DK144745B - ELECTRICAL CONDENSER - Google Patents

Description

146745

The invention relates to an electric capacitor containing a capacitor housing, a capacitor element present therein and a dielectric liquid, said capacitor element consisting of at least one pair of electrodes and a foil-shaped synthetic dielectric impregnated with chlorinated diphenyl liquid, which optionally contains a stabilizing additive agent.

The development of complicated electrical appliances and the need to achieve greater operational efficiency of existing appliances has resulted in stricter requirements for electrical capacitors in such appliances. There are e.g. there has been a strong need for high-power electrical capacitors with high efficiency, smaller size and lower price than the known ones. Capacitors, especially AC capacitors, with a high impact strength and greater corona start-up and shut-off voltages are particularly required to eliminate significant problems in the design and operation of new electrical appliances and to improve the operation of existing appliances.

Electric capacitors are known comprising a dielectric consisting of various foil-shaped materials such as paper sheets and cellulose acetate films impregnated with various impregnating agents such as liquid chlorinated diphenyl. However, the known combinations of insulators and impregnating agents have not resulted in the attainment of the desired properties and results required by the highly developed and complicated electrical apparatus now in use.

Polypropylene is distinguished by having good electrical properties such as ordinary dielectrics (excl. Capacitor elements), good temperature stability and good mechanical properties such as good workability and moldability, especially when forming thin films, but polypropylene films have so far been considered incompatible with the prior art impregnating agents, and as a result, no one has been used for the production of capacitors dielectrics. This is because polypropylene shows signs of being easily dissolved by halogenated organic compounds such as trichlorodiphenyl and it has also been claimed that the dissolution of polypropylene in a nonpolar liquid leads to a plasticizing effect such as swelling and low tensile strength. However, it has been found, according to the invention, that certain polypropylene materials, except at higher temperatures, are only soluble in halogenated aromatic materials and that this partial solubility, unsurprisingly, does not adversely affect the properties of a capacitor in which polypropylene U4745 2 are included. The partial solubility of polypropylene in trichlorodiphenyl under controlled temperature conditions proves to be a benefit and in particular promotes this partial solubility which, according to studies, occurs only after an initial penetration of the membrane, the movement of the impregnating agent into the film itself as well as into the gap. voids herein. This improvement of the impregnation is reflected in an extremely high corona starting voltage for the dielectric, even without the use of a wick or a porous layer on the sides of the polypropylene film. Studies of representative samples of a dielectric polypropylene film impregnated with trichlorodiphenyl and subjected to electric capacitor samples show a close relationship between the species and. the extent of the impregnation and the size of the corona starting voltage, and that a substantially complete and pervasive impregnation is particularly significant. The combination of polypropylene and a dielectric liquid of trichlorodiphenyl is favorable when a substantially complete impregnation is made. When the voids and gaps in the polypropylene are substantially filled with liquid impregnating agent and when the impregnation comprises both absorption of the impregnating agent in the material and a partial dissolution of the material in the liquid impregnating agent, the polypropylene is referred to as substantially complete impregnation. Representative studies with foils with varying degrees of impregnation show that very high and uniform corona initial stresses are obtained which are close to those calculated when the impregnation is extended or otherwise promoted to achieve a substantially complete impregnation. An impregnation, which is an example of a complete impregnation, consists of immersing the polypropylene foil in trichlorodiphenyl at a temperature of approx. 99 ° C.

Under these conditions, relatively stable conditions are achieved over 16 to 20 days, with approx. 1.0% by weight of polypropylene is dissolved in trichlorodiphenylene and approx. 11% by weight of trichlorodiphenyl is taken up in the polypropylene. The nature and extent of the impregnation can also be measured by the corona extinguishing voltage, with values approaching a maximum indicating a complete impregnation.

According to the invention, there is thus provided a capacitor which is characterized in that the synthetic dielectric consists of a swollen polypropylene film, prepared by impregnating the polypropylene film with a chlorinated diphenyl liquid and heat treatment at a temperature between 65 ° C and 100 ° C for a sufficiently long period. period 3 U4745 to obtain a completely impregnated film.

