CA1068894A

CA1068894A - Gaseous dielectric mixtures

Info

Publication number: CA1068894A
Application number: CA255,184A
Authority: CA
Inventors: Whitney H. Mears; Sabatino R. Orfeo
Original assignee: Allied Chemical Corp
Current assignee: Allied Corp
Priority date: 1975-06-23
Filing date: 1976-06-17
Publication date: 1980-01-01
Also published as: IT1070304B; SE7607128L; FR2317743A1; DE2627470A1; JPS5225299A

Abstract

INVENTION: GASEOUS DIELECTRIC MIXTURES
INVENTORS: WHITNEY H. MEARS and SABATINO R. ORFEO

ABSTRACT OF THE INVENTION
Carbon formation is suppressed by gaseous dielectric mixtures (I) containing SF6 and/or CO2 or mixtures (II) containing at least one of SF6 or CO2 in combination with halogenated alkanes which contain from 1 to 4 carbon atoms or mixtures (III) containing at least one of SF6 or CO2 in combination with perfluorinated ethers which contain from 2 to 6 carbon atoms. Moreover, certain of the gaseous dielectric mixtures evidence unusually high dielec-tric breakdown voltages. The gaseous dielectric mixtures are useful in high voltage coaxial lines, in transformers, in mini-substations, and the like.

Description

`` 10~8B94 GASEOUS DIELECTRIC MIXTURES
BACKGROUND OF THE INVENTION
I. Field of the Invention This invention relates to a process for the production of dielectric mixtures, useful for preventing or diminishing the formation of carbon in dielectric fluids during electrical discharges therein.
II. Description of the Prior Art During the operation of electrical equipment, such as switches, circuit breakers, transformers, and the like, arcing, sparking or glow discharges usually or occasionally occur, especially at higher voltages. Dielectric materials are common-ly employed to reduce or prevent the possibility of such arcing, sparking and glow discharges. For example, solid insu-lators, such as ceramics or resins, may be used to support or surround electrical conductors. Or, fluid dielectric materials, such as oils or gases, may be used to insulate electrical con-ductors.
A related problem involves the breakdown of carbon-aceous dielectric materials. During arcing, these materials - -tend to decompose and form carbon, which, being an electrical conductor, not only shortens the gap between conductors, but also eventually leads to carbon bridge short circuits, or carbon tracking. This is a serious problem which has plagued the electrical industry for years.
As used herein, arc interruption includes arc suppression and arc quenching, and refers to preventing or reducing arcing between electrodes. Carbon suppression refers to preventing the formation of carbon during arcing.
Sulfur hexafluoride (SF6) is well-known as an excel-lent gaseous dielectric. See, e.g., U.S. Patent 3,059,044, .

lQ68894 issued to R. E. Friedrich et al., October 16, 1962. It is unique in its electric arc interrupting properties. However, SF6 does have a few inherent disadvantages: insufficient vapor pressure at low temperatures, high freezing point, only fair thermal stability compared with fluorocarbons and high cost.
For some years, it has been known that certain electro-negatively substituted carbon compounds (halogenated alkanes) are al ~ highly useful fluid insulators in electrical apparatus. Typical examples are dichlorodifluoromethane (CC12F2), octafluorocyclo-butane (c-C4F8), hexafluoroethane (C2F6), octofluoropropane (C3F8), decafluorobutane (C4Flo), trichlorofluoromethane (CC13F), sym-dichlorotetrafluoroethane (CClF2CClF2), tetrafluoromethane (CF4), chloropentafluoroethane (CClF2CF3) and chlorotrifluoromethane (CClF3). While all of the above have reasonably good dielectric strengths, it is difficult to prevent spark-over or other electri-cal discharge from occurring in apparatus containing these materials when high voltage surges develop. The spark-over or other discharge typically leads to carbon track formation.
A patent issued to J.A. Manion, et al., U.S. Patent 3,650,955, issued December 9, 1966, teaches the use of CC12F2 combined with c-C4F8 as an arc interrupter gas. However, this combination has been observed to evidence extensive carbon tracking properties.
Mixtures of SF6 and CO2 have been suggested as a potential gaseous dielectric medium. See, e.g., U.S. Patent 3,059,044, issued to R. E. Friedrich et al., October 16, 1962.
However, no composition range having practical utility is dis-closed.
Perhalogenated fluids, including SF6 and perhalogen-ated alkanes, have been absorbed on molecular sieves (zeolites), which are then incorporated as fillers in organic insulators; see 10~8894 U. S. Patent 3,305,656, issued to J. C. Devins, February 21, 1967. During high voltage operation, voids in the insulation are filled by the perhalogenated fluid, which then serves as an arc interrupter.
Attempts have been made to develop gaseous dielectric compositions as carbon tracking suppressants. For example, B. J.
Eiseman, U.S. Patent 3,184,533, issued May 18, 1965, teaches the use of an oxygen-containing oxidizing agent, such as SO2, NO2 and N2O, to suppress carbon tracking of certain electro-negatively substituted carbon compounds, such as saturated poly-halohydrocarbon compounds, saturated perhalohydrocarbon compounds, saturated perfluoroethers and the like. However, none of these oxidizing agents is desirable because of their corrosive nature, toxicity, and/or chemical reactivity.
There remains in the art a need for efficient gaseous dielectric compositions that evidence superior carbon suppression properties.
SUMMARY OF THE INVENTION
In accordance with the invention, carbon formation in 20 a dielectric fluid during an electrical discharge from an electri- -cal conductor is suppressed by maintaining in contact with the electrical conductor during operation a gaseous dielectric mixture consisting essentially of (I) SF6 and CO2 or (II) at least one halogenated alkane plus at least one gas selected from the group consisting of SF6 and CO2, or (III) at least one perfluorinated ether plus at least one gas selected from the group consisting of SF6 and CO2.
Ratios of SF6 and CO2, of group I mixtures, useful in substantially eliminating carbon formation consist essentially of mixtures of SF6 and CO2 containing from about 1 to 50 mole percent of CO2, preferably about 1 to 30 percent CO2 and most preferably about 10 to 30 mole percent CO2.

10f~8894 Halogenated alkanes useful in the practice of the inven-tion are those which contain from 1 to 4 carbon atoms and at most one hydrogen atom, with the remaining hydrogen atoms replaced by at least one halogen selected from the qroup consisting of fluorine, chlorine and bromine. The halogenated alkanes desirably have a vapor pressure of at least about 100 Torr at 20C. These compounds are gaseous under operating conditions.
Perfluorinated ethers useful in the practice of the invention are those which contain from 2 to 6 carbon atoms and may be mono- or di-ethers, cyclic or acyclic. The perfluorinated ethers have a vapor pressure of at least about 100 Torr at 20C
and are gaseous under operating conditions.
The amount of SF6 and/or CO2 required to suppress carbon formation is unique to each mixture. In general, however, in group II mixtures containing halogenated alkanes for a binary mixtures and for multicomponent mixtures containing either SF6 or CO2, gaseous mixtures containing at least about 10 mole percent of SF6 or at least about 15 mole percent of CO2 are required to suppress carbon tracking. For multicomponent mixtures (ternary and higher) containing both SF6 and CO2, carbon formation is sup-pressed for compositions lying in regions rich in SF6 and CO2 on a ternary diagram defined by a line whose minimum extremities are defined by 1 SF6 - 15 CO2 - 84 halogenated alkane 10 SF6 - 1 CO2 - 89 halogenated alkane (the numbers are in mole percent).
Certain of these mixtures form novel compositions. Such novel compositions consist essentially of at least one halogenated alkane plus SF6 and CO2. The halogenated alkanes contain from 1 to 4 carbon atoms and at most one hydrogen atom, with the remaining hydrogen atoms replaced by at least one halogen selected from the -` 1068894 group consisting of fluorine, chlorine and bromine. The halo-genated alkanes desirably have a vapor pressure of at least about 100 Torr at 20C. The amount of SF6 and CO2 in the compositions is as given above.
Perfluorinated ethers useful in the practice of the invention in group III mixtures are those which contain from 2 to 6 carbon atoms and may be mono- or di-ethers, cyclic or acyclic.
The perfluorinated ethers have a vapor pressure of at least about 100 Torr at 20C. Such a vapor pressure limitation permits the use of certain perfluorinated ethers which are liquid at room temperature but which evidence at sufficiently high vapor pressure to be useful over a limited range of composition. Preferably, the perfluorinated ethers have a vapor pressure of at least about 400 Torr at 20C and most preferably, are totally gaseous (760 Torr) at room temperature and have a boiling point of less than about 5C.
Further in accordance with the invention, improved di-electric breakdown voltages that are unusally high are obtained by employing specific gaseous dielectric mixtures within the scope of -~ -20 this invention in certain critical proportions set forth below. -~
BRIEF DESCRIPTION OF THE DRAWING
__ __ ~
FIG. 1, on coordinates of breakdown voltage in kv-rms and concentration in mole percent, is a plot in a binary system A-B of the dielectric strength of various mixtures of components A and B
wherein A is a halogenated alkane and B is SF6 and/or CO2.
FIG. 2, on coordinates of breakdown voltage in kv-rms and concentration in mole percent, is a plot of various binary mixtures -of haloalkanes with SF6;
FIG. 3, on coordinates of breakdown voltage in kv-rms and -concentration in mole percent, is a plot of various binary mixtures of haloalkanes with CO2;

. ~ ... . . . .

