Note: Descriptions are shown in the official language in which they were submitted.
BACK~ROUND OF THE INVENTION
- Until recently, polychlorinated biphenyls were the
most widely used dielectrlc ~luids for capacitors and trans-
` ~ormers. However, due to their resistance to natural decom-
position, when spillage occurs they persist in the environ-
ment and find their way lnto the food chain, possibly pro-
ducing adverse effects on animals and people~ For these
reasons, thelr continued use is not desirable and may be
sub~ected to considerable restrictions in the United States
and elsewhere. They have already been outlawed in Japan.
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As a result, users of polychlorinated biphenyls
have been lookin~ for substitutes. A substitute, of course,
must be ecologically acceptable. It should also have good
dielectric ~roperties and be thermally stable and relatively
inexpensive.
But perhaps the most difficult requirement of an
ecologically acceptable die]ectric fluid is that it be non-
flammableg because while halogenated compounds are less
likely to be flammable than non-halogenated compounds, they
may also not biodegrade easily and eventually get into the
food chain. Theref`ore, it is difficult to find a dielectric
fluid which is both ecologically acceptable and non-flammable.
PRIOR ART
U.S. Patent 2,236,261 discloses a dielectric fluid
of trichlorobenzene and pentachlor phenyl benzoateO
UOS. Patent 3,754,173 discloses a dielectric f`luid
containing dioctyl phthalateO
U.S. Patent 3,786,324 discloses the use of per-
fluorinated hydrocarbons to decrease the flammability of
capacitor dielectrics such as dioctylsebacate.
U.SO Patent lg 935,595 discloses the use of tri-
chlorobenzene as a f`lame snuffer for mineral oil for use in
capacitors.
U.S. Patent 2,041,052 discloses the use of tri-
chlorobenzene with chlorinated napthalene as a dielectric
f`luid.
U.S. Patent 2,413,170 discloses dielectric fluids
containing tricholorobenzene
UOSO Patent 3,796,934 discloses dielectric fluids
for capacitors containing isopropyl biphenylO
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Japanese Application 28516~74 (lay open number11734g/75) and 34141/74 (lay open number 117350/75) disclose
the use of narrow foil with rounded edges alternat~ng with
straight foil in capacitors.
SUMMARY O~ TH~ INVENTION
.
I have found that a mixture of trichlorobenzene
and certain aromatic~ flammable hydrocarbons or esters makes
a dielectric fluid which is both non-flammable and ecologi-
cally acceptableO The flammable compound is biodegradable
because of its high hydrogen contentO Trichlorobenzene is
more inert, but it will not be persistent because it is
volatile and should be sufficiently biodegradable because it
is soluble in water.
The dielectric fluid has good dielectric pro-
perties, is thermally stable and compatible with many
dielectric materials, and is relatively inexpensiveO
~ESCRI~TION_OF THE INVENTION
Figure 1 is an isometric sectional view of a
certain presently preferred capacitor according to this
invention.
~igure 2 is a graph showing the results of experl-
ments described in Example 2~
In the drawing a container 1 holds a capacitor
wlnding of straight conducting foil 2 and conducting ~oil 3
which is narrower and has rounded edgesD These foils alter-
nate with layers of insulation 4, here shown as film 5,
paper 6, and film 7O A dielectric ~luid 8, according to
this invention, fills container 1 and impregnates the wind-
ing.
The dielectric fluid of this invention comprises
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about 40 to about 90 pbw (parts by weight) trichlorobenzene
and abouk 10 to about 60 pbw of' a flammable compound. Pre-
ferably, the dielectric fluid comprises about 40 to about 70
pbw trichlorobenzene and about 30 to about 60 pbw flammable
compound. This latter composition is preferred because at
less than about 40 pbw trichlorobenzene, f'lammability begins
to increase, and at more than about 70 pb~ trichlorobenzene
the pour point may be too higho
The trichlorobenzene is an essential component of
the dielectric fluid and I have not been able to find any
satisfactory substitute for it (However, up to about 50%
by weight of the trichlorobenzene may 'be replaced with
tetrachlorobenzene, and commercial trichlorobenzene normally
contains a minor portion of tetrachlorobenzene). It is
somewhat surprising that trichlorobenzene can be used in
film capacitors because to a greater extent than most di-
electric fluids it tends to attack film tpolypropylene)
causing it to shrink and dissolvea However, I have found
that film capacitors using the dielectric fluid of' this
invention have a commercially acceptable life (i.eO, at
least 20 years).
