Electrical bushing

An electrical bushing for providing electrical insulation of a conductor extending through the bushing is disclosed. The bushing includes: one conductive foil concentrically arranged around the conductor location; and one FGM part, made from a field grading material and partly arranged in the extension of part of a foil edge of a conductive foil. The FGM part and the conductive foil, in the extension of which the FGM part is arranged, are in electrical contact.

FIELD OF THE INVENTION

The present invention relates to the field of high voltage technology, and in particular to high voltage bushings for providing electrical insulation of a conductor.

BACKGROUND OF THE INVENTION

High voltage bushings are used for carrying current at high potential through a plane, often referred to as a grounded plane, where the plane is at a different potential than the current path. High voltage bushings are designed to electrically insulate a high voltage conductor, located inside the bushing, from the grounded plane. The grounded plane can for example be a transformer tank or a wall.

In order to obtain a smoothening of the electrical potential distribution between the conductor and the grounded plane, a bushing often comprises a number of floating, coaxial foils made of a conducting material and coaxially surrounding the high voltage conductor, the coaxial foils forming a so called condenser core. The foils could for example be made of aluminium, and are typically separated by a dielectric insulating material, such as for example oil impregnated or resin impregnated paper. The coaxial foils serve to smoothen the electric field distribution between the outside of the bushing and the inner high voltage conductor, thus reducing the local field enhancement. The coaxial foils help to form a more homogeneous electric field, and thereby reduce the risk for electric breakdown and subsequent thermal damage.

Such coaxial foils typically provide efficient capacitive grading of the electric field within the bushing. However, a local field enhancement in the vicinity of the foil edges typically remains. The enhanced field at the foil edges limits the operational voltage that can be applied between the high voltage conductor and the grounded plane.

Efforts to grade the electric field at the foil edges of a bushing condenser core are disclosed in U.S. Pat. No. 4,370,514. Here, double layer foils containing an electrically conducting layer and an insulating layer are coaxially arranged around a high voltage conductor, where the insulating layer has a high dielectric constant. At the foil edges, the double layer foils are folded so that the insulating layer encloses the electrically conducting layer in order to improve the ability of the bushing to withstand partial corona discharges and surge voltages. U.S. Pat. No. 4,370,514 also discusses the possibility of limiting the field stress around the foil edges by terminating the foils with a bead-like enlargement, in order to obtain a radius of curvature at the edge which is as large as possible.

The techniques for reducing the field stress at the foil edges discussed in U.S. Pat. No. 4,370,514 increase the radius of the condenser core, and therefore the radius of the bushing. As the electric power technology advances, higher voltages can be employed in various applications and bushings which may withstand higher potentials are therefore required. At the same time, the physical space available to a bushing is typically limited. Therefore, it is desired to find bushings that have an improved relationship between voltage-withstanding properties and bushing diameter.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a bushing having an improved relationship between voltage-withstanding properties and bushing diameter.

This object is achieved by an electrical bushing for providing electrical insulation of a conductor extending through the bushing. The bushing comprises at least one conductive foil concentrically arranged around the conductor location, and at least one field grading material (FGM) part, comprising (and typically made from) a field grading material and at least partly arranged in the extension of at least part of a foil edge of a conductive foil. The FGM part and the conductive foil, in the extension of which the FGM part is arranged, are in electrical contact.

The electrical field at the foil edge will thus be graded by the FGM part at local electric field strengths above the electric field threshold of the field grading material. Since an enhanced electric field strength at the foil edges is often limiting when attempting to decrease the dimensions of a bushing designed for a particular voltage, or when attempting to increase the nominal voltage for a particular bushing dimensioning, the field grading achieved by the FGM part at the foil edge allows for an improved relationship between voltage-withstanding properties and bushing diameter.

The field grading material can advantageously be a non-linear field grading material. When a non-linear field grading material is used, an FGM part will typically provide efficient field grading over a larger range of voltages.

The field grading material could for example be chosen such that an electrical field threshold of the field grading material, above which the field grading capability of the field grading material increases non-linearly with increasing electric field strength, lies above the local electric field strength expected at the foil edge at the nominal voltage of the bushing. Oftentimes, the field grading material will be chosen such that the electrical field threshold of the field grading material lies above the local electric field strength expected at the foil edge at twice the nominal voltage of the bushing. In some embodiments, a field grading material will be used that has an electric field threshold which lies below the local electric field strength expected at the foil edge at the nominal voltage of the bushing. By using an FGM part that provides field grading also at nominal voltage, aging effects around the foil edges may be mitigated.

In one embodiment, an extension distance over which an FGM part extends beyond at least part of the conductive foil edge substantially corresponds to the interfoil separation distance. Hereby can be achieved that the originally enhanced electric field strength at the foil edge can be reduced to a similar level to that found in the bulk of the condenser core.

The extension distance could for example be selected such that the electric field strength at the edge of the FGM part will be below the partial discharge inception threshold of the dielectric insulating material even for voltages above twice the nominal voltage of the bushing.