FIG. 4, on coordinates of concentration in mole percent, is a ternary plot of the system SF6-CO2-CC12F2, showing useful regions of carbon suppression and improved dielectric strength;
FIG. 5, on coordinates of concentration in mole percent, is a ternary plot of the system SF6-CO2-CHClF2, showing useful regions of carbon suppression and improved dielectric strength;
FIG. 6, on coordinates of concentration in mole percent, is a ternary plot of the system SF6-CO2-CBrF3, showing useful regions of carbon suppression and improved dielectric strength;
FIG. 7, on coordinates of concentration in mole percent, is a ternary plot of the system SF6-CO2-CClF2CClF2, showing useful regions of carbon suppression and improved dielectric strength;
FIG. 8, on coordinates of concentration in mole percent, is a ternary plot of the system SF6-CO2-CClF2CF3, showing useful regions of carbon suppression and improved dielectric strength; -FIG. 9, on coordinates of concentration in mole percent, is a ternary plot of the system SF6-CClF3-CHF3, showing useful regions of carbon suppression, - -FIG. 10, on coordinates of concentration in mole percent, is a ternary plot of the system CO2-CF3CF3-c-C4F8, showing useful :
regions of carbon suppression and improved dielectric strength;
FIG. 11, on coordinates of breakdown voltage in kv-rms and concentration in mole percent, is a plot of the binary system SF6-X, where X is a perfluorinated ether, showing proportions having improved dielectric strength;
FIG. 12, on coordinates of breakdown voltage in kv-rms and concentration in mole percent, is a plot of the binary system CO2-X, where X is a perfluorinated ether showing proportions having improved dielectric strength; and FIG. 13, on coordinates of breakdown voltage in kv-rms and concentrations in mole percent and is a plot of the binary 10~;8894 system SF6-CO2 showing dielectric breakdown as a function of CO2 addition to SF6.
DETAILED DESCRIPTION OF THE INVENTION
In accordance with the invention, carbon formation in a dielectric fluid during an electrial discharge from an electrical conductor is suppressed by maintaining in contact with the electric conductor during operation a gaseous dielectic mixture consisting essentially of a mixture of SF6 and CO2 in specified mole ratios (group I mixtures) or at least one of SF6 or CO2 in combination with at least one halogenated alkane tgroup II mixtures) or at least one of SF6 or CO2 in combination with at least one perfluorinated ether (group III mixtures).
Halogenated alkanes useful in the practice of the inven-tion for group II mixtures are those which contain from 1 to 4 carbon atoms, since compounds with a greater number of carbon atoms tend to possess undesirably low vapor pressures at desired operat-ing temperatures. ; ~
The halogenated alkanes contain at most one hydrogen ~ -atom, with the remaining hydrogen atoms replaced by at least one halogen selected from the group consisting of fluorine, chlorine and bromine. More than one hydrogen atom per molecule results in excessive carbon tracking.
The halogenated alkanes desirably have a vapor pressure -of at least about 100 Torr at 20C. The vapor pressure limitation --permits the use of certain halogenated alkanes, such as 1,1,2-tri-chloro-1,2,2-trifluoroethane (CC12FCClF2), which are liquid at room temperature but which evidence a sufficiently high vapor pressure to be useful over a limited range of composition. Preferably, the halogenated alkanes have a vapor pressure of at least ahout 400 Torr at 20C, and most preferably, are totally gaseous (760 Torr) at room temperature and have a boiling point of less than about 5C.

10~t3894 Examples of halogenated alkanes useful in the practice of the invention include chlorodifluoromethane (CHClF2), bromotri-fluoromethane (CBrF3), hexafluoroethane (CF3CF3) and cycloocta-fluorobutane (c-C~F8).
Unexpectedly, in many of these systems, improved break-down voltage characteristics that are unusually high are obtained by employing specific gaseous dielectric mixtures of this invention within certain critical proportions set forth in examples below.
Preferably, perhalogenated alkanes find use in applications such as high dielectric strength mixtures. Perhalogenated compounds are totally halogenated and include no hydrogen. Examples include chlorotrifluoromethane (CClF3), 1,2-dichloro 1,1,2,2-tetrafluoro-ethane (CClF2CClF2), and chloropentafluoroethane (CClF2CF3).
I. Binary Compositions of Group II Mixtures Binary compositions of group II mixtures consist essentially of mixtures of two components, where one component is selected from the group consisting of halogenated alkanes and the second component is selected from the group consisting of SF6 and CO2.
Examples of binary systems preferred as carbon formation suppressants include SF6-ccl2F2~ SF6-CHClF2~ SF6 CClF2CClF2~ SF6 3~
SF6-CClF2CF3, CO2-CC12F2 and CO2-CHClF2. While each combination evidences a unique useful range for carbon suppression, in general, at least about 10 mole percent of SF6 or at least about 15 mole percent of CO2 is required to obtain suppression. Many combina-tions may require somewhat more SF6 or CO2. Such a determination is easily within the ability of one skilled in the art, however, and in the Examples section below, details are set forth for determining optimum composition ranges and preferred examples are given; see also Table I below.
Gaseous dielectric mixtures which have a low tendency to form carbon when su,bjected to repeated electrical sparking (break-- , , 106889~

down) are desired for use as carbon suppression. This objective is attained by the addition of SF6 or CO2 diluent to halo-genated alkanes in proper quantities.
Table I summarizes the data developed for group III
binary mixtures which include SF6 or CO2. In Table I are the results of tests of various diluent compounds with potential carbon suppression capability, i.e., SF6, CO2, SO2, NO and air. The number listed in Table I in mole percent (suppression value), is the minimum quantity of the diluent component which will prevent carbon formation under the conditions of the tests described in Example 2 below. The stable inert gases, CF4 and N2, which also appear in Table I, serve as both diluents and blanks. Inspecting Table I, it is evident that by comparing the suppression values of SF6 and CO2 (and others) with those of N2 and CF4, the effective- ;
ness of carbon suppression gases and the tendency of various gaseous dielectrics to form carbon can be evaluated. In cases where carbon -~
suppression is most effective, inert diluents generally require a minimum of about 50 to 70 mole percent concentration to suppress carbon, compared with a minimum of about 10 to 40 mole percent concentration for suppression of diluents in accordance with the invention. The amount of diluent (SF6 or CO2) needed for carbon suppression varies, depending upon the particular halogenated alkane.

