GB1561765A

GB1561765A - Electrical apparatus having a dielectric system

Info

Publication number: GB1561765A
Application number: GB3742076A
Authority: GB
Original assignee: McGraw Edison Co
Current assignee: McGraw Edison Co
Priority date: 1976-09-09
Filing date: 1976-09-09
Publication date: 1980-03-05

Description

(54) ELECTRICAL APPARATUS HAVING AN IMPROVED DIELECTRIC SYSTEM (71) We, McGRAW-EDISON COM PANY, a Corporation organised and existing under the laws of the State of Delaware, United States of America, of 333 West River Road, Elgin, State of Illinois, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be per formed, to be particularly described in and by the following statement: The present invention provides an electrical apparatus, such as a capacitor, having an im proved dielectric system.A capacitor according to this invention typically includes alternate layers of metal foil and a dielectric material comprising a combination of polymeric film and paper, which is impregnated with a liquid dielectric composition composed of a mixture of mono-halogenated diphenyl oxide and an alkyl mono-halogenated diphenyl oxide where the alkyl group contains from 1 to 20 carbon atoms in the molecule. Capacitors are specifically described hereafter having improved corona characteristics and low dielec tric losses, and in which the liquid dielectric composition is substantially biodegradable.

In the construction of capacitors, such as power factor correction capacitors, the capacitor packs are formed of alternate layers of metal foil and a solid dielectric material which is impregnated with a liquid dielectric.

In the past, Kraft paper tissue was generally used as the dielectric material, and capacitors of this type had relatively high dielectric losses, limiting their use to capacitors having a capacity of 100 kvar and below.

The combination of paper and polymeric film, such as polypropylene film, has also been used as the dielectric layer in capacitors. The paper-film capacitor has substantially lower dielectric losses than the all-paper capacitor and increased reliability, thereby enabling the capacitor to have greater kvar capacity. The paper in the paper-film dielectric layer pre- sents certain limitations, but acts as a wicking material to increase the impregnation of the capacitor pack with the liquid dielectric.

For many years polychlorinated diphenyls have been used as a liquid dielectric in power factor correction capacitors, and this liquid dielectric produces an effective dielectric system which has made possible many improvements in capacitors. For example, during the past years, the casing volume per kvar has been reduced by a factor of about 8, the dielectric losses have been reduced by a factor of over 5, and the cost per kvar has been reduced by a factor of about 5. These improvements have been due at least in part, to the use of polychlorinated diphenyls as the liquid impregnant.

While polychlorinated diphenyls, such as for example, trichlorodiphenyl, produce an effective dielectric system for a capacitor, their usage has prwided certain ecological problems in that some of the polychlorinated diphenyls are virtually non-biodegradable, with the result that if leakage or rupture occurs in the capacitor casing, or if the capacitor is discarded as obsolete, the polychlorinated diphenyl will remain as a pollutant in the environment and will not degrade to any appreciable extent even over extended periods of many years. As a further disadvantage, the polychlorinated diphenyls tend to bioconcentrate in animals and have been shown to be toxic.

The invention relates to an electrical apparatus, comprising a pair of electrical conducting elements disposed in spaced relation with respect to each other and adapted to provide an electric potential therebetween, and a dielectric system interposed between said elements, said dielectric system comprising a dielectric material composed of polymeric film and cellulosic fiber material, and a liquid dielectric composition impregnated into said dielectric material, said dielectric composition com- prising a mixture of a mono-halogenated diphenyl oxide and an alkyl mono-halogenated diphenyl oxide where the alkyl group contains from 1 to 20 carbon atoms. The liquid dielectric composition in general, contains substantially 5% to 95% by weight of the monohalogenated diphenyl oxide and 95% to 5% by weight of the alkyl mono-halogenated diphenyl oxide.In addition, the dielectric com- position may contain from 0.01% to 10% of an epoxide compound which acts as a scavenger which will neutralize chlorine ions or other decomposition products generated or released from the impregnant or other materials in the capacitor during its operation.

The polymeric dielectric film can take the form of a material such as polypropylene, polyethylene, polyester, or the like, and the surface of the film and/or the contiguous surface of the metal foil can be formed with surface irregularities to achieve a wicking action for the liquid dielectric and provide thorough impregnation of the film by the liquid during processing.

