D E S C R I P T I O N

A FILM, A CAPACITOR, A VOLTAGE TRANSFORMER AND A METHOD OF USING A CAPACITOR

Field of the invention

The invention concerns a film according to the generic part of claim 1 and a capacitor comprising a film according to the invention. Capacitors of this type are typically employed in MV and HV electrical installations. In particular, the capacitor according to the invention can be part of a voltage transformer where it is employed as a high voltage capacitor in series with a low voltage capacitor. The invention also concerns a method of using a capacitor according to the invention.

Prior art

Films of the generic type with a polymer base and at least one longitudinally contiguous strip of electrically conductive coating, in particular, metal, are well known, see, e.g., EP 1 329 915 Al. According to this document, two such films are wound into a coil together to form a capacitor.

Similar capacitors where two or three such films are arranged one immediately above the other and wound together are known from EP 1 341 195 Al. Part of the electrically conductive coatings serve as electrically unconnected floating electrodes which subdivide the potential drop between two electrodes connected to opposite leads. Subseguent electrodes are always separated by an electrically insulating base. At least part of the coatings form tracks arranged side by side on the base of the respective film. However, the tracks are always contiguous

tracks, i.e. uninterrupted in the longitudinal direction. Also, two of the tracks in question each extend to one of the lateral edges of the film so they can be contacted there. Voltage differences and consequent electrical fields between tracks which are either arranged side by side at the same surface of the film or at opposite surfaces separated by the electrically insulating base of the film are of the same order of magnitude as the total voltage drop. As a consequence, the films in question and capacitors made from them are suitable for relatively low voltage applications only.

Summary of the invention

It is an object of the invention to provide a film of the generic type which is suitable for the formation of capacitors and similar devices which can withstand high voltages without a risk of surface flashovers or partial breakdowns .

It is a further object of the invention to provide a capacitor which comprises a film according to the invention or several such films. Capacitors of high quality and reliability can be produced from such films in relatively simple and cost-saving ways. In particular, time-consuming drying and impregnation and in some cases curing steps as required for the production of known capacitors can usually be avoided.

In a capacitor according to the invention the electric field between opposite contacts can be controlled with great precision and, in particular, the often large potential differences between them subdivided in a controlled manner into a relatively large number of small potential differences between mutually electrically insulated electrodes.

A further advantage of a capacitor according to the invention is the fact that the electrically conductive coating tends to evaporate around the location of a partial breakdown, that is, the capacitor is self-healing.

It is a further object of the invention to provide a capacitive voltage transformer of high accuracy and reliability which is at the same time relatively easy to produce. This object is achieved in that a capacitor according to the invention is employed as a high voltage capacitor where its capacity of withstanding high voltage differences is used to advantage while its relatively low capacitance does not constitute a drawback.

It is particularly advantageous that, due to the fact that the base of a film according to the invention can be made very thin and therefore a large number of electrodes can be provided in a gap of a given width, capacitors of the inventive type can in most cases be used in such a way that the potential differences between subsequent electrodes are fairly small, in particular, smaller than the minimum voltage of the Paschen curve. In this case, no partial discharges will occur even where the capacitor contains voids or gas inclusions.

Brief description of the drawings

In the following, the invention will be explained in more detail with reference to the following figures which show only embodiments .

Fig. 1 schematically shows a partial top view of a polymer film according to the invention,

Fig. 2 shows enlarged a cutout from Fig. 1 as indicated by II,

Fig. 3 shows an axial section through a voltage transformer according to the invention,

Fig. 4 shows an axial view of the voltage transformer of Fig. 3,

Fig. 5a schematically shows a partial longitudinal section of a film according to a first embodiment of the invention,

Fig. 5b schematically shows a cutout from a section of a capacitor wound from the film according to Fig. 5a,

Fig. 5c schematically shows a cutout from a section of a another capacitor wound from the film according to Fig. 5a,

Fig. 6a schematically shows a partial longitudinal section of a film according to a second embodiment of the invention,

Fig. 6b schematically shows a cutout from a section of a capacitor wound from the film according to Fig. 6a,

Fig. 7a schematically shows a partial longitudinal section of a superposition of two films, each according to the second embodiment of the invention,

Fig. 7b schematically shows a cutout from a section of a capacitor wound from the superposition of films according to Fig. 7a and

Fig. 8 schematically shows a cutout from a section of a capacitor zigzag-folded from a film similar to the film according to Fig. 6a.