TABL~3 I
CARBON FORMATION CONDITIONS
BINARY SYSTEMS
Compound SF6 CO2 N2 CF4 SO2 NO Air CClF3 10 CBrF3 10 15 20 10

2 2 10 15 50 50 CHClF2 35 45 75 75

3 3 20 35 30 25 CClF2CClF2 45 45 70 70 c-C4F8 35 55 70 75 20 20 40 From Table I,it is apparent that SF6 and CO2 are most effective sup-pressants with CClF3, CBrF3, CC12F2 and CHF3. Somewhat more sup-pressant is required for CF3CF3 and CClF2CF3 and even more suppressant is required for CC13F, CHClF2, c-C4F8 and CClF2CClF2.
In general, less diluent is required to suppress carbon formation when SF6 or CO2 is employed than when N2 or CF4 is employed.
With c-C4F8, it is possible to compare the effectiveness of SF6 and CO2 with the suppressor gases of SO2 and NO of U.S. Patent 3,184,533 and with air. The accuracy of suppression values is + 5 percent. Thus, SF6 is only somewhat less effective than NO or SO2.
Carbon dioxide has about the same effect as air.
Without subscribing to any particular theory, it is poss-ible that since SF6 is an inert diluent up to about 200C, and CO2 is an inert diluent up to about 300C, their action in carbon sup-pression is probably the formation of fluorine or oxygen atoms under ~ ~-arc conditions. These atoms then subsequently react with the -carbon-containing fragments of the arced halogenated alkanes, there- -1 0 - ' " ' by forming non-conducting decomposition products rather than electrically conducting carbon.
Sulfur dioxide (SO2) is a toxic, corrosive gas and is thus undesirable in a practical system. Nitric oxide (NO) in addi-tion is chemically unstable. Nitrous oxide (N2O), also chemically unstable, is an anesthetic. Air is undesirable since it tends to attack equipment components such as metals and plastics, particu-larly at the usual operating range of 120 to 250C.
Since halogenated alkanes vary in their carbon formation tendencies, the desired composition ranges are conveniently based upon the carbon suppression values of Table I. That is, for SF6 mixtures, the broad range of compositions useful as dielectric gases varies from the minimum diluent necessary to suppress carbon tracking up to about 99 mole percent of SF6. For CO2 mixtures, compositions having a -breakdown voltage of greater than about 10 kv-rms (kilovolt-root mean square) are considered useful, except in certain special appli-cations. Generally, compositions containing at least the minimum amount of CO2 necessary to suppress carbon tracking, but less than about 65 to 80 mole percent of CO2, depending on the particular gaseous mixture, are considered useful.
Of course, operating at voltages considerably less than the breakdown voltage at which carbon formation appears would permit use of a somewhat broader range of compositions. Preferred composi-tions are those that retain about 90~ of the breakdown voltage of the higher of the two components.
Within the broad range disclosed above, many combinations of halogenated alkanes with SF6 and with CO2 evidence an unex- -pected enhancement of dielectric strength, as measured by breakdown -voltage, using a standard cell as described by ASTM D2477-66T. ~ -Examples of such systems include SF6-CC12F2, SF6-CBrF3 and CO2-CBrF3. It would be expected that for most binary compositions, 10~8894 breakdown voltage would vary linearly with composition. However, for some compositions, an unexpected enhancement of breakdown vol-tage is observed. This may take the form either of a moderate positive deviation from linearity or of a significant positive deviation from linearity to the extent that over some range of com-position, the observed breakdown voltage is equal to or greater than that of either of the two end members. The latter condition is referred to herein as a synergistic effect. It is not possible to indicate general composition ranges. However, such a determination is easily within the ability of one skilled in the art. The Examples section sets forth further details and lists preferred examples; see also Tables IV and V, below.
An example of both carbon suppression and improved di-electric strength in accordance with the invention is shown in FIG. 1, which is a plot of breakdown voltage in kv-rms as a function of composition in mole percent for a binary system of com-ponents A and B. Carbon formation appears over the range indicated by the dotted portions of the curves. In this example, component B is chloropentafluoroethane (CClF2CF3). Component A is variously SF6 (curve 10); CO2 (curve 11); and CF4 (curve 12). Where component A is SF6 (curve 10), there is not only a positive deviation from linearity (cf. line 13), but an actual enhancement such that a -the mixture over a range of composition evidences a breakdown ~ ~ -voltage greater than that of either of the two end members. Where component A is CO2 (curve 11), there is a positive deviation from linearity. Where component A is CF4 (curve 12), both extensive carbon formation and little deviation from linearity are observed.
Line 13 depicts the expected linear behavior of breakdown voltage with composition variation. Such results for binary mixtures are typical of many of the mixtures of halogenated alkanes with SF6 and C2 disclosed herein. Such mixtures tend to exhibit both low carbon , 10~8894 formation and enhanced breakdown voltage characteristics.
FIGS. 2 and 3 depict preferred group II binary mixtureswith SF6 and CO2, respectively. In FIG. 2, the following curves represent the breakdown voltages of the listed compositions with SF6: curve 20, dichlorodifluoromethane (CCl2F2); curve 21, chloro-difluoromethane (CHClF2); curve 22, 1,2-dichloro-1,1,2,2-tetra-fluoroethane (CClF2CClF2); curve 23, chlorotrifluoromethane (CClF3);
and curve 24, chloropentafluoroethane (CClF2CF3). In FIG. 3, the following curves represent the breakdown voltages of the listed compositions with CO2: curve 30, CHClF2 and curve 31, CCl2F2.
II. Multi-Component Compositions of Group II Mixtures Ternary compositions of group II mixtures consist essentially of mixtures of three compounds, A, B and C, at least one of which is selected from the group consisting of halogenated alkanes and at least one of which is selected from the group con-sisting of SF6 and CO2. Examples of ternary systems preferred as carbon suppressants include SF6-CO2-CCl2F2, SF6-CO2-CHClF2, 2 2 6 C2 CClF2CF3, SF6-CO2-CBrF SF CCl and CO2-CF3CF3-c-4F8. While each combination evidences a unique useful range for carbon suppression, in general, for multicomponent mixtures containing either SF~ or CO2, gaseous mixtures con-taining at least about lO mole percent of SF6 or at least about 15 mole percent of C02 are required to suppress carbon formation.
For multicomponent mixtures containing both SF6 and C02, carbon formation is suppressed for compositions lying in regions rich in SF6 and C02 on a ternary diagram defined by a line whose minimum extremities are defined by 1 SF6 - 15 CO2 - 84 halogenated alkane 10 SF6 - 1 CO2 - 89 halogenated alkane.
Many combinations may require somewhat more SF6 and/or CO2. As before, such a determination is within the ability of one skilled . , : .:

10~8894 in the art. The Examples section below sets forth the details fordetermining such ranges and lists preferred examples.
Within the broad range of compositions useful for carbon suppression, many ternary mixtures evidence an unexpected enhance-ment of dielectric strength. Preferred examples of these systems 6 2 2F2' SF6 C2-C~ClF2' SF6-CO2-CClF CClF
SF6-CO2-CClF2CF3, SF6-CO2-CBrF3, 5F6-CHF3-CHClF2 and CO2-CF3CF3-c-C4F8. Mixtures possessing this property are also listed in the Examples section.
An example of both carbon suppression and improved dielec-tric strength in accordance with the invention is shown in FIG. 4, which is a plot of breakdown voltage in kv-rms as a function of composition in mole percent for the ternary system SF6-CO2-CC12F2.
Carbon formation appears for compositions rich in CC12F2, defined by a line whose extremities are defined by (b) 1 SF6 - 15 CO2 - 84 CC12F2 (c) 10 SF6 - 1 CO2 - 89 CC12F2.
This system evidences useful dielectric behavior within an area on the ternary diagram defined by a polygon a-b-c-d-e-a having at its 20 corners the points defined by: -(a) 1 SF6 - 65 CO2 - 34 CC12F2 (b) 1 SF6 - 15 CO2 - 84 CC12F2 (c) 10 SF6 - 1 CO2 89 CC12F2 (d) 98 SF6 - 1 CO2 - 1 CC12F2 (e) 24 SF6 - 75 CO2 - 1 CC12F2.
There is a synergistic BDV effect within an area on the ternary diagram defined by a polygon f-g-d-h-f having at its corners the points defined by ..
, . ~

10~894 (f) 30 SF6 ~ 25 CO2 45 CC12F2 (g) 30 SF6 - 1 CO2 - 69 CC12F2 (d) 98 SF~ - 1 CO2 - 1 CC12F2 (h) 74 SF6 - 25 CO2 - 1 CC12F2 See also Examples 2 and 21, below.
Other preferred examples are depicted in FIGS. 5 through 10. The figures are associated with the following systems, which are explained in further detail in the Examples section below:
FIG. 5, SF6-CO2-CHClF2 (Example 22); FIG. 6, SF6-CO2-CBrF3 (Example 23); FIG. 7, SF6-CO2-CClF2CClF2 (Example 25); FIG. 8, SF6-CO2-CClF2CF3 (Example 26); FIG. 9, SF6-CClF3-CHF3 (Example 27) and FIG.
10, CO2-CF3CF3-c-C4F8 (Example 34).
Quaternary and higher compositions within the above definition may also be formulated in accordance with the invention. One such example is SF6-CO2-CC12F2-CClF2CF3.
III. Group III Mixtures Examples of perfluorinated ethers useful in the practice of the invention for group III mixtures include perfluorodimethyl ether, (CF3)2O, perfluorodiethyl ether, (C2F5)2O, perfluoro-1,4-dioxane, O
F2C fF2 F2C\ /CF2 perfluoro-1,2-dimethoxy ethane, CF3O(CF2)2OCF3, perfluoro-1,2- ;
epoxyethane, F2C-CF2, perfluoro-1,3-epoxypropane, F2C-CF2, and F C-O
perfluoro-2,3-epoxy-2-methylbutane, (CF3)2-C-CFCF3.

Mixtures of the perfluorinated ethers with SF6 may contain from about 1 to 99 mole percent of SF6. The presence of . . .