The bushing may comprise a plurality of concentrically arranged conductive foils, each conductive foil having two outer foil edges. In one embodiment, an FGM part is arranged in the extension of substantially every outer foil edge, for example in the extension of every outer foil edge at which the local field would otherwise be considerably enhanced. In some geometries, the local field enhancement at some foil edges, for example the edges of the innermost foil, may not experience as strong local field enhancement as the majority of the conductive foils. By equipping substantially every outer foil edge of the bushing with an FGM part, the risk of bushing failure due to a local enhancement of the electrical field at outer foil edges can be minimized for situations when the stress is evenly distributed among the foil edges, such as for example at nominal voltage or withstand voltage.

A conductive foil of an electric bushing may have inner edges, such as for example edges of an opening in the conductive foil through which conductive leads can be arranged, or edges between by two cylindrical and axially displaced conductive foil parts forming the conductive foil. In one embodiment, an FGM part is at least partly arranged in the extension of at least part of an inner foil edge. Efficient field grading can thus be achieved also around such inner foil edges.

In order to further improve the field grading properties of the FGM part, the outer edge of the FGM part can be of a field grading geometrical shape.

The FGM part could for example be made from a tape of field grading material having non-linear electric properties.

Alternatively, the FGM part could for example be formed by field grading material that has been applied to at least part of a dielectric insulator arranged to provide insulation between adjacent conductive foils.

Further aspects of the invention are set out in the following detailed description and in the accompanying claims.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1schematically illustrates a bushing100comprising a hollow, elongate insulator105through which a conductor110extends. At each end of the conductor110is provided an electrical terminal112for connecting the conductor110to electrical systems or devices. Bushing100ofFIG. 1furthermore comprises a condenser core115. InFIG. 1, the conductor110has been shown to form part of the bushing100. However, some bushings100do not include a conductor110, but include a pipe-shaped hole in the conductor location in which a conductor110may be inserted.

The condenser core115ofFIG. 1comprises a number of foils120which are separated by a dielectric insulator123. The dielectric insulator123is typically made of a solid insulating material, such as oil- or resin impregnated paper. The foils120are typically coaxially arranged, and could for example be made of aluminium or other conducting material. The foils120could be integrated with the dielectric material, or separate from the dielectric material. Integration of the foil with the dielectric material could for example be achieved by means of a vacuum metallisation process, or by applying conductive ink to the dielectric material. A condenser core115can for example be in the shape of a cylinder or of a cylinder having a conical end part as shown inFIG. 1. The foils are often of cylindrical shape. Oftentimes, the axial length of an outer foil120is smaller than the axial length of an inner foil120so as to maintain a similar area of the different foils120in a condenser core115.

The bushing ofFIG. 1further comprises a flange125to which the insulator105is attached. The flange125can be used for connecting the bushing100to a plane130through which the conductor110is to extend. The flange125is often electrically connected to the outermost conductive foil120, as indicated inFIG. 1by connection135. Plane130may be connected to ground, or can have a potential which differs from ground. However, for ease of description, the term grounded plane will be used when referring to the plane130.

When the bushing100is in use, the condenser core115acts as a voltage divider and distributes the field substantially evenly within the condenser core115.

While the conductive foils120efficiently serve to capacitatively grade the electric field within the bushing100, the electrical field in the vicinity of the conductive foil edges is locally enhanced due to boundary effects. Typically, the electric field enhancement at foil edges is stronger the thinner the foils120are (in the limit of extremely thin foils120, the electric field strength at the edges formally tend to infinity). Since high electric field strengths at the foil edges may cause failure in terms of for example partial discharge or flashover, field grading would be beneficial.

According to the present technology, field grading at a foil edge may be achieved by arranging a Field Grading Material (FGM) part (at least partly) in the extension of at least part of an edge of a conductive foil120so that the FGM part is in electrical contact with the conductive foil, the FGM part being made from a field grading material.

An FGM part may be designed so as to provide efficient field grading for a certain range of voltages across the bushing100in the radial direction. For example, the FGM part may be designed so as to provide efficient field grading at and/or above a voltage where the local enhancement of the electric field strength at an edge of a conductive foil would be dimensioning for the bushing100unless field grading measures were taken. A critical voltage condition, corresponding to a particular voltage across the bushing100above which the most efficient field grading is desired (such voltage here referred to as the critical voltage), could advantageously be selected. Depending on the design of the bushing100, the critical voltage could for example be the nominal voltage of the bushing; a withstand voltage of the bushing, i.e. a voltage higher than the nominal voltage which the bushing100should be capable of withstanding during a longer period of time (typically twice the nominal voltage); a voltage occurring at a lighting impulse (e.g. the Basic Insulation Level, BIL, also referred to as the basic impulse withstand voltage), or a high frequency or transient voltage (at a magnitude of for example 3-5 times the nominal voltage).

