Capacitor

It is known to use aluminum foil electrodes of equal width in high voltage capacitors used to correct the AC power factor or for DC applications such as energy storage and discharge or filtration. Due to the width tolerances of the aluminum foil rolls and the manufacturing tolerances allowed when wrapping aluminum foil electrodes with insulating spacers into a helical capacitor part, the edges of the foil electrodes are not precisely extended from one end to the other. It is not considered practical to exercise as much care as would be necessary to ensure a complete extension. This results in an electrode system in which the edges of the foils are displaced relative to each other to some extent. Normally, at one end of the capacitor, the first foil extends laterally outwardly less than the second foil. At one end of the capacitor, the second foil extends laterally less

671 51 far than the first. During use, the voltage load is relatively high at the edge of the retracted foil. Thus, there are points at each end of the capacitor where the load is relatively high. This higher voltage load causes the breakdown voltage to be lower and the partial discharge start voltage to be lower than if the foils were exactly in extension.

Significant advances in power capacitor structures are described in U.S. Patent 3,857,073 and Japanese Patent Publication Nos. 28516/74 and 34141/74. These publications describe a capacitor comprising a layer of capacitor-grade paper and a layer film, in particular a polypropylene film, as a composite insulator between two electrode foils, of which the foil in direct contact with the paper is narrower at both edges than the foil in direct contact with the film . These structures, impregnated with an electrically insulating medium, have been shown in the overvoltage test to perform clearly better than otherwise similar capacitor structures in which the foil electrodes are of the same size and intended to be in extension but susceptible to normal manufacturing deviations from accurate extension. The advantage of this arrangement is that the narrower electrode is on both edges mainly of the paper layer, which alleviates the loading problem compared to the case where the electrodes are of the same width and intended to be in extension but not actually in extension due to manufacturing deviations.

An improved electrode fit arrangement is further explained. Namely, one in which one foil is not only offset from the other at both edges, but the edges of this narrower foil are also made rounded and smooth, for example by double-folding the extreme side portions of the foil. It has been found that an improvement in the overvoltage characteristics is thus achieved with respect to otherwise similar capacitors in which the edges of the narrower foil are directly cut.

Various capacitor-grade papers and various synthetic plastics are used as capacitor insulators as films of polymeric hydrocarbons, including members of the polyolefin group, of which polypropylene is the most widely used in power capacitors.

All kinds of paper, all kinds of transparencies and combinations of paper and transparencies have been used. Polychlorinated biphenyls (PCBs) have been most commonly used as the insulating liquid impregnating agent. The use of these fluids is currently discontinued because they are considered environmental pollutants. Various alternative fluids are used today and others are being considered. Some of these look promising as common substitutes for PCBs. However, their electrical properties are not the same as those of PCBs and the scope of their use and testing has been so limited compared to PCBs that full assurance of satisfactory operation over long periods of time is not available in a binding manner. Therefore, capacitor designers and manufacturers are facing a fundamental change in a tried and tested system for one element, namely the insulating fluid, leading to further improvements in other aspects of capacitor design that could provide the highest margin for safe, reliable operation and good surge handling and long life. .

The present invention is largely the result of a better understanding of the mechanism of overvoltage breakdown in relation to electrode fit, solid insulator components and liquid impregnant properties, leading to novel capacitor structures and giving the capacitor designer greater ability to make

The present invention is characterized in that the cutting edges of the aluminum foil material are bent substantially towards the center line of the foil.

The invention is explained in the following, by way of example, in connection with the accompanying drawing, in which:

Figures 1, 2 and 3 are cross-sectional views of prior art capacitor structures;

671 51 Figures 4, 5 and 6 are cross-sectional views of capacitor structures in accordance with embodiments of the present invention;

Fig. 7 is a cross-sectional view of a capacitor electrode suitable for use in embodiments of the present invention; and Figure 8 is a sectional view of another embodiment.

Figures 1, 2 and 3 show capacitors according to the prior art.

Figure 1 shows a capacitor structure having two electrode diffusion layers 10 and 12 of the same size separated by an insulating layer or layers 14 consisting of capacitor-grade paper or polymer film or combinations of one or more layers of both paper and film. The normal manufacturing practice is that the two foil electrodes are assembled with an insulator between them, and the second insulating layer is further in contact with the other foil electrode as shown in the figure. When the stack thus assembled is wrapped, there is an insulating material 14 on each side of each electrode. These structures, and all the structures described below, are normally and preferably impregnated with an insulating liquid. In the past, this has normally been some kind of halogenated biphenyl. The problem with the structure according to Figure 1 is that it is difficult to obtain foil electrodes of the same size for precise extension, which leads to a decrease in the electrical load resistance at the edges, as has been said above.

