying structure and a resin based composition, and the fiber structure being impregnated with and essentially coated by the resin composition, characterized in that the resin based composition at least in the internal surface areas of the casing comprises a particulate filler with a resistivity dependent of the electrical field its subjected to. 2. An electric device according to claim 1, characterized in that the particulate filler exhibit a non linear resistivity dependent of the electrical field its subjected to. 3. An electric device according to claim 1 or 2, characterized in that the resin based composition comprises a particulate filler with an intrinsic resistivity at from 10 4 to 10 8 ohm m at 25 °C. 4. An electric device according to claim 1, 2 or 3, characterized in that the resin based composition comprises a particulate filler comprising chromium oxide, Cr 2 O 3 , iron oxide, Fe 2 O 3 , metal phtalocyanine, silicon carbide, doped zinc oxide or a mixture of at least two of these substances. 5. An electric device according to claim 4, characterized in that the resin based composition comprises a particulate filler at a level of 10 % by volume or more. 6. An electric device according to any of the preceding claims, characterized in being a gas or air insulated high voltage electric alternating current device. 7. An electric device according to any of claims 1 to 5, characterized in being a gas or air insulated high voltage electric direct current device 8. An electric device according to any of the preceding claims, characterized in that the casing exhibits a tubular form. 9. An electric device according to any of the preceding claims, characterized in that the casing comprises a wound or pultruded fiber structure which is impregnated with a resin based composition and that at least the resin contained in the wound or pultruded layer adjacent to the internal face of the tube comprises the particulate filler, with a resistivity dependent of the electrical field it is subjected to. 10. An electric device according to any of the claims 1 to 7, characterized in that the casing comprises a wound or pultruded fiber structure which is in full impregnated with a resin based composition comprising the particulate filler, with a resistivity dependent of the electrical field it is subjected to. 11. A method for manufacture of an electric device with a fiber composite casing (15) according to any of the preceding claims, wherein the casing comprises a mechanically load carrying structure of associated and connected fibers that is impregnated with and essentially coated by a resin composition, characterized in that a particulate filler, with a resistivity dependent of the electrical field it is subjected to, is added at least to the resin composition that is used for impregnation of outmost areas of the type, such that at least the inner and outer surface of the tube comprises said particulate filler, with a resistivity dependent of the electrical field it is subjected to,. 12. A method according to claim 11, characterized in that the fiber composite casing in full is impregnated with a resin comprising a particulate filler, with a resistivity dependent of the electrical field it is subjected to. 13. A method according to claim 11, characterized in that a tubular casing is wound or pultruded from a tape or sheet comprising a coherent fiber web, that the tape prior to or in association with the winding is impregnated with a resin based composition and that a particulate filler, with a resistivity dependent of the electrical field it is subjected to, is added at least to the resin composition which is impregnated into the outmost and innermost layer of the wound or pultruded fiber composite tube, such that at least the inner and outer surface of the tube comprises said particulate filler, with a resistivity dependent of the electrical field it is subjected to. 14. A method according to claim 11, characterized in that a tubular casing is wound or pultruded from a tape or sheet comprising a coherent fiber web, that the tape prior to or in association with the winding or pultruding is coated with a resin based composition and that a particulate filler, with a resistivity dependent of the electrical field it is subjected to, is added at least to the resin composition which upon winding is impregnated into the outmost and innermost layer of the wound or pultruded fiber composite tube, such that at least the inner and outer surface of the tube comprises said particulate filler, with a resistivity dependent of the electrical field it is subjected to.

Inventors: WINDMAR DAN, WINDMAR, DAN
Applicants: ASEA BROWN BOVERI, WINDMAR DAN, ASEA BROWN BOVERI AB, WINDMAR, DAN
Application year: 1998
Main classification: H01B3
IPC codes: H01B3, H01B17