The field grading material can advantageously be a non-linear field grading material, the design thereby providing efficient field grading in a larger range of voltage situations. A suitable non-linear field grading material has electric properties that depend on the local electric field strength E to which the material is exposed, in a manner so that a high amount of field grading is achieved at high electric fields, while the impact on the field distribution is small or negligible at lower electric fields. The non-linear field grading property of the field grading material is a result of the material having a conductivity or permittivity that depends non-linearly on the electric field.

Non-linear field grading materials are typically associated with a (material dependent) electric field threshold Eb, above which the field grading properties of the material changes rapidly with increasing electric field, while for electric fields having a magnitude below the threshold Eb, the field grading effect obtained by the field grading material is considerably lower or negligible. Due to the changes of the electrical properties of the material with variations in electric field, an inhomogeneous electric field distribution wherein the electric field (at least) locally exceeds the electric field threshold Eb, will, in the presence of an FGM material, become more uniform than in the absence of FGM, since the electric stress in the region/spots where the electric field strength originally exceeded Ebwill be reduced. Depending on the composition of the field grading material, the electric field threshold Ebcan be more or less sharp.

Field grading materials can for example be polymer composites where an insulating polymer is filled with particles giving rise to non-linear electric properties. The non-linear electric properties can for example be achieved by an intrinsic non-linearity of the material of the filler particles, as a grain-boundary effect, or as a combination of the two. The filler particle size could for example lie within the range of 10-150 μm, or 10-100 nm, or any other suitable particle size could be used. All filling particles could be of the same material, or a mixture of particles of different composition could be used. A non-linear field grading material can have non-linear resistive properties (non-linear varistor properties), so that the conductivity increases non-linearly with increasing electric field strength, or non-linear capacitive properties, so that the dielectric constant increases non-linearly with increasing electric field strength.

Typical non-linear resistive field grading materials have a low and almost constant conductivity σ0below an electric field threshold Eb, while the conductivity increases rapidly with increasing electric field for electric fields higher than Eb. Below Eb, non-linear resistive field grading materials typically have electric properties closer to those of insulators, depending on the amount of filler in the field grading material. Above Eb, the current-voltage-relation can typically be described as I∝Vα+1, where α>0. Examples of materials which could be used in filling particles to achieve non-linear resistive properties of the field grading material are SiC, ZnO, TiO2, SnO2, BaTiO3, carbon black or semi-conducting polymer fillers. Non-linear capacitive field grading materials have a low and almost constant dielectric constant ∈rbelow an electric field threshold Eb, while the dielectric constant increases rapidly at electric fields of magnitude higher than Eb. An example of a material which could be used in filling particles to achieve non-linear capacitive properties of the field grading material is BaTiO3.

The insulating polymer of the field grading material can for example be an elastomer such as ethylene propyle diene monomer (EPDM) or silicon rubbers; a thermoplastic polymer such as polyethylene, polypropylene, polybutylene terephthalate (PBT), polyethylene terephthalate (PET), polycarbonate (PC), acrylonitrile butadiene styrene (ABS), polystyrene (PS) or nylon; a thermosetting polymer such as epoxy or polyurethane resin; an adhesive such as those formed based on ethylene-vinyl-acetate; a thermoplastic elastomer; a thixotropic paint or gel; or a combination of such materials, including co-polymers, for example a combination of polyisobutylene and amorphous polypropylene. In order to achieve other desired properties of the field grading material, for example in terms of mechanical properties, further components may be included, as described for example in EP1975949 and U.S. Pat. No. 4,252,692.

By arranging an FGM part in the extension of at least part of an edge of a conductive foil, local field grading at conductor foil edges is achieved when the magnitude of the local electric field, at the location of the FGM part, reaches above the electric field threshold Ebof the field grading material. The FGM part thus operates to grade a local electric field at the conductive foil edge when the voltage in the radial direction of the bushing takes a magnitude above a voltage threshold. The FGM part could for example be designed so that such voltage threshold corresponds to the critical voltage.

FIG. 2illustrates results from simulations of the electric field E in the vicinity of a conductive foil edge205at which an FGM part200in the form of an FGM tape has been arranged. The conductive foil edge205in the extension of which an FGM part200has been arranged is shown, as well as two adjacent conductive foil edges205A, which do not have any FGM part200(here referred to as conventional foil edges205A). The electric field E at a particular voltage has been illustrated by equipotential curves210in a conventional manner. For purposes of illustration, an (imaginary) plane215which is perpendicular to the foils120has been drawn at the foil edge205, to indicate where the conductive foil120having an FGM part200ends. Furthermore, the edge of the FGM part200has been indicated by reference numeral220. As can be seen in the figure, the electric field is highly homogeneous between the conductive foils120at a distance from the foil edges. However, locally at the conventional foil edges205A, the electric field is enhanced. At the foil edge205having an FGM part200, on the other hand, the equipotential curves are distributed along the length of the FGM part200, and in particular along the part of the FGM part200which extends beyond the foil edge205.