The structure of U.S. Patent No. 3,857,073 shown in Figure 2 wherein one of the foil electrodes 11 is made narrower than the other 12, and the insulating material comprises at least a first layer of paper 16 in contact with the narrower foil electrode 11 and a second layer of film 18, especially polypropylene film, in contact with wider electrode 12 . Such structures provide even better electrical load resistance compared to otherwise similar structures in which the foil electrodes are the same in size but inherently prone to be out of extension.

Figure 3 shows a narrower foil electrode 13 of a wide and narrow foil electrode combination, the edges of which are rounded and smoothed, for example by folding, and in which the insulator 14 'consists of three layers of film.

In general, it can be shown that in a given insulation system, the displaced electrodes cause a higher voltage load than the electrodes in the extension, and the voltage load increases up to a certain limit as the displacement increases. A difference of about 250 [mu] m in the dimensions of the foil on each side allows to be made without additional care, for example a situation where one edge of the narrow foil is about 200 or 230 [mu] m from the edge of the wide foil and the other edge of the narrow foil is 279 or 305 [mu] m. from the edge of the wide foil is satisfactory and much better than a similar variation in the case of foils of the same size. The invention takes this into account and takes advantage of it to provide capacitor structures with intentional electrode displacement and control of the nature of the insulating material in contact with the narrower electrode. Controlled stress loading at the edge of the narrower foil without extreme care in assembly is thus permissible.

Figure 4 shows a structure using a combination of a wide and a narrow foil with the edges directly cut, while Figure 5 shows the same structure with the edges of the narrower foil being rounded and smoother. In either case, the insulator is in particular a combination of selected materials, in which the insulating constant of the first layer 26 in contact with the narrower electrode 11 or 13 is higher and preferably also the resistivity lower than the second layer in contact with the wider electrode foil 13. The result of this in terms of electrical properties is that when a voltage is applied between the foil electrodes, the voltage load is distributed across the insulating sheets in inverse proportion to their insulating constants. When the material 26 has a high insulating constant in contact with the narrow foil, the load on this high insulating constant material is relatively low, and thus the load is also low at the edge of the narrow foil.

It happens to be that the insulating structure shown in Figure 2 above usually satisfies this with respect to the desired electrical properties. So. normally, layer 16 of capacitor-grade paper impregnated with an electrically insulating liquid provides a higher insulation constant and lower electrical resistance than layer 18 of polypropylene film with normal properties. What was not taken into account was that the improved properties of the structure presented in it are related to the ratio of the electrical properties of the different parts of the composite insulation. Now that the importance of this in terms of electrical properties is understood, it becomes possible to provide composite insulators other than paper and polypropylene, which provide even better performance in wide-narrow-foil electrode fitting. For example, the following table provides examples of insulator combinations that can be used together, including combinations in which a polypropylene film is in contact with wide foil electrodes and insulators other than capacitor grade paper are used in contact with narrower foil electrodes. Also in combinations where capacitor grade paper is used as the insulator in contact with the narrower foil, materials other than polypropylene are used in contact with the wider foil. When impregnated, all of these structures provide good electrical load resistance.

Table of insulation material examples

<td colspan="2">Floor 26 narrower next to the electrode Insulation constant</td><td>Floor 28 wider next to the electrode</td><td>Insulation standard</td>

<td>Polyester age</td><td> 3,2</td><td>with polypropylene</td><td> 2,2</td>

<td>Polyphenylene oxide film</td><td> 2,6</td><td>polypropylene film with</td><td> 2,2</td>

<td>Cyanoethyl ± cellulose</td><td> 18,0</td><td>polypropylene film with</td><td> 2,2</td>

<td>Capacitor grade kraft paper</td><td> 6,2</td><td>polyethylene film with</td><td> 2,2</td>

<td>Capacitor quality kraft paper</td><td> 6,2</td><td>polycarbonate film with</td><td> 3,1</td>

<td>Capacitor grade kraft paper</td><td> 6,2</td><td>polyamide film with</td><td> 4,6</td>

<td>Capacitor quality kraft paper</td><td></td><td>polyamide film with</td><td> 4,5</td>

<td colspan="2">The following are examples</td><td>capacitor tests,</td><td>that</td>

illustrate the improvement achieved by using a material with a higher insulation constant in contact with a narrower electrode and compare them to the opposite situation where a material with a higher insulation constant is in contact with a wider foil electrode. Examples 1 and 2 were impregnated with mineral oil and Examples 3 and 4 were impregnated with polychlorinated biphenyl.

