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BACKGROUND OF THE INVENTION
Dielectric gases have found increasing use in high voltage systems,
especially over about 100 kilovolts, with the most widely used material
being sulfur hexafluoride. Sulfur hexafluoride has been used in both
devices with uniform fields, such as compressed gas insulative devices,
and in devices with non-uniform fields, such as circuit breakers and
transformers. The rating of a particular device depends upon its
configuration, the gas pressure, the dielectric gas used, the degree of
freedom of the gas from moisture and other contamination, and other
conditions. Nevertheless, there is a continuing need for dielectric gases
of increased dielectric strength under comparable conditions that permit a
given device to merit a higher voltage rating or permit alterations in
other parameters with the maintenance of a rating.
Various gases, especially electronegative gases, have been proposed as
additives to sulfur hexafluoride or alternates for sulfur hexafluoride.
Some such gases also contain sulfur while others do not. The proposed
substitutes and alternates to sulfur hexafluoride which contain sulfur,
have one or more sulfur atoms at valence state 6 or 4 or otherwise bonded
with four or six electron pairs. Exemplary in U.S. Pat. No. 3,674,696
(issued July 4, 1972 to Griffiths) wherein compounds are disclosed as
dielectric gases with S at valence state 4 such as SN(CF.sub.3)F.sub.2,
SN(C.sub.2 F.sub.5)F.sub.2, SN(C.sub.3 F.sub.7)F.sub.2 or S at valence
state 6 such as SN(CF.sub.3)OF.sub.2, SN(C.sub.3 F).sub.7 OF.sub.2,
S(NCF.sub.3).sub.2 F.sub.2, S(NCF.sub.3)F.sub.2 and S(NC.sub.2
F.sub.5)(NC.sub.3 F.sub.7)F.sub.2. It has hitherto been thought, howver,
that sulfur at valence state 2 was too easily oxidized to offer high
dielectric strength in a dielectric gas.
BRIEF DESCRIPTION OF THE INVENTION
The present invention includes an improvement in a high voltage electrical
apparatus having at least two electrical conductors separated by an
insulative dielectric gas subjected to an electrical field, in which
improvement the insulative gas comprises about 0.5 to 100 mole% of a
divalent sulfur compound selected from the group consisting of
tetrafluorothiirane, hexafluorothietane, bis(trifluoromethyl) sulfide,
perfluoromethyl ethyl thioether, perfluorodiethyl thioether,
trifluoromethyl thiocyanate and mixtures thereof and 0 to about 99.5 mole%
sulfur hexafluoride. The electrical device may be of the type wherein the
dielectric gas is subjected to a uniform field or of the type wherein the
dielectric gas is subject to a non-uniform field.
The present invention also includes as a novel composition of matter a
dielectric gas comprising between about 10 and about 90 mole% of the above
divalent sulfur compound and between about 10 and about 90 mole% sulfur
hexafluoride. The preferred divalent sulfur compound for both the present
electrical apparatus and the present composition of matter is bis
(trifluoromethyl) sulfide.
DETAILED DESCRIPTION OF THE INVENTION
The present divalent sulfur compounds are of three types:
bis(prefluoroalkyl) sulfides of the formula (R).sub.2 S where R is
CF.sub.3 -- or C.sub.2 F.sub.5 --, perfluoro-cycloalkylsulfides of the
formula
##STR1##
(called herein tetrafluorothiirane) and
##STR2##
(called herein hexafluorothiethane) and the compound trifluoromethyl
thiocyanate CF.sub.3 --S--CN.
The three bis (purfluoroalkyl) sulfides may be considered perfluorinated
thioethers using the following nomenclature:
1. CF.sub.3 --S--CF.sub.3 can be called bis (trifluoromethyl) sulfide or
perfluorodimethyl thioether;
2. CF.sub.3 --S--C.sub.2 F.sub.5 can be called perfluoromethyl ethyl
sulfide or perfluoromethyl ethyl thioether; and
3. C.sub.2 F.sub.5 --S--C.sub.2 F.sub.5 can be called bis (perfluoroethyl)
sulfide or perfluorodiethyl thioether.
Each of these three compounds are known, with methods of synthesis and
certain physical properties being described in Vol. 14 of Inorganic
Synthesis (McGraw Hill, 1973), submission by D. T. Sauer and J. Shreeve
beginning on page 42 at pages 44-45 for the bis (trifluoromethyl) sulfide
and in D. T. Sauer and J. M. Shreeve, "Bis (perfluoralkyl) Sulfur
Difluorides and Bis (Perfluoroaralkyl) Sulfoxides," Journal of Fluorine
Chemistry Volume 1, pages 1-11 (1971-1972), especially at pages 9 and 10.