A method of processing an electrical apparatus suitable for introducing the dielec tric liquid into the capacitors of this invention is described and claimed in our co-pending application No. 37423/76. (Serial No.

1,554,583).

In a typical method of processing or fab ricating the capacitor, the capacitor casing con taining the solid dielectric layer is dried under vacuum conditions at a temperature be low 600C and preferably at room temperature for a period of time sufficient to eliminate water vapor and other gases from the interior of the capacitor. The liquid dielectric is cir culated or otherwise agitated under vacuum to remove gases from the liquid. Following the separate gas sing of the capacitor and the dielectric liquid, the liquid is introduced into the capacitor. With the capacitor filled, a vacuum is either drawn on the liquid or a superatmospheric pressure is applied to the liquid, while maintaining the capacitor at a temperature below 600C and preferably at room temperature. After impregnation, the vacuum is released and the capacitor is sealed.

Capacitors according to preferred embodiments of the invention exhibit low dielectric losses and have superior corona characteristics over the temperature range of --400C to +9ooh. As a further advantage, the liquid dielectric composition is less of a hazard to the environment than polychlorinated diphenyls in that it is less toxic, bioconcentrates less in living organisms and is more biodegradable than the polychlorinated diphenyls.

The dielectric system, as processed according to the method of the invention to eliminate gases from the dielectric system, is capable of operating under electrical stress at elevated temperatures up to 1250C without degradation of the polymeric layers or the liquid dielectric.

The increased stability at elevated temperatures enables the dielectric system to be used in large power factor correction capacitors which generally have an operational temperature range (case temperature), of -400C to +70 C, as well as in the smaller ballast or speciality capacitors that may be subjected to operational temperatures up to 1000 C'.

As a further advantage, the method of the invention eliminates the need for the post cure which was often necessary when using conventional processing techniques. Eliminating the post cure, which generally required a period of up to 72 hours, substantially shortens the overall processing time for the capacitor, thereby reducing manufacturing costs.

The invention will be illustrated by the following description with reference to the accompanying drawings of specific embodiments of the invention.

In the drawings: Fig. 1 is a perspective view of a typical capacitor constructed in accordance with the invention; Fig. 2 is a perspective view of a capacitor pack; Fig. 3 is a graph showing the pour point of the mixture of various ratios of mono-chloro- diphenyl oxide and dodecyl monochloro diphenyl oxide; and Fig. 4 is a graph showing the discharge inception voltage at various temperatures of a paper-film capacitor made in accordance with the invention as compared to a paper-film capacitor using a chlorinated diphenyl as the impregnant.

Fig. 1 illustrates a typical capacitor comprising an outer casing 1 having side walls 2, a bottom wall 3, and a cover 4. In service, the casing 1 is hermetically sealed and is provided with a small seal hole 5 through which the dielectric liquid is introduced into the casing during fabrication. In addition, a vacuum line can be connected to hole 5 for vacuum drying of the capacitor during fabrication. A pair of terminals 6 project through the cover and are insulated from the cover.

A series of capacitor packs 7 are disposed within the casing 1 and each capacitor pack, as illustrated in Fig. 2, includes wound layers of metal foil 8 separated by a dielectric layer 9.

Electrodes 10 are connected to the foil layers 8 and the electrodes of the various packs are connected together in series for final connection to the terminal 6.

The foil layers 8 may be formed of any desired electrically conductive material, generally a metallic material such as aluminum, or the like. The layers 8 may be in the form of flat sheets, or the layers can be provided with surface irregularities, such as a series of deformaticns formed by indentations on one side of the foil and corresponding elevations on the other side, as disclosed in United States Patent 3,746,953.

The solid dielectric layers 9 are composed of a combination of polymeric film, such as polypropylene, polyethylene, polyester, or polycarbonate, and cellulosic fiber material, such as Kraft paper, blotting paper, absorbent wood pulp fiber, or the like. The polymeric film can take the form of smooth-surfaced strips or can take the form of a polymeric strip, such as polypropylene, having a layer of fine polyole fin fibers adhering to the surface, as disclosed in United States Patent 3,772,578.

The polymeric film generally comprises up to 90% by weight of the dielectric layer, and in most cases from 30% to 90% by weight, and preferably from 70% to 85% by weight.