<td></td><td>In touch</td><td>In touch</td><td>Alternating current-</td><td></td>

<td>Example</td><td>narrow electrical</td><td>wide electronic</td><td>testing-</td><td></td>

<td>nto</td><td>with din</td><td>with din</td><td>voltage</td><td>Score</td>

<td> 1</td><td>18, um</td><td>25yUm: n</td><td rowspan="2"> 4750</td><td>Visible</td>

<td rowspan="3"></td><td rowspan="3">polyester</td><td rowspan="3">polypropylene</td><td rowspan="3">ronap eruption, breakthrough</td>

<td></td>

<td> 5750</td>

<td> 2</td><td>2 5, um: n polypropylene</td><td>18.um: n polyester</td><td> 4000</td><td>Visible corona discharge,</td>

<td></td><td></td><td></td><td> 5000</td><td>breakthrough</td>

<td> 3</td><td>3 sheets of 23yUm</td><td>18yUm: n</td><td rowspan="2"> 7000</td><td rowspan="2">No visible corona</td>

<td rowspan="3"></td><td rowspan="3">vo imapaper ia</td><td rowspan="3">polyethylene</td>

<td></td><td>folder,</td>

<td> 7500</td><td>breakthrough</td>

<td> 4</td><td>3 sheets of 23 μm kraft paper</td><td>18yum: n polyethylene</td><td> 5250</td><td>No visible corona discharge,</td>

<td></td><td></td><td></td><td> 5500</td><td>breakthrough</td>

Examples 1 and 3 are examples of a primary structure in which a material with a higher insulation constant is located closest to the narrow electrode and shows an approximately 15-30% improvement in breakdown voltage, indicating a significantly improved voltage load resistance.

The relative porosity of the different layers of insulation 26 and 28 and the relative insulation constants of the insulating liquid used for impregnation relative to the insulation constant of the sheets may also be relevant. Thus, in general, it is preferred to use a relatively non-porous insulating sheet 28 in contact with the wide foil 12 and a more porous insulating sheet 26 in contact with the narrow foil 11 or 13 and to use a liquid with a relatively high insulating constant for impregnation, which increases the insulating constant of the sheet 26. voltage load on. Examples of suitably porous insulating sheets (for layer 26) are:

kraft paper;

synthetic papers of polymeric fibers such as polypropylene fibers;

synthetic papers made porous or cellular by stretching polymeric films filled with inorganic powders such as calcium carbonate.

Examples of suitable non-porous insulating sheets (for layer 28) are:

Insulation constant

Polyester film 3.2 Polyphenylene oxide 2.6 Polypropylene 2.2 Polystyrene 2.5 Polyethylene 2.2 Polycarbonate 3.1

The insulation constant of a porous sheet can be increased by impregnating it with a liquid having a high insulation constant (at least about 5) such as:

Insulation constant

Chlorinated biphenyl 5.8

Tolyl-xylyl-sulfone 20.0

Diethyl hexyl phthalate 5.2

The above discussion of the ratio of layer insulation constant to electrical resistance is relevant for capacitors for both AC and DC applications. However, for AC applications, the ratio of insulation constants is most important, while for DC applications, the ratio of resistances is most important. The above examples mainly concern the ratio of insulation constants that can be used to make good AC capacitors. For DC applications where relative resistivity is more important, the following are examples of suitable materials that can be used. Examples

19 for sheet materials with a high resistivity (approximately lo - 10 ohm cm):

polyester film;

polyphenylene oxide film;

polycarbonate film; and a polyolefin (e.g., polypropylene or polyethylene) film.

Examples of sheets with low resistivity (approximately IO · *⁰ - 10¹⁴ ohms cm):

kraft or synthetic paper, impregnated with low-resistance liquids such as:

mineral oil;

diethylhexyl phthalate;

671 51 isopropylated diphenyl; or alkalized naphthalene, preferably in combination with liquid additives such as:

tinatetraphenyl;

cadmium carboxylate;

beta-methyl anthraquinone; or an epoxide, which additives further reduce the resistivity of the liquid.

