CA1155939A - Electrical apparatus having improved dielectric system - Google Patents

Abstract

Abstract of the Disclosure An electrical apparatus, such as a capacitor, having an improved dielectric system. The capacitor includes alternate layers of metal foil and dielectric material which is impregnated with 1,1-bis(3,4-dimethylphenyl)ethane. The capacitor has improved corona characteristics and low dielectric losses. The liquid dielectric is biodegradable and environmentally acceptable.

Description

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Background of the Invention In the construction of capacitors, such as power factor correction capaci~ors, the capacitor packs are formed of a~ternate layers of metal foil and a solid dielectric material such as paper or polymeric film, which is impregnated with a liqu.id dielectric. In the past, polychlorinated biphenyl has seen wide use as a dielectric fluid for impregnating the paper or polrmeric film dielectric material, While the~poly-: chlorinated biphenyls, such as or example, trichlorddiphenyl, produce an effective dielectric system for a capacitor, ~heir ~;~ usage has pro~ided certain ecology problems in that the poly-chlorinated biphenyls are virtually non-biodegradable, with the result that if leakag~ or ruptur~ occurs in the capacitor~casing, ~: or if the capacitor is discarded as obso~ete, the polychlorinated ~iphenyl w 11 remain as a pollutant in the environment and will not degrade to any appreriable exkent, even over extended periods of years.
As a conse~uence of the pollution problem presented by the~polychlorina~ed biphenyls, a number of diele~ric liquids have~more recently been used as replacements for polychlorinated biphenyls, including such materials as isopxopyl biphenyl, phthalate esterJtrichlorobenzene, and buylated monochloro-diphenyl oxide. :
Of these fluids, the phthalate e~terJtrichloroben~ene : 25 mixture has come under question because of its environmental acceptability. Trichlorobenzener for example~ has~been shown to form, in the presence of sunlight, polychlorinated biphenyl, ~: and thus there is an implication that the exposure o~ this compound-to the environment may generate the undes.irable poly-chlorinated biphenyls~
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01 Isopropyl biphenyl performs satisfactorily from an 02 electrical standpoint, but the flammability properties are below 03 that of the polychlorinated biphenyls, being only equivalent to 04 regular transformer oil in its flash and ~ire properties.
05 Summary of the Invention ~06 The invention relates to an electrical apparatus, such ~ ; .
07 as a capacitorl having an improved dielectric system. In 08 accordance with the invention, the solid dielectric material 09 which can be paper, polymeric film, or a combination of paper and polymeric film, is impregnated with a liquid 1,1-bis~3,4-dimethyl-11 phenyl)ethane. The capacitor of the invention exhibits low 12 dielectric losses and has improved corona characteristics compared 13 to conventional dielectric fields. The increase in corona 14 characteristics allows the capacitor to operate at higher electrical stresses and this, in turn, permits a more compact 16 capacitor design which results in lower material costs per 17 capacitor.
;18 As a further advantage, the dielectric liquid is 19 substantially completely biodegradable, with the result that the 20 ~ compound will decompose if exposed to the atmosphere through 21 leakage or rupture of the casing and thus, there will be no 22 adverse effects on the environment.
23 More particularly, the invention is a dielectric system 24 comprising layers o~ dielectric material berween layers of electrically conducting material, and a dielectric liquid 26 impregnated into the dielectric material which consists 27 essentially of 1,1-bis(3,4-dimethylphenyl)ethane.
28 According to a further embodiment, the invention is an 29 electrical capacitor, comprising a sealed casing and having ~ 2 -s~

