GB2298962A - Metallized dielectic film and relative capacitor - Google Patents

Abstract

A metallized dielectric film 10 is described, used for manufacturing self-healing electrical power capacitors. The surface 30 of electrically conducting material (metallization) of the film 10 is divided into sectors 51, 52 having a particular geometrical structure (essentially honeycomb) such that when under critical operating conditions (discharges, pulsating currents, defects, faults), the capacitor presents greater reliability and lesser losses than known capacitors. The sectors are interconnected by fuses 60. Films 10 can be combined to define more complex electrode patterns. One film edge 40 is preferably thickened by metallization and the other 20A is clear of metallization.

Description

METALLIZED DIELECTRIC FILM AND RELATIVE CAPACITOR This invention relates to a metallized dielectric film used for forming electrical power capacitors.

In the case of dielectric yielding, which occurs at structural defects present within it, a capacitor formed from a dielectric film with metallized surfaces is known to be able to isolate a possible fault zone. A capacitor of this type is usually known as "self-healing" because the energy released consequent on discharge removes the surrounding metallization so as to isolate the area of the short circuit, so enabling the capacitor to continue to operate, although its capacitance is less than the capacitance under rated operating conditions.

On the other hand, in a power capacitor, intense short-duration currents can fuse the zone of contact between the metallized part of the dielectric film and the terminal parts of sprayed metal for forming the contacts. Generally this fusion commences at the weakest points and proceeds until the terminal parts of the capacitive element are totally disconnected, with possibly dangerous thermal effects leading to the complete destruction of the component. In certain cases (high voltage, high capacitance), the discharge can be so violent as to compromise the capacitor dielectric through several successive layers. In this case the self-healing mechanism no longer functions correctly, the result being the appearance of a large region of fused material about the discharge zone, leading in a short time to the destruction of the component.

To reduce the deleterious effects deriving from a high-energy discharge, the film can be divided into "segments" which reduce the energy stored by an elementary capacitor (as described in European patent application 0056010). To make the intervention mechanism more reliable the terminal part of the film, consisting of sprayed metal, can be substantially reduced to create electrically conducting (and fusible) channels which more easily isolate the "segment" in the case of an internal fault. Other alternative solutions include dividing the metal coating of the film into numerous zones of various shape bounded by insulating material and connected together by fusible conducting elements, as in the preceding case.These zones can be of mosaic form (as described in national patent applications DE 4010753 and FR 2651602 and in European patent application 0611179) or of crenelated profile (as described in European patent application 0438344).

Dividing the capacitor into a number of elementary capacitors connected together by fuses, ie the use of segmented film rather than mosaic or crenelated film has the advantage of preventing complete destruction of the component which, in the case of a fault, tends to reduce its capacitance until isolation, with a small gas production and the absence of film fusion or burning (this also reducing the risk of explosion). However, in such devices the losses due to the Joule effect tend to be higher than in the traditional solution. Hence in this case the capacitor is generally unable to tolerate the passage of high-intensity current.

An object of the present invention is to provide a metallized dielectric film usable as the conductive electrode of a power capacitor of self-healing type which obviates the drawbacks deriving from the aforementioned film, ie to provide a metallized dielectric film which at one and the same time is reliable with regard to pulsating current passage retention (and hence reliable with regard to explosion risk), while maintaining losses due to the Joule effect low compared with traditional self-healing capacitors.

A further object is to provide a power capacitor which, in the case of local damage following discharge, undergoes a capacitance reduction which, for equal conditions, is less than the capacitance reduction encountered in traditional self-healing capacitor.

A further object is to make the formation of the metallized dielectric film and the manufacture of the capacitor simple and economical, without the need to use complex technology.

These objects are attained by a metallized dielectric film in accordance with claim 1.