The flammable compound must be miscible with the
trichlorobenzene or it will not give a homogeneous dielec-
tric fluid. Also, the f'lammable compound must be less
volatile than trichlorobenzene~ This is because the non-
flammability of the dielectric fluid is achieved by vola-
tilizing more of the trichlorobenzene than of the flammable
compoundO The non-flammable trichlorobenzene absorbs heat
in volatilizing and its vapors do not support a flame and
would not ~erm~t a flame to continue burning which results
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~rom great heat such as an electric arc. 'rhe ~lammable
compound must be flammable because non-flammable compounds
other than trichlorobenzene which might be o~herwise suit-
able are generally not ecologically acceptableO The flam-
mable compound must also be a liquid between 20C and 215C
(the boiling point of trichlcrcbenzene) for other~ise the
pour point o~ the fluid may be too low or its vapor pressure
too high. Finally, it must be aromatic for good corona
resistance to permit usage at high voltageO
Preferably, the flammable compound is non-halo-
genated because halogenated compounds, even if flammable,
tend to be ecologically less acceptable Preferably, the
flammable compound has good dielectric propertiesO ~hese
include a power factor of less than 5% at 100C after
purification, a dielectric constant between 2 and 9 at room
temperature, and a dielectric strength of at least 25 k~.
The ~lammable compound is a hydrocarbon, an ester,
or a mixture thereofO If an ester is used, it is preferably
a phthalic ester as phthalic esters are more stable and have
better corona resistanceO Phthalic esters with the best
thermal and hydrolytic stabilities are believed to be
diisononyl phthlate and diethyl-hexyl-phthlateO Other
suitable flammable compounds include isopropyl biphenyl,
isopropyl naphthalene, castor oil, sebacate and adipate
esters, and various phosphate esters such as tr~cresyl
phosphate. At the present time the pre~erred fllammable
compound is isopropyl biphenyl because o~ its particularly
good corona, resistance, thermal stability, and functional
temperature range.
To improve its dielectric properties, the die-
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lectric fluid preferably contains about Ool to about 0.5 pbw
of an anti-oxidant and about 0.2 to about 1 pbw of a hydro-
gen acceptor to improve corona resistance. The preferred
anti-oxidant is di-tert-butyl-p-cresol although other com-
pounds such as other sterically hindered phenols (eOg., as
di-tert-butyl-phenol) can also be usedO A preferred hydro-
gen acceptor is ~-methyl anthraquinone although other
compounds can also be usedO The optimum amounts o~ anti-
oxidant and hydrogen acceptor seems to be 0O2 and Q.5 pbw~
respectively. About 0O5 to about 2 pbw glycidyl phenyl
ether is also preferably present to reduce deleterious
corona effects.
In addition to paper, filmg and film-paper capa-
citors, the dielectric fluid of this invention can be used
in transformers, cables, and other electrical apparatus. In
capacitors, it would be particularly effective, and pre-
ferred, with 100% film windings because of their low power
factor and consequently low hot spot temperatures, which
would allow them long life in hot ambients. However, it
could be very useful in film and paper composite windingsg
which are used in power capacitors. It could also be used
with all paper windings such as those used in power coupling
capacitorsO It should be used ~ith caution where explosions
may be a problem because its non-flammability characteris-
tics may not be sufficient to overcome the explosive effects
of a combustible gas generated by a failure arc.
The following examples further illustrate this
invention.
EXAMPLE 1
-
A d~electric fluid o~ 50% (by weight) d~isononyl
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phthlate - 50% trichlorobenzene was ~ound to have the ~ol~
lowing properties:
Dielectric constant, ' r, at 100C 4.0 (+.1)
Power factor, %, at 100C
Viscosity, cs, at 20C <50
Flash and Fire points, 135C, and no
(Clevelan~ Open Cup Test~ sustained flame up to
ASTM D-92) boiling, at ~,240C
Vapor Pressure, Torr ~ 5 at 100C
~ 2 at 85C
Shrinkage of polypropylene film
at 125C therein 18%
For comparison, shrinkage of
polypropylene film in
trichlorobiphenyl 24%
Small test capacitors with two layers of 005 mil
polypropylene ~ilm and other test capacitors with 005 mil
polypropylene film and ~.6 mil 0.7 density paper were easily
impregnated with the above-described dielectric fluid, the
flooding occurring at 80C. The capacitances of both sets
of capacitors was about 0.2/~F. The following table shows
the electrical characteristics of these capacitorsO
100% film film-paper-film
Corona Discharge inception
voltage, kV 2.4 301
Corona Discharge extinction
voltage, kV 1.9 2.5
Power factor, %, at 85~C with
1600 volts applied 0002 0.04
Tiny capacitors specially designed for low tempera-
ture screening of dielectric fluids, containing two layers
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of 0.5 mil polypropylene film, were impregnated with the
above-described dielectric fluido When cooled overnight
to -40C, the capacitors retained a hl.gh discharge inception
voltage of 2.3 kV, compared to 3O3 kV at 25C.
