SE1500498A1 - Method of manufacturing high voltage bushing - Google Patents

Abstract

Method of manufacturing a condenser core body for an electric bushing comprising the steps. i. ) rolling up the non-impregnatable insulating film with electrodes. ii. ) secure the whole rolled up package by heat shrink tape or any other heat shrink material iii. ) heat the rolled up package with heat shrink material to a sufficient high temperature so that adjacent turns of non-impregnatable insulating film bond together forming a void free body.Fig. 2 for publication

Description

METHOD OF MANUFACTURING A HIGH VOLTAGE BUSHING Technical field The present invention relates to the field of high voltage technology, and in particular to a method of manufacturing a condenser core for a high voltage bushing.

Background 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. 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 condenser core. A condenser core is a body which typically comprises a number of floating, coaxial electrodes made of a conducting material, where the electrodes are separated by a dielectric spacing material. The dielectric spacing material is often oil impregnated or resin impregnated paper. Application PCT/EP2013/075649 describes a condenser core with at least two electrodes which are separated by a dielectric part. At least one of the electrodes is arranged to be at a floating potential so as to control the electric field around the conductor. The dielectric part comprises at least one turn of at least one non-impregnatable electrically insulating film between two neighbouring electrodes. The electrodes are bonded to adjacent turns of non-impregnatable insulating film, and turns of nonimpregnatable insulating film which are adjacent to each other, if any, are bonded to each other, so that the turns of non-impregnatable insulating film and the electrodes form a solid body.

Summary The present invention relates to a method of producing an insulating body arranged to provide electrical insulation of a conductor which extends through the device. The insulating body can for example be used in a bushing or a cable termination. The insulating body comprises at least two electrodes which are separated by a dielectric part. At least one 1 of the electrodes is arranged to be at a floating potential so as to control the electric field around the conductor. The dielectric part comprises at least one turn of at least one nonimpregnatable electrically insulating film between two neighbouring electrodes. The electrodes are bonded to or placed on adjacent turns of non-impregnatable insulating film.

The non-impregnatable insulating film with electrodes are rolled up around the conductor or a dye and the whole rolled up package is heated to a sufficient high temperature so that adjacent turns of non-impregnatable insulating film bond together, so that the turns of nonimpregnatable insulating film and the electrodes form a solid body.

Before heating the whole rolled up package of non-impregnatable insulating film with electrodes is secured by heat shrink tape that is wound on top of the non-impregnatable insulating polymer film. When the whole rolled up package is heated the heat shrink tape will exert stabilizing pressure.

The steps of the manufacturing process are; rolling up the non-impregnatable insulating film with electrodes secure the whole rolled up package by heat shrink tape or any other heat shrink material heat the rolled up package with heat shrink material to a sufficient high temperature so that adjacent turns of non-impregnatable insulating film bond together.

The heat shrink tape can also be in the form of a heat shrink sleeve enclosing the rolled up package, a number of bands of heat shrink loops threaded around the rolled up package, a number of strings of heat shrink material or any other form of material that will stabilize the rolled up structure before heating and that will exert a bonding pressure during heating.

Heat shrink material, is a material made up of polymer plastic and when heat is applied, it shrinks tightly over whatever it is covering. The heating of whole rolled up package with heat shrink material will guaranty a void free insulating body.

The pressure is obtained through a heat shrink material that is wound on top of the non- impregnatable insulating polymer film and heat is applied using an oven. The external pressure obtained through the heat shrink material is crucial to obtain a sufficient joining of the polymer film layers and also help to stabilize the structure during heating. The 2 temperature during lamination is controlled to achieve lamination between the film layers without melting the whole structure which is important in order to secure positioning of the conductive layers.

An alternative process where the steps of the manufacturing process are rolling up the non-impregnatable insulating film with electrodes secure the whole rolled up package (possibly by heat shrink material but other solutions are possible) submit the rolled up package to a isostatic pressure and heat by, for example, a heated liquid under pressure, e.g. oil, to sufficient high temperature and pressure so that adjacent turns of non-impregnatable insulating film bond together.

In the alternative process, one is not relying exclusive on the pressure generated by the heat shrink material wrapped around the rolled up package, to bond insulating film together.

The iso static pressure on the rolled up package by liquid under pressure is easily controlled and maintained constant over time. The temperature and pressure during lamination is controlled to achieve lamination between the film layers without melting the whole structure which is important in order to secure positioning of the conductive layers.