Advantageously, the electrically conducting fusible elements are situated distant from the thick metallization region which forms the contact electrode of the capacitor. Compatibly with the requirements of the deposition process for the layers, these are located in correspondence with those zones of conducting material through which current of low intensity flows. These zones are identified by laboratory experiments, taking account of the flow diagrams for the currents flowing between the conductive electrodes. In this manner a reduction in Joule effect losses is obtained (with respect to the known art) within the elementary capacitor.

The surface of the conducting material coating the dielectric film is divided into sectors, such that the fusible elements act to isolate a sector of film if several layers of the dielectric are perforated, but not following self-healing discharges. The choice of the geometry of the sectors of the conductive electrode is determined by the results of experiments (conducted by laboratory tests) on the behaviour of the fusible conducting elements following the passage of current. All this, combined with the fact that the reduction in capacitor capacitance due to the discharge is less, for equal conditions, than that encountered in in capacitors made from segmented or mosaic film of the state of the art, makes the invention substantially reliable even for power capacitors operating with large currents.Finally, by extending segmentation of the conducting material as far as the thick metallized region used for the contacts, fusion of said region in the case of a fault, and beginning at a single point, is prevented from proceeding along the entire length of the film, with consequently greater retention of pulsating currents.

The characteristics of the invention will be more apparent from the description of one embodiment thereof given hereinafter by way of non-limiting example with reference to the accompanying drawings in which: Figure 1 is a schematic plan view of a portion of a metallized dielectric film according to the invention; Figure 2 is a schematic plan view of a portion of a second embodiment of the film of Figure 1, which, when combined with the structure of Figure 3 forms an electrical circuit equivalent to two capacitors in series; Figure 3 is a schematic plan view of a portion of a non-segmented metallized dielectric film, used as a conductive electrode within a self-healing power capacitor, according to the known art; Figure 4 is a schematic plan view of a portion of a third embodiment of a metallized dielectric film according to the invention; ; Figure 5 is a perspective view of the metallized dielectric film portion of Figure 1.

With reference to Figures 1 and 5, the metallized dielectric film according to the invention is indicated overall by 10 and comprises a substrate 20 of dielectric material, an insulating edge 20A and a surface 30 of electrically conducting material.

The insulating edge 20A is usually of plastic or paper. The surface 30 terminates in regions 40 of thicker metallization, in correspondence with the terminal parts formed from sprayed metal where the capacitor contact electrodes are provided. Said surface 30 is divided into sectors 51, 52 of substantially pentagonal shape. The sectors 51, 52 are obtained by forming "segments" 11, 12, 15, all consisting of dielectric material.

A first series of "segments" 11, all parallel to each other, isolate the sectors 51 from each other. Each "segment 11 is positioned in contact with the thick metallization region 40 and extends towards the interior of the conducting surface 30 as far as the vertex 14, where a further two "segments" 12 meet each other. Said two "segments" 12 and the "segment" 11 meet at the vertex 14, forming three substantially equal angles.

A second series of "segments" 15, also parallel to each other and parallel to the "segments" 11, isolate the sectors 52 from each other and are located in contact with the insulating edge 20A.

Each "segment" 15 is located in an intermediate position between two consecutive "segments 11, on the opposite side of an imaginary horizontal axis 16 passing substantially through the centres of the "segments" 12 and dividing the film 10 into two parts. The "segment" 15 encounters the two "segments" 12 at the vertex 21, to form three substantially equal angles.

Each sector 51 is connected to two sectors 52 via two fusible conducting elements 60, which intervene in the case of discharge to isolate from current passage that sector 51, 52 involved in the fault. The fusible conducting elements 60 are located substantially at the centre of the "segments" 12 and are spaced from the thick metallization region 40 to reduce Joule effect losses when current flows. The distance between the vertex 21 and the upper edge of the thick metallization region 40 is less than one half of the total height of the film 10 (this latter indicated by "h" in Figures 1 and 5 and comprising the height of the insulating edge 20A, the height of the surface 30 and the height of the thick metallization region 40).The distance between the lower edge of the thick metallization region 40 and the vertex 14 of each sector 51 is also less than one half the total height "h" of the film 10. By extending the constituent dielectric material of the "segments" 11 of the sectors 51 into the thick metallization region 40 of the film 10, fusion of said region 40, beginning at a point, is prevented from propagating and extending along the entire length of the capacitor conductive electrode.

