Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROUND OF THE INVENTION
This invention relates to a dielectric liquid impregnant for electrical
devices and more particularly to an electrical capacitor utilizing an
improved non-toxic dielectric liquid impregnant theretofore.
Liquid impregnants for electrical capacitors should have a high
dielectric constant, maintain a low dissipation factor, and be compatible
with the other materials utilized in capacitor structure. At the same
time, the impregnant must withstand elevated and fluctuating temperature,
pressure, and voltage stress conditions with excellent electrical
characteristics for a long operative life of the capacitor.
The polychlorinated biphenyls as capacitor impregnants meet these
requirements and they were eminently satisfactory for several decades. The
polychlorinated biphenyls are broadly referred to as PCBs. The poly-
chlorinated biphenyls have recently been associated with ecological
problems, restrictive use limitations, and rising costs. These problems
have spurred the search for a suitqble non-toxic replacement capacitor
impregnant which would have some advantageous impregnant characteristics
comparable to those of the chiorinated diphenyls, and still provide
outstanding electrical and compatibility performance with the two most
important present day capacitor solid dielectrics, paper and polypropylene.
In the search of a non-toxic liquid impregnant, consideration must
also be given to gas absorbing properties and offensive odor which may
evolve when the capacitor is used for prolonged periods at elevated
temperatures. Good gas absorbant properties prolong capacitor life.
FEATURES_OR OBJECTS OF THE INVENTION
It is, therefore, a feature of the invention to provide a dielectric
impregnating fluid that is non-toxic. Another feature of the invention is
to provide such an impregnating fluid that includes soybean oil. Another
feature of the invention is the provision of such a dielectric impregnating
fluid that includes an additive which prevents the evolution of offensive
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odor. Still another feature is the provision of such an additive which
includes butylated hydrotoluene. Another feature of the invention is the
provision of such a dielectric impregnating fluid that includes an
additive providing good gas absorbing properties for the soybean oil.
Yet still another feature of the invention is the provision of such an
additive that includes an olefin such as decene, dodecene and tetradecene.
Yet another feature of the invention is the provision of such an additive
that includes o~ -dodecene-tetrodecene. These and other features of the
invention will become apparent from the following description taken in
conjunction with the accompanying drawings.
SUMMARY OF THE INVENTION
Generally speaking, the present invention pertains to a capacitor
having a dielectric fluid which includes soybean oil and a minor amount
of an additive of butylated hydrotoluene. In addition, the dielectric
fluid could include an additive of an olefin taken from the class
consisting essentially of decene, dodecene, and tetradecene, the preferred
additive being c~ -dodecene-tetradecené.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGURE 1 of the drawing illustrates a convolutely would a-c capacitor
body partially unwound to show the dielectric plastic film and foil
electrode structure.
FIGURE 2 of the drawing illustrates the convolutely would a-c capacitor
body of FIGURE 1 to which leads are attached.
FIGURE 3 of the drawing illustrates the a-c capacitor body suitably
encased and sealed in a housing.
DISCUSSION OF THE INVENTION
Referring now to Figure 1 of the drawing, there is illustrated a
convolutely wound a-c capacitor body 10. The capacitor body 10 includes
a pair of dielectric film layers 11 and 12 and overlying electrodes 13 and 14
respectively. Electrodes 13 and 14 are of a suitable metal such as
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aluminum for example. Margins 15 and 16 are provided for preventing
electrical short circuit between the electrodes. As shown in Figure 2,
suitable terminal leads 23 and 24 are attached to the electrodes 13 and
14 from opposite ends 21 and 22 of the capacitor body 10. Suitable
lead material could be solder coated copper for example.
The dielectric plastic films or layers 11 and 12 should have high
dielectric strength, low dissipation factor to prevent thermal runaway,
broad temperature range capabilities, resistance to the dielectric fluid,
and freedom from pinholes and conductive particles. Suitabie dielectric
10 film layers 11 and 12 may be paper, a paper and polymeric film combination, or
polymeric films.
In Figures 2 and 3, a capacitor body 10 is preferably sealed to a
can or housing 40 to form a capacitor 41. Capacitor 41 includes the
metal housing or can 40 and terminals 42 and 43. The capacitor body 10
is contained within the housing 40 and terminal lead 23 makes electrical
contact with terminal 42 and the remaining terminal lead 24 makes electrical
contact with terminal 43.