Different examples of an FGM part200arranged in the extension of a conductive foil edge at an end of the condenser core115are shown inFIGS. 3a-c. A conductive foil edge205at an end of the condenser core115will be referred to as an outer conductive foil edge205. High electrical stress typically occurs locally in the region around the outer conductive foil edges205, both during transient and in-service AC or DC voltage.

InFIGS. 3a-c, the contours of the FGM part200are indicated by unbroken lines, while the contours of the conductive foil120are indicated by dashed lines. The FGM parts200ofFIGS. 3a-cextend a distance dEalong an (imaginary) extension foil (not shown), where the imaginary extension foil extends from the foil edge205in a (continuous) set of extension directions, which are perpendicular to the foil edge205and parallel to a plane which is tangent to the conductive foil120. An example of an extension direction is indicated inFIG. 3a-cby an arrow310. The distance dEthat an FGM200extends from a foil edge205into the space on the outer side of the imaginary plane215in an extension direction310will be referred to as the extension distance dEin this direction.

In the example shown inFIG. 3a, the FGM part200is formed as a cylinder which is arranged in the extension of the outer conductive foil edge205in a manner so that the FGM part200partly covers the conductive foil120.

In the example ofFIG. 3b, the FGM part200is formed as a cylinder which is arranged in the extension of the outer conductive foil edge205in a manner so that part of the FGM part200is enclosed by the conductive foil120. In the example ofFIG. 3b, the conductive foil120covers part of the FGM part200.

In the examples shown inFIGS. 3aand3b, the FGM part200and the conductive foil120overlap by an overlap distance do.

In the example ofFIG. 3c, the FGM part200is formed as a cylinder which stretches along the entire length of the cylindrical conductive foil120, and which extends beyond the outer conductive foil edges205. Hence, in this example, the overlap distance docorresponds to the entire length of the conductive foil120. The FGM part200ofFIG. 3cis shown to be arranged to cover the conductive foil120. An FGM part200which stretches along the entire length of the cylindrical conductive foil120could alternatively be arranged on the inside of the conductive foil120.

The FGM parts200shown inFIGS. 3a-care examples only, and alternative embodiments of an FGM part200arranged in the extension of at least a part of a conductive foil edge may be used. For example, an FGM part200could be folded over the conductive foil edge205to cover the conductive foil edge205at both the inside and the outside. Furthermore, for illustrative purposes, the FGM parts ofFIGS. 3a-chave been shown as cylinders of smooth lateral surfaces and straight, perpendicular base edges. However, other shapes of the FGM parts200may be used. For example, an FGM part200arranged in the extension of at least a part of a conductive foil does not have to be confined to the imaginary extension foil, but could occupy the space beyond the foil edge205in other directions as well. An FGM part200which is arranged in the extension of at least part of a conductive foil edge205extends, at least partly, beyond an imaginary plane215which is tangential to at least part of the foil edge205and perpendicular to the foil120, into the space on the outer side of the imaginary plane215(i.e. the side which is not occupied by the foil120). In one embodiment, the part of the FGM part200which is arranged in the extension of at least part of a conductive foil edge205is arranged substantially along the imaginary extension foil.

FIGS. 3a-cshow different examples of FGM parts200arranged in the extension of an outer conductive foil edge205at one end of a condenser core115. Typically, an FGM part200would be arranged in the same manner at the outer conductive foil edge205at the other end of the condenser core115. In one embodiment, substantially every conductive foil120of a condenser core115is equipped with an FGM part200at every outer edge205, providing efficient smoothening of the electric field at the outer foil edges205. In this embodiment, it may be that every outer edge205is equipped with an FGM part200, or that that all but one (e.g. the innermost) conductive foil120, or all but a few, such as two or three conductive foils, are equipped with an FGM part200at the outer foil edges205. An embodiment wherein substantially every conductive foil120is provided with an FGM part200is suitable where the electric field stress is approximately the same at the edges205of the different conductive foils120. Oftentimes, the electric field varies throughout the bushing100. An even electric field stress can then for example be achieved by varying the interfoil separation distance such that at locations of high electric field, the distance between adjacent foils120is smaller than at locations of lower electric field.

Further embodiments, wherein the conductive foils120which have been equipped with an FGM part200have been selected in a different manner, may also be contemplated. For example, there may be situations where the electrical stress is unevenly distributed between the foil edges. This may for example be the case when the bushing is subjected to high frequency transients. When the FGM part(s)200of a bushing100are designed to reduce the stress in such situations, the application of FGM part(s)200could for example be limited to those foil edges where high stress would be expected in such situations. One example of such a situation is where the field grading material serves to reduce the field stress in case of a fast, transient impuls which effects the outermost foil the most. In this situation, it may be sufficient to provide an FGM part200at the edges of the outermost foil.