Description of the preferred embodiments

A film 1 which may be used for producing a capacitor, in particular for a voltage transformer as shown in Fig. 3, 4 comprises (Fig. 1) a base 2 comprising electrically insulating polymer material, with a first surface which is partially covered by a coating of electrically conductive material. The film 1 can be wound or folded in longitudinal direction of the film 1. The longitudinal direction of the film 1 is that direction, in which the curvatures induced by winding or folding occur consecutively. The film 1 forms a long strip of usually but not necessarily constant width. Its thickness is preferably between 2 μm and 20 μm. The material of the base 2 is, e.g., polyethylene, polystyrole, polypropylene, polycarbonate, polyimide, polyethyleneterephtalate (PET) , polyethylenenaphtalate

(PEN) , polyester, epoxy resin, polysulfone or a mixture of such polymers possibly also comprising additives and suitable for forming a thin film with high dielectric strength. The coating is in the form of a single sequence of at least four but usually a much greater number of rectangular coated areas 3 which follow one upon the other in a longitudinal direction. The lateral boundaries of the said coated areas 3 are spaced from the lateral edges of the film 1, i.e. the margins of the base 2, so the latter exhibits non-coated strips adjacent to its margins which extend over virtually the whole length of the film 1 without interruption. Subsequent coated areas 3 are in each case separated by a transverse interstice 4 or gap in the coating. As a consequence, the said areas 3 are electrically insulated from each other.

The coating comprises preferably metal, and is in particular a metallization applied by vapour deposition, but use of composition of materials or other electrically conductive

materials beside metals, e.g., carbon or a semiconducting material, is also possible. The electrically conductive coating can be patterned as known per se (s. EP 0 225 822 Al), that is, it can comprise e.g., quadratic, patches 5, 5' (s. Fig. 2 which shows an enlarged cutout II from Fig. 1) with adjacent patches 5, 5' separated by uncoated strips 6 and only connected by thin bridges 7 of coating at the corners. If a partial breakdown occurs at one of the patches, e.g., patch 5', the surrounding bridges 7 evaporate, thereby interrupting the electrically conductive connections of the patch 5' in question with its neighbours. Alternatively, a graded or uniform conductive coating can be used. The electrical conductivity of the coating may be relatively low as larger currents will normally be perpendicular, that is, its surface resistivity will usually be at least IωQ (Ohm-square) .

In another preferred embodiment the film (1; Ia; Ib) is windable or foldable such that subsequent film layers of the at least one film (1; Ia; Ib) are arranged one on top of the other, and in that at least one sector exists, which extends through all film layers and over the whole width of the film (1; Ia; Ib) and which sector contains only non-coated interstices (4; 4a, 4a'; 4b, 4b')- The longitudinal extension of the conductively coated areas (3, 3' ; 3a, 3a' ; 3b, 3b' ) and/or the longitudinal extension of the non-coated interstices (4; 4a, 4a'; 4b, 4b') may vary.

In the case of a wound film (1; Ia; Ib) the variation can be done in such a way that the longitudinal extension of consecutive non-coated interstices (4; 4a, 4a'; 4b, 4b') increases in the longitudinal winding direction such that .the sector has a cone-like shape. In an alternative, the variation is done in such a way that the longitudinal extension of the non-coated interstices (4; 4a, 4a'; 4b,

4b' ) is constant and in that the longitudinal extension of conductively coated areas (3, 3'; 3a, 3a'; 3b, 3b') increases such that the sector has a rectangular shape (Fig. 5c) .

