Note: Descriptions are shown in the official language in which they were submitted.
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47,971
CAPACITOR HAVING DIELECTRIC FLUID
WITH HIGH DI-ISOPROPYL ~IPHENYL CONTENT
BACKGROUND OF THE INVENTION
Recent developments of hydrocarbon dielectric
fluids ~or high voltage stress lnsulation~ such as capaci-
tors, were based on the use of high levels of aromaticity to
resist the e~fects of corona generating overvoltages. These
flulds are essentlally alkylated aromatics, such as i80-
propyl blphenyl, or solutions of an aromatlc in an alkyl-
aromatic e~ter, such as di-isononylphthalate. The alkyl
substitutes glve the fluid a large temperature range of
~luidlty, particularly at low temperatures and in low vapor
processes; however, they cause the corona reslstance to be
poor wlth respect to aromatlc substltutes because hydrogen
is generated and persists, whlch causes ~urther discharges.
Thls ls probably because the hydrogen is not stron~ly bound
in the dielectric fluid~ and is therefore more easily and
coplously generated. Moreover, those flulds do not absorb
hydrogen as aromatics apparently do. The alkyl content o~ a
hydrocarbon dielectric fluid should therefore be ~ust high
enough to achieve the necessary fluid properties, but should .
not adversely affect the electrical properties. ~he tendency,
therefore, is to favor fluids with low amounts of alkyl and
with high amounts of aromatic. In optimizing isopropyl
biphenyl dielectric fluid for capacitors, 20% di-isopropyl
biphenyl with the remalnder mono~isopropyl biphenyl was
believed to be the maximum tolerable amount of alkyl. (See
U.S. Patent 4,054 2 937.) T~e corona extinctlon voltage at
low temperatures was found to begin to decrease at a di-iso-
propyl biphenyl content of about 20 percent. Also, the pour
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3 ~ ~g ~ 47,971
-2~
points were ~ound to increase at a di~isopropyl biphenyl
content of about 20 percent.
PRIOR AR~
U.S. Patent 4,054,937 discloses a capacitor con-
taining 20% di-isopropyl biphenyl and 80% mono-isopropyl
biphenyl.
U.S. Patent 2,837,724 discloses a dielectric fluid
for transformers which can be a methyl tertiary butyl
biphenyl.
~ electric fluid sold under the trade designation
"MCS1238" was analyzed as containing 17% sulfone and the
rest isopropyl biphenyl, of which 31.1% was di-isopropyl
biphenyl and the rest was mono-isopropyl biphenyl. The
fluid is covered by U.S. Patent 3,796,934.
~UMMARY OF THE INVENTION
Contrary to the implications of U.S. Patent
4,05~,937, we ha~e found that high concentration~ of di
isopropyl biphenyl in an isopropyl biphenyl dielectric fluid
~or capacitors is not as harmful to electrical properties as
was expected. Specifically, the overvoltage propertles and
the corona resistance, while not as good as in dielectric
fluids containing large amounts of mono-isopropyl biphenyl,
is nevertheless within acceptable commercial limits.
In addition, isopropyl biphenyl dielectric fluids
with high concentrations of di isopropyl biphenyl resist
crystallization at low temperatures better than do isopropyl
biphenyl dielectric fluids ~ith high concentrations of mono-
isopropyl biphenyl unless the meta isomer content in the
mono-isopropyl biphenyl ls also high.
Finally, isopropyl biphenyl dielectric fluids with
47~971
--3~
high concenkrations o~ dt-i~opropyl blphenyl are les~ expen-
sive than are isopropyl blphenyl dielectric fluids with high
concentratlons of mono isopropyl biphenyl~
DESCRIPTION OF THE INVENTION
The aGcompanying drawing is a partial isometric
sectional ~iew o~ a certain presently preferred capacitor
winding according to this lnvention.
In the drawing, a container 1 which is hermetic-
ally sealed, holds one or more windings containing strai~ht
conducting foll 2 and a conducting foil 3 which is narrower
and has rounded edges. These foils alternake with layers of
insulatlon 4, here shown as film 5, paper 69 and film 7. A
dielectric ~luid 8, accordin~ to this invention~
container 1 and impregnates the winding. Electrode termina~
tions may be provided in accordanc~e with conventional prac-
tice. Also, the dlelectric layers will tend to conform to
the spaces available so that in practice a large Bpace RS
shown ln the drawing wlll be substantially reduced.