A preferred impregnating agent is a diphenyl compound having from 1 to 5 chloro groups. In particular, triphenyl is particularly suitable as this compound has a large corona start and corona off voltage.

The combination of a dielectric liquid of trichlorodiphenyl and a non-porous polypropylene membrane as dielectric gives the best results, as mentioned.

Preferred polypropylene films for use in the capacitor of the present invention generally consist of isotactic polypropylene. This is a high molecular weight stereoregular crystalline material which, in addition to the dominant crystalline phase, contains an amorphous or non-crystalline phase. In some known isotactic polypropylenes, the amorphous phase may comprise up to 30% of the total polyolefin. Films of this polypropylene may e.g. are made by rolling, extrusion, pressing, solvent casting and melt casting. In order to improve the mechanical properties, it is common to produce some form of ordered structure in such films by stretching or heat curing. A stretch is preferably carried out in different directions, e.g. both in the longitudinal and transverse directions of the web, so that the web is forced into a biaxial orientation. Thus, a preferred film is an isotactic biaxially oriented polypropylene film.

In the films, only small amounts and preferably no contaminants should be present which can give the product formed a large power factor, i.e. give rise to a great loss of power. The contaminants may be materials that are absorbed during the film treatment operations or catalyst residues. These contaminants can be removed by an appropriate solution of the polyolefin and removal of the contaminants by precipitation, extraction and adsorption methods.

The invention will now be described in more detail with reference to the drawing, in which fig. 1 is an enlarged cross-sectional view of a portion of a dielectric for use in a capacitor according to the invention; FIG. 2 shows a perspective view of a partially unfolded capacitor according to the invention produced by winding, 4

U474S

FIG. 3 shows a fully assembled capacitor of the device shown in FIG. 2; FIG. 4 is a cross-sectional view of a portion of a capacitor according to the invention comprising one impregnated polypropylene film; FIG. 5 is a cross-sectional view of a portion of another electrical capacitor according to the invention comprising several impregnated polypropylene films; 6 is a cross-sectional view of a portion of a further electrical capacitor according to the invention comprising a relatively thick impregnated polypropylene film; FIG. 7 is a cross-sectional view of a portion of a modified capacitor of FIG. 6 and comprising several impregnated polypropylene films, and FIG. 8 is a cross-sectional view of a portion of a further electrical capacitor according to the invention with only one impregnated polypropylene film.

In FIG. 1, there is shown a dielectric 10 which comprises a solid dielectric material 11 in the form of a polypropylene film having numerous small discontinuities 12, which may also be referred to as holes, voids and gaps, the presence of which is a generally recognized characteristic of the polypropylene film. The material is referred to as non-porous, having at least some pores or channels connecting or passing through the side surfaces and which could allow the movement of an impregnating fluid from one side surface to the other. The polypropylene is impregnated with a dielectric liquid impregnating agent which permeates the material itself and fills the said discontinuities, and the resulting product forms a coherent albeit heterogeneous dielectric system. An impregnation increases in one respect the electrical insulating properties of the dielectric by introducing into the polypropylene an impregnating material having a greater impact strength than air or gas which is in the discontinuities described.

With a dielectric shown in FIG. 1, unexpected good properties have been obtained when, e.g. is used in a high voltage AC current capacitor.

A preferred impregnating agent is trichlorodiphenyl which is commercially available under the designation "PYRANOL". This material has a large corona start-up and corona-off voltage.

In FIG. 2, as mentioned, there is a wound capacitor 14 comprising separate electrode foils or coatings 15 and 16 and intermediate dielectric foils 17 and 18. Two clamping shoes 19 and 20 have relatively large surfaces (only one clamping shoe surface 21 is shown) which in contact with the electrode foils 15 and 16. The electrode foils 15 and 16 may consist of one or more of a variety of materials, generally metallic, including e.g. aluminum, copper and tantalum.