The structure of Figure 5 using a rounded-smooth-edged narrow foil electrode offers advantages over the structure of Figure 4. It should be noted that in cases where the smooth edges are obtained by folding, it is not critical in which direction the folding is made or which surface of the foil is outside the folding. Normal aluminum foil, available for use in capacitors, has an inherently glossy surface and a matte surface with some difference in roughness. It has been found that if the matte surface is outside the fold, there is some improvement compared to the case where the clear surface is outside the fold. In general, however, rounding and ironing of the edges of the narrower foil can be accomplished in many other ways than folding, for example, assuming that the foils are directly cut as in Figure 4, the edges of the narrower foil can be treated by coating, flame treatment, chemical treatment and electrical discharges. from sharp points, causing these points to burn off. It is also advantageous to use thick foil materials. This is not essential, but a better result is obtained. As a narrower, rounded-edged foil, a thick foil made of 25 μm material is used than if a 6 μm thick material is used.

Yet another structure is shown in Figure 6. The structures of Figures 1-5 have two electrodes outside the capacitor. The structure of Figure 6 has two narrow foils 13 and 13 ', preferably with rounded edges, between which are two groups of capacitor insulating layers as discussed in connection with Figures 4 and 5, each with a wider electrode foil 22 between the two groups. This wider foil 22 in this combination is intended as a floating conductor which is not gently connected to any voltage outside. Its voltage is determined by the voltage distribution within the insulation system. The floating conductor 22 reduces the thickness between the actual electrodes 13 and 13 'and the floating conductor 22 by half the total thickness between the electrodes 13 and 13'. The ratio of the edge load to the frame load decreases as the thickness between the electrodes decreases. Thus, the overvoltage resistance per unit is increased when a floating conductor 22 is used. According to the present invention, the floating conductor 22 is made wider than the two actual electrodes 13 and 13 ', the edges of the two electrodes 13 and 13' are preferably rounded and the floating conductor and the insulator between the electrode is a combination of layers selected according to their electrical properties as described above.

Figure 7 shows a rounded edge foil electrode 23 used as a narrow electrode. The rounded edges of FIG. Thus, towards each of the adjacent insulating materials, the electrode 23 has turned its rougher surface, which further facilitates the impregnation of the adjacent insulating with the insulating liquid.

Fig. 8 is a partial view of a further development generally consistent with Fig. 6, in which the floating conductor comprises two separate electrically conductive strips 22a and 22b, both arranged to cover the edge portion of the capacitor electrode 13, which electrode 13 is substantially in extension with the second electrode 13 '. In both Figures 6 and 8, the floating conductor may consist of separate foil layers 22 or layers 22a and 22b, or may be formed by immediately depositing a metal material on one of the insulating sheets, i. into one or two metal layers Especially when this metallization is used, the structure according to Figure 8 provides the desired advantages in saving material.

Options for insulators for use in the structures of Figures 4-8 include film materials that have rough surfaces or are otherwise made suitable as absorbent and absorbent core materials. These materials include biaxially oriented polypropylene or polyethylene films or films of polyester, polycarbonate, amide, or other plastics. All nft films may have a modified surface on one side, i.e. rough, or may be formed by co-extruding with a thin film a copolymer embossed to give a rough surface on one side of the base film, or a second layer of polypropylene may be applied to one surface of this base film. composite fibers to act as a lymphatic layer, or a film layer may be used, containing a filler and made by a stretching treatment to obtain a paper-like internal, absorbent cellular structure which may be termed synthetic paper.

Thorough impregnation of all cavities in the insulator layer of the capacitor is necessary for use in capacitors of the AC or energy storage type with high voltage loads. This can be achieved by using any of the above-mentioned materials, using one of these materials as a single-layer film layer together with one or more standard capacitor-grade films which in themselves have a higher electrical load resistance and a lower cost. Since some deterioration in electrical load resistance is likely to occur in the case of such modified films compared to unmodified films, it becomes all the more important that the insulating impregnation fluid is well selected for its insulation constant and resistivity and contains some additives such as epoxide or beta-methyl anthraquinone.

The invention provides designers with an even wider range of capacitor structures with composite insulators than has previously been available. However, it is expected that prior art structures in which the insulating sheets are of the same material (e.g., all paper or all of the film) or at least the same material in contact with both electrodes (e.g., a film-paper-film combination) and have foil electrodes of the same size or width, remain in use, so that the present invention now provides new alternatives for the considered choice of materials, electrodes and their arrangement.

TT ”

671 51