CA1191564A - Polyglycol dielectric capacitor fluid - Google Patents

Abstract

POLYGLYCOL DIELECTRIC CAPACITOR FLUID
Abstract of the Disclosure A polyglycol dielectric fluid is described for use in electrical capacitors where the fluid has a moisture content below about 75 PPM and a molecular weight above about 1000, and includes additives such as a hydrogen gas absorber, an anti-oxidant and a voltage stabilizer.

Description

POLYGLYCOL ~IELECTRIC CAPACITOR FLUID

Background of the Invention This invention relates to polyglycol dielectric fluids for use in electrical devicesg and more particularly to polypropylene ylycol fluids for use as a dielectric fluid in metallized elec-trical capacitors.

Description of the Prior Art Liquid impregnants for electrical capacitors should have a high dielectric constant, maintain a low dissipation factor, and be comp~tible with th~ other materials in the capacitor structure.
At the same time, the impregnants mus~ withstand elevated and fluctuating temperature, pressure, and voltage stress condi~ions with excellent electrical characteristics for a long operative life of the capacitor. Ease of processing, impregnating and other such physical character-istics are also much desired.
There are a great number of differ2nt k;nds o-f dielectric liquid impregnated capacitors which have been developed over the years to meet specific application requirements. Broadly speak-ing~ among the larger capacitors are ~ound high voltage (above 660 volts AC), and low voltage power capacitors, which also ~ay be denoted as energy storage capacitors, induction heating cap-acitors, and power factor correction capacitors. Sma~1 cap~
acitors are usually ~ound in application categories as motor start and run capacitors and lighting capacitors.

In some instances different capacitors use different fluids although there are common desirable fluid characteristics as well as common capacitor operating environmen~al characteristics such as vacuum dried fluid filled and sealed units. With the advent of small capacitors where the solid dielectric is under very high electrical stresses, and particularly where the solid dielectric is a plastic film coated with an evaporated metal layer, i.e~, a metallized capacitor, the prior fluids used have been found not to be optimum. For example, in some capacitors it is either not necessary or desirable ~or the fluid to swell the plastic, or the penetration of the fluid in~o the plastic dielectric is undesir-able. In yet other metallized capacitors there is a deleterious relationship between the fluid and the metallized layer which contributes to deficiencies such as electrode degradation and corrosion, and a resultan~ capacitance loss in the capaci~or.
These problems in metallized capacitors have led to a search for new and improved dielectric fluids and specialty dielectric fluids for metallized capacitors.
Summary of the Invention It has been discovered that a certain class of fluids known as the polyglycols have certain desirable characteristics which, when properly treated and used, will mitigate the foregoing elec-trode problems. Electrical capacitors utilizing metalli~ed poly-propylene films, when impregn~ted with polypropylene glycol demonstrate superior performance in resistance to electrode corrosion and electrode clearing effects.
The Drawings This invention will be better understood when taken in connection wi~h the following description and drawings in which -~\
- 3 - 36~C~-3522 FIG. 1 is an illustration of a metallized polypropylene capacitor roll most adaptable for utiliziny the glycol fluid this invention.
FIG. 2 is an illustration of the capacitor roll section of FIG. 1 assembled in a casingO
FIG. 3 is an illustration of a dif~erent metallized capacitor which is adaptable Eor the use of the fluid of the invention~
FIG. 4 is an illustration of an all-film dielectric power capacitor adapted to utilize -the fluids of this invention .
FIG. 5 is an illustration of an assembled capacitor which uses the rolls of FIG. 4 and utilizes the fluids of this invention.
Description of Preerred Embodiments The polyglycol components of this invention are essen-tially linear polymers having the following generalized formula where R, R' and R" can be hydxogen and/or an alkyl group. ~
R0- CH -CH-0 -R"