The electric foils 17 and 18 consist of impregnated polypropylene foils. The dielectric foil 17 and the metallic electrode foils 15 and 16 together form a capacitor element.

In FIG. 3, an assembled capacitor 23 is shown in which a wound capacitor of the type shown in FIG. 2nd

The assembled unit also includes a container 24, a hermetically mounted cover 25 which includes a small dielectric fluid fill hole 26 and a set of clamping shoes 27 and 28 which penetrate the cover 25 and are insulated therefrom. In the container 24, the clamping shoes 27 and 28 are connected to the clamping shoes 19 and 20 shown in FIG. 2. The capacitor 23 shown in FIG. 3, although not shown, further comprises a dielectric liquid which occupies the space in the container 24 which is not occupied by the capacitor element and which also impregnates the dielectric films 17 and 18 shown in FIG. 2nd

In a known impregnation method, the capacitor units or enclosed means such as the capacitor 23 of FIG.

3, generally for the removal of residual moisture. The drying temperature varies depending on the length of the drying cycle, but usually ranges from approx. 60-150 ° C. If too low a temperature is used, the drying period becomes too long, while too high a temperature may result in a decomposition of any paper component in the dielectric used. The hole 26 allows moisture to be removed from the interior of the container or housing 24 during the drying process.

The impregnating dielectric fluid is introduced into the capacitor through hole 26, preferably while still holding the dried member under vacuum in a suitably evacuated chamber. Generally, sufficient impregnation liquid is introduced to at least keep the capacitor element submerged in liquid in the container. The pressure in the chamber is then increased to atmospheric pressure and the member is allowed to soak for a number of hours to obtain a thorough penetration with the liquid impregnating agent. After impregnation, the capacitor unit can be closed by applying a certain amount of a suitable solder to the hole 26.

In addition to the above-mentioned method, other methods may also be used, whereby heat or pressure or both are generally used to promote the impregnation process.

Heat and pressure can improve the impregnation ability by changing the relative, wettability and viscosity and solubility of the materials.

Furthermore, single expansion and contraction of the individual components of the system, which may be the result of the application of heat and pressure, may act as a driving force for generating a fluid migration into the solid dielectric, especially when the hole 26 is closed.

Good results have been obtained by repeating a heating cycle after the impregnation or closure to produce or secure higher impregnation rates or a substantially complete impregnation. Coiled and assembled capacitors are impregnated e.g. first by heating and evacuating the assembled capacitors and then filling them with or immersing them in a liquid mass of the dielectric impregnating agent which may be preheated or later heated to enhance impregnation.

After this treatment, the combined and impregnated capacitors are sealed and the sealed units are heat treated for a long period of time. A preferred heating cycle in carrying out the impregnation is referred to as a controlled heating period, using elevated temperatures within a range between 55 and 95 ° C.

i. a period of approx. 4 to approx. 16 hours. Variations in the treatment, utilization of pressure differences and additives may shorten this period. High voltage AC capacitors for which a composite product of a polypropylene foil and a dielectric paper piece as well as a dielectric liquid impregnating agent, e.g. "PYRANOL" 1499, and which has been heat treated at a temperature between 85 and 95 ° C for from 4 to 16 hours, has been found to have a persistently high corona starting voltage.

The temperature conditions are regulated such that a partial solution of the polypropylene takes place in the dielectric liquid and the dielectric impregnant dissolves in the propylene to produce a substantially complete impregnation. An increased penetration of the polypropylene foil is obtainable as a result of the fact that some of the amorphous or low molecular weight or bite amorphous and low molecular weight portions of the polypropylene can be dissolved in the liquid at a temperature of approx. 85 to 95 ° C. More uniform capacitors with improved corona initial voltage characteristics have been produced by the above impregnation process.