1~6889~

SF6 serves to increase the vapor pressure of the mixture and reduce its cost. An increased vapor pressure is desirable, since as temperature is decreased, the gas density correspondingly is decreased, with an accompanying decrease in breakdown voltage.
Increased vapor pressure serves to maintain a high breakdown voltage.
Mixtures of the perfluorinated ethers with CO2 may con-tain from about 1 to 75 mole percent of CO2. For CO2 mixtures, only compositions having a breakdown voltage of greater than about 10 kv-rms are considered useful. Typically, use of greater than about 75 mole percent of CO2 in these mixtures results in a break-down vo~tage of less than about 10 kv-rms. The combination of CO2 ~-with perfluorinated ethers serves the same function as SF6.
Mixtures of one or more of the perfluorinated ethers with ~ ~-SF6 plus CO2 also result in further reduction in cost with little sacrifice in the desirable properties described above. Examples of such compositions include SF6-CO2-(CF3)2O and SF6-CO2-(C2F5)2O.
Preferred mixtures of the perfluorinated ethers with SF
and/or CO2 are those which retain 90% of the dielectric breakdown voltage of the highest component. For example, employing the methods described by ASTM D2477-66T, a mixture of SF6 and (C2F5)2O
over the range of about 1 to 60 mole percent of SF6 evidences a dielectric breakdown voltage of at least 20.0 kv-rms (kilovolt-root mean square), which is 90% of the dielectric breakdown voltage of pure (C2F5)2O, the component having the higher dielectric break-down voltage.
With some combinations of perfluorinated ethers and SF6 and/or CO2 for group III mixtures within the proportions disclosed above we have unexpectedly found some evidence of enhancement of dielectric strength, as measured by breakdown voltage. Examples f ch systems include SF6-(CF3)2O, SF6 (C2 5)2 2 2 - , , , , , ~ . . . . . . .

106889~

It would be expected that for binary compositions, breakdown voltage would vary linearly with composition. However, for some compositions, an unexpected enhancement of breakdown voltage is observed. This may take the form either of a moderate positive deviation from linearity or of a significant positive deviation from linearity to the extent that over some range of composition, the observed breakdown voltage is equal to or greater than that of either of the two end members. The latter condition is referred to herein as a synergistic effect. Although it is not 10 possible to define general composition ranges, such a determi- -~
nation is easily within the ability of one skilled in the art.
For example, employing the methods described by ASTM D2477-66T, a mixture of SF6 and (C2F5)2O over the range of about 1 to 55 mole percent of SF6 evidences a dielectric breakdown voltage of at least 22.1 kv-rms (kilovolt-root mean square). At 40 mole ~-percent of SF6, this value is observed to rise to 24.3 kv-rms.
The dielectric breakdown voltages of pure SF6 and pure (C2F5)2O
are 16.79 and 22.1 kv-rms, respectively. -~
Similarly, a mixture of CO2 and (CF3)2O over the range of about 1 to 45 mole percent of CO2 and a mixture of CO2 and (C2F4)2O over the range of about 1 to 50 mole percent of CO2 both evidence a synergistic effect. In FIGS. 11 and 12 the breakdown voltage of some perfluorinated ethers with SF6 and CO2, respec-tively, are shown. It is apparent that there is deviation from linearity, ranging from a slightly positive deviation for curve 10 to a substantial deviation for curves 11 and 21. The curves are plotted from the data of Table II.

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IV. Group I Mixtures Mixtures of SF6 and CO2 within the contemplation of group I mixtures of the invention contain from about 1 to 50 mole percent of CO2. The presence of CO2 serves to increase the vapor pressure of the mixture, lower its freezing point below -50C, and reduce its cost. An increased vapor pressure is particularly desirable, since as temperature is decreased, the gas density correspondingly is decreased due to condensation or solidification of various components, with an accompanying decrease in breakdown voltage. Thus, increased vapor pressure serves to maintain a high breakdown voltage.
The effect of CO2 addition to SF6 on vapor pressure at various temperatures is given in Table III below. There, it is seen that addition of about 10 to 30 mole percent of CO2 results in a substantial increase in vapor pressure over that of pure SF6 over the same temperature range. This increase is considerably qreater than that predicted by Raoult's law. Such mixtures are particularly useful in high voltage apparatus exposed to sub-freezing temperatures. Also, the stability of CO2 at high tempera-tures (200C and higher) indicates that these mixtures are alsouseful in high temperature applications.

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Unexpectedly, mixtures of SF6 and CO2 within the pro-portions of about 1 to 50 mole percent of CO2 retain at least 90%
of the dielectric breakdown voltage of pure SF6, which is con-sidered to be substantially equal to the breakdown voltage of SF6.
Over the range of about 1 to 30 mole percent of CO2, the observed breakdown voltage is approximately equal to or greater than that of SF6. The latter condition in these group I mixtures is referred to herein as a synergistic effect. For example, employing the methods described by ASTM D2477-66T and a gap of 0.1 inch between electrodes, a mixture of SF6 and CO2 over the range of about 1 to 30 mole percent of CO2 evidenced dielectric breakdown voltages of about 16.2 to 16.5 kv-rms (kilovolt-root mean square). The dielec-tric breakdown voltages of pure SF6 and pure CO2 were 16.32 and 6./16 kv-rms, respectively.
An increase in pressure and an increase in gap distance for compositions over the range of about 10 to 30 mole percent of C2 results in breakdown voltages that are still within 90% of that of pure SF6. These results are listed in Table IV below.

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10~8894 FIG. 13 depicts the breakdown voltage of mixtures of SF6 and CO2 in group I mixtures at room temperature. It is apparent that there is significant deviation from linearity.
These results, together with the effects of temperature and pressure given above and the lack of carbon formation observed for these mixtures, indicate that mixtures of SF6 and C02 within the proportions disclosed form excellent gaseous dielectric com-positions suitable for use over a wide range of temperatures and pressures.
The breakdown voltage data for mixtures of SF6 and CO2 are listed in Table V below. From the data given, the useful range for gaseous dielectric behavior may be determined. The mixtures over the range of about 1 to 50 mole percent of CO2 evidenced breakdown voltage values at least about 90% of that of SF6. Mix-tures over the range of about 1 to 30 mole percent of C02 evidenced unusually high breakdown voltage values compared with the SF6.
Since the normal expected behavior is a linear dependence with com-position, such unusual behavior is considered to be a synergistic -effect, and such mixtures are preferred.
No carbon formation was observed within the limits of -the test.

~BLE V
Breakdown Voltage, kv-rms, as a Function of CO2 Addition (mole ~ercent) to SF
_ _ 6 16.32 16.47 16.36 16.19 15.78 15.24 13.28 11.33 9.83 8.08 6.16 All compositions disclosed herein have utility as gaseous dielectric mixtures for carbon suppression. As such, they have application in electrical apparatus, especially high voltage power --30 equipment, such as transformers, capacitors, coaxial lines and minisubstations, having a chamber in which electrical arcing .. , .. : : ~

10~8894 occasionally occurs and which includes the gaseous dielectric mix-ture. Some of the mixtures are particularly useful in certain specific areas, such as for extreme temperature conditions, when high dielectric strength is required, which are indicated in examples set forth below.
The considerations in choosing a particular system in-clude the cost of the components, the temperature performance desired (low or high), the electrical properties desired, and the relative safety of the total mixture.
EXAMPLES
I. Description of Test Procedure Breakdown voltage (BDV) was measured by equipment which included a glass breakdown voltage cell as described by ASTM D2477-66T, a 50 kv-rms (kilovolt-root mean square), 60 Hz, 5 kva trans-former and suitable accessory circuits. A vacuum manifold with Bourdon Tube type manometer, solenoid valves and controls was also used.
The cell had an 0.75 inch sphere and a 1.5 inch plane elec-trodes. The breakdown cell filling manifold, using solenoid valves, furnished connections to the cell, the manometer, various gas inlets and the vacuum pump. The manometer was a Wallace and Tiernan model 62A-4D-0800, ranging in two rotations of the indicator needle be-tween 0 and 800 Torr absolute. A simple control panel governed the solenoid valves used to admit the various gases of the mixtures in the BDV cell. The BDV measurement conditions were 60 Hz, 0.100 inch gap, 760 Torr total pressure and ambient room temperature. Compo-sitions were prepared in terms of partial pressure, accurate to + 0.5 Torr, and converted to mole percent assuming ideal gas law behavior. -The electrodes had to be polished prior to taking BVD data.
They were polished with E5 emery grit, soaked in xylene for 30 min, ~: ' ' .: .

~0~8894 rinsed with petroleum ether and dried at 100C for 15 min. A few preliminary breakdown voltage shots were necessary prior to taking data to condition the electrodes. Even so, the BDV of pure compo-nents, such as SF6, was observed to vary slightly from one experi-ment to the next.
For measuring carbon suppression, there were two levels of testing. In the first, any carbon appearing after 5 BDV shots was monitored as a "go-no go" test. For a more severe exposure test, 50 successive BDV shots were taken in the same manner.
Carbon tetrafluoride, CF4, the most stable fluorocarbon known, and nitrogen, N2 served as inert diluents and blanks. In the test for carbon formation, the measurements started at high SF6 or CO2 concentrati~ns. These were gradually reduced until carbon appeared. Carbon was usually observed to form on the grounded plane electrode.
Example 1. - Procedure for Measurement of Breakdown Voltage This Example demonstrates the breakdown voltage measure-ment by the ASTM D2477-66T method, using various dielectric mix-tures. The equipment included a vacuum manifold, the glass breakdown voltage cell, 0 to 50 kv test set rated at 5 kva and 40,000 ohms of 250 watt current limiting resistors. The manifold had valved connections to air, to the vacuum pump, to the manometer and to three cylinders which contained components A, B or C. -An air gap of two 12.5 cm diameter brass spheres served -for a peak voltage calibration standard. Prior to measurements, the transformers voltmeters were calibrated with this gap using the BDV methods of ASTM D2477-66T, i.e., averaging 5 successive spark breakdowns at set gap distances. The meters were accurate to 0.5 kv, or within calibration.
In preparing a test sample, the ideal gas law was used and pressure percent was assumed equal to mole percent. The desired , . , , , - , 10~889~
mole percent of each component was calculated as the number of Torr compared to 760 Torr (1 atmosphere), which yielded the desiréd mole percent. Prior to make up of the composition the test cell was evacuated to less than 1 Torr. During make up of the composition the component to be present in the smallest amount was admitted first, until it attained the desired partial pressure. This was followed by the component with the next highest percentage and finally by the component present in the largest mole fraction.
Table VI below presents the pressures used for some SF6-CC12F2 mixtures, together with the breakdown voltage of each composition and its standard deviation (SD).