~ ~ x ~See Synthetic Lubricants, Edited by Gunderson & Hart, Reinhold Publishing Corporation, New York, New York, 1962, Chapter 3, Polyglycols, Pages 61~102, and Encyclopedia of Polymer Science and Technology, Vol. 6, 1967, John Wiley &
Sons, Library of Congress Card #6422188.) These materials have, at various times, also been called glycols, polyethers, polyalkylene glycols, or polyoxyalkylene glycols. This extensive nomenclature has given rise to the generally accepted term polyglycol whether the actual product is a diol, triol, etc., a monoether, a diether, or an ester. While various polyglycol fluids are applicable to the practice of this invention it has been discovered that the polyglycols having a significant poly-propylene component and preferably triol type polypropylene glycols derived from a glycerine base, have provided ~ excellent results in the practice of this inven-tion.
.~

A descriptive formula for a triol type polypropylene glycol fluid of this invention is shown as follows:

H2 C~ O C - C - O C C ~0 - H
H H _ ~y _ H H
_ _ _ _~
H H H CH~
Hl C~ O C ~ C--O C - C -O ~ H
H H _ x H H _ y H2 Cc~ C C- O C~ C - O ~ H
H H H H
Where X = O or ~Yx- is 3-5. The first bracket represents ; ethylene oxide and the second bracket propylene oxide com-ponents.
Among the presently co~mercially available fluids which have been utilized in thi invention are the following:
1. Union Carbide NIAX 16~46 - ethylene oxide/polypropylene oxide triol type with a glycerine starter and available from Union Carbide Company.

2. Union Carbide NIAX LG-56 polypropylene oxide triol type with a glycerine star-ter and available from Union Carbide Company.

3. Dow P2000 - polypropylene oxide diol type and available from Dow Chemical Company.

4. Dow 112-2 ethylene oxide/propylene oxide block copolymer and triol type with a glycerine starter and available from Dow Chenlical Company.
The molecular weight of the polyglycol fluids of this inven-tion, are above about 1003 and preferably in the range of 3000-4000 and above. Other desirable characteristics are dielectric constants in the range of about 4.5 to 5.5 measured at 100C, ease of refining to reduce water content below abou~ 75 parts per million ~PPM) and a power factor of less than abou~ 10%
measured at 100 and lOO Hz~

6~

It has been unexpectedly discovered that polypropylene glycol has un;que electrically related properties as a dielectric fluid when used in electrical capacitors where the electrodes are metallized layers on polypropylene film, Also polypropylene glycol has been found to be advantageous as a capacitor fluid in metall;zed capacitors because its chemical and physical properties are quite adaptable and compatible with polypropylene films. For example, ;t is a rela~ively non^spreading, non-pene~rating, vis-cous fluid with low swelling characteristics in polypropylene film. These properties are particularly advantageous in very tightly wound, hard roll, metallized capacitors of the kind disclosed and described in U.S. Patent 3,987,348 Flanagan et al -assigned to the same ass;gnee as the present inventton and illustrated in FIGS. 1 and 2 of this invention.
Referring now to FIGo 1 there is disclosed one preferrPd embodiment of this invention as a capacitor rull section 10.
Roll section 10 comprises a pair of dielectric material strips 11 and 12 of polypropylene which have been metallized as illus-trated by aluminum metalliziny surfaces or coatings 13 and 14.
As is the usual practice, the strips 11 and 12 are metallized in a manner which leaves metal free margins 15 and 16 along opposite edges of roll 10. In the winding process the roll 10 is wound on a core member 17, and the strips 11 and 12 are laterally offset with respect to each other in order that each roll edge or end will display an offset with its metalli~ed coatings exposed at the edge of the strip. Thereafter, suitable electrical leads 18 and 19 may ~e attached to the exposed me~al coat;ng through utilization of the well-known schooping process to provide a metal layer 20, and the roll 10 is then placed in a can or casing as illustrated in FIG. 2. In FIG. 2, the capacitor 21 includes a cas;ng or can 22 which contains a single roll 10, and the leads 18 and 19 from the roll connect to the terminals 23 and 24. Casing 22 is filled with a dielectric fluid 25 such as a polyglycol fluid of this invention and then sealed.

