DE7336917U - ELECTRIC CAPACITOR - Google Patents

Description

Folder 23 334 - Dr. Κ / by
Case P 23800

IMPERIAL CHEMICAL INDUSTRIES LTD. London, UK

Electric capacitor

I The innovation relates to electrical capacitors,

I a spacer made from a modified thermopla-

% stic polymeric dielectric film.

I In the past, dielectric spacers, especially spacers

v Stand holder for use in capacitors, from power

j? Paper or other cellulosic materials. In

I recently have this films of synthetic Har-

, zen either alone or together with kraft paper sheets

■ used. Synthetic resin films in general

j and films made of polypropylene in particular are dielectrics

': superior to cellulose sheets because they are higher

Withstand tensions per unit of strength because they are less too Defects, e.g. small holes or conductive particles,

r tend to decrease the electrical strength and because

I they have better Lelsturigsfactor clear characteristics. Because

| - These and other properties can be used by capacitors

; before having dielectrics made from a polypropylene film

: the same capacitance and the same voltage resistance can be made smaller than those made with paper dialectrics. However, thin, oriented films made of polypropylene have the disadvantage that they are tight to both

733891710.1.74

each other as well as the electrode foils or other sheet materials with which they are used in the manufacture of capacitors be wrapped, adhere. The resulting sticking (commonly referred to as "blocking") of the polypropylene film surfaces on "adjacent surfaces made of polypropylene film or a metal foil makes it difficult to to impregnate such capacitors after winding. Furthermore, the tendency to stick causes the finished wound Capacitor has air bubbles, and the tendency to stick prevents an easy flow of a dielectric Fluid to all parts of the winding. As a result, the electrical strength is greatly reduced, begins the corona discharge at lower voltage and the capacitors are subject to premature breakdown and have a shorter useful life. Furthermore, the relatively non-porous nature of the polypropylene film makes it difficult to ensure adequate impregnation in a reasonable time. A bad Impregnation has free pores and bubbles in the spaces between the film and an adjacent non-porous one Surface area, thereby degrading the capacitor and causing premature breakdown can.

A method that is already known to improve the impregnation of capacitors, which polyolefin dielectrics is to use a porous material such as Kraft paper with the film. In these composite or layered dielectric capacitors, the porous layer is due to the suction effect a better and more uniform impregnation of the capacitor by the dielectric fluid. However the use of a paper layer reduces the possibility of the dielectric having high voltages per unit thickness endures. It also increases both the energy factor and the size of the unit, so that im

I * · ♦ ♦

generally the useful results are lost again to an economically significant extent.

Another known method known for improving the impregnation of polyolefin films is by placing particles of a finely divided insulating material on the surface of a pre-formed thin Applies sheet material from which laminated dielectric bodies can be fabricated.

It is also known to manufacture a capacitor in which the dielectric spacer is made from a sheet a synthetic resin film wherein particles of alumina having an average particle diameter dispersed by less than 1 µ. However, with particles of this size it has been difficult proven to achieve rapid impregnation of the capacitor.

It has now been found that improved capacitors are obtained when incorporated into the thermoplastic film comprising the dielectric spacer represents an inert particulate dielectric material having a relatively large particle diameter is incorporated.

So according to the innovation, an electrical capacitor is proposed that has a pair of conductive electrodes dielectric spacers arranged between the electrodes and an impregnation in the spacer comprising a dielectric fluid, the spacer being formed from a film of a thermoplastic polymeric material consists of 0.01 to 1.0 checked tf, based on the Weight of film, an inert particulate dielectric material having an average particle diameter from 1 to 5 µ which forms protrusions on at least one surface of the film.

The thermoplastic polymeric films that are suitable for the

Suitable for use as dielectric spacers in capacitors according to the innovation are, for example, films made from any thermoplastic materials are made that have the required electrical properties and can be made into a film by an ordinary film-making process. Suitable materials are e.g. polycarbonates, polyamides such as polyhexamethylene adipamide or polycaprolactam, polyesters such as polyethylene terephthalate and polyethylene-1,2-diphenoxyethane-4,4 · - dicarboxylate, polysulfones and polyolefins. A special one preferred material is a high molecular weight stereoregular, predominantly crystalline polymer of propylene, and either in the form of a homopolymer or in the form of a mixed polymer with smaller amounts, for example up to 10% by weight of other unsaturated monomers, such as ethylene.