The process proposed here is simpler than a process where the non-impregnatable, electrically insulating film is bonded during the rollup, since there is no need for heating/cooling or glue application during winding and a simpler winding machine without integrated lamination step can be used.

Advantages by using the suggested manufacturing method (insulating polymer film melted together using a heat shrink material): Simplified/shortened process and handling; no impregnation/curing step necessary Improved breakdown strength due to multi-thin-layer structure Narrow tolerance of film thickness, therefor distance between conductive layers can be reduced for better field distribution allowing a smaller diameter of the bushing improving cooling performance Different and simpler manufacturing compared to a process where lamination is integrated during winding 3 The non-impregnatable electrically insulating film could for example comprise a thermoplastic material. Many thermoplastic materials, exhibit higher dielectric strength than oil-or resin impregnated paper. This is especially true when the material thickness is low, such as in the dielectric part between two electrodes. Hence, at a given rated voltage of the device, the use of such materials allows for a smaller device diameter.

Non-impregnatable films can typically be made considerably thinner than a layer of oil- or resin impregnated paper, and electrodes can hence be placed at a smaller distance from each other. With a smaller distance between the electrodes, the dielectric strength of the material increases, the dielectric strength being a measure of the highest electric field which can be maintained in the material before an electric breakdown occurs. This also allows for a reduced diameter of the electric device as the distance between electrodes is reduced.

A suitable distance between neighbouring electrodes typically lies within the range of 4-5000 gm. Oftentimes, the electrode distance will lie within the range of 50-1000 ii,M, for example within the range of 50-300 ,tin or 100-250 gm.

Typically, the average number of turns of non-impregnatable insulating film between two neighbouring electrodes lies within the range of 1-100, although an even higher number of turns may be used. Oftentimes, the average number of turns between two neighbouring electrodes will lie within the range of 1-50, and for example within the range of 1-20.

Also, the precision in the thickness of the non-impregnatable insulating films is typically considerably higher than the precision in the thickness of conventionally used impregnated paper. Such improved precision in the film thickness results in an improved precision in the distance between electrodes. This is particularly beneficial in manufacturing methods wherein separate electrodes are introduced during the winding. Both the fixed positions of the electrodes, and an improved precision in the distance between electrodes, are factors which will improve the predictability of the field grading properties of the device. An improved predictability of the field grading properties of the set of electrodes also allows for reduction of the device diameter. 4 A reduced diameter provides the advantages of less material being used during production of the device, as well as less weight and less space occupancy, both at transportation and during installation. Furthermore, a reduced diameter typically results in enhanced transportation of heat from the centre of the electrical device, thus reducing the risk of thermal damage of the electric device.

The electrodes can be formed from a conductive material which has been printed or painted onto at least one of the at least one non-impregnatable insulating films. Printed or painted electrodes can be very thin, so that a fine grading of the electric field with high precision in field distribution can be achieved. Electrodes can also be formed from foils of conductive material which have been inserted between turns of non-impregnatable film. The thickness of the electrodes could for example fall within the range of 10 nm -300 pm.

The electric device is oftentimes arranged so that at least two of said electrodes have a different length in the axial direction of the electric device, and so that at least one end edge of at least one electrode is not covered by any outer electrode. In some electric devices according to this aspect of the invention, none of the end electrode edges are covered by an outer electrode, whereas in other electric devices according to this aspect, some (at least one) of the electrodes have at least one end edge (and typically two end edges when the device is a bushing) which is not covered by an outer electrode, while other electrodes have end edges which are covered by outer electrodes. Here, an outer electrode is said to cover an end edge of an inner electrode if the outer electrode extends to, or beyond, the axial position of the end edge. The term end edge is here used to refer to an edge which defines a plane that is more or less perpendicular to the conductor, as opposed to an axial edge, which is parallel to the conductor.

By arranging the electrodes so that at least one end edge of an electrode is not covered by any outer electrodes, the electric field around the conductor will be efficiently graded. In this configuration, the electric field, at the end edges which are not covered by any outer electrodes, will have significant components in both the radial and axial directions. The axial electric field components can give rise to undesired treeing and/or partial discharge, unless the interior of the electric device is basically free from voids. By means of the manufacturing method described above, electric devices can be obtained wherein the extension of any voids is less than 101AM, or smaller. Hereby, electric devices which can operate in the high voltage range can be achieved, for example in the range of 36 kV — 1100 kV, or higher.

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

Brief description of the drawings Fig. 1is a schematic cross sectional view of example of a bushing having a condenser core.