' ~

When the capacitor of FIG. l is impregnated with prior ~luids, and operated at high voltage, corona discharge occurs at the roll end or edges and also in the firs~ few turns o~ ~he roll. It is in these areas where most corona occurs and from which most capaci~ors Fail. But for the fact that a metallized capacitor has inherent clearing effects the corona discharge would cause an early failure of the capacitor. The clearing effect is also knswn as self-healing and occurs when there is some electrical discharge between metallized electrode layers. The heat of the electrical arc or discharge vaporizes an increasingly larger area of the metallized layer, thus increasing the arc length until i~ extinguishes.
Alternatively described, should a dielectric faul~ and puncture occur, the very thin film of metallizing will burn back and away from the failure site, isolating the fault. The self-healing feature will then permit the capacitor to continue to function following the clearing~and the dielectric system can be used at voltage stress levels higher ~han conventional unmetallized capacitors. Clearing is therefore an instrumental factor in pre-venting early failure of a metallized capacitor. However, repetitive clearings, which have been described, remove some of the electrode area and accordingly reduce the rated capacitance of the capacitor. This is a major disadvantage in metallized capacitors and a limitation on their applications.
It has been discovered that polypropylene glycol fluids act to suppress clearing in the favorable sense of localizing the clearing and not contributing excessive gases and arcing products which would preserve or extend the clearing action and cause excessive eYaporation of the electrode metal. This major clearing advantage of polypropylene glycol fluids is evidenced by its reaction to the many minor electrical clearing effects which take place in metallized capacitors~ Polypropylene glycol has a high oxygen content which is a faYorable factor in the ability of an aluminum metallized layer to clear without the build up of conductive elemental carbon. This build up~ which is typical of metallized dielectric systems contributes to maintaininy the arc during clearing, with the adverse result of excessive elec-trode evaporation, loss of capaci~ance and the development o~ a high resistance short circuit.
In a liquid filled tightly wound capacitor as described, it is not desirable to have any liquid penetrate significantly into the roll. It is therefore desirable to limit liquid pene~ration to the roll edges where significant corona may occur. The pre-sence of a high dielectric cons~ant fluid such as polyglycol is additionally favorable at the roll edge. In fac~, the rclls are very tigh~ly wound and given a prebaking treatment at elevated temperatures ~o hea~ shrink the roll. In these capacitors, if the liquid penetrates and swells the resin it will cause th~
metallized layer to lose its bond to the resin. The layer-~h-~rapidly deteriorates in those areas. Further, the presence of the liquid deeper into the roll interacts in the electrical field to further cause corrosion of the metal layer. Electrode corrosion is also a signi~icant factor in the loss of capacitance in a metallized capacitor. The physical char~cteristics of polyglycol fluids in these capaci~ors, i.e. their high viscosity and low swelling with respect to polypropylene filmg made them highly desirable dielectric ~luids for metallized capacitors.
The loss of capaeitance due to the two factors of clearing and corrosion is at present the most serious disadvantage in metallized capacitors and a limiting factor on their useful life The following examples show the benefits, in this respect, from the use o~ polyglycol fluids of this invention in metallized capacitors.
EXAMPLE I`
In this example six identical aluminum metallized capaoitors generally following the Flanagan patent disclosure were assembled and impregnated wi:th different capaci~or fluids as shown. Se~ere tests were carried out at 410 VAC and 470 VAC ~volts alternating current) at the temperature noted. Ordinarily these capacitors would be rated at 330 VAC and 90C and 370 VAC, 70C respect-ively. Measurements were made as the units were thermal cycled to 100C and 80C respectively. ~ ~ C is the percent net change in capacitance. F is a capacitor failure. Geconol is a fluid used in the commercial production of other capaci~
tors, and comprises di-ethyl hexyl phthalate ester, an epoxide and IonolO A Eurther description of this fluid is found in U.S. Paten-t 3,754,173, issued August 21, 1973 to Eustance, assigned to the same assignee as the present invention. Ionol refers to a commercially available anti-oxidant comprising 2.6 di tert-butyl-p-cresol and its capacitor use is described in U.SO Patent 4,117,579, issued October 3, 1978 to Shaw et al Polypropy]ene glycol (PPG) was commercially available as Union Carbide NIAX 16-46. In this example, the polypropylene glycol was first column refined with fuller's earth until the water content was reduced to below about 100 PPM ancl the power factor, measured at room temperature was below about 10.0%.
Test results are given in the following tahles:
'41-0-Volts AC470 Volts AC
%~ C Af't'er Time T. ~ C'~fter Time T.
Fluid I'n HbursIn Hours ~' -'' 300 500800 1600 300 50010001500 PPG NIAX 16-46 0.5 2.6 7.4 2.13.95.7 Cotton Seed Oil -~0.05% Ionol 1.3 All-F 2.77.011.7 Corn Oi] 1.3 All-F 2.76.08.6 +0.05% 2-F
Geconol 7.4 24.6 All-F 3.310.016.8