The impregnation can be further improved by modifying the physical properties of the components of the impregnated dielectric system. More particularly, mixtures of dielectric liquids or additives may be introduced into the dielectric liquid impregnating agent, or the solid dielectric material may be treated in such a way as to impair the impregnation ability of the system. Eg. For example, an additional dielectric liquid such as "PYRANOL" 1499 containing mainly trichlorobenzene may be added to the dielectric liquid of "PYRANOL" ® 1499 in an amount e.g. 25%. Other dielectric fluids which can be successfully used in admixture with e.g. "PYRA-NOL" is liquid mineral oil and silicone oil.

The impregnation of the aforementioned film-shaped dielectric gives this an improved impact strength, low energy loss and a high corona starting voltage. The nature and extent of the impregnation regulates the corona starting voltage that can be achieved in the system. An increased impact strength is important because it results in a more efficient dielectric and also allows the use of a smaller volume or a smaller amount of weight of the solid dielectric to withstand a given voltage in a system. Energy loss is important because an energy loss in the system adversely affects the electrical efficiency of the unit and can cause a physical breakdown of the structural materials in the unit due to the lost energy being converted to heat.

These significantly improved properties can be utilized in AC capacitors which are suitable for use in high voltage applications. AC capacitors have been assembled which have a long service life at voltages of over 1200 volts per second. 0.025 mm impregnated dielectric and a corona starting voltage of approx. 750 volts to over 3000 volts. The development of high voltage AC capacitors has so far been limited by the relatively short life of the dielectric under high voltage effects. A previously manufactured AC capacitor for long-term operation is e.g. only applicable within a normal voltage range of up to less than approx. 500 volts and for short pulses of up to only approx. 750 volts.

8 to 5 In FIG. 4 shows a portion 29 of a capacitor using a type of composite dielectric spacer designated as a "full sandwich". The capacitor portion shown comprises an impregnated dielectric polypropylene film 11 interposed between a pair of impregnated porous dielectric sheets 30 and 31 and a pair of electrode sheets 15 and 16. The sheets 30 and 31 may be a well-known paper product such as kraft paper, and may also be suitably impregnated with a liquid dielectric, e.g. the impregnating medium used to prepare the impregnated polypropylene film. The term "porosity" refers, in connection with said paper, to a substantial number of continuous channels or pores in and through the paper which are capable of passing the liquid impregnating agent through the paper from one side to the other. The corona initial stress properties of an impregnated polypropylene foil are, for the most part, dependent on the effective impregnation of the holes in the foil as well as the gaps between the foil's surfaces and any adjacent material. The corona starting voltage of a dielectric of the type referred to as a "full sandwich", such as the dielectric shown in FIG. 4, is increased by the use of an adjacent material, e.g. of paper.

In FIG. 5, a dielectric 32 is shown, they are based on a different structure. This is referred to as a "reverse sandwich". This composite dielectric body comprises a single sheet of impregnated porous material 30 or 31 inserted between a pair of impregnated polypropylene sheets 11 and 11 '. The composite dielectric itself is inserted between a pair of electrode sheets 15 and 16. Examples of capacitors of this kind included a single sheet of kraft paper having a thickness of 0.017 mm to form the porous material between the two polypropylene sheets having a thickness of 0.013 mm. This generally difficult to impregnate system was impregnated without difficulty, and from this a capacitor unit with a capacity of 0.9 mF and a corona starting voltage of over 2650 volts AC can be produced, effective value.

This advantageous combination of polypropylene and "PYRA-NOL" 1499 makes it easy to impregnate, even in a tightly wound roll, where hitherto with the known combinations it was necessary to rely on loosely wound rolls for better impregnation. . An important advantage of the combination of polypropylene and "PYRANOL" in connection with this as well as other structures U4745 9 is that the polypropylene permits penetration of the impregnating agent so that it can reach hitherto difficult to reach voids and spaces in locations far away from the point of departure of the impregnating agent, and especially to such areas which are along or near the interface between the foil or electrode and the foil.

A dielectric 33 for a capacitor and similar to that shown in FIG. 3 is shown in FIG. 6. This dielectric 33, designated as a "half sandwich", differs from the "reverse sandwich" in that one of the impregnated polypropylene sheets 11 or 11 'is omitted.