Fig. 2is a schematic cross sectional view of an example of a condenser core according to an embodiment of the invention.

Fig. 3aillustrates a film of a single layer of a non-impregnatable insulating material.

Fig. 3billustrates a double layer film of two different non-impregnatable insulating materials.

Fig. 3cillustrates a triple layer film of at least two different non-impregnatable insulating materials.

Fig. 4schematically illustrates an example of a dielectric part of a condenser core being manufactured from two separate non-impregnatable insulating films according to an embodiment of the manufacturing process.

Fig. process steps of the proposed method Detailed description Fig. 1 schematically illustrates an embodiment of a bushing 100 wherein a conductor 1 extends through a condenser core 115. The conductor 110 could form part of the bushing 100, or could be separate to the bushing 100. Fig. 1 is a cross sectional view along the axis of the bushing 100. At both ends, the conductor 110 is provided with a terminal 112 for connecting the bushing 100 to electrical devices such as cables, transformers etc. The condenser core 115 operates as a voltage divider and distributes the field along the length of the bushing 100, thereby providing a smoothening of the electrical potential distribution. 6 The condenser core 115 comprises at least two (and often a plurality of at least three, or more) electrodes 120 which are separated by a dielectric part 125 of a dielectric spacing material. The dielectric part 125 serves to separate the electrodes 120 from each other. The electrodes 120 are typically coaxially arranged, where the radius of an inner electrode is smaller than the radius of an outer electrode. In order to obtain an efficient grading of the electric field, the axial length of an outer electrode 120 is often smaller than the axial length of an inner electrode 120, so that a similar area of the different electrodes 120 is achieved. Hence, the end edges 127 of the electrodes 120 typically form steps, so that the end edges 127 of an electrode 120 are not covered by any outer electrode 120, as shown in Fig. 1. The term end edge 127 is here used to refer to an edge which typically forms a more or less circular (spiral) shape, and which defines a plane which is more or less perpendicular to the conductor 110, as opposed to an axial edge, which is substantially parallel to the conductor 110.

If desired, a condenser core 115 could additionally or alternatively have at least some electrodes 120 arranged so that an inner electrode 120 extends a shorter distance in the axial direction than an outer electrode 120 at at least one of the condenser core 115 ends, so that steps are formed by the electrode edges 127 in a manner opposite to that shown in Fig. 1, in a direction from a condenser core end towards the centre of the condenser core 115.

In such arrangements, where the end edges 127 of the electrodes 120 form steps at a condenser core end, the local electric field at the end edges 127 of the electrodes 120 will be considerably higher than the electric field in the interior of the bushing 100, and will have significant components in both the radial and the axial direction. However, a varying axial length of the electrodes 120 further results in an axial distance between end edges 127, the axial field at the end edges 127, as well as between the end edges 127, thereby being reduced.

Two electrodes 120, between which there is no further electrode 120, so that the two electrodes 120 are separated by the dielectric part 125 only, will here be referred to as neighbouring electrodes 120. 7 The bushing 100 of Fig. 1 further includes an elongate insulator 130 surrounding the condenser core 115, as well as a flange 135, which can be used for electrically connecting the busing 100 to the grounded plane 140, typically via the outermost electrode 120 of the condenser core 115, or via some of the outer electrodes 120 of the condenser core 115. It should be noted that the grounded plane 140 does not have to be connected to ground, but may have a potential which differs from ground. However, the grounded plane 140 will have a potential which differs from the potential of the conductor 110, when in use, and the term grounded plane will hereinafter be used for ease of description.

Typically, the outermost electrode 120 is connected to a flange 135, or other part, which is at the potential of the grounded plane 140. In some bushings 100, the innermost electrode 120 is arranged to be at the potential of the conductor 110, whereas in other bushings 100, the innermost electrode 120 is arranged to be at a floating potential. An electrode 120 located between the innermost and outermost electrodes 120 is typically arranged to be at a floating potential, although a bushing may have one (or more) electrode 120 which is located between the innermost and outermost electrodes, and which is arranged to be at a fixed potential, the fixed potential differing from the potential of the conductor 110 and the potential of the grounded plane 140.