The film can also be folded or wound in such a way that at least two film layers of the folded or wound film have a contiguous conductively coated area extending in longitudinal direction of the at least two film layers without any separation of the coated area by non coated- interstices (4, 4a, 4a ^f , 4b, 4b' ) within these at least two layers.

The leftmost of the areas 3 in Fig. 1 which is electrically conductively connected to a lead 8 is relatively long. The lateral extension of the coated areas 3 is constant throughout whereas their longitudinal extension decreases, in particular, as outlined above. Alternatively, the lateral extension of the coated areas varies, e.g., increases by a constant increment from one area to the next. The rightmost of the areas 3 is connected to another lead 9.

The film 1 is suitable for forming - with its partially conductively coated surface facing inside - a spiral winding which can be used as a high voltage capacitor in a capacitive voltage transformer as shown in Figs. 3, 4. This capacitor 10 is carried by a tube-shaped support 11 of electrically insulating material, e.g., fiber-enforced plastic on which the film 1 has been wound and which in turn surrounds a conductor rod 12. A contact ring 13 which also surrounds the conductor rod 12 and contacts the same with elastic contact fingers is electrically conductively connected to the innermost of the conductively coated areas 3 via lead 9. The capacitor 10 also comprises as a separate first terminal electrode a measuring electrode 14

surrounding the winding and comprising metal, in particular, aluminium foil. It is electrically conductively connected to the outermost of the conductively coated areas 3 by lead 8. Alternatively, it is possible to have the partially conductively coated surface facing outside, if an appropriate electrical connection to the innermost of the conductively coated areas 3 via lead 9 is established. In that case, the outermost of the conductively coated areas 3, which is electrically contacted via the lead 8 is preferably surrounded by at least one film layer, which is electrically insulating, i.e. which is non-coated. The capacitor 10 may comprise a sector containing only non-coated interstices as described above.

In another alternative the electrodes are formed by at least two subsequent film layers arranged one on top of the other.

Alternatively, lead 8 is omitted and the measuring electrode is merely capacitively coupled to the outermost electrode of the capacitor. In priciple it is possible to omit lead 9 as well but this may compromise measuring accuracy. A further possibility is to wind the capacitor directly onto the conductor rod with lead 8 and contact ring 13 omitted.

The measuring electrode 14 is surrounded by a further tube- shaped support 15 carrying a further capacitor, a low voltage capacitor 16. It may be of a conventional design, e.g., as described in EP 1 329 915 Al, comprising two films wound on the support 15 together, each with an electrically conductive coating at an inner surface or outer surface, respectively , which may be a contiguous coating, the coating covering an area extending essentially over the whole length of the winding in each case, where with a first one of the said films the said area extends to a first lateral edge which forms a first face of the winding and at

the second film to an opposite second lateral edge which forms the opposite second face of the winding.

The first face is covered by a contact ring 17 which contacts the conductively coated area of the first film and is electrically conductively connected to the measuring electrode 14 whereas the second face is covered in the same manner by a contact ring 18 which contacts the conductively coated area of the second film and is grounded. The capacitance of low voltage capacitor 16 is much larger than that of capacitor 10, preferably by a factor of between 100 and I ¹ 000 '000, in particular between 100 and lOO'OOO, in particular between 100 and 10' 000, which is at the same time the conversion factor of the voltage transformer. The measuring electrode 14 is electrically conductively connected via the contact ring 17 to an evaluation circuit 19 which may be of known design and delivers an output signal corresponding to the potential of the measuring electrode 14 which reflects the potential of conductor rod 12.

It is advantageous to use in both the high voltage capacitor 10 and the low voltage capacitor 16 the same material for the film or at least for its base, as in this case the influence, which variations of parameters like temperature or voltage may have on the electrical properties of the said capacitors, will be substantially the same and the conversion factor will remain practically unaffected, in particular, where they are arranged in close proximity of each other, e.g., immediately adjacent to each other. As a consequence, high measuring accuracies can be achieved which allows application of the transformer in fields with particularly stringent respective requirements like metering. The voltage transformer can be encapsulated in a

casing of electrically insulating material for mechanical stabilization and protection.