Briefly, CF.sub.3 SCl is reacted with AgOC CF.sub.3 or AgOCC.sub.2 F.sub.5
to produce CF.sub.3 SOCCF.sub.3 or CF.sub.3 SOCC.sub.2 F.sub.5 and this
product is decarboxylated with ultraviolet light to produce CF.sub.3
SCF.sub.3 or CF.sub.3 SC.sub.2 F.sub.5. Perfluorodiethyl thioether may be
similarly prepared from AgOCC.sub.2 F.sub.5 and C.sub.2 F.sub.5 SCl or by
the reaction of SF.sub.4 with C.sub.2 F.sub.4.
Tetrafluorothiirene CF.sub.2 --CF.sub.2 is a known compound, with a method
for its synthesis reported by W. R. Brasen et al. in volume 30 of the
Journal of Organic Chemistry, beginning on page 1488 (1965), especially
page 4190. Hexafluorothietane
##STR3##
is believed to be a novel compound which may be prepared by the method
described in Example 1, below.
Trifluoromethyl thiocyanate CF.sub.3 --S--CN may be prepared by the method
described in Journal of the Chemical Society, 1963, pages 1272-1274 which
comprises the reaction of trifluoromethyl sulfonyl chloride with silver
throcyanate.
The present divalent sulfur compounds may be present as the sole dielectric
gas, as a mixture of two or more such gases, as a mixture with sulfur
hexafluoride or as a mixture of two or more such gases and sulfur
hexafluoride. The dielectric gases preferably are free of any ingredient
or impurity, other than above dielectric sulfur compounds, that will lower
the dielectric strength to any substantial extent, such as to less than
about 90% of the strength of pure divalent sulfur compounds or pure
mixture of dielectric sulfur compounds. In particular, the dielective gas
should not contain appreciable amounts of water vapor or metal
particulates. The present invention contemplates, however, additional
ingredients which enhance or do not materially detract from the dielectric
strength of the gas. For example, especially in uniform field devices
where sulfur hexafluoride is a part of the dielective gas composition,
materials such as carbon dioxide, perhalogenated hydrocarbons, nitrogen or
air may be used to enhance or dilute without weakening the sulfur
hexafluoride; see U.S. Pat. Nos. 4,052,555 and 4,071,461 and pending
application of W. H. Mears et al. Ser. No. 767,717, filed Feb. 11, 1977.
Similarly, as described in a copending application of M. J. Mastroianni
and S. R. Orfeo Ser. No. 919,338, filed June 26, 1978, noble gases may be
present, especially when combined with sulfur hexafluoride in dielectric
gases for uniform field devices.
When the present divalent sulfur compounds are mixed with sulfur
hexafluoride, it is preferred that the mixture contains between about 90
and about 10 mole% sulfur hexafluoride and between about 10 and 90 mole%
of one or more of the present divalent sulfur compounds. More preferred is
about 40 to 90 mole% divalent sulfur compound. Of the several divalent
sulfur compounds, especially preferred for mixture with sulfur
hexafluoride are the three bis (perfluoroalkyl) sulfides. Preferred
additives to these compositions include nitrogen, air, carbon dioxide,
perhalogenated hydrocarbon gases and noble gases.
The present dielectric gas compositions may be present in any high voltage
electrical device of the type now using a dielectric gas such as sulfur
hexafluoride, with either a uniform of non-uniform field configuration.
Examplary of uniform field devices are compressed gas insulative
transmission lines as described in A. H. Cookson, COMPRESSED GAS INSULATED
TRANSMISSION SYSTEMS: THE PRESENT AND FUTUTRE (Westinghouse Electric
Corporation 1978). Exemplary of non-uniform field devices are generators,
transformers, circuit breakers and the like. It should be appreciated that
in applications such as circuit breakers, the present gases are to be used
as the insulating or padding gas and not as the electrical energy
absorbing material used to extinguish the arc. The present dielective gas
compositions may also be used in other devices where sulfur hexafluoride
has been proposed such as the fluidized bed transformers of U.S. Pat. No.
3,889,042 (issued June 10, 1975 to Mears et al.).
EXAMPLE 1 -- Preparation of Hexafluorothietane
Into a 1 Hastelloy autoclave cooled to -78.degree. C. is condensed 82g (1.0
moles) of thiocarbonyl fluoride (prepared according to W. J. Middleton, E.