A main reason for providing electrodes 120 in a bushing 100 is to geometrically shape the electric field around the conductor 110 around the location of the grounded plane 140, so as to avoid flashover between the conductor 110 and the grounded plane 140. In the interior of the bushing, the electric field between two neighbouring electrodes 120 will mainly be in the radial direction of the bushing 100. At the end edges 127 of the electrodes 120, however, the electric field will have significant components in both the axial and radial directions. The axial field gives rise to special requirements in terms of avoiding voids which extend in the axial direction of the bushing: The amount of voids, e.g. gaps/bubbles of air, other gases, or vacuum, needs to be kept to a minimum in a high voltage bushing. In the presence of voids which extend in the axial direction, the axial field can cause charges to move between the electrodes 120, and the risk of treeing will increase. Treeing might cause adverse changes in the electric field, and can ultimately cause electric breakdown. Furthermore, the presence of voids can cause partial discharge, which would, apart from causing aging of the dielectric material, also give rise to electric 8 signals. In case the bushing is connected to equipment which needs to be monitored, e.g. a transformer, such electric signals can disturb the monitoring measurements. Hence, there is a desire to minimise the presence of voids in a bushing 100.

A condenser core 115 is conventionally wound from sheets of dielectric material, such as paper or non-woven plastic, which will form the dielectric part 125. The electrodes 120 are conventionally entered into the winding at suitable positions during the winding process. After winding, the dielectric material is conventionally impregnated with an electrically insulating impregnant such as oil or thermoset polymer (e.g. resin). By use of an impregnant, a dielectric part 125 can be obtained which has basically no voids in terms of gaps/bubbles of air, other gases, or vacuum.

The post-winding processing in the manufacturing of a condenser core 115 having impregnated paper as a dielectric spacing material is very time consuming and therefore costly. The paper is typically first wound around the conductor. The paper is then dried, impregnated and cured (in the thermoset polymer case) or dried and impregnated (in the oil case). This post-winding processing of the condenser core in the form of drying/- impregnation/curing often takes around a week, or more. Hence, there is a strong desire to find improved manufacturing methods which are less time consuming, but which nevertheless provide bushings having adequate electrical and mechanical properties.

According to the invention, an electric device comprising at least two electrodes 120 which are separated by a dielectric part 125 can be obtained by forming the dielectric part from at least one turn of at least one non-impregnatable, electrically insulating film. Any adjacent turns of non-impregnatable insulating film are bonded to each other, and electrodes 120 are bonded to adjacent turns of non-impregnatable insulating film, so that a solid body is formed.

Here, a film is referred to as being non-impregnatable if it cannot be impregnated by an electrically insulating impregnation fluid, such as oil, resin, ester oil or an electrically insulating gas. An impregnatable film, on the other hand, has a structure such that openings exist on one side of the film, such openings being connected to openings on the other side of the film via connections/voids, here referred to as channels, in which impregnation fluid 9 may be transported from one side of the film to the other. When an impregnatable film has been impregnated, such channels will be filled with impregnation fluid (cured or not). Hence, a film is here referred to as being impregnated if it has channels filled with an electrically insulating impregnant such as oil, a cured resin, an ester oil or an electrically insulating gas. Consequently, a turn is referred to as being impregnated if there are channels filled with an electrically insulating impregnant which lead through the turn (often in a meandering manner). A non-impregnatable film, on the other hand, does not have such channels. If a non-impregnatable film were to be exposed to an impregnation process, there would be no channels in the film structure through which the impregnant could be transported. In some circumstances, diffusion may act to facilitate for an impregnant to enter also a non-impregnatable film. In some cases, a non-impregnatable film may contain for example 5 weight% of an electrically insulating impregnant. However, diffusion is a much slower process than an impregnation process and does not result in channels which are filled with the impregnant, and thereby not in an impregnated film.

Oftentimes, the dielectric part 125 is formed from more than one turn, so that a multi-turn dielectric part is formed.

By forming the dielectric part from turns of at least one non-impregnatable insulating film which are bonded into a solid body, no impregnant will be required and the post-winding processing of the condenser core 115 can be significantly reduced or eliminated. Typically, no material which has been impregnated with an electrically insulating fluid will be present in the condenser core 115 (although in some circumstances, a pre-impregnated film can be used, which is non-impregnatable at the time of winding of the dielectric part 125, thus resulting in a condenser core 115 in which turns of impregnated film are present). By using the proposed method, the dielectric part 125 can be made essentially free from voids in terms of gaps/bubbles of air, other fluids, or vacuum. Partial discharge can thus be avoided without any impregnation of the condenser core 115. At the same time, the condenser core 115 will obtain suitable mechanical properties in terms of force-absorption and prevention of fluids from migrating through the bushing 100. The solid condenser core 115, obtained by bonding the electrodes and turns of non-impregnatable insulating film into a solid body, can serve as a plug which seals the flange 135 and stops any oil or gas from passing between the two sides of the grounded plane 140. This property is typically useful for bushings 100 which are used for connecting oil or gas filled electrical equipment, such as an oil filled transformer.