Another structure of a solid dielectric is illustrated in FIG. This structure, which comprises a composite dielectric 34 referred to herein as a "modified half sandwich", comprises two contiguous impregnated polypropylene sheets 11 and 11 'and an impregnated porous sheet 30 adjacent thereto. As in the other structures, this composite dielectric is inserted between a set of electrode sheets 15 and 16. The purpose of placing the sheets 11 and 11 'up to one another is to prevent a dielectric failure due to an irregularity in one of the sheets. Adjacent foils will seek to delineate any irregularity in one of the foils, thereby preventing a failure. FIG. Figure 7 shows a greatly improved dielectric in which a set of adjacent films provides impregnation properties which in many respects are more favorable than for a single film of similar thickness.

In FIG. 8, a dielectric 35 is shown with two adjacent impregnated sheets 11 and 11 'interposed between a pair of electrode sheets 15 and 16. Two adjacent thin sheets 11 and 11' are used instead of one film which is twice as thick for the same reasons as highlighted above in connection with FIG. 7. Another important feature of the structure shown in FIG. 8, there is no porous sheet, such as 30 or 31 (Fig. 4), which acts as a wick or impregnation-promoting layer.

It should be understood that in each of the structures shown in Figures 4-6, many minor modifications can be made.

Eg. For example, metallized barriers may be applied to the outer surfaces of the composite dielectrics which serve as electrodes in place of the shown electrode sheets 15 and 16. Similarly, the films may consist of either self-supporting films or supported films which in the form of a coating. or a layer is formed on another element of the structure, such as an electrode foil or a porous dielectric sheet.

In the constructions shown in Figures 5-8, at least one side of the polypropylene foil adjoins a relatively non-porous surface such as an electrode foil or other plastic foil. According to the prior art, it is difficult to obtain a sufficient or complete impregnation of a foil surface when the surface of the foil borders a relatively non-porous layer. However, by using the films described above and the impregnating agent mentioned, this difficulty is eliminated. 5, as a result, structures such as those shown in Figures 4-8 can be provided with relatively high corona initial values.

To illustrate the improved properties of the capacitors of the present invention, a significant number of capacitor elements illustrated in Figures 1-8 were collected and subjected to various tests of electrical capacitors, as well as comparisons of operating time conditions and measurements.

It is well known that synthetic plastic materials have an extremely high intrinsic impact strength. An impregnated polypropylene film used in accordance with the present invention has e.g. an operating average power of over 1200 volts per 0.025 mm, although its inherent breakthrough power can be over 20000 volts per second. 0.025 mm based on an area of approx. 6.45 mm. Impregnated paper, which is currently the most common dielectric material for use in AC capacitors, has an operating average strength of approx. 400 volts per 0.025 mm. The extent to which the use of higher impact polypropylene dielectrics reduces the amount of dielectric required in different types of a given system can be illustrated by comparing the experimental results of a series of similarly impregnated dielectrics to electrical capacitors. These types, which include paper alone, i.e. that only sheets of paper are disposed between the electrodes, composite paper products and one or more polypropylene sheets, and one or more sheets alone, i.e. only one or more polypropylene sheets between the electrodes are listed in Tables 1 and 2 below.

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Έ α cn. o) 5 i- * c _>> Ό c cn ζ <D CD £ c £ ® 3 -C -CO CO 3 CO - - = tho ^ 144745 14 In Table II the same kinds of dielectrics are given together with the thickness, which is required to withstand a total voltage of the system in 1200 volts. For the purposes of calculation, the thicknesses of each sheet and sheet of paper were considered to be the same in a given kind of composition. An even more efficient structure for use for a variety of purposes would consist of a paper layer of minimal thickness and slightly thicker foils are used to compensate for this. The results in Table II indicate the diminished amount of dielectric material required to withstand a given voltage as the content of plastic material in the dielectric increases. Table II also indicates the volumetric efficiency which can be expected to be present in capacitors with dielectrics of specified types. These values are 3 in microfarad per day. 6.45 cm dielectric spacer.