As noted previously, fluid or liquid dielectric impregnants for
electrical capacitors should have a high dielectric constant, maintain a
20 low dissipation factor and be compatible with the other materials used in
capacitor structure. According to the present invention, the dielectric
fluid which is used to impregnate capacitor body 10 not only satisfies
these quirements, but is substantially non-toxic. The dielectric
fluid of the present invention includes refined soybean oil. A useful
soybean oil for the present invention would be of a low acid grade with
a flash point of about 325C which is available in commercial quantities.
In order to prevent the soybean oil from becoming rancid when
capacitors are used at elevated temperatures for long periods of time
and giving off offensive odors, it has been found that a minor amount of
30 butylated hydrotoluene is added to the soybean oil. The amount added
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is less than 1% by weight of the total weight of the dielectric fluid
with 0.1% or 0.2% by weight of the total weight being preferred.
Another problem with dielectric fluids in capacitors is that of
gas generation which likely occurs at high stress "hot spot" areas
which eventually leads to capacitor failures. Therefore, an additive
of a good gas absorbing medium needs to be provided to the dielectric
impregnate in the capacitor. The main gases generated in capacitors
of the type described and using soybean oil as the dielectric impregnant are
peroxides and hydrogen. It has been found that a good gas absorber under
these conditions are olefins such as decene, dodecene, tetradecene and
hexadecene. Two particularly useful additives to soybean oil has been found
to be cx -dodecene-tetradocene in an amount of from 1-20% by volume of the
total amount of dielectric fluid with 10% being preferred or ~ -tetrahexadecene
in the same amount.
One thousand hour life tests were performed on 16 electrical
capacitors of the type described in Figures 1-3 using soybean oil as a
dielectric fluid impregnant. Eight of the capacitors used a dielectric
fluid which included an additive of 0.2% by weight of the total weight of
the dielectric fluid of butylated hydrotoluene (Group I) and eight of the
capacitors included an additional additive of 10% by volume of the total
volume of the dielectric fluid of 0~ -dodecene-tetradecene (Group II).
The results are shown in Table I which presents average capacity and
dissipation factors. There was one capacitor failure in Group I from
an unknown cause.
_-- . . ... .
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TABLE I
ELECTRICAL CAPACITOR PROPERTIES
Capacitance and % Dissipation Factor (DF) Stability
During Life Test
25.0 uf, 370 VAC rated capacitors operated at 495 VAC, 80C Life Test
Dielectric - 2 layers x 0.45 mils + 2 layers x 0.40 mil paper
GROUP I GROUP II
Capacity-uf: Initial500 1000 Hrs. Initial 500 1000 Hrs.
25C 19.2 19.4 19.5 19.2 1~.2 19.3
65C 19.0 19.2 19.2 18.7 19.0 19.0
85C 19.0 19.1 19.1 18.6 18.6 18.9
DF %:
-
25C 0.2750.271 0.280 0.257 0.257 0.258
65C 0.2270.210 0.228 0.210 0.210 0.213
85C 0.2040.210 0.216 0.205 0.210 0.208
It is seen that over a life test of 1000 hours, the capacitor exhibitedgood stability. There was no evidence of offensive odors or "hot spots"
due to gas generation.
One thousand hour life tests were also performed on 16 electrical
capacitors of the type described in Figures 1-3 using soybean oil and
additives of O.lYO by weight of the total weight of the dielectric fluid
of butylated hydrotoluene and 10% by volume of the total volu~e of the
dielectric fluid of ~-tetrahexadecene. The results are shown in Table
II which presents average capacity and dissipation factors.
.:............................................ . . ..
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TABLE II
ELECTRICAL CAPACITOR PROPERTIES
Capacitance and % Dissipation Factor Stability
During Life Test
370 VAC/25 uf Capacitors on 495 VAC
100C Life Test
Dielectric - 2 layers x 0.45 mils + 2 layers x 0.40 mil paper
Capacitance -uf Initial 1000 Hrs
25C 19.824 19.870
60C 19.599 19.586
~5C 19.520 19.473
100C 19.407. 19.364
% DF
25C 0.204 0.221
60C 0.166 0.152
85C 0.147 0.144
100C 0.160 0.192
Again, it is seen that over a life test of 1000 hours, the capacitor
exhibited good stability. There was no evidence of offensive odors or
"hot spots" due to gas generation.