Examples of suitable non-impregnatable insulating materials include thermoplastic materials. Examples of suitable thermoplastics include polypropylene (PP), polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polyphenylene sulfide (PPS), polyether sulphone (PES), polytetrafluoroethylene (PTFE), polyamide (PA), polycarbonate (PC), etc.

Thermoplastics are typically less harmful to the environment than the thermosetting polymers or oils commonly used as impregnants of impregnatable dielectric materials such as paper or non-woven plastic. However, films of thermosetting plastics could also be used to form non-impregnatable insulating turns in a dielectric part 125, if desired.

The method of forming the condenser core 115 from turns of non-impregnatable insulating films and electrodes, which are bonded in a separate step to form a solid body, facilitates for the use of insulating materials which have better electric and/or mechanical properties than the commonly used impregnated paper. For example, many thermoplastics exhibit a significantly higher dielectric strength than epoxy-or oil impregnated paper under the circumstances in a bushing 100, wherein the material thickness (determined by the distance between neighbouring electrodes 120) is comparatively low. Hence, by use of a thermoplastic, a condenser core 110 of a smaller diameter can typically be used for a given voltage, than if impregnated paper were used in the dielectric part 125. Thus, the space occupied by the bushing, as well as transportation costs to the installation site, can be reduced. Furthermore, many insulating materials, which are suitable for use in a nonimpregnatable insulating film, exhibit similar thermal conductivity than the traditional insulating materials, such as oil-or resin impregnated paper. Thus, a bushing of smaller diameter would also result in the advantage of lower temperatures within the bushing 100.

Fig. 1 illustrates a cross sectional view along the axial direction of a bushing according to an embodiment of the invention. Fig. 2 schematically illustrates a cross section of an example of a condenser core 115 according to an embodiment of the invention, where the 11 cross section is taken perpendicularly to the axis of the condenser core 115. The dielectric part 125 of Fig. 2 is formed from bonded turns 200 of a non-impregnatable insulating film 205. In the schematic drawing of Fig. 2, the condenser core 115 comprises three electrodes 120. The number of electrodes 120 could take any number greater than one. In many implementations, the condenser core 115 comprises a higher number of electrodes 120, for example two, three, five, ten, twenty, a hundred or more. The boundary between different turns 200 of the insulating film 205 is indicated by reference numeral 210. There will be at least one turn 200 of the non-impregnatable insulating film between two electrodes 120. In the example illustrated in Fig. 2, the number of turns 200 between two electrodes 120 is 2- 3. The average number of turns 200 between two neighbouring electrodes 120 could for example lie within the range of 1-100. However, in some circumstances, a higher number of turns 200 can be used between neighbouring electrodes 120, for example in the order of hundreds or thousands of turns 200. By using a lower number of turns 200, the number of interfaces within the dielectric part 125 can be kept low. On the other hand, if the non- impregnatable insulating film has some defects, it can be advantageous to use at least two turns between neighbouring electrodes 120, since the risk of a defect occurring in two turns at the same position of the bushing is small. The number of turns 200 between two neighbouring electrodes will often fall within the range of 1-50, for example within the range of 1-20 turns.

The electrode arrangement shown in Fig. 2 is an example only. For example, in Fig. 2, all electrodes 120 are electrically separated, and the two axial edges of each electrode 120 exhibit a small overlap. Other electrode arrangements may be used. Two or more neighbouring electrodes 120 could for example be short circuited; each electrode 120 could be arranged so that there is no, or a larger, overlap, etc.

In an internal bonding process in a separate step from the rolling process wherein a film 205 itself enters an adhesive state, the adhesive state of the film in an internal bonding process could for example be a liquid state, so that the material which provides the bonding is melted upon bonding; or a semi-liquid state, which can occur for example in amorphous thermoplastics, depending which material(s) are present in the film 205. 12 A non-impregnatable and electrically insulating film 205 could be a single layer film having a single layer 300, a double layer film having two layers 300 of different materials, or a film 205 of three or more layers 300 of at least two different materials. Examples of a single layer film 205i, a double layer film 205ii and a triple layer film 205iii are shown in Figs. 3a-3c, respectively. When more than one layer 300 is used in a film 205, the materials of the different layers 300 could be selected to have different properties, so that the film 205 will benefit from properties of different materials. In Figs. 3a-3c, examples of different designs of non-impregnatable insulating films 205 are shown, the three examples here referred to by use of reference numerals 205i, 205ii and 205iii, respectively. When jointly referring to these film designs, or to non-impregnatable insulating films 20in general, the reference numeral 205 is used. Similarly, different layers 300 of the same film 205 are indicated in Figs. 3a-3c by reference numerals 300i, 300ii and 300iii. When referring to a film layer in general, the reference numeral 300 will be used. A film 205 comprising more than one layer 300 will be referred to as a layered film 205.