This also increases the reliability of the film 10 from the explosion aspect, in the case of large currents of pulsating type.

The conducting material coating of the surface 30 is usually obtained by depositing metal layers, whereas the segmentation of the sectors 51, 52 and the formation of the fusible conducting bridges 60 are achieved usually by localized ablation or during metallization.

The capacitor manufacture consists of superposing two metallized dielectric films 10 such that the substrate 20 and the metallization region 40 are inverted to each other and are slightly offset in the vertical direction. The films 10 formed in this manner are rolled together along the shorter side. A cylinder is obtained, this constituting the actual capacitive element.

The metallized dielectric film 10 of Figure 1 can also be advantageously employed when two or more capacitors connected in series have to be used. By ideally joining the film portion indicated by 80 in Figure 2, along a geometricai axis 17 (positioned substantially along the centre of the film 25), to a second film portion 80' having the same structural characteristics as the portion 80, and combining the structure 25 obtained in this manner with the metallized dielectric film 90 of Figure 3, an electrical circuit is obtained equivalent to two capacitors in series. The structure 25 is scheduled to operate at a potential difference equal to i the potential difference used for the operation of the capacitor having the film 10 of Figure 1 as its conductive electrode.

With reference to said Figures 2 and 3, 20'A indicates the insulating edge, 30' indicates the coating of electrically conducting material, 40' indicates the thick metallization region containing the contacts and 12' indicates the "segments" of the sectors 51', 52' of the structure 25, the seat of the fusible conducting elements 60'.

The electrical circuit equivalent to two capacitors in series can also be obtained by ideally coupling two portions of metallized dielectric film 10, along the imaginary axis 19, as shown in Figure 4. With reference to this figure, 26 indicates a film portion overall, 20"A indicates an edge of insulating material, 30" indicates a surface coated with conductor material, and 40" indicates a thick metallization region, the seat of the capacitor contacts. The elements indicated by 11", 12", 14", 21", 51", 52", 60" are analogous to and have the same structural and functional characteristics as the elements indicated respectively by 11, 12, 14, 21, 51, 52, 60 in Figure 1 and the elements indicated by 11', 12', 14', 21', 51', 52', 60' in Figure 2.

The characteristics of the metallized dielectric film of the present invention are apparent from the aforegoing description, as are the resultant advantages.

The metallized dielectric films 10, 25, 26 according to the invention are particularly suitable for manufacturing power capacitors, in which intense short-duration currents flow which can fuse the contact zone between the thick metallization regions 40, 40', 40" and the metal sprayed on the terminal parts for forming the contacts.

The sectored structure 51', 52', 51", 52" can also be used in films 25, 26 for power capacitors connected in series, using the method of the present invention for metallized dielectric films 25, 26 subjected to a tension of i the tension used for a single capacitor, the second film (forming one of the conductive electrodes of the capacitor) being a non-segmented metallized dielectric film 90. In these cases it has been noted experimentally, during laboratory tests, that the fuses 60" can be positioned along the isolating "segments" 12" (ablating the conducting material 30"), and, in particular, substantially in a central position, so as to considerably reduce Joule effect losses.

Modifications can be made to the embodiment of the invention without leaving the principles on which the inventive idea is based, and moreover in the practical implementation of the invention the materials and dimensions can be chosen according to requirements.