It is greatly preferred that the surface of the polymeric film and/or the contiguous surface of the metal foil 8 may have surface irregularities or deformations so that the two contiguous surfaces are not in continuous intimate contact. The surface irregularities provide a wicking or capillary effect for the liquid dielectric, enabling the liquid to thoroughly impregnate the film 9 during processing.

The dielectric layers 9 are impregnated with a liquid dielectric composition which consists of a mixture of mono-halogenated diphenyl oxide and a mono-halogenated alkyl diphenyl oxide, with the alkyl group containing from 1 to 20 carbon atoms.

The monohalogenated diphenyl oxide is employed in the mixture in an amount of about 5% to 95% by weight, with the mono halogenated alkyl diphenyl oxide comprising the balance. For most applications, the monc- halogenated diphenyl oxide is employed in an amount of 10% to 70% by weight of the composition with the balance being the mono halogenated alkyl diphenyl oxide.

In both components, chlorine is preferred as the halogen although other halogens, such as bromine, may be utilized. The processing used in manufacturing results in the halogen atom being usually located at the para position in each compound and in the typical process for preparing the compounds about 80% to 100% of the halogen atoms are in the para position, while the remaining 0% to 20% are in the ortho position.

The alkyl group in the mono-halogenated alkyl diphenyl oxide preferably contains 3 to 16 carbon atoms and can either be a branch chain or a straight chain, and the particular position and number of branches is not critical to the invention.

Specific examples of the dielectric composition to be used in the capacitor of the invention in weight percent are: 50% mono bromodiphenyl oxide and 50% dodecyl mono chlorodiphenyl oxide; 30% mono - chlorodiphenyl oxide and 70 /O dodecyl mono - chlorodiphenyl oxide; 80% mono - chiorodiphenyl oxide and 20% hexyl mono - chlorodiphenyl oxide; 40% mono-chlorodiphenyl oxide and 60% butyl mono-chlorodiphenyl oxide; 20% mono-chlorodiphenyl oxide and 80% propyl monochlorodiphenyl oxide; 35% monochlorodiphenyl oxide; 17% mono-chiorodi- phenyl oxide and 83 butyl mono-chlorodiphenyl oxide.

The dielectric composition can also include from 0.01% to 10% by weight, and preferably from 0.2% to 1.5% by weight, of an epoxide scavenger which will act to neutralize decomposition products that are released from or generated from the liquid impregnant and other materials in the capactior during its operation. The neutralizing agents or scavengers can take the form of 1,2 - epoxy - 3 - phenoxypropane; bis(3,4 - epoxy - 6 - methylcyclohexylmethyl) adipate; l - epoxyethyl - 3,4 epoxycyclohexane; 3,4 - epoxycyclohexyl methyl - 3,4 - epoxycyclohexanecarboxylate; 3,4 - epoxy - 6 - methylcyclohexylmzthyl 3,4 - epoxy - 6 - methylcyclohexanecarboxylate; and mixtures thereof.The epoxide compounds are effective to rapidly neutralize decomposition products, thereby improving the dielectric properties and service life of the capacitor.

The mono-halogenated diphenyl oxide can be prepared by conventional processes in which diphenyl oxide is halogenated by use of an aluminum halide, such as aluminum chloride, or other protonic acids to produce a mixture of o- and p-halodiphenyl oxide, as described in United States Patent 2,022,634.

Similarly, the alkyl mono-halogenated diphenyl oxide can be prepared by known techniques in which a halo-diphenyl oxide is treated with a small proportion of aluminum chloride after which an alkyl halide or an olefin is gradually introduced while maintaining the mixture at a reaction temperature, as disclosed in United States Patent 2,170,989.

No special procedures are required for mixing the two components together and they are miscible at room temperature or elevated ternperatures. Alternately, the mixture can be obtained through the alkylation process in which the alkylation is terminated at a time sufficient to obtain the desired ratio of the alkyl mono-halogenated diphenyl oxide and the mono-halogenated diphenyl oxide. When using this method to obtain a lower alkyl alkylation, some small percentage of di - alkylation may occur.