EXAMPLE 2
A dielectric fluid was prepared of 50% (by weight)
isopropyl biphenyl and 50% trichlorobenzene and included
0.5% ~based on the fluid weight) of ~ -methyl anthraquinone
and 0.2% (based on the fluid weight) of di-tert-butyl-
paracresolO The fluid was used to impregnate two types ofcapacitors. The first type consisted of aluminum foil, 75
gauge polypropylene film, 45 gauge paper, 75 gauge poly-
propylene film, and aluminum foil (AFpFA). The second type
was identical except khak the second aluminum foil had
rounded edges and was narrower (AFpFar), as illustrated in
Figure 1.
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The power factor (100 tan ~ ) of these capacitors
was measured at the temperature of var~ous conditions im-
posed in sequence:
Power Factor
(100 tan ~ ?
Test
Condition Voltage AFpFA AFpFar
(v)
Initial at 100C 50 ~22 ~16
1800 o12 o 04
3000 ~17 ~05
3 kV for 1 day at 100C 3000 o16 ~07
0 kV for 2 hours at 100C 50 o17 ~12
1800 ~23 ~09
3000 ~21 oO8
0 kV ~or 1 day at 85C 3000 . 23 o16
3 kV for 1 day at 85C 3000 o15 oO6
The above table shows that the power factor was
unacceptably high after being without voltage at 100C and
at 85C~ and, in the case of the capacitor with rounded foil
edges, it increased rapidly with 3 kV at 100C. However,
quite surprisingly3 the power factor decreased substantially
under 3 kV at 85Co A capacitor which can be operated at
85C would be acceptable.
Encouraged by these results, additional tests on
these capacitors were performed to deter~ine whether the
power factor would decrease still furtherO Figure 2 shows
the results of testing up to 36 days at 85C and 3 kV.
Figure 2 shows that as the capacitors are used in service~.
their power factors will decreaseO (The lines in Figure 2
are drawn approximately to fit the data.) The power factors
of these capacitors are w~thin acceptable limits. Even
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lower power ~actors are expected when the dielectric fluid
is used in large capacitors.
This example also illustrated the greatl~ decreased
power factor which results from usin~ rounded foilO
EXAMPLE 3
An extensively used test ~or ~lammability is the
Cleveland Open Cup Test (ASTM D-92). In that test the fluid
is gradually heated and after every 5C rise in its tempera-
ture, a ~lame is passed over the fluido The temperature at
which the fluid flashes and the (higher) temperature at
which lt fires (burns for at least 5 seconds) are recorded.
According to this test, a fluid is o~`ten adJudged non-
flammable if it has a fire point higher than 250C even
though it burns above that fire pointO
Almost every material, even polytetrafluoroethy-
lene, will burn at scme temperatureO Moreover, electrical
arcs are at a very high temperatureO Thus, if the ~luid is
exposed to the air~ such as by a failure rupture, it may be
ignited by an electric arc and burn even though it is not
flanmable by the Cleveland Open Cup Test.
For that reason, whether a fluid is "flammable" as
used herein is determined by whether the fluid will burn un-
assisted by exposure to a flame or arc in an open cup at
temperatures below its boiling pointO (A fluid cannot be
heated above its boiling point~) It may be ignited at its
boiling point, but it must stop burning when the flame or
arc is removed, if it is to be 7'non-flammableO" Such a
"non-flammable" fluid would not spread a fire when spilled
or thrown from an electrical device which has failed with
h~gh current arcing.
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Various fluids were gradually heated and tested to
determine their temperature at ignition by exposure to a
flame. The temperature an~ time at which they ceased burn-
ing during coolin~ was also noted, The fsllowing table
~ives the results.
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The above table shows that only pentachlorobi-
phenyl, trichlorobiphenyl and its mixtures with up to 10%
mineral oil, and isopropylbiphenyl with at least 50% tri-
chlorobenzene failed to burn below their boiling point and
therefore were not flammable, The mixture o~ isopropyl-
biphenyl and 40% trichlorobenzene was o~ marginal non-flam-
mability.
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