TABLE VI
BREAKDOWN VOLTAGE OF SF6-CC12F2 MIXTURES i-~

P, P, BDV + SD, Mole %Torr Mole % Torr kv-rms* kv-rms*

100 760 0 0 17.43 0.33 608 20 152 17.74 0.51 456 40 304 18.53 0.29 -~

20 40 304 60 456 18.46 0.51 152 80 608 17.90 0.43 0 0 100 760 17.28 0.32 *rms = root mean square value, i.e. BDV rms = 0.707 BDV peak.

Synergism is indicated in the magnitude of about 1 kv-rms greater than the breakdown voltage of SF6 over the range of about 40 to 60 mole percent of SF6; see also FIG. 2 and Example 4, below.

Example 2 - Process for Measurement of Carbon Formation Suppression This Example describes the method of measuring carbon suppres- -sion, using SF6, CO2 and CC12F2. The equipment of Example 1 was used for the tests. The compositions were again made up using pressure percent (mole percent) at one atmosphere total pressure.

10~889~

To evaluate carbon formation, a given sample of definite composi-tion was repeatedly sparked, as in Example 1, and BDV observéd.
There were two levels of exposure, 10 sparks and 50 sparks, all applied successively to the same gas sample. If carbon appeared, the BDV cell was disassembled and the electrodes cleaned and condi-tioned again.
Table VII presents the pressures and compositions of the samples, the observed breakdown voltages and the number of shots which did, or did not, produce carbon. With these mixtures, a 5 per-cent change in composition caused a large increase in carbon forma-tion suppression: at 90 CC12F2 - 10 CO2, carbon formed after 20 sparks, whereas at 85 CC12F2 - 15 CO2, no carbon appeared after 50 sparks. Similarly, pure CF2C12 formed carbon after 10 sparks, while at 95 CC12F2 - 5 SF6, no carbon appeared after 50 sparks. A detailed study of this system is shown in FIG. 4 and is discussed below in further detail in Example 21. In FIG. 4, the breakdown voltages of compositions in the system SF6-CO2-CC12F2 are depicted on a ternary -plot as a function of mole percent.

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106885~4 II. Binary Group II Mixtures A. SF Binary Mixtures The breakdown voltage data for binary group II mixtures which included SF6 is listed in Table VIII. From the data given, both the minimum amount of SF6 useful in suppressing carbon formation and the useful range for gaseous dielectric behavior may be determined. Many binary mixtures evidenced breakdown voltage values within about 90~ of that of the higher end member over a range of compositions; such mixtures are preferred.
Certain binary mixtures evidenced unusually high breakdown vol-tage values compared with the values of either end member. Since the normal expected behavior is a linear dependence with compo-sition, such unusual behavior is termed a synergistic effect, and such mixtures are also preferred. Following Table VIII is a discussion of some of the binary mixtures including SF6 and their utility.

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Example 3. System SF -CCl F

A BDV of 23.7 kv was observed for CC13F, compared with a value of 18.1 kv for SF6. The system evidenced useful dielectric behavior over the range of about 30 to 99 mole percent of SF6. The BDV was at least 90% that of CC13F over the range of about 30 to 70 mole percent of SF6. At least about 30 mole percent of SF6 was required to suppress carbon formation in CC13F.
The combination of SF6 and CC13F is an inexpensive dielectric mixture. The preferred operating temperature range is greater than ambient but less than 150C.
Example 4. System SF6-CC12F2 (FIG. 2, curve 20) This system evidenced useful dielectric behavior over the range of about 10 to 99 mole percent of SF6. There was a synergistic BDV effect over the range of about 40 to 80 mole percent of SF6. At least about 10 mole percent of SF6 was required to suppress carbon formation in CC12F2.
The combination of SF6 and CC12F2 is an inexpensive dielectric mixture for use in units such as underg~ound or underwater high voltage coaxial lines, capacitors and in gas filled transformers.

Example 5. System SF -CClF3 (FIG. 2, curve 23) This system evidenced useful dielectric behavior over the range of about 15 to 99 mole percent of SF6. The BDV was at least 90~ that of SF6 over the range of is about 60 to 99 mole percent of SF6. There was a synergistic BDV effect over -~
a narrow range of about 75 to 85 mole percent of SF6. At least about 15 mole percent of SF6 was required to suppress carbon formation in CClF3.
This system is useful in raising the vapor pressure of SF
without substantially decreasing the BDV. Hence, it is suitable for low temperature use in transformers and capacitors.

Example 6. System SF -CBrF

This system evidenced useful dielectric behavior over the .. ... . . . . .
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10~8894 range of about 10 to 99 mole percent of SF6. The BDV was at least 90% that of SF6 over the range of about 20 to 99 mole percent of SF6. There was a synergistic BDV effect over the range of about 60 to 85 mole percent of SF6. At least about 10 mole percent of SF6 was required to suppress carbon formation in CBrF3.

Example 7. System SF -CHClF (FIG. 2, curve 21) An SF6-CHClF2 azeotrope existed at 90 mole percent of SF6.
This system evidenced useful dielectric behavior over the range of about 35 to 99 mole percent SF6. A synergistic effect was observed over a narrow range of about 40 to 50 mole percent of SF6. At least about 35 mole percent of SF6 was required to suppress carbon ..
formation in CHClF2.
The combination of SF6 and CHClF2 is an inexpensive dielectric mixture with only a slight compromise in SF6 BDV and vapor pressure.
Example 8. System SF6 CHF3 This system evidenced useful dielectric behavior over the range of about 15 to 99 mole percent of SF6. The BDV was at least 90% that of SF6 over the range of about 65 to 99 mole percent of SF6. At least about 15 mole percent of SF6 was required to suppress carbon formation in CHF3.
The use of CHF3 can increase the vapor pressure of SF6 -without substantial SF6 BDV decrease, either alone or together with an azeotropic mixture of CClF3. These gaseous dielectric mixtures are useful in low temperature applications, such as gas filled trans- -.
formers which are exposed to winter conditions.

Bxample 9. System SF -CClF CClF (FIG. 2, curve 22) A BDV of 21.8 kv was observed for CClF2CClF2, compared :

with a value of 16.6 kv for SF6. The system evidenced useful di- ~ -30 electric behavior over the range of about 45 to 99 mole percent of :
SF6. The BDV was at least 90% that of CClF2CClF2 over the range : . .

of about 45 to 85 mole percent of SF6. At least about 45 mole per-cent of SF6 was required to suppress carbon formation in CClF2CClF2.
This system evidenced a substantial BDV improvement over SF6 alone.
The relatively high boiling point of CClF2CCLF2 (3.6C) limits low temperature uses of this system, but at ambient room temp-erature or above, it is a satisfactory dielectric mixture at only moderate cost.

Example 10. System SF -CClF CF (FIGS. 1, curve 10 and 2, curve 24) ~ 2- 3 This system evidenced useful dielectric behavior over the range of about 25 to 99 mole percent of SF6. There was a synergistic effect over the range of about 25 to 90 mole percent of SF6, and a substantial synergistic effect (greater than about 1 kv) over the range of about 30 to 60 mole percent of SF6. At least about 25 mole percent of SF6 was required to suppress carbon formation in CClF2CF3.
The combination of SF6 and CClF2CF3 is an inexpensive gaseous dielectric mixture having the same or higher dielectric strength than SF6 alone.

Example 11. System SF -CF CF

This system evidenced useful dielectric behavior over the range of about 20 to 99 mole percent of SF6. At least about 20 - ~ -mole percent of SF6 was required to suppress carbon formation in The low boiling point of CF3CF3 (-78C) and its good thermal stability make it a desirable diluent for SF6 for low temperature applications. Also, since CF3CF3 is a very ther-mally stable gas, the addition of sufficient SF6 to suppress car-bon formation (about 20 mole percent) should lead to a more desirable high temperature gaseous dielectric mixture for use in transformers~
Example 12. System SF6-c-C4F8 A BDV of 19.9 kv was observed for c-C4F8, compared with a value of 17.7 kv for SF6. The system evidenced useful .