In a preferred embodiment, the low voltage capacitor 16 comprises at least two films, each with a base comprising an electrically insulating polymer and an electrically conductive solid foil, in particular made of metal, metal alloy or carbon, which solid foils are separated from .each other by a base. The at least two films with a solid foil arranged on each film are arranged in form of alternating layers in a winding with the solid foils forming two electrodes, each extending contiguously over the length of the base and exceeding the length of the base on one side.

One solid foil comprises a first contact area, which exceeds the base in the longitudinal winding direction at the most inner turn of the winding of the low voltage capacitor 16 and the other solid foil comprises a second contact area, which exceeds the base in the most outer turn of the winding of the low voltage capacitor 16. One electrode is configured to be contacted at the first contact area, which exceed the bases, i.e. one electrode is to be contacted at the solid foil at the most inner turn of the winding of the low voltage capacitor and the other electrode is to be contacted at the second contact area at the most outer turn of the winding of the low voltage capacitor. With such an arrangement, the solid metal foils can be easily contacted at the most inner turn of the winding of the low voltage capacitor 16 and the most outer turn of the low voltage capacitor 16, respectively. The low voltage capacitor can also comprise two films, each with a base comprising an electrically insulating polymer and at least two electrically conductive coatings insulated from each other by a base. For manufacturing the low voltage capacitor, the solid foils can be applied to base by winding them between

two base layers, i.e. the two bases are wound and during the winding process the solid foils are inserted, each on top of (or each below) a base. The winding process is continued so that the solid foils in the fabricated low voltage capacitor lie between two layers of wound base. Another possibility is to fix the solid foils to the base before or during winding, e.g. by gluing the solid foil on the base or by fixing them together by a heating process, in which base and solid foil are laid above each other and heat is applied, by which at least one of the materials, i.e. the material of the base and / or the solid foil at least partially melts or weakens and thereby forms a connection with the other material .

It is also possible that only one film has two coatings on both sides of the base and the other film having either no coating or one coating or also two coatings. In another preferred embodiment, one film has one coating on one side, and the other film has either one coating on one side or two coatings on both opposite sides.

With the lengths of the areas 3 of the film 1 forming the capacitor 10 following the rule indicated above, every one of them extends over a certain constant angle, in particular, over slightly more than two turns of the capacitor 10, forming an electrode comprising two essentially parallel electrode layers. Subsequent interstices 4 are slightly staggered (see Figs. 5a, 5b). As a consequence, radial components of the electric field are small in the neighbourhood of the interstices 4. Also, throughout every second winding of the film 1 both surfaces of the base 2 are adjacent to electrode layers pertaining to the same electrode as formed by one of the coated areas 3. Therefore the base 2 is not subjected to a strong and permanent electric field which is a further obstacle to an extension of any partial breakdown.

The width of the interstices 4 will have to be chosen large enough to prevent surface flashovers at the expected voltages. The interstices may, however, be much wider than this requirement necessitates and can e.g. extend over a complete turn of the winding or more. The risk of flashovers at the margins is generally quite small, the coated areas 3 being spaced from the lateral edges of the film 1. It is, however, possible to seal the winding there by welding or by covering it with resin or some other suitable material in order to protect the winding against humidity.

The width of the electrodes formed by the coated areas 3 is constant as indicated by dashed lines in Fig. 3. However, it is possible to arrange the lateral extensions of the coated areas in such a way that they follow some non-constant, e.g., linearly decreasing function of the radius in order to influence, in particular, equalise the electric field distribution between the conductor rod 12 and the measuring electrode 14.

Beside the measuring electrode 14, the capacitor 10 may comprise a second terminal electrode which may also comprise metal, in particular, aluminium foil and which surrounds the support 11 and is electrically conductively connected to contact ring 13. The coupling between a terminal electrode and an adjacent electrode of the winding can be via a lead or other electrically conductive connection or merely capacitive. On the other hand, a separate measuring electrode can be omitted and the outermost electrode of the winding directly connected to contact ring 17 and evaluation circuit 19.