The dielectric ~luid of thi~ invention con~i~$s
~ `
20 essentially o~ about 25% to about 100% ~all percentage3
herein are by weight ba~ed on the dielectric fluid wel~ht,
unless otherwise lndicated) of di-isopropyl biphenyl, up to
o
about ~ mono~isopropyl biphenyl, and up to about 20~ tri-
i opropyl biphenyl. Preferably, the dielectric f~uid con-
sists essentially o~ about 25% to about 80% dl-isopropyl
biphenyl, about 20% to about 7~% mono-l~opropyl biphenyl,
and up to àbout 15% tri isopropyl biphenyl. The dielectric
fluid may also comprise about 35% to about 100% di~ opropyl
biphenyl, up to about 65% mono isopropyl biphenyl and up to
about 20% tri~isopropyl biphenyl. Preferably, the dielec-
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47~971
tric ~luid comprises about 35~ to about 80~ di~isopropyl
biphen~l~ about 20% to about ~5% mono-isopropyl bip'len~l~
and~up to about 15% tri~isopropyl biphenyl. Xf less than
isopropyl biphenyl ls used, ~he ~ielectrlc fluid may
become impractical for low cost capacitors. Isoprop~l
biphenyl dielectric fluids having about 25% to about ]00~
di~isopropyl blphenyl have very good low temperature proper-
. tles.
Due to the method of p-reparation (alkylation of
biphenyl); the dielectric fluid may contain some biphenyl.
Biphenyl is irritatln~ and volatile, and there~ore it ls
desirable that it not be presènt a~ more ~han 1%. Its
concentration would be considerably below this when the
level of diisopropyl biphenyl is high, because biphen~l
would be greatly con~erted in the first p].ace at a high
degree of alkylation o~ blphenyl, and it would be further
reduced on fractio~ation of the dialkyl constituent because
o~ its considerably greater volatllity.
The dielectric fluid preferably includes up to
about 1% o~ an antioxidant for thermal stability. The
pre~erred amount is about 0.01% to about 0.2%, and the
pre~erred antioxidants are di-t-butyl-paracresol, di-t-
butyl~pherlol, or mixtures thereofO
~he fluid also pre.~erably contains llp to about 2~ -
and preferably about 0~1% to about 0.5% of a hy~rogen accep-
tor for imp-roved corona resistance. An anthraqulnone~ such
as B-methylanthraqulnone, anthraqulrlone, or ~chloroanthra~
quinone may be used. Because it ls more readil~ a~ailable
and more soluble~ ~methylant~raquinone is pre~erred.
The antioxldants and h~drogen acceptors seem to
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47~971
~5
lnteract to produce a loss of corona re~isGance wherl eithe
one i3 used in high concentrat:l.ons ~ ThereI'ore 3 preferabl~
neither one ls used at more than ].~. A compositlon in whlch
both are effective contains about 0.2% dl--tertlary butyl
paracresol and about 0O5% ~ methylanthraquinone. Although
not necessarlly preferred3 the fluid may include up to about
2% and pref'erabl.y about 0.05% to about 2% of an epoxy~ such
as glycidyl phenyl ether, for corona resista.nce~
The capacitors are preferably film (e.~.~ poly~
propylene)-paper or 100% fil.m because a hi~h level of di~
electric constant of the fluid ls not important in these
types of' capacitors, but may also be 100% paper.
The followin~ e~amples ~urther i.:Llustrlte thls
lnvention~ ,
FXAMPLE 1
Tests were made with sma.11. lmpregnat,ed capacitors
o~ abou-t 12 microfarads containing polypropylene film and
paper, the same dielectric used in high volt,age power capa
cltorsO The windings consisted of 75 gauge ~llm and 45
gauge paper in a film-paper-film conflguration with one of
the foils narrower than the other a.nd foldecL at the edges.