A preferred method of making a capacitor of the invention will now be described. The interior of the capacitor casing containing the capacitor pack is initially subjected to a vacuum or subatmospheric pressure for a period of time sufficient to remove water vapor and other gases from the interior of the capacitor. The vacuum is drawn on the interior of the casing 1 by a line connected between a vacuum head and the hole 5. A vacuum less than 100 microns and preferably below 30 microns, is normally employed, and a vacuum drying period of longer than 40 hours is customary although the time period depends on the magnitude of the vacuum.

To prevent molecular expansion of the polymeric film, the temperature should be maintained below 600C and the vacuum drying is preferably carried out at a temperature below 430C, such as room temperature. The polymeric film is impregnated with the liquid dielectric by means of diffusion in which the molecules of the liquid dielectric enter the film and migrate from regions of high concentration to regions of low concentration until equilibrium is achieved. It has been found that heating of the polymeric film adversely effects the rate of diffusion due to the expansion of the molecular structure by heating, so that it is important to avoid heating the polymeric film to a temperature above 600C during the drving of the capacitor.

The liquid dielectric is separately subjected to a vacuum drying treatment to remove gases from the dielectric liquid. To provide the degassing treatment, a vacuum less than 500 microns is utilized, with a vacuum less that 50 microns being preferred. The liquid is subjected to the vacuum drying for a period of time sufficient to remove gases from the liquid.

To accelerate the degassing, it is preferred to agitate the liquid either by circulating the liquid or subjecting it to a stirring or mixing action. The time of degassing depends on various factors, such as the viscosity of the liquid, the magnitude of the vacuum, the type of agitation and other factors. In general, the liquid is subjected to the vacuum drying treat ment for a period in excess of 12 hours.

During the vacuum drying treatment the liquid is preferably maintained at room temperature. Heating can be used, but the degassed liquid should be at a temperature of below 600C and preferably below 430C when introduced into the capacitor. The degassed liquid dielectric is introduced into the capacitor casing 1 through the line attached to hole 5 while maintaining the vacuum. After filling of the capacitor casing, a vacuum of less than 100 microns and preferably less than 30 microns is maintained on the liquid for a period of time sufficient to thoroughly impregnate the solid dielectric layers with the liquid dielectric. For most operations an impregnation period in ex cess of 24 hours is utilized.During this period the temperature of the solid dielectric layers 9 in the capacitor and the dielectric liquid are maintained less that 60or, and preferably at a temperature below 430C, such as room tem - perature.

~ Alternatively, after filling, a positive pressure in the amount of 1 to 4 psig can be applied to the liquid dielectric in the capacitor to aid in impreganation of solid polymeric layers. The pressure is normally maintained on the liquid dielectric for a period in excess of 30 minutes. The manner of applying the pres sure to the liquid is not critical although it is preferred not to utilize a pressurized gas in direct contact with the liquid for the gas may be absorbed in the liquid, and the absorbed gas could have an adverse effect on the dielec tric properties of the system.

Following the impregnation or soaking period, the vacuum or pressure, if used, is re leased and the capacitor is sealed.

In the past, a post cure operation was often utilized in which the sealed capacitor was heated to a temperature of about 85 C for a period of up to 72 hours to improve the impregnation and obtain better reliability. The post curing operation is not required with the method of the invention, although the incorporation of the post curing could be employed without adverse results, but it would substantially increase the overall processing time. By eliminating the post cure, a substantial reduction in the time of processing is achieved and this is important from a production standpoint.

It has been found that the liquid dielectric composition used in the capacitor of the in vention will more readily impregnate the polymeric film than conventional impregnants, such as trichlorodiphenyl. This increase in the rate of impregnation is related to the surface energy of the composition and is also dependent, at least in part, on the relatively low viscosity of the composition. An increase in the rate of impregnation can result in a substantial time saving in fabrication of the capacitor.

The liquid dielectric composition to be used in the electrical apparatus of the invention has a substantially lower relative dielectric constant than that of polychlorinated diphenyls; as for example the relative dielectric constant of 17% mono-chlorodiphenyl oxide and 83% butyl mono-chiorodiphenyl oxide is about 4A, while that of capacitor grade trichlorodiphenyl is about 5.9. This substantial difference in relative dielectric constants would lead one to expect that the dielectric composition of the invention would be a poor choice in comparison with capacitor grade polychlorinated diphenyls, because the relative dielectric con stant ordinarily directly influences the capacitance and thus a lower relative dielectric constant would normally be expected to result in a larger volume per kvar.However, it has been unexpectedly discovered that the use of the dielectric composition of the invention, in combination with a paper-film solid dielectric, does not result in any significant decrease in capacitance over a similar capacitor impregnated with polychlorinated diphenyls.