10~8894 dielectric behavior over the range of about 35 to 99 mole percent of SF~. At least about 35 mole percent of SF6 was required for carbon suppression.
The SF6-c-C4F8 system has a higher BDV than SF6 alone.
It is not suitable for use below 0C due to the high boiling point of c-C4F8 (-6C). On the other hand, c-C4F8 can be a component in high temperature gaseous dielectric mixtures; see also its use with CO2 in Example 20, below.
B. CO2 Binary Mixtures The breakdown voltage data for binary group II mixtures which included CO2 are listed in Table IX. From the data given, both the minimum amount of CO2 useful in suppressing carbon forma-tion and the useful range for gaseous dielectric behavior may be determined. As before, mixtures evidencing at least 90% of the breakdown voltage of the higher of the two components are preferred, as are synergistic compositions. Following Table IX is a discussion of some of the binary mixtures including CO2 and their utility. -In general, while CO2 binary mixtures tended to evidence less BDV synergism than did the SF6 binary mixtures, they evidenced good carbon suppression properties. Except in special applica-tions, such as low voltage use, mixtures evidencing breakdown voltages of less than about 10 kv-rms are not considered to be as useful as those greater than about 10 kv-rms.

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Example 13. System CO -CCl F (FIG. 3, curve 31) This system evidenced useful dielectric behavior over the range of about 15 to 65 mole peecent of CO2. The BDV was at least 90% that of CC12F2 over the range of about 15 to 35 mole per-cent of CO2. At least about 15 mole percent of CO2 was required to suppress carbon formation in CC12F2.
At about 20 mole percent of CO2, this system has a BDV
of 14.9 kv, compared with a BDV of 16.1 kv for pure SF6. This system is an inexpensive gaseous dielectric mixture suitable for operation in the range of about -20C to 150C.
Example 14. System CO -CBrF

This system evidenced useful dielectric behavior over the range of about 15 to 65 mole percent of CO2. At least about -15 mole percent of CO2 was required to suppress carbon formation in CBrF3.
This binary system is useful in high temperature applica-tions.
Example 15. System CO -CHClF (FIG. 3, curve 30) This system evidenced useful dielectric behavior over the range of about 45 to 70 mole percent of CO2. There was a synergistic effect over this entire range. At least about 45 mole percent of CO2 was required to suppress carbon formation in CHClF2.
This system is an inexpensive dielectric mixture for low voltage uses when SF6 is not economically practical.
Example 16. System CO2-CHF3 This system evidenced useful dielectric behavior over the range of about 15 to 99 mole percent of CO2. There was a synergistic effect over a narrow range of about 15 to 25 mole percent of CO2.
At least about 15 mole percent of CO2 was required to suppress carbon formation in CHF3.

. .

10~889~
Example 17. System CO2-CClF2CClF2 This system evidenced useful dielectric behavior over the range of about 45 to 85 mole percent of CO2. At least about 45 mole percent of CO2 was required to suppress carbon formation Example 18. System CO2-CClF2CF3 (FIG. 1, curve_ll) This system evidenced useful dielectric behavior over the range of about 25 to 70 mole percent of CO2. The BDV was at least 90~ that of CClF2CF3 over the range of about 25 to 35 mole percent of CO2. At least about 25 mole percent of CO2 was required to suppress carbon formation in CClF2CF3.
This system is suitable for dry type transformers up to about 250C and is relatively inexpensive compared with SF6.

Example 19. System CO -CF CF

This system evidenced useful dielectric behavior over the range of about 35 to 50 mole percent of CO2. At least about 35 mole percent of CO2 was required to suppress carbon formation 3 3' Example 20. System CO -c-C F

This system evidenced useful dielectric behavior over .
the range of about 55 to 75 mole percent of CO2. At least about 55 mole percent CO2 was reqùired to suppress carbon formation c C4F8- -This system can be used in formulating multi-component mixtures which do not contain SF6 and which are suitable for ~ -operating temperatures up to about 300C.
III. Multicomponent Group II Mixtures Data for these group II mixtures are most conveniently represented on ternary diagrams expressed in mole percent.

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Example 21. System SF -CO -CCl F ( FIG . 4 ) This system evidenced useful dielectric behavior with-in an area on a ternary diagram defined by a polygon a-b-c-d-e-a having at its corners the points defined by:
(a) 1 SF6 - 65 CO2 - 34 CC12F2 (b) 1 SF6 - 15 CO2 - 84 CC12F2 ( c ) 10 SF6 - 1 C2 - 89 CCl 2F2 (d) 9~ SF6 - 1 CO2 - 1 CC12F2 (e) 24 SF6 - 75 CO2 - 1 CC12F2.
There was a synergistic BDV effect within an area on the ternary diagram defined by a polygon f-g-d-h-f having at its corners the points defined by (f) 30 SF6 - 25 CO2 - 45 CC12F2 (g) 30 SF6 - 1 CO2 - 69 Ccl2F2 (d) 98 SF6 - 1 C2 - 1 CC12F2 (h) 74 SF6 - 25 CO2 - 1 CC12F2 Carbon formation was suppressed for compositions lying in -:~
regions rich in SF6 and CO2 defined by a line b-c whose extremities ~.
are defined by (b) 1 SF6 - 15 CO2 - 84 CC12F2 (c) 10 SF6 - 1 C2 - 89 CC12F2.
This system is an inexpensive gaseous dielectric mixture suitable for coaxial lines exposed to temperatures down to -30C.
Example 22. System SF6 CO2-CHClF2 ( FIG 5 ) This system evidenced useful dielectric behavior within an area on a ternary diagram defined by a polygon a-b-c-d-e-a having at its corners the points defined by (a) 1 SF6 - 70 CO2 - 29 CHClF2 (b) 1 SF6 - S0 CO2 - 49 CHClF2 IC) 35 SP6 - 1 C2 - 64 CHClF2 (d) 98 SF6 - 1 CO2 - 1 CHClF2 (e) 24 SF6 - 75 CO2 - lCHClF2.
There was the synergistic BDV effect within an area on the ternary diagram defined by a polygon c-d-f-c having at its corners the points defined by (c) 35 SF6 - 1 CO2 - 64 CHClF2 (d) 98 SF6 - 1 CO2 - 1 CHClF2 (f) 64 SF6 - 35 CO2 - 1 CHClF2.
Carbon formation was suppressed for compositions lying in regions rich in SF6 and CO2 defined by a line b-c whose ex-tremities are defined by(b) 1 SF6 - 50 CO2 - 49 CHClF2 (c) 35 SF6 - 1 CO2 - 64 CHClF2.

Example 23. System SF -CO -CBrF (FIG 6) This system evidenced useful dielectric behavior within an area on a ternary diagram defined by a polygon a-b-c-d-e-a having at its corners the points defined by -(a) 1 SF6 - 65 CO2 - 34 CBrF3 -(b) 1 SF6 - 15 CO2 - 84 CBrF3 (c) 10 SF6 - 1 CO2 - 89 CBrF3 Id) 98 SF6 - 1 CO2 1 CBrF3 (e) 24 SF6 - 75 CO2 - 1 CBrF3.
There was a synergistic BDV effect within an area on the ternary diagram defined by a polygon f-g-h-d-f having at its corners the points defined by (f) 50 SF6 - 49 CO2 - 1 CBrF3 (g) 50 SF6 - 5 CO2 - 45 CBrF~ -(h) 54 SF6 - 1 CO2 - 45 CBrF3 ~ :
(d) 98 SF6 - 1 CO2 - 1 CBrF3~
Carbon formation was suppressed for compositions lying :~.
in regions rich in SF6 and CO2 defined by a line b-c whose ex-tremities are defined by -39- :

.

(b) 1 SF6 - 14 CO2 - 85 CsrF3 (c) 14 SF6 - 1 CO2 - 85 CBrF3.
Example 24. System SF -CO -CC1 FCClF

This system evidenced useful dielectric behavior with-in an area on a ternary diagram defined by a polygon having at its corners the points defined by 1 SF6 - 74 C02 - 25 CC12FCClF2 25 SF6 - 50 CO2 - 25 CC12FCClF2 38 SF6 - 1 C02 - 61 CC12FCClF2 98 SF6 - 1 CO2 - 1 CC12FCClF2 25 SF - 74 CO - 1 CC1 FCClF
Carbon formation was suppressed for compositions lying in regions rich in SF6 and C02 defined by two lines whose extremities are defined by 1. 1 SF6 - 74 C02 - 25 CC12FCClF2 25 SF6 - 50 CO2 - 25 CC12FCClF2 2- 25 SF6 ~ 50 CO2 ~ 25 CCl FCClF
38 SF6 - 1 CO2 - 61 CC12FCClF2.
Example 25. System SF6 CO2-CClF2CClF2 (FIG. 7) This system evidenced useful dielectric behavior within an area on a ternary diagram defined by a polygon a-b-c-d-e-a hav-ing at its corners the points defined by (a) 1 SF6 - 85 CO2 - 14 CClF2CClF2 (b) 1 SF6 - 45 CO2 - 54 CClF2CClF2 (c) 45 SF6 - 1 CO2 - 54 CClF2CClF2 (d) 98 SF6 - 1 C02 - 1 CClF2CClF2 (e) 24 SF6 - 75 CO2 - 1 CClF2CClF2 There was a synergistic BDV effect within an area on the ternary diagram defined by a polygon f-c-d-g-f having at its corners the.points defined by (f) 11 SF6 - 35 CO2 - 54 CClF2CClF2 ~068894 (c) 45 SF6 - 1 CO2 - 54 CClF2CClF2 (d) 98 SF6 - 1 CO2 - 1 CClF2CClF2 (g) 64 SF6 - 35 CO2 - 1 CClF2CClF2.
Carbon formation was suppressed for compositions lying in regions rich in SF6 and CO2 defined by a line whose extremi-ties are defined by (b) 1 SF6 - 45 CO2 - 54 CClF2CClF2 (c) 45 SF6 - 1 CO2 - 54 CClF2CClF2.