In some bushings100, one or more conductive foils120may have further edges than the outer edges205at the condenser core ends. This could for example be the case if an electrical tapping is connected at a conductive foil120for current and/or voltage sensing purposes. In order to connect to an inner conductive foil120(i.e. a conductive foil120which is surrounded by the outermost conductive foil120), a tapping lead has to go through an opening in the outermost conductive foils120(and possibly further conductive foils120, depending on which inner conductive foil120is to be connected to the tapping). Hence, such bushing100will have conductive foil edges inside the condenser core115, here referred to as inner conductive foil edges. Due to resonances, formed by an interaction between the bushing100and the system/device to which the electrical terminals112of the conductor110are connected, over voltages can be induced along such inner foil edges, thus making such inner foil edges a potentially vulnerable part of the bushing100.

An FGM part200could be applied to such inner foil edges in order to lower the electrical field stress and thereby mitigate the risk for partial discharge or breakdown. An example of two concentrically arranged conductive foils120aand120bare shown inFIG. 4, where the outer conductive foil120asurrounds the inner conductive foil120b. Measuring taps400aand400bare arranged on the conductive foils120aand120b, respectively. Outer conductive foil120aofFIG. 4has been opened in order to reach the inner conductive foil120bwith leads connecting the measuring tap400b, thus creating an inner edge405.

An FGM part200has been arranged in the extension of two different parts of the inner edge405(alternatively, the FGM part200ofFIG. 4can be seen as two FGM parts200, each arranged at a part of the extension of the inner edge405). The FGM part200ofFIG. 4extends from the conductive foil120along a direction which is perpendicular to the inner foil edge405and tangential to the conductive foil120, i.e. along an extension direction. InFIG. 4, outer conductive foil120ahas been divided into two parts, interconnected with a bridge410which ensures that the two parts will be at the same electrical potential. Other ways of opening an outer conductive foil120amay be employed.

Inner conductive foil edges405may appear in a condenser core115for other reasons than connecting measuring taps400. For example, in some bushings100, some or all of the conductive foils120(for example all but the outermost foil120) are divided into two parts, which are of the same diameter and displaced in relation to each other in the axial direction of the bushing100. Thus, such conductive foils120will have two outer edges205and two inner edges405. An example of a bushing having conductive foils arranged in this manner is disclosed in U.S. Pat. No. 3,659,033.

The FGM part200and the conductive foil120should be in electrical contact in order to achieve efficient field grading at the foil edge205/405. Electrical contact could for example be achieved by applying conductive glue between the FGM part200and the conductive foil120, or by tightly arranging the FGM part200and the conductive foil120etc. In embodiments where the conductive foil120is used to provide mechanical support to the FGM part200, the overlap distance doshould preferably be chosen such that sufficient mechanical support can be provided. In other cases, it might be sufficient for the FGM part200and the conductive foil120to touch, in order to provide for electrical contact between the two.

For a given bushing application, the design of the FGM part200involves the selection of a suitable field grading material and designing the dimensions of the FGM part200, including determining a suitable extension distance dE. Furthermore, a critical voltage, corresponding to a particular voltage across the bushing100above which the most efficient field grading is desired, could advantageously be selected. The field grading material could for example be chosen such that the electric field threshold Eblies below or at the local electric field strength expected at the foil edge205/405at the critical voltage. The threshold Ebcould for example be selected to approximately correspond to the local electric field strength expected within the bulk of the condenser core115at the critical voltage.

The critical voltage could for example be set so that the FGM part200would protect against transient voltages which would occur across the bushing100in case of failure, the FGM part200thus reducing the impact of any such transient voltages. A suitable critical voltage could then for example be set within a range of 2-4 times the nominal voltage of the bushing100(the nominal voltage being the maximum operating voltage for which the bushing is designed). The critical voltage could alternatively be set to, for example, the nominal voltage of the bushing100, thus reducing the risk for partial discharge during normal operation of the bushing. Alternatively, the critical voltage could be set to a withstand voltage, for example at approximately twice the nominal bushing, or the BIL voltage. Other ways of defining the critical voltage condition may alternatively be used when suitably dimensioning the FGM part200.

For a given field grading material, the extension distance dEcould be chosen to be sufficiently long for the potential drop from the foil edge205to the edge220of the FGM part200to be distributed over a sufficient distance when the bushing100is exposed to the critical voltage. The extension distance dEcould for example be selected such that the stress in the vicinity of the FGM part200will be kept below the partial discharge inception threshold of the dielectric insulating material in the voltage range for which field grading by the FGM part200is desired.

In one embodiment, the extension distance dEapproximately corresponds to the radial distance between two adjacent conductive foils120, also referred to as the interfoil separation distance, dI. A suitable field grading material having suitable non-linear electric properties could in this embodiment for example be selected such that at the critical voltage, the electrical potential difference between the foil edge205/405and the edge220of the FGM part200will be of the same order of magnitude as the voltage between the conductive foil120and the adjacent conductive foils120.