0l alternate layers of an electrically conductive material and a 02 dielectric material disposed within the casing, and a dielectric 03 liquid impregnated into the dielectric material. The dielectric 04 liquid consists essentially of l,l-bis(3,4-dimethylphenyl)ethane.
05 The electrically conductive material is connected to electrical ~06 terminals extending through the casing.
07 Other objects and advantages will appear in the course ~08 of the following description.
~Q9 Description of the Drawings ~;10 The drawings illustrate the best mode presently ~`~ll contemplated of carrying out the invention, ~12 In the drawings:
13 Fig. l is a perspective view of a typical capacitor 14 constructed in accordance with the invention; and Fig. 2 is a perspective view of a capacitor paclc.
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Description of the Preferred Embodiment Fig. 1 illustrates a typical capacitor comprising an ~` outer casing l having side walls 2, bottom wall ~ and a cover 40 In service, ~he casing 1 is hermetically sealed ~nd is provided with a small seal hole 5, through which the dielectric li~uid 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 rom the cover.
A series of capacitor packs 7 are disposed-within the casing 1 and each capacitor pack, as illustrated in FigO ~, includes wou~d layers of metal foil 8 separated by a dielectric ~; layer 9. Electrodes 10 are connected to the foil Iayers 8, and the electrodes o the various pàcks are connected togethex in series for final ~onnection to the:terminals 6.
The foil layers 8~may be formed of~any electrical conductive material, generally a me!tallic material,~Guch as ; aluminum, copper,.or the li~e. Th;e layers 8:may be in ~he ~orm : of:flat`sheets, or :the layers can be provided wi~h suxface ~20 irregularities, such as a seri s of deformations formed by indentations on one~side of th:e foil and corrasponding elevations . OD the other side,: as disclosed in United~States Patent 3,746~,9530 The dielectric layers 9 can be composed of paper~
~: such as Kraft paper, or polyme~ic f~lm, such as polypropylene, polyethylene, polyester, polycarbonate, or the like. The . dielectric layers 9 can be i~ the form of smoo~h surface sheets, or ln the caseof polymeric film, the layers can be form~d of polyolefin film, such as polypropylene, having a layer of fine polyolefin fibers adhering to the surface, as disolosed in United States Patent No.3,772 r579.

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It is preferred that the surface of the polymeric film 9 and/or the contiguous surface of the metal foil 8, have surface irregularities ox deformations, so ~hat the two con-tiguous surfaces are not in continuous intimate contact. The surface irregularities provide a wicking or capillary effect for the li~uid dielectric,enabling the liquid to thoroughly impregnate the film 9 during processing.
In accordance with the invention, the dielectric layexs 9 are impregnated wi~h 1,1-bis(3,4-dimethylphenyl)ethane which has the following structural formula~

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~3 ~ 33 I5 The dielectric liquid, 1,1-bis(3,4-dimethylphenyl~ethane has a ~1ash point of 170C, a fire point of ~88C, a pour:point . , .
- of -20C, a ViSCOslty at 40C of 10.43 centistokes, and a visco~ity at 100C of 2.43 centistokes.
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~ bis(3,4-dimethylphenyljethane is commercially . "
available and sold under the trade designation '~DXE" by the Gulf Oil Company.
The dielectric liquid can al50 include fro~ 0.01 to L0~0% by weight and preferably from about 0.2~ to 1.5~ by weight of an epoxide scavenger which will act to neutralize decomposition products .hat are generated from the liquid impregnant and other materials in the capacitor during its operation. The neutralizing agen.s or scavengers can take the ~` form of 1,2,-epoxy-3-phenoxypropane, bis(i,4-epoxy-6-methylcyclo-hexylmethyl) adiptate; l-epoxyethyl-3,4-epoxycyclohexane;

3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexanecarboxylate;

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~15~9 3 ~ 4-epoxy-6-methylcyclohe~:ylmethyl-3 r 4-epoxy-~-methylcyclo-he~anecarboxylzte; and mixtures thereof. The ep~ide compounds are effective to rapidly neutralize decomposition products, thereby improving th~ dielectric properties and service life ~f the capacitor.

The capacitor of the invention can be processed by the method set orth in U.S. Pat. 4~1~7,327 issued Feb. 5, :~ 1980, to McGraw Edison Company. In general, the processing consists of subjecting the interior of the casing containing the capacitor packs to a vacuum for a period of time sufficient ~ to remove water vapor and other gases from the interior of ~he : ~ :
`;I casing. A vacuum less than 100 microns, and preferably~below . 30 microns, is normally employed for a period longer than ; 4D hours.
~:15 The liquid dielec~ric is separately subjected to ~.
: vacuum to remove gases, and a vacuum less than 50C microns:is ~: u~ilized with a vacuum less than 50 microns being preferred~:
To accelerate the degasslng it is prelerred to agitate ~he ~