Exam~le 26. System SF -CO -CClF CF (FIG. 8) 6 2 2 ~ 3 This system evidenced useful dielectric behavior within an area on a ternary diagram defined by a polygon a-b-c-d-e-a having at its corners the points defined by :~
(a) 1 SF6 - 70 CO2 - 29 CClF2CF3 (b) 1 SF6 - 25 CO2 - 74 CClF2CF3 (c) 25 SF6 - 1 CO2 - 74 CClF2CF3 (d) 98 SF6 - 1 CO2 - 1 CClF2CF3 (e) 24 SF6 - 75 CO2 - 1 CClF2CF3.
There was a synergistic effect within an area on the ternary diagram defined by a polygon f-c-d-g-f having at its corners the points defined by (f) 35 SF6 - 20 CO2 - 45 CClF2CF3 (c) 25 SF6 - 1 CO2 - 74 CClF2CF3 (d) 98 SF6 - 1 CO2 - 1 CClF2CF
(g) 79 SF6 - 20 CO2 - 1 CClF2CF3.
Carbon formation was suppressed for compositions lying .
in regions rich in SF6 and CO2 defined by a line b-c whose ex- ~ ~:
tremities are defined by (b) 1 SF6 - 25 CO2 - 74 CClF2CF3 (c) 25 SP6 - 1 CO2 - 74 CClF2CF3. ,:- -Example 27. System SF6-CClF3-CHF3 (FIG. 9) The system evidenced useful dielectric behavior within 106~894 an area on a ternary diagram defined by a polygon a-b-c-d-a having at its corners the points defined by (a) 10 SF6 - 89 CClF3 - 1 CHF3 (b) 25 SF6 - 37.5 CClF3 - 37.5 CHF3 (c) 10 SF6 - 1 CClF3 - 89 CHF3 (d) 98 SF6 - 1 CClF3 - 1 CHF3.
Carbon formation was suppressed for compositions lying in regions rich in SF6 defined by two lines, a-b and b-c, whose extremities are defined by 1. (a) 10 SF6 - 89 CClF3 - 1 CHF3 (b) 25 SF6 - 37.5 CClF3 - 37.5 CHF3 2- (b) 25 SF6 - 37.5 CClF3 - 37.5 CHF3 (c) 10 SF6 - 1 CClF3 - 89 CHF3.
This system is useful in gas filled transformers operating under winter conditions and in circuit breaker controls.
Example 28. System SF6 CHF3-C~ClF2 This system evidenced useful dielectric behavior within an area on a ternary diagram defined by a polygon having at its corners the points defined by 20 SF6 - 79 CHF3 - 1 CHClF2 44 SF6 ~ 1 CHF3 - 60 CHClF2 98 SF - 1 CHF - 1 CHClF .

There was a synergistic BDV effect within an area on ~:
the ternary diagram defined by a polygon having at its corners the points defined by 45 SF6 ~ 5 CHF3 - 50 CHClF2 49 SF6 ~ 1 CHF3 - 50 CHClF2 98 SF6 ~ 1 CHF3 - 1 CHClF2 S 6 5 CHF3 1 CHClF2.
Carbon formation was suppressed for compositions lying in regions rich in SF6 defined by a line whose extremities are defined by ~ . . , :

10f~889~

20 SF6 - 79 CHF3 - 1 CHClF2 44 SF6 ~ 1 CHF3 - 60 CHClF2 Example 29. System SF -CCl F -CClF CClF

This system evidenced useful dielectric behavior within an area on a ternary diagram defined by a polygonal having at its corners the points defined by 6 89 CC12F2 - 1 CClF2CClF2 47 SF6 - 1 CCl F - 52 CClF CClF
6 2 2 1 2CClF2.
Carbon formation was suppressed for compositions lying in regions rich in SF6 defined by a line whose extremities are defined by 6 2 2 2CClF2 47 SF6 - 1 CC12F2 - 52 CClF2CClF2.
Example 30. System SF6-CC12F2-CClF2CF
This system evidenced useful dielectric behavior within an area on a ternary diagram defined by a polygon having at its -~
corners the points defined by : -11 SF6 - 88 CC12F2 - 1 CClF2CF3 26 SF6 - 1 CC12F2 - 73 CClF2CF3 -~ -98 SF6 - 1 CC12F2 ~ 1 CClF2CF3 There was a synergistic BDV effect within an area on the ternary diagram defined by a polygon having at its corners ...
the points defined by 30 SF6 ~ 55 CC12F2 ~ 15 CClF2CF3 30 SF6 - 1 CC12F2 - 69 CClF2CF
98 SF6 - 1 CC12F2 - 1 CClF2CF
44 SF6 - 55 CC12F2 - 1 CClF2CF3.
Carbon formation was suppressed for compositions lying 30 in regions rich in SF6 defined by a line whose extremities are ~-~
defined by ~.

. .

10~8894 11 SF6 - 88 CC12F2 - 1 CClF2CF3 26 SF6 - 1 CCl F2 ~ 73 CClF CF
Example 31. System SF6-CClF3-CClF2CF3 This system evidenced useful dielectric behavior within an area on a ternary diagram defined by a polygon having at its corners the points defined by 98 SF6 ~ 1 CClF3 - 1 CClF2CF3 14 SF6 - 85 CClF3 - 1 CClF2CF3 There was a synergistic BDV effect within an area on the ternary diagram defined by a polygon having at its corners the points defined by 30 SF6 ~ 15 CClF3 - 55 CClF2CF3 S 6 CClF3 69 CClF2CF3 98 SF6 - 1 CClF - 1 CClF2CF
84 SF6 - 15 CClF3 - 1 CClF2CF3.
Carbon formation was suppressed for compositions lying in regions rich in SF6 defined by a line whose extremities are defined by 1 SF - 95 CClF - 4 CClF CF
27 SF6 - 1 CClF3 - 74 CClF2CF3 Example 32. System SF -CBrF -CClF CClF
6 3 ~ -2 2 This system evidenced useful dielectric behavior within an area on a ternary diagram defined by a polygon having at its corners the points defined by ~
14 SF6 - 85 CBrF3 - 1 CClF2CClF2 -45 SF6 - 1 CBrF3 - 54 CClF CClF
98 SF6 - 1 CBrF3 - 1 CClF2CClF2.

Carbon formation was suppressed for compositions lying in regions rich in SF6 defined by a line whose extremitiès are ~0~j889~