FIG. 5ais a graph showing results from simulations of the magnitude of the electric field E in the extension direction310of a bushing100. Simulated values of this magnitude at the underside of a conductive foil120, and, in its extension, at the underside of the corresponding FGM part200, are plotted as a function of distance x in the extension direction310for five different values of the extension distance dE. The following relation was assumed to apply to the conductivity σ of the FGM material:

The following parameters were used in the simulations: Thickness of FGM part: 0.25 mm; thickness of conductive foils: 0.03 mm; interfoil distance dI: 1.57 mm; low-field conductivity σo: 10−8S/m; electric field threshold Eb: 1 kV/mm; exponent α: 4. The foil edge205was, in the simulations, located at x=0 mm. The material parameters used in these simulations correspond to a typical SiC-based FGM material to which conductive particles have been added in order to increase the value of σo. The same material properties were used in the simulations by whichFIG. 2was obtained.

The five different values of the extension distance dEfor which simulations are shown inFIG. 5aare: 0.32 dI, 0.96 dI, 1.59 dI, 2.23 dIand 2.87 dI. In addition, the result when there is no FGM part200is also shown. As can be seen inFIG. 5a, a peak500appears at the foil edge205/405when no FGM part200is applied. The use of an FGM part200drastically reduces the peak at the foil edge205/405, the remaining peak at the foil edge205/405indicated by reference numeral505. When an FGM part200is applied at the foil edge205/405, the height of the remaining peak505is basically independent of how far the FGM part200extends—a similar magnitude of the remaining peak505is obtained regardless of the extension distance dEof the FGM part200.

As expected, an additional peak510appears when an FGM part is introduced, this additional peak appearing at the edge220of the FGM part200. This additional peak510is considerably lower than the peak500appearing at the foil edge205/405when no FGM part is used. The magnitude of this additional peak510partly depends on the field grading properties of the FGM material, and partly on the increased geometrical field grading properties due to the greater thickness of the FGM part200than of the conductive foil120. As can be seen inFIG. 5a, for the FGM material and geometry at hand, dE≈1.6 dIprovides the most efficient field grading. For higher values of the extension distance dE, the magnitude of the additional peak510at the edge220of the FGM part200will be lower than the magnitude of the remaining peak505at the foil edge205/405. This further reduction of the electric field at the edge220of the FGM part200will not improve the electric stress situation for the bushing100, and any further extension of the FGM part200beyond dE≈1.6 dIcan thus be considered unnecessary. For lower values of the extension distance dE, in the other hand, the potential of the field grading material is not fully exploited in that the additional peak510at the edge220of the FGM part is higher than the remaining peak505at the foil edge205/405.

The optimal ratio of the extension distance dEto the interfoil distance dIwill vary somewhat depending on the properties of the FGM material, as well as on the ratio of the thickness of the foil120to the thickness of the FGM part200. InFIG. 5b, results are shown of simulations of a further bushing100, having an FGM part200with a higher value of the low-field conductivity than the FGM part200ofFIG. 5a. The other parameters of the bushing are the same as in the simulations shown inFIG. 5a. The low-field conductivity of the FGM material has been increased to σo=1.4 10−7S/m, i.e. an increase of nearly 15 times. FromFIG. 5bit can be concluded that, for the FGM material and geometry for which the simulations shown inFIG. 5bwere performed, an extension distance, dE≈4.1 dIprovides the most efficient field grading. The FGM material of the simulation shown inFIG. 5bcan be considered non-standard, since it combines high conductivity with a significant non-linearity.

As can be seen from a comparison ofFIGS. 5aand5b, the reduction in the magnitude of the remaining peak510due to the increase in the conductivity of the FGM material is comparatively small. Any further increase in the low-field conductivity σ0will only contribute the reduction in magnitude of the remaining peak in a minor way, and thus, for a geometry wherein the ratio between the foil and FGM part thicknesses is that used in the simulations shown, there is generally no need of further increasing the extension distance beyond approximately four times the interfoil distance. We therefore conclude that a ratio of dEto dIwithin the range of 0.3-4 will, in most cases, provide efficient field grading at an edge of a foil205/405. For a typical SiC-based material similar to the one used in the simulations illustrated inFIG. 5a, an extension distance dEwithin the range of [0.7 dI; 3 dI], or [0.9 dI; 2 dI] will often provide efficient field grading. As the low-field conductivity σ0is increased, the optimal ratio of dEto dIwill typically increase somewhat. However, even for the more extreme materials, like the one simulated inFIG. 5b, an extension distance of four times dIor lower will typically be sufficient.

A decrease in the ratio of the thickness of the FGM part200to the thickness of the conductive foil120would increase the optimal extension distance dEand vice versa, since a reduction in FGM part thickness would increase the magnitude of the additional peak510, and a decrease in foil thickness would decrease the magnitude of the remaining peak505. However, in most cases, an extension distance dEof four times dI, or lower, will be sufficient. If, in an application, a thickness ratio is desired which yields an optimal extension distance considerably exceeding four times dI, geometrical field grading could be applied at the edge220of the FGM part200. This could for example be the case if further savings on FGM material are desired, or if a thicker foil120is required. An example of such geometrical field grading is shown inFIG. 6below.