Capacitors according to the invention have been built on 50 and 150 kVAR (kilovolt lamps reactive), and these have been used for long periods, including thousands of hours of lifetime studies.

These capacitors are designed for and used at voltage values which result in an influence of the plastic component of the dielectric system approaching 1200 volts per minute. 0.025 mm. The size and weight of these capacitors shows that the potential improvements illustrated in Tables I and II have been realized.

Eg. takes up a 50 kVAR capacitor with a "reverse sandwich" polypropylene dielectric and paper impregnated with a "PYRANOL" ® 1499 dielectric liquid and with an epoxy compound stabilizer, 40% less volume than a known construction using paper alone . In other words, such a capacitor is slightly more than half the size of the known construction. If a 50 kVAR capacitor according to the invention was as large as a known 50 kVAR capacitor, the capacitor of the present invention would have a significantly larger rated output.

In comparison, a unit containing only paper and of approximately the same size as a 50 kVAR capacitor containing plastic films has a nominal output of approx. 30 kVAR.

A correspondingly significant weight reduction is also achieved.

When comparing a 150 kVAR capacitor with a trichlorodiphenyl impregnated polypropylene paper dielectric and a 100 kVAR capacitor with a same impregnated paper dielectric, the former is smaller and weighs 0.318 kg per

144745 15 kVAR. The latter, which represents a typical known capacitor, weighs 0.568 kg per kVAR.

Although the total weight and volume of electric capacitors with a given rated output can be reduced according to the present invention, it should be noted that in some cases there may be a maximum practical size for an electric capacitor and the invention can also be used for manufacture of capacitors of this size and with a larger rated output. In all of the above cases, the diminished weight and increased volumetric efficiency of the units can be attributed to the use of a combination of materials which results in a stress effect in the plastic component approaching the component's maximum practical capacity to withstand stresses.

There are many uses for the present capacitors, such as those using high voltage power capacitors, whereby it is highly desirable to reduce the energy loss in the dielectric system to the greatest possible extent. The impregnated resin-containing dielectric systems of the present invention are particularly advantageous for these applications. The power factor of the systems according to the invention is usually between approx. 0.05 and approx. 1.5% at rated voltage, even substantially above room temperature. This represents a significant improvement over typical known impregnated systems with power factors of from 0.2 to over 0.5% and further enables a reduction of up to 40% in size over the typical capacitors mentioned.

As an example of a reduced energy loss in impregnated plastic-containing dielectric systems according to the invention, a test was carried out with a 50 kVAR capacitor with a polypropylene dielectric constructed as a "reverse sandwich" and impregnated with "PYRANOL" 1499. This capacitor is 40% smaller than above. a corresponding 50 kVAR capacitor containing paper alone. The amount of energy lost in this capacitor was expressed by a temperature rise in the dielectric, i.e. the measured temperature rise in the capacitor dielectric relative to the ambient temperature.

In this experiment, a temperature rise of 25 ° C in the dielectric in the foil capacitor was measured compared to a temperature rise of 48 ° C in a corresponding unit in which paper alone was used. In addition, after a 5000 hour life study at temperatures of 55 to 70 ° C, the loss factor of the investigated capacitor was approx. 0.05% 16 144745 compared to approx. 0.2% for the capacitor in which paper alone was used.

As an example of the stability of the power or loss factor in a polypropylene dielectric system and "PYRANOL" 1499, a study was conducted in connection with a group of electrical capacitors with dielectrics constructed as a "half sandwich" and comprising adjacent films of 0.012 mm polypropylene and 0.010 mm kraft paper impregnated with a dielectric liquid of "PYRANOL" 1499 and containing approx. 1% by weight of 1-epoxy-ethyl-3,4-epoxycyclohexane. These capacitors were examined for temperature changes and aging, and the following loss factor measurements were made at rated voltages for the capacitors, 460 volts AC, 60 Hz.