Films are suitably made from these materials in any convenient manner, for example by rolling, extruding, pressing, solvent casting, or melt casting. They are preferably oriented by stretching the film to increase its strength. A particularly suitable film is made by melt-extruding polypropylene in the form of a tube from an annular die, inflating the tube by the so-called ^M bubble ^{method 11} to introduce a transverse orientation, and at the same time stretching the tube longitudinally to stretch the film longitudinally orientate. The film is then thermoformed at a temperature below its melting point.

The particulate material incorporated into the dielectric film spacers is any suitable inert, finely divided material that maintains its particulate nature during and after film production. By the term "inert" it is meant that the particulate material is integral with both the thermoplastic film, in which it is dispersed as well as with any

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The impregnation agent subsequently applied to the film is largely chemically unreactive and is an electrical one represents insulating material. The particulate Material must also be compatible with the polymeric film-forming material in order to ensure uniform dispersion of the particles to ensure. Suitable particulate materials are barium sulfate and metal or metalloid oxides, e.g. Alumina, titania, zirconia, magnesia and silica. Silica is a particularly suitable im Commercially available material.

The average particle diameter of the particulate material should not be less than 1μ to ensure that a sufficiently rough or wavy profile is achieved on the surface of the film to the flow of the Impregnation agent to ease over the surface, especially if the film is with another sheet or laminated to another film or wound in intimate contact. In practice it has been shown that a appropriate upper limit for the average particle diameter of the inert material is 5 ρ, since about this limit value, the breakdown voltage of the spacer is worsened. It is therefore preferred to be particulate To use materials with an average particle diameter of 3 to 5 μ. Particularly preferred Particles have an average diameter of about 4 μ.

At least 0.01 wt -.% Of the particulate material, based on the weight of the film is required to make a suitable improvement of the impregnation property to achieve the incorporated into the renewal proper capacitors film, while the degree of improvement is gradually decreased when the amount of 1 % By weight. Suitably the amount of particulate material incorporated into the film constitutes 0.2% by weight of the film.

The particulate material becomes the polymeric film-forming material Material incorporated prior to film production by any convenient method. So can the particulate Material are suitably dry mixed with the polymer in powder form or in chip form, for example by tumbling the components of the mixture in a suitable mixer, whereupon the mixture is extruded will. Alternatively, the pellet-shaped material and the polymeric material (in suitable proportions) the Feed hoppers (or separate feed hoppers) one Screw extruder are fed, the two components intimately together during the extrusion process be mixed.

In addition to the particulate material, the dielectric Films contain other additives commonly found in thermoplastic dielectric films are. For example, the dielectric film spacer can be an antioxidant and / or an anticorrosive agent, such as calcium stearate.

Thickness The thermoplastic films can / their thickness from 4 to 250 μ vary, but to facilitate incorporation of the films into wound capacitors, a film thickness between 6 and 25 μ, in particular between 8 and 20 μ, are preferred.

Optionally, the film itself can be metallized, after which a pair of such films can be wrapped together to form one conventional regenerative multi-plate capacitor. A metallized film can be made by conventional techniques suitably by vacuum deposition using a metal such as aluminum, zinc, silver, Gold or nickel, is evaporated onto the surface of a film which is placed in a chamber containing the is kept under high vacuum. Suitably the film surface is chemical or physical prior to metallization Subjected to treatments, such as an oxyda

tion with a mineral acid or a treatment by a Ccrona discharge to the adhesion of the metal layer To improve the film substrate, it has been observed that the incorporation of the inert particulate material in the films according to the invention reduces the blocking properties of the film, making the film more intense Corona delamination treatment can be exposed than is the case with a "clean. Film. In addition, this in the subsequent metallization of the film, which has been intensively treated in this way, a greatly improved bond obtained between the metal layer and the film substrate.