Examples of properties which could vary between the layers 300 of a film 205 include adhesive properties, electrical insulation, mechanical stability, heat resistance, cost etc.

In one embodiment of the invention, the materials in a layered film 205 are selected so that the temperature dependency of the mechanical properties of the materials is such that there exists a temperature range within which the difference in the mechanical properties of the different layers 300 is more pronounced. Upon bonding of the different turns 200, the film 205 could for example be heated to reach a temperature within such temperature range. In a first implementation of this embodiment, a first material provides better adhesion within a temperature range than the other material(s). In this implementation, such first material could advantageously be facing at least one of the surfaces of the film 205. The first material could then contribute to the bonding between turns of film 205 and/or between electrodes 120 and adjacent turns of film 205, if the film 205 is heated to a temperature within this range. In another implementation, the materials are selected so that a first material provides better mechanical stability than the other material(s) in a temperature range. In yet another implementation, a first material provides the best adhesive properties, while a second material provides the best mechanical stability in a temperature range to which the film 205 is heated during the bonding process. In one example, a first material is 13 in an adhesive state, while a second material is in a solid, non-adhesive state within this temperature range. The second material would then ensure mechanical stability of the film 205 during the bonding process, while the first material would contribute to the bonding.

Material combinations could for example include a combination of different thermoplastic materials; a thermoplastic material and a glass material; a thermoplastic material and a ceramic material, etc. Examples of suitable material combinations include polyethylene & polyether sulfone (PES), where polyethylene can provide adhesion and is typically of lower cost, while the PES material provides mechanical stability; or polypropylene & polyphenylene sulfide (PPS), where the polypropylene is of lower cost and the PPS is more resistant to heat, so that the PPS provides better mechanical stability during operation of the bushing in environments of higher temperatures; or a glass and polyvinyl butyral (PVB). These combinations are given as examples only, and there are many further suitable material combinations.

Some thermoplastics, occur in a glass state rather than an ordered solid state. The term solid state is here used to refer to both the ordered solid state and the glass state of materials.

Fig. describes the process steps of the proposed manufacturing process are; 10 rolling up the non-impregnatable insulating film with electrodes. 20 secure the whole rolled up package by heat shrink tape or any other heat shrink material 30 heat the rolled up package with heat shrink material to a sufficient high temperature so that adjacent turns of non-impregnatable insulating film bond together In all embodiments of the manufacturing method, electrodes 120 will be introduced between turns 200 of film 205 at suitable positions during the process of forming the turns 200 (if the condenser core 115 only includes two electrodes 120, one will typically be introduced between two turns 200 and the other will typically be introduced on top of the (last) outermost turn 410). The electrodes 120 could for example be made of foils of aluminium, copper or any other conducting material, which are inserted at suitable positions during the formation of the turns 200. This way of introducing the electrodes 120 between turns 200 is illustrated in Figs. 4 and 5a. Alternatively, the electrodes 120 could be 14 formed from a metallized insulating layer, where the metallization is achieved for example by printing or painting metallic material onto an insulating film. Electrodes 120 made of conducting, non-metallic materials such as carbon black or graphite could also be used. Electrodes 120, metallic or not, could for example be printed or painted directly onto a non-impregnatable insulating film 205 forming the turns 200 of the dielectric part 125, or on a separate sheet of insulating material which is inserted between turns 200. When printed or painted directly onto a film 205 forming the turns 200, the printing/painting could be made prior to forming the turns, or on the most recently added (outermost) turn 410 during the formation of the dielectric part 125. Electronic printing is well known in the art and described for example in Chapter 1.3 of "Bit Bang — Rays to the Future", edited by Yrjo Neuvo & Sami Ylonen, Helsinki University Print, 2009. Printing techniques include for example screen printing, flexography, gravure, offset lithography and inkjet printing. Roll-to-roll processing could also be used. Other possible techniques for depositing the electrodes onto an insulating film include Physical Vapour Deposition techniques, for example sputtering, and Chemical Vapour Deposition techniques.