The electric field between two adjacent foils120is around 5 kV/mm in the simulated scenarios shown inFIGS. 5aand5b. Thus, the electric field peak magnitude obtained by means of the FGM part200is of the same order of magnitude as the electric field between two adjacent foils120.

We have realized that there is generally no need for the extension distance dEof an FGM part200to be larger than around four times the interfoil separation distance. If the extension distance is large, the electrical stress at the foil edges205will be lower than the electrical stress in the bulk of the condenser core115. Thus, in order to avoid an unnecessary usage of field grading material, an efficient extension distance typically lies within the range 0.3-4 interfoil separation distances. A larger extension distance will involve unnecessary costs, since the additional field grading material will not contribute significantly to the desired field grading.

By selecting the extension distance of an FGM part within the range of approximately four times the interfoil separation distance or less, the cost of the bushing can be reduced in that less FGM material will be used than if FGM parts of larger extension distance were used.

If desired, the extension distance dEcould vary along a conductive foil edge250/405—for example, as shown inFIG. 4, an FGM part200could be arranged in the extension of only part of a conductive foil edge205/405. Smaller and/or more local variations of the extension distance dEalong a foil edge205/405may also be employed.

In an implementation wherein the interfoil separation distance varies throughout the bushing100, as discussed above, and wherein more than one conductive foil120is equipped with an FGM part200, the extension distance dEcould be constant for all FGM parts200, or could be shorter for foils120at a location where the interfoil separation distance is smaller, the interfoil separation distance being the radial distance between the conductive foil, in the extension of which the FGM part is arranged, and an adjacent conductive foil. When the extension distance takes the same value for all FGM parts200, such value could for example be selected in dependence on the largest extension distance of the bushing, so that the FGM part200lies within the range of four times the largest extension distance or less.

The dimension of the FGM part200in the radial direction of the bushing, here referred to as the thickness of the FGM part200, will often be selected to be smaller than the extension distance dE. A smaller thickness means lower costs for the material. Furthermore, in some applications, it might be necessary to consider the thermal properties of the field grading material and/or the dielectric insulating material when selecting a suitable thickness of the FGM part200. A thinner FGM part200will dissipate less heat than a thicker FGM part200of the same field grading material, and a thinner FGM part200is therefore desirable for thermal reasons.

If the part of the FGM part200that extends beyond the foil edge205/405is assumed to be in the shape of a cylinder at a radial distance Drfrom the longitudinal axis of the bushing100, and assumed to have a length dEand a thickness t, the losses Pfgmoccurring in the FGM part200can be described as:

Pfgm=Ifgm2⁢Rfgm=(Vfgm)2Rfgm≈2·π·(Vfgm)2⁢σfgm⁢Dr⁢tdE,(2)
where Vfgmis the potential difference between the foil edge205/405and the edge220of the FGM part200, Rfgmis the resistance of the FGM part200and σfgmis the conductivity of the FGM part200. In an FGM part200having non-linear resistive properties, the conductivity σfgmwill typically vary along the extension of the FGM part200for electric fields above the electric field threshold. However, by using the highest expected value of σfgmwhen estimating the thermal losses, an upper limit for the losses can be obtained. Furthermore, when an FGM part200is arranged at several concentric conductive foils120, the radial distance Drfrom the longitudinal axis of the bushing will typically be larger for the FGM parts200arranged at the outer conductive foils120. By using the largest value of the radial distance Dr, a maximum value of the losses may be estimated. An estimated maximum value of the losses Pfgmcould be compared with the highest losses that are thermally acceptable, and the dimensions of the FGM part could be selected accordingly. When dimensioning the FGM part200, it is also advantageous to consider that there is often a (material dependent) minimum thickness, relating to the finite size of the filler particles, beyond which the field grading material no longer exhibits the non-linear electric properties of the bulk material. Hence, the thickness of the FGM part200could preferably exceed this minimum thickness. For finer particle sizes, the minimum thickness is typically lower. However, very fine particle sizes typically lead to increased manufacturing costs.

An FGM part200could for example be made from a tape of a suitable field grading material, such as for example a ZnO tape as disclosed in EP1736998. An FGM tape used to form an FGM part200could be non-adhesive, or could be adhesive in order to stick to the conductive foil120. A conductive adhesive, such as e.g. thixotropic paint, could for example be used. An FGM part200made from a tape could for example cover only an area in the vicinity of a foil edge205/405, for example as shown inFIGS. 3a-cand inFIG. 4.