In contrast, a capacitor having an all-paper dielectric layer and impregnated with the dielectric composition used in the invention, acts predictably in that the capacitance is reduced in comparison with an all-paper capacitor impregnated with polychlorinated diphenyl. Thus, an all-paper capacitor impregnated with the dielectric composition of the invention would have to be constructed with about a 20 larger volume than an all-paper capacitor impregnated with polychlorinated diphenyl, or the electrical stress would have to be raised about 10%, providing heat dissipation is not a factor, to be comparable to the all-paper polychlorinated diphenyl capacitor.

It has been found that when polymeric film, such as polypropylene, is impregnated with a polychlorinated diphenyl, such as trichlorodiphenyl, the relative dielectric constant of the polymeric film is slightly increased. However, quite unexpectedly, the relative dielectric constant of the polymeric film is substantially increased when impregnated with the dielectric composition used in the invention, so that the relative dielectric constant of the film impregnated with the dielectric composition of the invention is actually slightly higher than the relative dielectric constant of the same film impregnated with the polychlorinated diphenyl, even though the relative dielectric constant of the dielectric composition of the invention is only about 4.4 compared to that of about 5.9 for the polychlorinated diphenyl.Even the addition of 25% to 30% paper to the dielectric system using the liquid of the invention results in a decrease in capacitance of only about 1% over that obtained with an identical system using capacitor grade polychlorinated diphenyl. This compares to an expected decrease in apacit- ance of about 5% to 6% based on calculations involving the relative dielectric constants of the materials. Thus, even though the relative dielectric constant of the liquid of the inven tion is substantally less than that of poly chlorinated diphenyl, the capacitance of the paper-film capacitor impregnated with the di electric composition used in the invention is comparable to that using polychlorinated di phenyl.

The liquid composition used in the invention, even though inonochiorinated, produces a stable dielectric system. Traditionally, stability has been associated with increased chlorination, but it has been found that the dielectric liquid composition used in the invention produces a stable system, comparable to that of trichlorodiphenyl and tetrachlorodi- phenyl. Moreover, the lesser chlorination is a distinct advantage in making the composition more readily biodegradable, less toxic, and less apt to bioconcentrate in living organisms.

The liquid dielectric composition is sub stantially fully biodegradable, meaning that if the dielectric composition should be exposed to the environment because of leakage or rupture of the casing, or through discarding of obsolete capacitors, the liquid dielectric will decompose into harmless compounds and there will be no significantly adverse environmental effect On the other hand, capacitor grade poly chlorinated diphenyls have been found to be persistent in the environment and will not readily biodegrade.

Polychlorinated diphenyls have also been found to bioconcentrate in fish and animals and are toxic. By comparison the dielectric liquid in accordance with the invention is less of a hazard to the environment for it will bioconcentrate to a lesser degree in living organisms and is less toxic to both fish and mammals. For example, capacitor grade polychlorinated diphenyls will bioconcentrate in fish at levels 100 to 200 times greater than a mixture of 17% by weight of monochlorodiphenyl oxide and 83% by weight of butylmonochlorodiphenyl oxide. Moreover, the acute oral toxicity of capacitor grade polychlorinated diphenyls is about ten times as great as that of the dielectric liquid composition of the invention.

One of the important considerations in capacitor design is the partial discharge inception voltage (DIV). It is important that a dielectric not be permitted to operate under conditions of partial discharges, for the dielectric system, in the presence of partial discharges, will degrade and severely shorten the life of the system.

The curve in Fig. 3 illustrates pour point data of various mixtures of mono-chlorodiphenyl oxide and dodecyl mono - chlorodiphenyl oxide which was obtained under test conditions where the temperature was stabilized for several hours before readings were taken.

As shown in this curve, the pour point of dodecyl chlorodiphenyl oxide alone is about OOC and the crystallization point of mono chlorodiphenyl oxide alone is approximately - 180C. However, contrary to expectations, a mixture of the two has a pour point lower than either of the individual components. For ex ample, the pour point of a 5:50 mixture of the two components is approximately 450C and for most mixtures the pour point is well below --200C.