liauid, by circulating:the liquid or subjecting i~ to stirring 2D or~mixing action. In general, the liquid is subjected to the : vacuum drying trea,ment for a period in e~cess of 12 hours .
:; ~The;liquid is~preferably:maintained~ at room temperature, or at:
a temperature below 6~C,~;~during~degassing and:wh n introduced into the capacltor casiny~
~i ~5 ~fter the degassed liquid is introduced into the ~
capacitor casing~ a p~sitive pressure, g~nerally in the range of 1 to 5 psig, is applied to the liguid dielec~ric to aid in impregnation of the solid dielectric iayers and to bulge the casing so that subse~uently, a~ter sealing and exposure to lowèr iemperatures, the shrlnkage in volume of the dielectric _5_ .
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1 ~5~39 liquid will not produce a negative pressure in the casing.
The casing is then sealed.
As an example of the processing of the capacitor of the inventionJ six miniature capacitors were constructed, each having a dielectric layer composed of two sheets of polypropylene ~ilm with a total thickness of 1 mil and aluminum foil having a thickness of 0.285 mil. The capacitor casings were degassed by subjecting the casings to a vacuum of less than lOO micro~s for a period of a~out 120hours. The liquid dielec~ric 1,1-~is:(3,4-dimethylphenyl)ethane was separately degassed by stirring the li~uid and subjecti~g it to a vacuum of approx-imately SO microns for a period of 48 hours. The degassed liguid was then introduced into ~he casin~ and maintained under a vacuum of less than 100 micxons for a period of 96 hours to impxegnate the polypropylene film with the liquid.
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: After *he impregnation period the capacitor casings were ~ealed.
The capacitor samples were tested at room temperature : to determine:t~e electric losses and the samples show an average loss of 0.0~3% at an operating voltage of ~400 volts per mil ; ~ after 1000 hours.
The~average discharge inception voltage (DIV) at room . temperature of the sample capacitors was determined to be 3~87 ~v/mil, while the average DEV was determined to be 2.79 Kv/mil.
Both the DIV and DEV were approximately 25% higher than capaci tors utilizing polychlorinated biphen~l as the dielectric liquid.
The capacitor using 1,l-bis~3,4--dimethylphenyl) ethane as the dielectric liquid exhibits low dielectric losse~ and~has a higher DIV and DEV than conventional dielectric li~uids, thereby providing good corona characteristics and resulting in an increased safety margin insuring that the electrical .~.i ``..,1 ~5~33~
stress will be well below the DIV, or alternately, permitting an increase in the electrical stress without a decrease in the safety marginl.
The liquid dielectric is substantially fully bio-5 degradable, meaning ~hat if the li~uid dielectric should beexposed ~o the enviro~ment, because of leakage or rupture of ~`~ the casing or through discarding of obsolete capacitors, the liquid dielectric will readily decompose in o harmless com-ponents and there will be no significant adverse environmental effect.
While the dielectric liquid has a bur~ point it will not ~urn as r~adily as other conventional dielectrics, such as mineral oil and vegetable oils and isopropyl biphenyl, all of - which have seen use as dielec~ric liquids.
While the above descrip~ion has shown the dielectric system as used in an electrical capacitor, it is contempIated : that the dielectric system can also be utilized in other electrical apparatus, such as switch gears, txans~ormers, and ~ -~ the llke.
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Claims (6)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEDGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. In an electrical apparatus including layers of electrically conductive material, a dielectric system comprising layers of dielectric material between the layers of electrically conductive material, and a dielectric liquid impregnated into said dielectric material, said dielectric liquid consisting essentially of 1,1-bis(3,4-dimethylphenyl) ethane.

2. The electrical apparatus of claim 1, wherein the dielectric material is polyolefin film.

3. The electrical apparatus of claim 2, wherein the polyolefin film is polypropylene.

4. An electrical capacitor, comprising a sealed casing and having alternate layers of an electrically conductive material and a dielectric material disposed within the casing, and a dielectric liquid impregnated into said dielectric material, said dielectric liquid consisting essentially of 1,1-bis(3,4-dimethylphenyl) ethane, the electrically conductive material being connected to electrical terminal means extending through sald casing.

5. The capacitor of claim 4, wherein the dielectric material is polymeric film.

6. The capacitor of claim 4, wherein one of said materials is provided with surface irregularities so that said materials are in non-contiguous contact.