Since the electrodes 120 can be applied onto a film 205 in the solid state, or even onto the solid body formed by the presently added turns 200, electrodes of very low thickness can be used. A fine grading of the electric field with high precision in field distribution can thus be achieved. The thickness of the electrodes 120 could for example in the range of 1- 10 1,tm, or be as small as 10 nm, or smaller. Thicker electrodes could also be used. The thickness of the electrodes 120 typically falls within the range of 10 nm-300 1,tm.

The manufacturing of a condenser core 115 having a dielectric part 125 formed from bonded turns 200 of non-impregnatable, electrically insulating film(s) 205 will be considerably less time consuming than the manufacturing of condenser cores 115 having a dielectric part formed from impregnated material, such as paper, or from layers of impregnated plastic nonwoven as described in US6452109. The main reason for this reduction in production time is that post-winding steps, such as impregnation, curing, or post-heating, can be eliminated.

The use of non-impregnatable insulating films which are bonded together leads to a possibility of improved precision in the distance between two neighbouring electrodes 1 in a condenser core 115, since many non-impregnatable materials can be made into thinner films 205 than paper can. Paper typically has a thickness of around 100 gm or more. Moreover, the paper used in resin impregnated condenser cores 115 has to be crepped in order to allow the resin, which is of high viscosity, to flow into the inner parts of the condenser core 115. The crepping prohibits a high precision in the thickness of a turn of paper, and typically increases the average thickness of the paper to around 300 gm. Hence, the distance between neighbouring electrodes 120 will be at least 300 gm when impregnated paper is used as the insulation material.

Many non-impregnatable films 205, such as thermoplastic films, can be made as thin as 4 gm or less, and the distance between neighbouring electrodes 120 can hence be controlled with much better precision than when using impregnated paper, for example in steps of 4 pm. Hence, the distance between electrodes can be smaller in a bushing comprising turns of non-impregnatable insulating film instead of impregnated paper. Hereby, a bushing 100 of smaller diameter can be used at a particular voltage, or a bushing of a particular diameter can be used for higher voltages, since the field grading can be more efficient if the distance between electrodes can be better controlled. Furthermore, the dielectric field strength of a material increases when the thickness of the material decreases. As thin films will allow for a smaller distance between neighbouring electrodes, the dielectric field strength of the dielectric part can be increased, and the diameter of the bushing can thereby be reduced. A reduced diameter means that space savings can be made, both during transport and at the installation site.

Typically, the thickness of the non-impregnatable film lies within the range 4-600 i_un, for example in the range of 4-500 gm.

In fact, the manufacturing method described above is suitable for manufacturing of condenser cores 115 of any length: If it is desired to obtain a condenser core 115 of an axial length, which exceeds the width of a film 205, two or more films 205 can be applied side by side. This applies to the manufacturing method using extrusion, as well as to the method wherein solid films 205 are used to form the turns 200. When two or more films 205 are placed side by side, part of a turn 200 is formed by a first film 205a, and another part of a turn 205 is formed by a second film 205b, and so forth. Fig. 4 schematically 16 illustrates an example of a condenser core 115, of which the dielectric part 125 is formed from two separate solid films 205a and 205b, which are placed side-by-side.

In order to improve the dielectric strength of the dielectric part 125, the position of the joint can vary during the forming of the turns 205, so that the distance between the joint and an end of the condenser core 115 will vary between and within different turns 205.

In a condenser core 115, which has been formed from two or more separate nonimpregnatable films 205 placed side-by-side, at least some of the electrodes 120 can also be divided into two or more parts along the length of the condenser core, so that an electrode 120 at a particular radial distance from the conductor 110 comprises at least a first part at the first end of the condenser core 115, and a second part at a second end of the condenser core 115, where the first and second parts are not electrically connected. If desired, electrodes 120 can be divided into such parts also in condenser cores 115 which are formed from a single film 205.

Since the length of the electrodes 120 decreases as the radial distance from the conductor 110 increases, the outermost turn(s) 200 can oftentimes be formed from a single film 205, if desired, even in a condenser core 115 of large length.

As mentioned above, the axial length of the electrodes 120 is typically smaller for outer electrodes 120 than for electrodes closer to the conductor 110. The solid body formed from the bonded turns of non-impregnatable film and the electrodes 120 could, if desired, have conical or tapered ends. Conical or tapered ends are often used in order to reduce the weight of the conductor core 115, and/or to save on non-impregnatable film material.