As the capacitors may be subjected to extremely low ambient temperatures in use, it is desirable that the dielectric liquid have a low pour point. The depressed pour point provided by a mixture of the two components insures that the liquid dielectric will not crystalize and will be in a liquid state throughout the entire operating temperature range of the capacitor.

The dielectric liquid of the invention has a lower pour point than capacitor grade polychlorinated diphenyls. This results in the capacitor of the invention being usable down to -450C, and even at temperatures as low as -600C, depending on the blend of the components utilized in the liquid dielectric.

It has been found that most dielectric systems exhibit poor operating reliability at low temperatures. Dielectric systems permitted to stabilize at very low temperatures under electrical stress are particularly prone to premature failure particularly if the system is switched into service at very low temperatures.

Because of this, it is important that the dielectric system exhibit high losses at low temperatures, for the high losses force operation at higher dielectric temperatures where the reliability is known to be better Thus, the dielectric loss characteristics of the dielectric system of the invention meet the basic re quirements of being high at low temperatures, thereby improving reliability. This characteristic, in combination with the low pour point of the dielectric liquid, provides an excellent low temperature dielectric system.

The DIV of a film-paper capacitor used in the present invention impregnated with. the dielectric liquid of the invention is higher over normal operating temperature range than a similar capacitor using polychlorinated diphenyl as the impregnant. In this comparison, sets of capacitor samples were constructed using two sheets of 0.0005 inch electrical grade polypropylene film separated by one sheet of 0.00035 inch Kraft tissue as the dielectric layers (74% polypropylene film and 26% Kraft tissue). This laminated dielectric was located between aluminum foil having a nominal sheet thickness of 0.00025 inch.

Sets of capacitor samples were impregnated with capacitor grade polychlorinated diphenyl (Aroclor 1016 which is primarily a blend of trichlorociphenyl and tetrachlorodiphenyl: "Aroclor" is a Registered Trade Mark, plus 0.5% by weight of an epoxide additive bis (3,4 - epoxy - 6 - methylcyclohexyl - methyl) adipate (ERL-4289) as a scavenger.

Additional sets of capacitor samples were impregnated with the dielecuic liquid used in the invention which was a mixture of 17% by weight of monochlorodiphenyl oxide and 83% by weight of butyl monochlorodiphenyl oxide, plus 0.5% of the same epoxide additive.

All of the capacitor samples were vaccum dried in an oven at 1050C for 48 hours at a vacuum of less than 100 microns. The dielectric liquid, in each case, was separately vacuum dried at 200C for 48 hours at a vacuum of less than 100 microns. The capacitors were cooled to 60 C, and the dielectric liquids were admitted to the casings and permitted to soak at a temperature of 6000 for 72 hours while maintaining a vacuum of less than 100 microns on the liquid. After this period of soaking, the vacuum was releassed on each sample and the capacitor was sealed.

The capacitor samples in all cases were operated under conditions of electrical stress at room temperature for a period of 24 hours at 1800 volts per mil. After this period of operation the DIV was determined as various temperatures from --450C to t90?c and the results of this test are shown in Fig. 4.

Fig. 4 is a showing of the DIV values as a function of temperature for the film-paper capacitor samples impregnated with polychlorinated diphenyl broken line and samples impregnated with the dielectric composition used in the invention (unbroken line).

As shown in Fig. 4, the sample capacitors made in accordance with the invention have a higher DIV over the normal operating temperature range than the sample capacitors using polychlorinated diphenyl as the impregnant. Furthermore, the characteristic dip at abo;ut --200C to 0 C, which is exhibited by the capacitor samples using the polychlorinated diphenyl is not present in samples impregnated with the dielectric composition of the invention. This lack of the characteristic dip is important in that it provides an increased safety margin, insuring that the normal operational stress is well below the corona voltage, or conversely, if the safety factor of the system is maintained about the same as that which is practiced with polychlorinated diphenyl, a higher stress may be utilized.This results in a smaller volume capacitor with the same kvar ratting.

These tests illustrate that the capacitor of the invention can be constructed to have a DIV greater than 1.5 kv/mil at all temperatures Erom --400C to +90 C.