5-F 2-F
Epoxidized Soybean Oil 0.30 1.3 - 5.2 0.7 2.24.1 Soybean Oil 0.6 2.8 - F 0.6 2~45.9 Ac'etylated All-F 0.6 2.05.1 Castor Oil l-F 5-F
As can be seen from the above table the ~ ~ C
change in capacitance for the polypropylene ylycol impregnated capacitors is consistently and markedly less than -the % ~ C change associa-ted _ g .~
with the other fluids. Note the average percent ~C for the 410 volt unit was 0.5 as compared to 1.3 and 7.4 for the other fluids.
Translated into more practical terms this reduc~ion ma~ mean as much as 2 to 3 times the predic~ed operating life of this capaci-tor, compared to a Geconol impregnated capacitor.
EXAMPLE II
In this example a number of similar capacitors were made up and subjected to an applied voltage of 400 VAC which is correlated to the capacitor design to produce immediate and positi~e clear-ings and clearing times were compared. Clearing time is thenumber of micro secands~s, over which the electrical discharge from a clearing extends~ PXE is phenyl xylyl ethane and M0 is mineral oil, mJ is milli joules.
Applied Voltage 400 VAC
Fluid ~ PPG GEC PXE M0 Cleariny Time,~sec. 9 8 10 14 12 Energy mJ 440 380 360 380 360 This table shows that PPG has a shorter clearing time than the other fluids. Microscopic examina~ion of clearing sites showed a cleaner clearing for PPG. It is believed the high oxygen content of PPG is a contributing factor to the shorter clearing ~imes.
EXAMPLE III
Tests were undertaken to compare the physical properties of polypropylene glycol (PPG) and Di ethylhexyl phthalate capacitor fluid DEHP, a widely utilized fluid, with the results showing polypropytene glycol to be superior. For example, a physical analysis of DEHP impregnated capacitor rolls and polypropylene glycol impregnated capacitor rolls showed that fluid penetration with polypropylene glycol was significantly less than that of ~EHP. Note the following comparison characteristics which support the results of these examinations.

3~-CA-3522 Contact ~ngle oE
100 Swelling Fluid On Untreated Viscosity Of PP Vol. ~ Polypropylene(D'ey'rees) 25C
PPG C 1% 43 '~ 2 525 cs DEHP 7.2% 19 ~ 2~ 57 cp The described advantages of the polyg]ycol fluids are primarily useable to -their best advantage in metallized synthetic resin capacitors. They may also be used in metal-lized capacitors requiring full impregnation/ i.e. double metallized paper elec-trode polypropylene film dielectric capacitors, composite dielectrics of paper and polypropylene film with aluminum foil electrodes, and all polypropylene film dielectrics with aluminum foil electrodes. These capacitors are described as follows:
Referring now to FIG~ 3 the exemplary capacitor roll section 26 comprises a core member 27 on which is a tightly wound series of synthe-tic resin strips 28 and metallized paper electrodes 29. The roll section 26 is wound with the electrodes in offset relationship to each other so -that the metallized edges 30 of one electrode are exposed at one end 31 of the roll section and the exposed edges 32 of -the other metallized electrode are exposed at the other end 33 of the roll section. A suitable metal such as aluminum or zinc is sprayed at each end of the roll sec-tion -to form a coating 34 and electrode leads 35 and 36 are joined to coatiny 3~.
The metallized paper electrodes 29 comprise a thin, high density paper strip on which is a layer or coating of aluminum. A number of materials may be employed for the paper including woven and non~woven polymeric material or other porous and wicking materials which will permit -the ingress of dielectric fluids therein and therealong. ~Iowever, in the practice of this invention, capacitor tissue is preferred which is about 1.0 density. Such tissue is commercially available as Kraft capacitor tissue.
The paper strips are coated with a metal layer on bo~h '?