The dielectric fluid used in the manufacture of a capacitor according to the invention can be any fluid Impregnation agent commonly used in the electrical industry. Examples of suitable Impregnating agents are mineral oil, castor oil, cottonseed oil, silicone oil, polybutene and preferably one of the various halogenated aromatic hydrocarbon compounds. Examples of halogenated aromatic compounds sixid the chlorinated diphenyls, the chlorinated diphenyl ketones, pentachloronitrodiphenyl and its alkyl derivatives, chlorinated benzene and benzene derivatives.

A particularly suitable impregnating agent is trichlorodiphenyl. Pressurized gases such as dry nitrogen or sulfur hexafluoride can also be used will.

For the impregnation of the capacitors according to the innovation, the usual impregnation techniques can of course be used there are used for the impregnation of capacitors.

The innovation is explained in more detail by the accompanying drawings. In the drawings show: Figure 1 is a side view of a thermoplastic film,

containing an inert particulate material; Figure 2 is a plan view of part of a similar film; Figure 3 is a perspective view of a partially opened, wound capacitor; and FIG. 4 is a perspective view of a capacitor from FIG Type of Figure 3 which is in a suitable container.

Figure 5 is a graph showing the impregnation

fast
explained, which are obtained in capacitors when a silicon oxide-filled polypropylene film A or an unfilled polypropylene film B is used as the dielectric spacer.

The dielectric spacer (not drawn to scale) of Figures 1 and 2 comprises a biaxially oriented polypropylene film from both surfaces of which particles of silicon oxide 2 project having an average particle diameter of about 4 µm. These particles are randomly distributed through the polypropylene film so that a statistical variation in the depth of the valleys between adjacent projecting particles is obtained on the surface. Thus, a statistically rough surface is obtained, which facilitates the impregnation of capacitors under!, Amination an m or produced more of the film spacer first

The capacitor 10 of FIG. 3 can be wound on top of one another of strips 11 of a metal foil, such as aluminum, with a dielectric spacer interposed the latter being made of a polypropylene film 12 having a surface roughness which is caused by silica particles protruding from at least one surface of the film, as shown in FIGS. 1 and 2.

Electrical contact with the electrodes can be made by inserted eyelets or lines 13 and 14, the ends of which are from

7338917 \ ii 74

projecting an end of the wound capacitor can be created, can eyelets which are welded to the electrodes, if desired, to achieve zn to better contact, and the lower part of each of the eyelets 13 and 14 may be enlarged, as at 15 can be seen to create a larger contact area with the associated electrode.

Before impregnation, the assembly of electrode and spacer is usually placed in a container, such as in the metal container 21 of Figure 4, whereupon a lid 22 is hermetically sealed on the container is imposed.

Although not shown, in the unit 20 shown in Fig. 4, a dielectric liquid takes place the remaining part in the container 21 which is not occupied by the combination of electrode and spacer is. The dielectric spacer 12 is with

f impregnated the dielectric liquid.

Before the lid is sealed onto the container the contact lugs 13 and 14 on the connections 23 and 24

that extend through the lid and are isolated therefrom. About the moisture extraction and the air extraction

>: to allow from the compilation and to allow the impregnation

[Introduce approximately liquid, is a small hole 25 in the

£ 22 cover provided.

'' Before the impregnation, the capacitors are usually

se vacuum dried to remove residual moisture.

The drying temperature depends on the duration of the drying, but is in the general range from about 60 ^° C. to about 150 ^° C. If the temperature is too low, the drying time is excessively long, while if the temperature is too high, the dielectric spacer will decompose can.

The impregnating dielectric liquid is preferably also let into the capacitor through the hole 25 when the dried condenser is still in a suitable evacuated space under vacuum, usually sufficient impregnation liquid is introduced that at least the capacitor element is submerged in the container is. The pressure in the room is then increased to atmospheric pressure, and the composition becomes several Allow to stand for hours to allow thorough penetration of the liquid impregnant. After Impregnation is used to seal the capacitor, for example by applying a suitable amount of solder to it Hole 25. In addition to the above methods, other techniques, usually involving heat, can be used and / or pressure is used to aid the impregnation process.

The innovation will now be explained in more detail using the following examples.