As the electrodes in capacitor 10 are electrically insulated from each other they are on usually different floating potentials. It is advantageous if the distances between

subsequent electrodes are such that, with the expected maximum overall potential difference between the conductor and ground, the potential difference between the said electrodes is always below the minimum of the Paschen curve as in this case partial breakdowns of an insulating layer between two subsequent electrodes cannot occur.

According to a second embodiment (Fig. 6a, 6b) of a film according to the invention both surfaces of the film 1 are partially conductively coated surfaces, i.e., each is partially covered with an electrically conductive coating in the manner explained above. The film 1 is used to form a capacitor 10 like the one used in the voltage transformer of Figs. 3, 4. The coated areas 3, 3' extend over slightly more than one turn of the winding which forms the capacitor 10. So, apart from a small overlap, every electrode consists of only one electrode layer. The interstices 4, 4' are staggered by the same amount and the coated area 3 and interstice 4 of the upper surface of a turn is congruent with the coated area 3' and the interstice 4' of the lower surface of a subsequent turn. Here, each electrode layer comprises two coated areas 3, 3' which are arranged one on top of the other. They are substantially congruent and virtually everywhere in mechanical and electrically conductive contact. As a consequence, eventual air inclusions between subsequent turns of the film 1 do not entail any risk of partial discharges as such inclusions are never exposed to a substantial electric field.

According to Figs. 7a, 7b two films Ia, Ib, each conforming to the second embodiment described above, are arranged one on top of the other with bases 2a, 2b carrying congruent coated areas 3a' , 3b and interstices 4a' , 4b to form a superposition which again is wound into a capacitor 10 like the one apparent in Figs. 3, 4. In a manner similar to the

second embodiment, the coated areas 3a and interstices 4a on a first partially conductively coated surface on the upper side of the superposition, i.e., on the first such surface of the upper film Ia, of a turn are congruent with the coated areas 3b' and the interstices 4b' , respectively, on a second partially conductively coated surface on the lower side of the superposition, i.e., on the second such surface of the lower film Ib, of the subsequent turn in each case.

There are, of course, many possibilities of forming capacitors in the form of a winding within the scope of the invention. E.g., several films, comprising at least one film according to the invention and not excluding films without electrically conductive coating, may be arranged one on top of the other to form a superposition and the superposition wound into a capacitor according to the ^• invention.

Apart from windings, it is possible to superpose subsequent sections of a film according to the invention or of a superposition of films comprising at least one film according to the invention into an inventive capacitor in some other way, e.g., by zigzag-folding of the film or superposition. In this way, an, e.g., parallelepipedic capacitor can be formed with the straight film layers forming a stack as indicated in Fig. 8 where a film of fundamentally the same structure as the one shown in Fig. 6a is used. However, all the coated areas 3, 3' have the same length, being somewhat shorter than a section such that in each case an interstice 4 or 4' extends over the fold connecting subsequent film layers and areas adjacent to it. Here again every electrode comprises two substantially congruent coated areas 3, 3 or 3', 3' which are arranged one on top of the other. The film of Fig. 5a or the superposition of Fig. 7a and other films according to the

invention or superpositions comprising such film may, with appropriate modifications, also be used in this way.

The capacitors are preferably dry, that is, they do not contain impregnation fluids applied to the film during the manufacturing of the capacitor and having penetrated its base. Of course, the dimensions, shapes and arrangement of the coated areas and their electrical properties as well as the dimensions and properties of the base may vary widely depending on the specific purposes of the capacitor or other configuration for the production of which the film is to be used.

List: of reference symbols

1, Ia, Ib film

2, 2a, Ib base 3, 3', 3a, 3a' , 3b, 3b' coated area

4, 4', 4a, 4a', _. 4b, 4b' interstice

5, 5' patch

6 uncoated strip

7 bridge 8, 9 lead

10 (high voltage) capacitor

11 support

12 conductor rod

13 contact ring 14 measuring electrode

15 support

16 low voltage capacitor

17, 18 contact ring

19 evaluation circuit