TABLE III% Loss Factor

Time (hours) 25 ° C 65 ° C 85 ° C

0 0.143 0.113 0.119 519 0.120 0.091 0.096 1524 0.119 0.094 0.093 5008 0.113 0.084 0.090

These results show the highly stabilized nature of the loss factor in the system through a temperature range pi from 25 to 85 ° C and for more than 5000 hours of use.

Since impregnation is important in preventing the occurrence of a corona discharge in a solid dielectric, the impregnation properties of the dielectric systems used in capacitors according to the invention are an important factor. For some applications, e.g. such that using high voltage power capacitors, corona starting voltages which are well over 2000 volts are required. Although many physical properties of both the resin material and the dielectric liquid impregnant can be instrumental in determining the impregnation ability of the overall system, the permeability of the resin to the liquid is related to the solubility of the resin in the liquid. This ratio has been demonstrated by a test whereby a certain amount of dielectric liquid impregnating agent consisting of "PYRANOL" ® 1499 is filled into a bag or sheath made of a non-porous polypropylene film and the bag placed in an oven at a temperature of ca. 75 ° C. Penetration through the bottom of the bag was chased by passing the bottom of the bag over a microscope glass into it. In the cases where the dielectric fluid had permeated the bag, an overgrowth of the microscope glass was produced. Using this test, it has been found that the polypropylene film is not permeated by the dielectric liquid of "PYRANOL" 1400 after many hours at room temperature. A penetration can only be observed after a few hours, when the system temperature is raised to 75 ° C and above.

When a temperature impregnation is combined with a pressure effect, such as an external pressure effect or an internal pressure effect during heating, a substantially complete impregnation is obtained, as expressed in corona starting voltage values which are above 2500 volts for difficult impregnation. -justable devices. In a large, tightly wound capacitor with the dielectric system disposed up to a non-porous material, such as an electrode metal foil, the boundary regions of the system allow only limited access for the liquid dielectric to the dielectric system, and for this reason it is desirable to use pressure and temperature to produce an optimal impregnation. It is noteworthy that the effect of the dielectric liquid on the dielectric film both in the bag penetration test and in the capacitor impregnation test with the dielectric liquid of "PYRANOL" 1499 differs substantially at room temperature from the effect at temperatures between 75 and 85 ° C.

To illustrate the uniformly high corona starting voltage values produced in capacitors according to the invention, three wound 40 kVAR film capacitors were impregnated with a "reverse sandwich" structure, each comprising a paper layer having a thickness of 0.076 mm. placed between 0.012 mm polypropylene foils with a dielectric liquid consisting of "PYRANOL" ® 1499 which contained a small amount of an epoxy type stabilizer. These capacitors were 26.7 cm wide and had a starting-to-corona starting voltage of 750 to 1050 volts alternating voltage. The units were heated to 100 ° C for several hours and a substantially complete impregnation was obtained, which was evidenced by corona initial voltage values above 3000 volts. After examining other electrical properties, the corona starting voltage value was re-determined. The results of this experiment are summarized in Table IV.

TABLE IV

Sample No. Initial corona- Corona start- Gene controlled start voltage voltage after corona start through voltage heating 1 1050 3100 3100 2 750 3050 3100 3 950 3100 3100

The extremely high corona starting voltage of these capacitors together with the uniformity with which it has been obtained is taken as an indication that a substantially complete impregnation has been provided. Another indication that a substantially complete impregnation has been obtained is that the measured values of the corona initial voltage approach the final value which could be expected based on mathematical calculations.

The "PYRANOL" 1499 impregnated polypropylene paper dielectrics are significantly more resistant to corona damage than known impregnated paper dielectrics. Specifically, it has been found that sample capacitors, e.g. of the kind shown in FIG. 5, exposed to 300% overvoltage (i.e., three times the specified voltage capacity) for 30 seconds, exhibited relatively minor corona damage and, in effect, improved power factors. These sample capacitors included a polypropylene paper dielectric impregnated with "PYRANOL" 1499. Known paper and other paper-mold fabric capacitors, which are similarly examined for comparison, showed a significant increase in the power factor and a significant corona damage. Corona damage was assessed in both cases by performing a disassembly and visual examination of the dielectrics mentioned.