'~

31 ~''-3~

sides thereof, a combination referred to as doubly metallized paper. Preferably the metal is aluminum which is vacuum de-posited on the paper by well-known ~acuum deposition to provide a uniform high purity metal layer. Such layers are measured in terms of their ohms resistance per square of electrode foil and a range for the present invention is from about 4.0 to about 7.0 ohms/square.
The synthetic resin strips 28 may be single or multiple strips of one or moreof the more common dielectric resins such as the polyolefinsD polycarbonates and polyamines, etc.~ and homo-polymers and copolymers thereof. However, a resin comprising electrical capaci~or grade polypropylene is a preferred resin strip for this invention. Capacitor grade polypropylene film is a higher purity, smoother, polypropylene film of enhanced die-lectric characteristics.
The polypropylene strips 28 and metallized paper electrodestrips 29 are wound together in roll form as illustrated in FIG. l, inserted in a round can similar to can l9 of FIG. 2, subjected to an elevated temperature ancl a vacuum drying process to remove moisture, and vacuum impregnated with a suit able polypropylene glycol fluid 25 of this invention. In the practice of this invention the capacitor rolls may be wound or flattened in a somewhat oval section, both configurations being adaptable to polypropylene glycol fluids. Because of the use of paper in this capacitor the polypropylene glycol fluid impregnates the entire roll structure. The advantageous use of polypropylene glycol fluid in -the FIG. 3 capacitor is illus-trated by ~he fol10wing example.
EXAMPLE IV
In this example a number of identical capacitors were made up following the FIG. 3 structure and impregnated wi th a 50/50 by volume blend of polypropylene glycol and phenyl xylyl ethane known as PXE dielectric ~luid. The capacitors were then sub-jected to very high AC and DC voltages to tes~ khe breakdown strength of polypropylene glycols against the known excellent performance of the 50/50 volume PXE/Geconol blend~ The following results were noted:
DC (Kilovolts) AC (Kilovolts) Fluid Average Of 12 Units Averaye of 13 Units PPG/PXE 3.0 2.4 Geconol/PXE 2.5 2.0 Geconol 3.6 2.4 PPG 4.4 3.0 The results oF the above tests indicate that polypropylene glycol provides all of the good characteristics of known commer-cially used dielectric fluids while contributing its own com-patibility and clearing advantages.
The polypropylene glycol fluid of this ;nvention may also be employed in other impregnated capacitors such as mixed polypropy-lene/paper dielectric capacitors as disclosed and claimed in U.S~
patent 3,3633156-Cox, as well as the all-film dielectric capacitor of the same patent. In an all-film capacitor as illustrated in FIG. 4, a longer roll 37 comprises film strip 33, 39, 40 and 41 which are wound together with separate aluminum foil strips 42 and 43 in a roll form similar to roll 10 of FIG. 1 or, a composite dielectric of alternate polypropylene strips and paper s~rips are wound together with aluminum foil strips in a roll form. An assembled capacitor utilizing the flattened and elongated rolls 37 and electrode leads 44 and 45 is shown in FIG. 5.
In FIG. 5 capacitor 44 comprises a casing 45 containing a plurality of rolls 37 suitably electrically connected to terminals 46 and 47. Rolls 37 are submerged in polyglycol fluid oF this invention. Such a capacitor is denoted as a power capacitor and typically may have a rating of 100-300 Kvar and range upwardly to about 13,000 volts.