The film-paper capacitor of the present invention also is stable under operating conditions. The stability and reliability of the system is shown by the data set forth in the following Table I. In these tests a series of sample capacitors were fabricated utilizing two sheets of 0.00075 inch polypropylene film and one sheet of 0.00035 inch Kraft tissue as the dielectric material and this laminated structure was sandwiched between aluminum foil having a nominal thickness of 0.00025 inch.

The sample capacitors were impregnated with a mixture of 17% mono - chlorodiphenyl oxide and 83% butyl mono - chlorodiphenyl oxide. In addition 0.5% of an epoxy additive (ERL4289) was utilized. The filmpaper capacitor samples were processed in the manner previously described in connection with the DIV-temperature tests.

TABLE I Initial DIV DIV After Opera- DIV After Opera Sample Reading tion for 192 Hrs tion for 192 Hrs No. in kv/mil at 1800V in kv/mil at 2100V in kv/mil a 1.86 2.14 2.35 b 2.11 2.19 2.35 c 1.83 2.05 2.21 d 1.62 2.16 2.35 c 2.05 2.22 2.38 f 2.03 2.30 2.38 ave. 1.92 2.17 2.34 Table I shows the initial DIV reading, the DIV after operation for 192 hours at 1800 volts in kv/mil, and the DIV after operation for 192 hours at 2100 volts in kv/mil.

As can be seen from Table I, operation under electrical stress improved the DIV values and this is an important factor since it demonstrates that the system does not deteriorate, but actually improves when operated under conditions of electrical stress.

The capacitor made in accordance with the invention has a relatively high DIV over the entire prating temperature range, thereby providing good corona characteristics, and exhibits low dielectric losses. The higher DIV results in an increased safety margin, insuring that the volts per mil will be well below the DIV, or alternately permitting an increase in the volts per mil without a decrease in the safety margin.

The readers attention is drawn to our British Patent No. 1,509,681 which describes and claims an electrical capacitor, comprising a sealed casing, a capacitor pack in the casing and having a pair of electrically conductive strips and a pair of dielectric layers wound altemately to form the capacitor pack, and a dielectric liquid composition impregnating said dielectric layers characterized in that said dielectric composition comprises a mixture of a mono-halogenated diphenyl oxide and a monohalogenated alkyl diphenyl oxide where the alkyl group contains from 1 to 20 carbon atoms in the molecule and the halogen sub stitution is in the phenyl radical, and said di electric layers are composed of polymeric film.

WHAT WE CLAIM IS:- 1. An electrical apparatus, comprising a pair of electrical conducting elements disposed in spaced relation with respect to each other and adapted to provide an electric potentional therebetween, and a dielectric system inter posed between said elements, said dielectric system comprising a dielectric material com posed of polymeric film and cellulosic fiber material, and a liquid dielectric composition impregnated into said dielectric material, said dielectric composition comprising a mixture of a mono - halogenated diphenyl oxide and an alkyl mono - halogenated diphenyl oxide where the alkyl group contains from 1 to 20 carbon atoms.

2. Apparatus as claimed in claim 1, wherein said dielectric material is composed of 30% to 90% by weight of said polymeric film and 700at to 10% by weight of said cellulosic fiber material.

3. Apparatus as claimed in claim 2 wherein said dielectric material is composed of 70% to 85(ó by weight of said polymeric film and the balance is paper.

4. Apparatus as claimed in any preceding claim wherein the polymeric film is polypropylene film and the cellulosic fiber material is paper.

5. Apparatus as claimed in any one of claims 1 to 4 wherein the mono - halogenated diphenyl oxide is present in the amount of 5% to 95% by weight of the mixture and the alkyl mono - halogenated diphenyl oxide is present in an amount of 95% to 5% by weight of the mixture.

6. Apparatus as claimed in any one of claims 1 to 5 wherein the dielectric composition has a pour point below --200C.

7. Apparatus as claimed in any one of claims 1 to 6 wherein the composition includes an epoxide scavenger in an amount of 0.1% to 10% by weight of said mixture, and said epoxide scavenger is 1,2 - epoxy - 3 phenoxypropane; bis(3,4 - epoxy 6 - methyl- cyclohexylmethyl)adipate; 1 - epoxyethyl - 3,4epoxycyclohexane; 3,4 - epoxycyclohexylmethyl - 3,4 - epoxycyclohexanecarboxylate; 3,4 - epoxy - 6 - methylcyclohexylmethyl 3,4 - epoxy - 6 - methyl - cyclohexanecarboxylate or a mixture of two or more thereof.