When winding the conductor core 115 from one or more already existing films, as discussed in relation to Fig. 4, the width of the film could e.g. be cut prior to winding the film onto the condenser core 115, so as to decrease the width of the film prior to winding as the radius of the condenser core 115 increases. Alternatively, instead of the solid body having conical or tapered ends, the solid body could have the shape of a cylinder, or have ends of another shape, such as spherical ends. 17 The above described bushings, wherein the condenser core 115 is formed from turns of non-impregnatable, insulating films 205 and electrodes 120 which are bonded together to form a solid body, can be applied in both AC and DC applications. The bushings are particularly suitable for high voltage applications, for example in the voltage range of 36- 1100 kV, or higher, but could also be used at lower voltages.

By use of the manufacturing method described above, electric devices can be produced, wherein any voids in the dielectric part 125, or between the dielectric part 125 and the electrodes 120, are negligibly small. For example, electric devices which basically contain no voids of an extension larger than 15 pm can be obtained. Electric devices wherein the voids, if any, have an even smaller extension, such as 10 mn or IIM, or smaller, can also be achieved.

When the dielectric part 125 of a condenser core 115 is formed by bonding turns 200 of non-impregnatable insulating film 205 and electrodes 120 to form a solid body, there will be no need for an impregnation medium, and no need for a housing surrounding the condenser core 115. If desired, a housing could still be used to protect the condenser core 115 from dirt and wear, and/or to provide an increased creepage distance, etc. However, a housing is not necessarily required, since the condenser core 115 is solid.

Although described in relation to bushings 100, the technique of bonding turns 200 of nonimpregnatable, electrically insulating films 205 and electrodes 120 to form a solid body, in a separate step from the rolling step, can also be used for other electrical devices wherein at least two electrodes are separated by a dielectric part 125. Examples of such other equipment include capacitors, measurement transformers (also referred to as instrument transformers) and cable terminations. Cable terminations are typically used to provide electrical insulation of a conductor at a transition from a cable to equipment such as a transmission line, transformer bushing, busbar, etc. One type of cable termination comprises a condenser core 115 having at least one electrode 120 at a floating potential.

What has been said about the design of the bushing 100 in the above, can also be applied to the design of this type of cable termination. 18 The manufacturing methods discussed above have been described in terms of turns 200 of at least one non-impregnatable, insulating film 205 being arranged around a rotating conductor 110. This corresponds to rotating an inner part of the condenser core 115 around its axis of rotation. In the case of for example a capacitor, there is no conductor 1 present, but the inner part of the capacitor will be rotated when arranging the film 205 into turns 200. In the case of a capacitor, the inner part could, if desired, be of a non-cylindrical shape. The resulting capacitor could take a more elongate shape — for example an elliptical shape, or a parallelepiped shape, a triangular shape etc. Furthermore, in some cases, a condenser core 115 which does not include a conductor 110 may be desired, so as to allow a user of a bushing 100 to fit his own conductor 110 into the condenser core 115. When manufacturing an electrical device which does not include a conductor 110, an axial edge of the first turn 200 can be temporarily fixed to an axis of rotation, this axial edge of the first turn corresponding to the inner part of the device.

Although various aspects of the invention are set out in the accompanying 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. 19

Claims (10)

Claims

1. Method of manufacturing a condenser core body for an electric bushing (100) comprising the steps - rolling up a non-impregnatable insulating film on a conductor or a dye - bonding the rolled film to one void free body

2. The method of claim 1 comprising a step where a part of the surface non-impregnatable insulating film is conducting by for example conductive paint, carbon etc.

3. The method of claim 1 comprising a step where part of the surface non-impregnatable insulating film is covered by metallic foil, creating floating potential electrodes to control the electric field around the conductor.

4. The method of claim 1 further comprising a step where the rolled up non-impregnatable insulating film is covered with a heat shrink material.

5. The method of claim 1 where bonding is done separately from the rolling up of nonimpregnatable insulating film.

6. The method of claim 1 where bonding is done by heating the rolled up body of non- impregnatable insulating film surrounded by heat shrink material.

7. The method of claim 1 where bonding is done by heating and submitting the rolled up body of non-impregnatable insulating film to an isostatic pressure by a liquid.

8. The method of claim 6 where heating is done by placing the body i a temperature controlled oven.

9. The method of claim 7 where heating is done by heating the liquid generating the isostatic pressure.

10. The method of claim 1 where the non-impregnatable film comprises at least one layer of a thermoplastic material. UN n. t. •--\ 014 t") 914 2/4