G. Howard, and W. H. Sharkey, J. Am. Chem. Soc., 83, 2589 (1961), followed
by 100g (1.0 mole) of tetrafluoroethylene. The autoclave is heated to
150.degree. C. for 10 hours. At the end of this period the autoclave is
allowed to cool to room temperature and the contents are bled off into a
receiver, cooled in a Dry Ice-Acetone bath. Distillation of the product
gives the desired CF.sub.2 CF.sub.2 CF.sub.2 S in good yield along with
some higher molecular weight by-product.
EXAMPLE 2 -- Preparation of Perfluorodiethyl Thioether
A mixture of 22 mmoles C.sub.2 F.sub.4, 10 mmoles SF.sub.4 and 4g anhydrous
cesium fluoride was heated at 170.degree. C. for eight hours in a 75 ml
Hoke bomb. Separation of the volatile components by gas chromatography
gave C.sub.2 F.sub.5 SF.sub.2 C.sub.2 F.sub.5 in 40% yield. Also isolated
were C.sub.2 F.sub.5 SF.sub.3 (7%) and C.sub.2 F.sub.5 SSC.sub.2 F.sub.5
(15%).
EXAMPLE 3 -- Preparation of Bis (trifluoromethyl) sulfide
Bis(trifluoromethyl) sulfide was prepared by the reaction sequence
described in "Inorganic Synthesis" Vol. 14, pp. 42-47 (1975).
Two hundred twenty-four grams (1.64 moles) of CF.sub.3 SC1 were allowed to
react with excess silver trifluoroacetate (578g, 1.80 moles) at 25.degree.
C. for 3 hours in a 1 liter, 3-neck flask. The product was distilled from
the flask into a -78.degree. C. trap. About 182g. of crude product was
recovered. Distillation of this material gave 106g. (0.49 mole) of
CF.sub.3 SOCOCF.sub.3, boiling point 42.degree.-45.degree. C.
Photolysis of CF.sub.3 SOCOCF.sub.3 (106g., 0.49 mole) for 8 hours at
25.degree. C. through Pyrex glass with a Hanovia ultraviolet quartz lamp
(100 watts) produced 75g. of crude (CF.sub.3).sub.2 S. On distillation,
48g. (0.28 mole) of (CF.sub.3).sub.2 S, crystallization point -22.degree.
C., was recovered. The purity was determined to be 99.2% by gas liquid
chromatography.
EXAMPLE 4 -- Determination of Breakdown Voltage of (CF.sub.3).sub.2 S
The breakdown voltage of bis(trifluoromethyl) sulfide was determined by
injecting a sample of the material prepared in Example 3 into a 0.1 inch
plane to sphere gap at atmospheric pressure and progressively increasing
the voltage until breakdown occurred. As shown on the fourth line of Table
1, a value of 26 kV was noted, representing a 50% improvement over
SF.sub.6.
EXAMPLE 5
Example 4 was repeated for SF.sub.6 and mixtures as SF.sub.6 and
(CF.sub.3).sub.2 S in the proportions indicated in Table 1. The breakdown
voltage and percent improvement over pure SF.sub.6 are indicated in the
table.
TABLE 1
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Atmospheric Pressure
%
SF.sub.6 Mole %
(CF.sub.3).sub.2 S
(CF.sub.3)O(C.sub.2 F.sub.5)
BDV Improvement
______________________________________
100 -- -- 0
80 20 -- 12.
60 40 -- 26.
40 60 -- 38.
20 80 -- 45.
-- 100 -- 50.
______________________________________
EXAMPLE 5
Examples 3 and 6 were repeated for SF.sub.6, (CF.sub.3).sub.2 S and
mixtures as shown in Table 2 at 3 atmospheres pressure. The results are
displayed in Table 2.
TABLE 2
______________________________________
3 Atmospheres Pressure
SF.sub.6 Mole %
(CF.sub.3).sub.2 S
BDV % Improvement
______________________________________
100 -- 0
80 20 7.
60 40 18.
40 60 31.
-- 100 42.
______________________________________
EXAMPLE 6
Following each procedure of Example 4, breakdown voltages were measured at
1, 2 and 3 atmospheres for SF.sub.6, (CF.sub.3)S and (CF.sub.3)O(C.sub.2
F.sub.5). The results, displayed in Table 3, show that (CF.sub.3)S is
superior in breakdown voltage to this perfluoroether.
TABLE 3
______________________________________
Pressure (Atmospheres)
SF.sub.6
CF.sub.3 OC.sub.2 F.sub.5
(CF.sub.3).sub.2 S
______________________________________
1 17. 21. 25.
2 32. 37. 44.
3 44. 47. 62.
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