An FGM part200could alternatively be formed by applying the field grading material on the dielectric insulating material between different conductive foils120of the condenser core115(such dielectric material being for example paper). When applying a layer of field grading material on the dielectric insulating material, the FGM part200could be arranged to cover the vicinity of the foil edges205/405only, for example as shown inFIGS. 3a-band inFIG. 4, or the FGM part200could be arranged to extend along the entire conductive foil, as shown inFIG. 3c, or the overlap distance docould take any suitable value. The field grading material could for example be applied as a coating by means of spraying or painting.

In a method of forming the conductive foils120of a condenser core115wherein the conductive foils120are applied on the dielectric insulator123in the form of for example conductive ink (applied for example by means of spraying), the FGM part200could be applied to the dielectric insulator123in the same process as the conductive foils, or be applied separately.

The dielectric insulating material of a bushing200is often impregnated with oil or resin in order to improve the dielectric properties of the insulating material. In one implementation of the present technology, the field grading material, for example in the form of a powder, is mixed with the oil or resin before impregnating the dielectric insulating material. Hence, the impregnated dielectric insulating material will in this method form FGM parts200. When using this method of forming the FGM parts200, the dielectric losses in the bushing100upon use will often be higher than if the FGM part200is applied locally to the foil edges205/405, and furthermore, the amount of field grading material required will be larger. However, this method of forming FGM parts200is efficient in that the manufacturing steps will be simple. Hence, in an implementation wherein simple manufacturing is more important than the magnitude of the dielectric losses, this method can be suitable.

The use of at least one FGM part200as described above in a bushing100to grade a locally enhanced electric field could, if desired, be combined with other ways of obtaining local field grading. For example, geometrical field grading may also be used. If desired, an additional geometrical field grading arrangement could be employed, or the edge220of an FGM part200could be of a suitable shape to further improve the field grading properties. For example, a cross-section of the edge of the FGM part200could for example have a circular area of diameter larger than the thickness t of the FMG part200, or the edge of the FGM part200could be of another field grading curvature, such as an elliptic shape, or a rectangular shape with rounded corners. The combination of material dependent field grading obtained by the FGM part200with other means of field grading could for example be useful in situations when restrictions on the dimensioning of the FGM part200does not allow for a design which provides sufficient field grading at an acceptable heat loss (cf. expression (2)), or in order to save FGM material by making the main part of the FGM part200thinner. The FGM part200could then be designed such that partial field grading is provided at acceptable heat loss, while additional field grading could be provided by other means. Since the FGM part200will provide a considerable contribution to the local field grading, the diameter of the geometrical shape at the edge of the FGM part200could be smaller than if no FGM part200was employed, the geometrical shape at the edge thus contributing less to the bushing diameter. An example of a cross-section of an FGM part200having a circular cross-sectional edge220is shown inFIG. 6.

FIG. 7shows the simulation results ofFIG. 2in a graph where the magnitude of the electric field E in an extension direction310is shown as a function of position L, also referred to as the arc length, along a line in the radial direction of the bushing at the foil edge205. The dashed and solid curves denote, respectively, the electric field at foil edges without (cf. foil edge205A ofFIG. 2) and with (cf. foil edge205ofFIG. 2) an FGM part200. As can be seen in the graph, the electric field exhibits a peak at the foil edge both with and without an FGM part200. However, the peak in the case where the foil edge205has an FGM part200is considerably lower than the peak in the conventional case (by a factor ¼).

Although simulations are simplified, here for example in that no account has been taken for space charge effects in the insulating material, the simulations performed clearly show that a great reduction in electric field stress around conductive foil edges205can be achieved by the application of an FGM part200.

The decreased stress enhancement at conductive foil edges205/405which can be achieved by use of FGM parts200having a suitable electric field threshold allows for an increase in the average field between conductive foils120as compared to when no FGM parts200are employed. Hence, with maintained bushing dimensions, a bushing employing such FGM parts200can be rated for higher voltages. Alternatively, if the voltage rating is maintained, the dimensions of the bushing100can be reduced, resulting in a lower product cost and smaller physical space requirements for the bushing installation.

Furthermore, by use of FGM parts200at conductive foil edges205/405in a bushing100, the failure rate of the bushing can be reduced. The risk for flashovers, possibly causing insulation puncture, and for partial discharges, resulting in ageing and eroding of the surrounding insulation, is high at spots where the electric field is locally enhanced. By use of FGM parts200at conductive foil edges205/405, local field enhancement at the conductive foil edges205/405can be reduced, and hence, the rate of failure at the foil edges205/405can be reduced.

The present technology is suitable for use in high voltage bushings, as well as for low and medium voltage bushings. The technology can advantageously be used in AC voltage bushings as well as in DC voltage bushings.

Although various aspects of the invention are set out in the accompanying independent claims, other aspects of the invention include the combination of any features presented in the above description and/or in the accompanying claims, and not solely the combinations explicitly set out in the accompanying claims. One skilled in the art will appreciate that the technology presented herein is not limited to the embodiments disclosed in the accompanying drawings and the foregoing detailed description, which are presented for purposes of illustration only, but it can be implemented in a number of different ways, and it is defined by the following claims.