tWith this foil arrangement~ the stress on the foil edge is
less than wlth the usual one of two equal widths and sharp
edged foils.) A severe overvoltage ~est was applied to the
capacitors, in which the~ were contl}luously ener~ized at 2.7
kV, 10% above rated vc)ltage~ and sub~ected to 5 c~cles of
8.1 kV, 3~3 tlmes rated every three mirltltec. t~ne hlghest
over~oltage such a cap2citor would encourlter ln ser-rlce ls
three tlmes rated, and this would occur very rare:L.y~) The
following table give~s the results o:E' this test on such
,
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~6--
capacitors impregnated with the ~luid containlng 80% di-
isopropyl biphenyl and wlth the presently accepted one whlch
contains less than 5% di--isopropyl biphenyl and more than
95% mono-isopropyl biphenyl. lncluded in this table are
also results for capacltors impregnated with tri-chloro-
biphenyl which was u~ed until recently and mineral oil~ with
about 80% aliphatic type carbon3.
re~nt
80% di-isopropyl biphenyl
14% tri isopropyl biphenyl
and about 5% mono isopropyl 603~ 700 ,i
biphenyl + .2% dl-teriary-butyl ~:
paracresol + 0.5%~methyl
; anthraquinone
96% mono-isopropyl biphenyl,
3% dl~isopropyl biphenyl >1005,~1475, 1058,~1430,
0.2% di-tertiary-butyl paracresol,
0.5%~imethyl anthraquinone
trichlorobiphenyl ~ 0.3%~-methyl
anthraquinone 170, 240,
mineral 0.1 ~ 0.2~ di-tert-butyl
paracresol + 0.5% B-methYl anthra-
qulnone (not rounded ~oil) 9, 23-
; In regard to the above results, the high di-
i~opropyl biphenyl, although not performing as well as the
high mono-isopropyl biphenyl, did perform better than the
. ~ . tri-chlorobiphenyl, and is satlsfactory ~or commercial high
voltage capacitors.
EXAMPLE 2
This example shows that isopropyl biphenyl dielec-
tric fluids containlng high concentrations of dl-isopropyl
biphenyl are more fire resistant than those containing hlgh
conc~ntrations of mono~isopropyl biphenyl. The following
table gives the results of a Cleveland Open Cup Test for
~lash and fire points for various mixtures of mono- and di-
isopropyl biphenyl.
~3971
--7--
uid Composition C`~ 3~n~æ_C
Flash Point Plre Point
80% di-, 14% tri-, 5% mono-
isopropyl biphenyl 180 186
53% di-, 10% tri-, 36% mono-
lsopropyl blphenyl 160 170
27% di-, 5% tri-, 65% mono-
isopropyl blphenyl 157 171
1% di~, 95~ mono-isopropyl
biphenyl 150 160
EXAMPLE 3
Tests were performed to determlne the low tempera-
ture stability of di~isopropyl biphenyl dielectrlc flulds
containing various amounts of di~lsopropyl biphenyl.
A series of test tubes containing fluids of vary-
ing amounts of mono- and di-isopropyl biphenyl were sub-
~ected to six repeated cycles of two hours at -3~C and two
hours at -60C and observed for the development of opaque
crystals. Resistalloe to crystallization, by remaining
transparent in a glassy state, is necessary for low temper-
ature ~luid stability because gas-filled voids can develop
on crystallization that break down with high voltage. The
-30C and -60C cycling accelerates crystallization e~fects
in such fluids that are prone to this. For the solutionS
one of two stock ~luids contained 97.2% mono~isopropyl
biphenyl, with 54.8% para and 42.4% meta isomers which cause
it to be easily crystallized because of the greater fraction
of the para isomer. The other consisted of 80% dl-, 14%
tri and 5% mono-isopropyl biphenyl. The following table
gives the observation made of these solutions after the si~
low temperature cycles.
117~971
Fluid Composition
% mono- % di~ % tri- Appearance at -30C'and at ~60C a~er
i~gg~ ~=~b99o_ _ 6 cYcles at these tempera~ures
97.2 2 all opaque
88 10 - lower 2/3 opaque~ top transparent
79 17.5 3 lower 1/3 opaque, top transparent
61 34 5 all transparent
24.5 64.5 11 all transparent
6 80 14 all transparent
It can be seen that low temperature crystalli~a-
tion of mono-isopropyl biphenyl i6 e~fectively inhibited
wlth about 25% or more di-isopropyl biphenyl, and that the
composition with 80% dl-isopropyl biphenyl continues to
resist crystallization.
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