r~ ~, Polypropylene glycol may be used ~l.o~ or with certain addi-~ives which have been found bene~icial in a capaci~or environmer~
Polypropylene glycol may be subje~t to oxidation both in the hand-ling and impregnation system or in the capacitor environment.
For this reason it is advisable to use an an-ti-oxidant additive~
Ion~l being one exampleO Usually an anti~oxidan~ is added in amounts ranging from about 0001% by volume to about 10.0% by valume.
Epoxides have been found to be beneficial in a capacitor environment having either chlorinated fluids or ester fluids present. (See U.S. Patent 3,363~156-Cox and 3,754,173-Eustance) for a further description of epoxide use. A typical epoxi~e which may be used e~fectively in this invention is commercially available, Unox 221 a dicyclo diepoxy carboxylate. Epoxides are usually added in the range of about a.l% to 10~ by weight of the fluid.
The polypropylene glycols are not considered as aromatic fluids and accordingly are not good gas absorbers. In some oap-acitor applications gas absorption general ly, and often ti~,es rapid gas absorption9 is necessary to reduce deleterious corona discharge. In this instance certain gas absorbing additives may be used with the polypropylene glycol. One class of additives includes the alkene~ such as an aliphatic olefin, tetradecene being a good example. U.S. Patent 4,190~682-Shaw described the use of aliphatics as gas absorbers in combination with ester fluids. A
different class of materials, the anthraquinones may also be employed in this inventionO Alternatively a polyglycol may be chemically modified tc improve its gas absorbency by using starting materials which will provide carbon to carbon double bonds.
Where a pre-ferred fluid would romprise essentially polypro-pylene glyeol (with additives) this fluid may be blended with other fluids to provide spe~ial fluids for special needs, The blendin~
o~ ,luids is described for example in Can. App. Ser. No. 379,085, filed June 5, 1981-Grahame. In the,Grahame application certain fluids . 1 D, -such as Phenyl Xylyl Ethane (PXE) and Mono Iso Propyl B;phenyl (MIPB) are blended wi~h certain esters. For the present inven-tion the polypropylene glyool of this invention may be sub~
stituted ;n place oF the ester. It is preferred that the poly-propylene glycol be the major constituent in any blend~ e.g.,that it comprises about one~half or more of volume of the mix-ture. Alternative~y however, the advantages of the polyglycol may be enhanced or buttressed by chemically modifying or com bining other materials therewith. Suitable examples are ether and ester linkages. Included also are copolymers, random or block and polypropylene glycols containing ethylene oxide sub-units.
The fluids of this invention are those that remain in the capacitor through its effective life as a fluid as opposed to cured and solid compounds whioh may contain some combination of pol ygl ycol .
Other fluids tested included Soybean Oil, Acetylated Castor Oil, Soybean Oil~ and Polybutene. While in some limited tests, good results were obtained, ~hey were usually obtained at shorter 20 hours of life. Life tests at hours beyond about 1000 clearly show the predominance of the polyglycol fluids of this invention.
While this invention has been disclosed wîth respect to particular embodiments thereof, numerous modiFications may be made by those skilled in the art without departing from its true spirit and scope. Therefore, it is intended that the appended claims cover all such modifications and variations which come within the true spirit and sc3pe oF the present invention.

Claims (9)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. An improved dielectric fluid for electrical capacitors comprising:
(a) a polyglycol fluid refined to have a moisture content below about 75 PPM and a power factor of less than about 10% measured at room temperature and 100 Hz, said polyglycol fluid being a dielectric and having a molecular weight above about 1,000; and (b) a plurality of additives therein taken from the class consisting of:
1) a hydrogen gas absorber, 2) an anti-oxidant and 3) a voltage stabilizer.

2. The fluid of claim 1 wherein said plurality of additives comprises an anti oxidant and an epoxide and said polyglycol is a polypropylene glycol.

3. The fluid of claim 2 wherein said poly-propylene is a triol polypropylene glycol.

4. The fluid of claim 1 wherein the polyglycol fluid has the general formula wherein R' is CH3 and/or H; R is alkyl having from 1 to 7 carbon atoms or H; and R" is H.

5. The fluid of claim 4 wherein about 0.01%
by volume to about 10.0% by volume anti-oxidant is present in the polyglycol fluid.

6. The fluid of claim 5 wherein the anti-oxidant additive is 2,6-di-tert-butyl-4-methylphenol.

7. The fluid of claim 1 wherein the hydrogen gas absorber is selected from the group consisting of tetradecene and anthraquinone.

8. The fluid of claim 2 wherein the epoxide comprises about 0.1% to about 10% by weight of the polyglycol fluid.

9. The fluid of claim 2 wherein the epoxide is a dicyclodiepoxycarboxylate.