Claims (10)

1. A metallized dielectric film (10, 25, 26), of the type comprising at least one substrate (20, 20', 20") of dielectric material, at least a first insulating edge (20A, 20'A, 20"A) and a surface (30, 30', 30") of electrically conducting material which terminates in generally thick metallization regions (40, 40', 40") corresponding to the terminal part of the substrate (20, 20', 20"), in which said surface (30, 30', 30") of electrically conducting material is divided into a plurality of first sectors (51, 51', 51") and second sectors (52, 52', 52") which are isolated from each other overall except at electrically conducting fusible elements (60, 60', 60"), characterised in that said first sectors (51, 51', 51") and said second sectors (52, 52', 52") are obtained by forming a plurality of "segments" (11, 11', 11", 12, 12', 12", 15, 15', 15") consisting of dielectric material, of which: - a first series consists of mutually parallel equal-length first "segments" (11, 11', 11"), each of which is positioned with a first end in contact with said thick metallization region (40, 40"); - a second series consists of second "segments" (12, 12', 12"),of which at least to have a first end (14, l4',l4"! in common with each other and in common with the second end of at least one of said first segments" (11, 11', 11"), so as to form three angles;; - a third series consists of equal-length third "segments" (15, 15', 15") parallel to each other and parallel to and alternating with said first "segments (11, 11', 11"), and each positioned on that side of a geometrical horizontal axis (16, 16', 16") dividing the film (10, 25, 26) into two parts which is opposite to said first "segments" (11, 11', 11"), with a first end in contact with said insulating edge (20A, 20'A, 20"A), at least one of said third "segments" (15, 15', 15") having its second end (21, 21', 21") in common with the second end of at least two of said second "segments" (12, 12', 12") so as to form three angles, at least one of said first sectors (51, 51', 51") being connected to two of said second sectors (52, 52', 52") via at least two electrically conducting fusible elements (60, 60', 60") located in correspondence with said second "segments" (12, 12', 12").

2. A dielectric metallized film (10, 25, 26) as claimed in claim 1, characterised in that said first sectors (51, 51', 51") and said second sectors (52, 52', 52") are of pentagonal shape.

3. A dielectric metallized film (10, 25, 26) as claimed in claim 1, characterised in that the distance between the electrical contact of said metallization region (40, 40") and said second end (21, 21', 21") of said third "segment" (15, 15', 15") is less than one half the total height (h) of the film (10, 25, 26).

4. A dielectric metallized film (10, 25, 26) as claimed in claim 1 or 3, characterised in that the length of said first "segments" (11, 11', 11") is less than the distance between the electrical contact of said metallization region (40, 40") and the second end (21, 21', 21") of said third "segment" (15, 15', 15").

5. A dielectric metallized film (10, 25, 26) as claimed in claim 1 or 4, characterised in that the distance between the electrically conducting fusible elements (60, 60', 60") and the electrical contacts of said metallization region (40, 40") is greater tan the length of said first "segments" (11, 11', 11").

6. A dielectric metallized film (10, 25, 26) as claimed in claim 1, characterised in that said first end of said first "segments" (11, 11', 11") is inserted into the interior of the thick metallization region (40, 40").

7. A dielectric metallized film (25) as claimed in claim 1, characterised in that said first insulating edge (20A) is removed and said thick metallization region (40) is replaced by a second insulating edge (20'A), the film (80) formed in this manner being coupled to at least one second film (80') of the same type as said film (80) and having the same structural and functional characteristics, and to at least one metallized dielectric film (90) of self-healing type, so as to obtain an electrical circuit equivalent to series-connected capacitors.

8. A dielectric metallized film (26) as claimed in claim 1, characterised in that said film (10) is coupled to at least one second metallized dielectric film (10") of the same type and having the same structural and functional characteristics as said film (10), so as to obtain an electrical circuit equivalent to series-connected capacitors.

9. A capacitor of self-healing type, characterised by comprising at least one metallized insulating film (10, 25, 26) in accordance with at least one of the preceding claims.

10. A metallized dielectric film (10, 25, 26) and capacitor as substantially described and as illustrated on the accompanying drawings.