defined by 14 SF6 - 85 CBrF - 1 CClF CClF
45 SF - 1 CBrF - 54 CClF CClF
Example 33. System CO2-CBrF3-CClF2CClF2 This system evidenced useful dielectric behavior within an area on a ternary diagram defined by a polygon having at its corners the points defined by 2 3 2CClF2 45 C2 ~ 1 CBrF3 - 54 CClF CClF
85 CO2 - 1 CBrF3 - 14 CClF2CClF2 50 C2 ~ 49 CBrF3 - 1 CClF2CClF2.
Carbon formation was suppressed for compositions lying in regions rich in CO2 and defined by a line whose extremities are defined by 15 CO2 - 84 CBrF3 - 1 CClF2CClF2 45 C2 ~ 1 CBrF3 - 54 CClF2CClF2.
Example 34. System CO2 CF3CF3 c C4_8 ( This system evidenced useful dielectric behavior within an area on a ternary diagram defined by a polygon a-b-c-d-e-f-g-a having at its corners the points defined by (a) 35 CO2 - 64 CF3CF3 - 1 c-C4F8 ~
(b) 15 CO2 - 70 CF3CF3 - 15 c-C4F8 . :
(c) 15 CO2 - 50 CF3CF3 - 35 c-C4F8 (d) 55 CO2 - 1 CF3CF3 - 44 c-C4F8 :~
(e) 75 CO2 - 1 CF3CF3 - 24 c-C4F8 .
(f) 75 C2 ~ 15 CF3CF3 10 c C4F8 2 49 CF3CF3 - 1 c-C4F8.
There was a synergistic BDV effect with an area on the ternary diagram defined by a polygon h-c-d-j-i-h having at its ~.
corners the points defined by 10~8894 ( 2 3 3 4 8 (c) 15 CO2 - 50 CF3CF3 - 35 c-C4F8 ( ) 2 3 3 c C4 8 (j) S8 CO2 - 1 cF3cF3 - 41 C-C4F8 ( ) 2 3 3 4 8-Carbon formation was suppressed for compositions lying in regions rich in C02 defined by three lines, a-b, b-c and c-d, whose extremities are defined by 1. (a) 35 CO2 - 64 CF3CF3 - 1 c-C4F8 (b) 15 CO2 - 70 CF3CF3 - 15 c-C4F8 2. (b) 15 CO2 - 70 CF3CF3 - 15 c-C4F8 (c) 15 CO2 50 cF3cF3 35 c C4F8 3. (c) 15 CO2 - 50 CF3CF3 - 35 c-C4F8 (d) 55 CO2 - 1 CF3CF3 - 44 c-C4F8.
Example 35. System 90 SF6-10 CO2-CC12F2 CClF2CF3 This quaternary system, in which SF6 and CO2 were held in a constant ratio of 90/10, evidenced useful dielectric behavior within an area on a ternary diagram of SF6-CO2, CC12F2 and CClF2CF3 defined by a polygon having at its corners the points defined by 10 SF6-C02 ~ 89 CC12F2 ~ 1 CClF2CF3 98 SF6-CO2 - 1 CC12F~ - 1 CClF2CF3-There was a synergistic BDV effect within an area on the ternary diagram defined by a polygon having at its corners the points defined by 40 SP6-CO2 - 30 CC12F2 - 30 CClF2CF3 30 SF6-C2 ~ 1 CC12F2 ~ 69 CClF2CF3 -6 2 C12 2 1 CClF2CF3 69 SF6-CO2 - 30 CC12F2 - 1 CClF2CF3.
Carbon formation was suppressed for compositions lying in 10f~889~

regions rich in SF6-CO2 defined by a line whose extremities are defined by 10 SF6-CO2 - 89 CC12F2 - 1 CClF2CF3 6 C2 1 CC12F2 69 CClF2CF3.
The following examples are illustrative of perfluori-nated ether (group III) systems.
Example 36._ System SF6-(CF3)2O (FIG. 11, curve 10) Both pure (CF3)2O and mixtures with SF6 over the entire range of SF6 addition (about 1 to 99 mole percent of SF6) evidenced useful dielectric behavior. The BDV was at least 90% that of SF6 over the range of about 10 to 99 mole percent of SF6.
Example 37. System SF6 (C2F5)2O (FIG. 11, curve 11) Both pure (C2F5)2O and mixtures with SF6 over the entire range of SF6 addition (about 1 to 99 mole percent of SF6) evidenced useful dielectric behavior. The BDV was at least 90% that of (C2F5)2O over the range of about 1 to 60 mole percent of SF6. ~
There was a synergistic BDV effect from about 1 to 55 mole percent ~ -of SF6 Example 38. System CO2-(CF3)2O (FIG. 12, curve 20) Both pure (CF3)2O and mixtures with CO2 over the range of about 1 to 65 mole percent of CO2 addition evidenced useful dielectric behavior. The BDV was at least 90% that of (CF3)2O
over the range of about 1 to 50 mole percent of CO2. There was a -slight synergistic BDV effect from about 1 to 45 mole percent of CO2 ' ' Example 39. System CO2 (C2F5)2O (FIG. 12, curve 21) Both pure (C2F5)2O and mixtures with CO2 over the range of about 1 to 75 mole percent of CO2 evidenced useful dielectric behavior. The BDV was at least 90~ that of (C2F5)2O over the range of about 1 to 45 mole percent of CO2. There was a synergistic BDV

effect over the range of about 1 to 50 mole percent of CO2.

... . . ..
. "

Claims

What is claimed is:

1. A process for suppressing carbon formation in a dielectric fluid during an electrical discharge from an electrical conductor which comprises maintaining in contact with the electrical conductor during operation a gaseous dielectric mixture consisting essentially of (a) a mixture of SF6 and CO2 containing from about 1 to 50 moles of CO2 or (b) at least one halogenated alkane plus at least one gas selected from the group consisting of SF6 and CO2, said halogenated alkane containing from 1 to 4 carbon atoms and at most one hydrogen atom, with the remaining hydrogen atoms replaced by at least one halogen selected from the group consisting of fluorine, chlorine and bromine, or (c) at least one perfluorinated ether plus at least one gas selected from the group consisting of SF6 and CO2, said per-fluorinated ether containing from 2 to 6 carbon atoms and having a vapor pressure of at least about 100 Torr at 20°C.

2. The process of claim 1 in which the gaseous dielectric mixture consists essentially of at least one halogenated alkane selected from the group consisting of CHClF2, CCl2F2, CClF2CClF2, CClF3 and CClF2CF3 plus at least about 10 mole percent of SF6.

3. The process of claim 1 in which the gaseous dielectric mixture consists essentially of at least one halogenated alkane selected from the group consisting of CHClF2 and CCl2F2 plus at least about 15 mole percent of CO2.

4. The process of claim 1 in which the gaseous dielectric mixture consists essentially of at least one halogenated alkane selected from the group consisting of CCl2F2, CHClF2, CBrF3, CCl2FCClF2, CClF2CClF2 and CClF2CF3 plus both SF6 and CO2, the composition of the mixture, when plotted on a ternary diagram, lying in regions rich in SF6 and CO2 defined by a line whose minimum extremities are defined by 1 SF6 - 15 CO2 - 84 halogenated alkane 10 SF6 - 1 CO2 - 89 halogenated alkane.

5. The process of claim 1 in which the perfluorinated ether consists essentially of at least one compound selected from the group consisting of (CF3)2O and (C2F5)2O.

6. The process of claim 1 in which the gaseous dielec-tric mixture consists essentially of a mixture of at least one perfluorinated ether with from about 1 to 99 mole percent of SF6.

7. The process of claim 1 in which the gaseous dielec-tric mixture consists essentially of a mixture of at least one perfluorinated ether with from about 1 to 75 mole percent of CO2.

8. A carbon suppressant composition characterized in that the composition consists essentially of (a) a mixture of at least one halogenated alkane plus both SF6 and CO2, said halo-genated alkane containing from 1 to 4 carbon atoms and at most one hydrogen atom, with the remaining hydrogen atoms replaced by a least one halogen selected from the group consisting of fluorine, chlorine and bromine, and having a vapor pressure of at least about 100 Torr at 20°C, said composition, when plotted on a ternary diagram, lying in the regions rich in SF6 and CO2 defined by a line whose minimum extremities are defined by 1 SF6 - 15 CO2 - 84 halogenated alkane 10 SF6 - 1 CO2 - 89 halogenated alkane or (b) a mixture of at least one perfluorinated ether and a member selected from the group consisting of SF6 and SF6 plus CO2, said perfluorinated ether containing from 2 to 6 carbon atoms and having a vapor pressure of at least about 100 Torr at 20°C.

9. The composition of claim 8 in which the halogenated alkane consists essentially of at least one compound selected from the group consisting of CHClF2, CHF3, CCl3F, CCl2F2, CClF3, CBrF3, CClF2CClF2, CClF2CF3, CF3CF3 and c-C4F8.

10. The composition of claim 9 consisting essentially of CCl2F2, SF6 and CO2, the composition being defined by the area enclosed by the polygon a-b-c-d-e-a in FIG. 4 of the attached drawings.

11. The composition of claim 9 consisting essentially of CHClF2, SF6 and CO2, the composition being defined by the area enclosed by the polygon a-b-c-d-e-a in FIG. 5 of the attached drawings.

12. The composition of claim 9 consisting essentially of CBrF3, SF6 and CO2, the composition being defined by the area enclosed by the polygon a-b-c-d-e-a in FIG. 6 of the attached drawings.

13. The composition of claim 9 consisting essentially of CClF2CClF2, SF6 and CO2, the composition being defined by the area enclosed by the polygon a-b-c-d-e-a in FIG. 7 of the attached drawings.

14. The composition of claim 9 consisting essentially of CClF2CF3, SF6 and CO2, the composition being defined by the area enclosed by the polygon a-b-c-d-e-a in FIG. 8 of the attached drawings.

15. The composition of claim 9 having improved dielec-tric strength, in which the halogenated alkane consists essentially of at least one compound selected from the group consisting of CCl2F2, CHClF2, CBrF3, CClF2CClF2 and CClF2CF3.

16. The composition of claim 8 in which the perfluori-nated ether consists essentially of at least one compound selected from the group consisting of (CF3)2O and (C2F5)2O.