In addition to the solid dielectric materials and the dielectric liquid with which the former is impregnated, the capacitor of the present invention may comprise numerous other components.

19 U4745

It is especially often desirable to introduce a component which acts as a stabilizer in the capacitor. Generally, the purpose of having a stabilizer in the system is to neutralize certain contaminants or foreign substances which may be present in or which may form in the capacitor. Such contaminants may be catalyst residues or catalyst activators or neutralizers remaining from the resin-forming reaction. Another source of such contaminants may be degradation products which are produced by a chemical reaction induced by the environment or which are voltage induced. These undesirable contaminants and foreign substances have an adverse effect on the loss or power factor of the capacitor. Stabilizers have been found to be extremely effective in stabilizing the power factor of the capacitor.

Examples of stabilizers are dipentene dioxide and 1-epoxyethyl-3,4-epoxycyclohexane, which are more specifically described in U.S.A. patents Nos. 3,242,401 and 3,342,402. In particular, 1-epoxyethyl-3,4-epoxycyclohexane has been used in dielectric liquids in amounts which are generally from 0.001 to about 0.000. 8.0% by weight. A preferred range is from approx. 0.35 to 1.0% by weight using "PYRANOL" as a liquid impregnating agent.

A particulate inorganic material such as alumina may also be used as a stabilizer.

Another component commonly used in the capacitor of the invention is a porous dielectric sheet which is disposed up to the plastic film to act as a wick through which the dielectric liquid impregnating agent can, by capillary action, penetrate into the region which coincides with the contact area between the porous dielectric sheet and the plastic film. In a condenser where the polypropylene foil has a large surface area, at least one such impregnation-relieving porous layer is advantageous. It is particularly effective in e.g. relatively large, tightly wound capacitors requiring a substantially complete impregnation or an extremely high corona starting voltage.

The porous material used is preferably condenser kraft paper having a thickness of no greater than 0.025 mm and preferably approx. 0.076 mm. Such a paper has a breakthrough strength which is relatively good compared to other dielectrics, although substantially less than for most solid plastics materials. Furthermore, it has a relatively large dielectric constant which improves

Claims (2)

144745 the stress distribution in a composite system so that a greater part of the stress is placed on the plastic material with the greater impact strength. Synthetic resin or fiberglass paper can also be used as a wetting element according to the invention. The efficiency of the dielectric liquid impregnating agent in modifying the physical properties to improve impregnation and the resulting impregnation type and degree of impregnation has been demonstrated by experiments whereby dielectric capacitors consisting solely of foils having two 0.0081 mm polypropylene films, were impregnated with epoxy-modified "PYRANOL" 1499. Similar capacitors were impregnated with a mixture of the same impregnating agent together with another dielectric liquid, "PYRANOL" ® 1478, at a ratio of 3 parts "PYRANOL" ® 1499 pr. 1 part "PYRANOL" 1478. The latter impregnating agent is a commercially available dielectric liquid consisting mainly of trichlorobenzene. While the capacitors impregnated with "PYRANOL" 1499 had corona starting voltages of between 400 and 1000 volts alternating voltage, the capacitors impregnated with the mixed dielectric liquid impregnant had corona starting voltage values exceeding 1500 volts, that a significantly improved impregnation had been achieved. An electric capacitor containing a capacitor housing, a capacitor element present therein and a dielectric liquid, said capacitor element consisting of at least one pair of electrodes and a foil-shaped synthetic dielectric impregnated with chlorinated diphenyl liquid, optionally containing a stabilizing additive, provided with the synthetic dielectric consists of a swollen polypropylene film prepared by impregnating the polypropylene film with a chlorinated diphenyl liquid and heat treatment at a temperature of between 65 ° and 100 ° C for a sufficiently long period of time to obtain a fully impregnated film. Electrical capacitor according to claim 1, characterized in that as much as polypropylene is dissolved in the chlorinated diphenyl compound which can be dissolved during swelling.