EXAMPLES 1-6

A series of dielectric film spacers were prepared by dry blending powdered polypropylene with the desired amount of finely divided silica gel sold by J. Crosfield & Sons Limited under the trademark "Gasil" and allowing the mixture to form a Polypropylene tubing was melt extruded, which was inflated and stretched lengthways to produce a biaxially oriented polypropylene film about 15µ thick. Each film also contained 0.1 wt -.% 1,3,5-trimethyl-2,4,6-tris (3 ', 5'-di-t-butyl-4'-hydroxybenzyl) benzene as antioxidant and 0.1 wt .-? 6 calcium stearate as an anti-corrosion agent.

The properties of the different spacers that are used in capacitors each with a single spacer are shown in the table below.

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The following test procedure was used.

The surface roughness of the film was measured using a Rank Taylor Hobson "TaIysurf" meter ("Talysurf" is a registered trademark).

The degree of blocking was measured by the vertical The force required to separate two layers of film (152.4 25.4 mm) was determined pressed together at 56 C for 24 hours under a pressure of 2298 Nm " had been.

The coefficient of friction was determined by the slide slide method.

The loss factor was measured with an AC bridge on a 0.5 pF capacitor at 50 Hz and 100 ^° C. 1 min after a voltage of 500 V had been applied.

Trichlorodiphenyl was used as the impregnating agent.

EXAMPLES 7 AND 8

The procedure of the above examples was repeated except that a polypropylene film about 12 ρ thick was used. The results are given in the table.

The data in the table show that the addition of silica improves film handling properties (by reducing both the blocking properties and the coefficient of friction) and the impregnation, by creating a rough film surface without the loss factor of the impregnated capacitors is significantly affected.

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EXAMPLE 9 '' - "■ '

A series of polypropylene film dielectric spacers, each 12.7 yu in thickness, were prepared by the procedures of Examples 1 to 6, with the content of particulate silica having an average particle size of 4 yu in each spacer being 0.?. Wt -.% Was.

Twice the thickness of each spacer sandwiched between aluminum foil electrodes was wrapped in a capacitor, which was then impregnated with dichlorodiphenyl at room temperature, and the withstand voltage of the impregnated capacitor was measured using a Discharge Detector, Model 111, Type ERA, which by FC Robinson & Partners Ltd. is delivered. The effect on the withstand voltage ₉ obtained by increasing the duration of the impregnation is shown by curve A in Figure 5 of the accompanying drawings.

An identical set of polypropylene spacers, but not containing particulate silica, was made similarly processed into spacers, impregnated and tested, and the results obtained are shown as curve B. entered in the aforementioned Figure 5.

A comparison of curves A and B shows the greatly improved impregnation rate obtained with capacitors can be that gemälr the filled spacers of the innovation (curve A), compared to those that have unfilled spacers (curve B). So takes after impregnation for only 5 hours, the spacers of curve A have a breakdown voltage of approximately 2.3 kV. the comparable values for the capacitors of curve B are approximately 0.05 kV and 1.2 kV, respectively. The differences in breakdown voltages between capacitors with filled spacers

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733891710.1.14

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Examples 8 and 9 result partly from the difference in the impregnation temperature and also from the use a single thickness of spacer in Example 8, while a double thickness of spacer in Example 9 was used.

Claims (4)

PROTECTION CLAIMS 1. An electrical capacitor, consisting of a pair of conductive electrodes, a dielectric spacer arranged between the electrodes and a dielectric fluid impregnating the spacer, characterized in that the spacer consists of a film of a thermoplastic polymer material, which is 0.01 to 1 , 0% by weight , based on the weight of the film, of an inert particulate dielectric material having an average particle diameter of 1 to 5 µm which forms protrusions on at least one surface of the film. 2. Electrical capacitor according to claim 1, characterized in that that the spacer is made of a film of a propylene homopolymer or a mixed polymer of Propylene with up to 10% by weight of an unsaturated comonomer consists. 3. Electrical capacitor according to one of claims 1 or 2, characterized in that the particulate Material in the spacer consists of silicon oxide (silica or SiOg). 4. Electrical capacitor according to one of the preceding claims, characterized in that the impregnation fluid consists of trichlorodiphenyl. 15 - 7318917 itl η