8. Apparatus as claimed in any one of claims 1 to 7 wherein the halogen of said mono - halogenated diphenyl oxide is bromine and the halogen of said alkyl mono - halogenated diphenyl oxide is bromine and the halogen of said alkyl mono - halogenated diphenyl oxide is chlorine.

9. Apparatus as claimed in claim 8 wherein the monohalogenated diphenyl oxide is mono chlorodiphenyl oxide and said mono - halogenated alkyl diphenyl oxide is mono - chlorobutyldiphenyl oxide.

10. Apparatus as claimed in any one of claims 1 to 9 wherein said polymeric film is polypropylene, polyethylene, or polyester film and said alkyl group contains from 3 to 16 carbon atoms.

11. Apparatus as claimed in any one of claims 1 to 10 which apparatus is an electrical capacitor having a sealed casing.

12. A capacitor as claimed in claim 11, having discharge inception voltage greater than 1.5 kv/mil at all temperatures from --400C to +9000.

13. A capacitor substantially as hereinbefore described with reference to and as illustrated in Figure 1 and Figure 2 of the accompanying drawings.

14. A method of forming a capacitor as claimed in claim 11 comprising the steps of forming a capacitor pack by alterately winding strips of an electrically conductive material and a polymeric-paper dielectric material, introducing the pack into a capacitor casing, reducing the pressure in the interior of the casing to less than 100 microns for a period of time sufficient to remove substantially all of the gas from the interior of the casing while maintaining the temperature of the polymeric material below 600C; separately reducing the pressure above a liquid dielectric com

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. Table I shows the initial DIV reading, the DIV after operation for 192 hours at 1800 volts in kv/mil, and the DIV after operation for 192 hours at 2100 volts in kv/mil. As can be seen from Table I, operation under electrical stress improved the DIV values and this is an important factor since it demonstrates that the system does not deteriorate, but actually improves when operated under conditions of electrical stress. The capacitor made in accordance with the invention has a relatively high DIV over the entire prating temperature range, thereby providing good corona characteristics, and exhibits low dielectric losses. The higher DIV results in an increased safety margin, insuring that the volts per mil will be well below the DIV, or alternately permitting an increase in the volts per mil without a decrease in the safety margin. The readers attention is drawn to our British Patent No. 1,509,681 which describes and claims an electrical capacitor, comprising a sealed casing, a capacitor pack in the casing and having a pair of electrically conductive strips and a pair of dielectric layers wound altemately to form the capacitor pack, and a dielectric liquid composition impregnating said dielectric layers characterized in that said dielectric composition comprises a mixture of a mono-halogenated diphenyl oxide and a monohalogenated alkyl diphenyl oxide where the alkyl group contains from 1 to 20 carbon atoms in the molecule and the halogen sub stitution is in the phenyl radical, and said di electric layers are composed of polymeric film. WHAT WE CLAIM IS:-

1. An electrical apparatus, comprising a pair of electrical conducting elements disposed in spaced relation with respect to each other and adapted to provide an electric potentional therebetween, and a dielectric system inter posed between said elements, said dielectric system comprising a dielectric material com posed of polymeric film and cellulosic fiber material, and a liquid dielectric composition impregnated into said dielectric material, said dielectric composition comprising a mixture of a mono - halogenated diphenyl oxide and an alkyl mono - halogenated diphenyl oxide where the alkyl group contains from 1 to 20 carbon atoms.

prising a mixture of 5% to 95 Ó by weight of a mono-halogenated alkyl diphenyl oxide where the alkyl group contains from 1 to 20 carbon atoms and 95 í) to 5% by weight of a mono-halogenated diphenyl oxide, to less than 500 microns while agitating the to less than 500 microns while agitating the liquid for a period of time sufficient to remove substantially all gas from the liquid while maintaining the temperature of the liquid below 60or; introducing the degassed liquid into the interior of the casing, mair- taining a pressure above the liquid in the cas- ing of less than 100 microns while maintaining the temperature of the polymeric material below 600C, and thereafter sealing the casing.

15. A method of forming a capacitor as claimed in claim 14 and substantially as hereinbefore described.