Note: Descriptions are shown in the official language in which they were submitted.
lQ46754
Background of the Invention
Liquid dielectric compositions are used in various types
of electrical apparatus, such as capacitors, condensor bushings,
transformers, cables and the like. In the construction of
capacitors, such as power factor correction capacitors, capacitor
packs are formed of alternate layers of metal foil and a dielectric
material which is impregnated with the liquid dielectric It is
tesirable that the liquid dielectric have a high dielectric con-
stant, maintain a low dissipation factor and be compatible with
other materials in the capacitor structure Furthermore, the liquid
dielectric must be capable of withstanding elevated and fluctuat-
ing temperature, pressure and voltage stress conditions over the
entire operational life of the capacitor.
In the past, various types of liquid dielectrics have
been used in capacitors such as polychlorinated diphenyl, castor
oil, mineral oil, silicone oil, and the like. Of the commonly
used liquid dielectrics, polychlorinated diphenyl, such as tri-
-chlorodiphenyl, has been the most widely used in capacitor
applications~
While the polychlorinated diphenyls produced an effect-
ive electrical dielectric system for a capacitor, their usage ~as
provided certain ecological problems in that the polychlorinated
diphenyls are virtually non-biodegradable with the result that
if leakage or rupture occurs in the capacitor casing, or if the
capacitor is discarded as obsolete, the polychlorinated diphenyl
w~11 remain as a pollutant in the environment and will not degrade
to any appreciable extent even over extended pexiods of time.
Because of the ecological problems there has been
increased actlvity in an attempt to develop a replacement for the
polychlorinated diphenyls as a capacitor impregnant. Any sub-
1~46754stitute impregnant should be biodegradable and have dielectric
properties and performance characteristics comparable to those
of the polychlorinated diphenyl.
While higher molecular weight polybutene, having a
molecular weight of over 800 has had some limited use in the past
as a liquid dielectric in electrical cables, polybutene has not
been used in high stress electrical capacitors for use in power
factor correction applications for several reasons. First, the
relative dielectric constant of polybutene is about 2.10 to 2.25
compared to about 5.9 for trich?orodiphenyl. In a capacitor using
all paper, or a combination of paper and polyolein film, as the
solid dielectric material, a substantial amount of capacitance
i~ lost due to the lower relative dielectric constant of the
polybutene.
Secondly, polybutene tends to generate gas under con-
ditions of aging at temperatures above room temperature and the
development of gas tends to shorten the life of the capacitor by
giving rise to corona discharges. It is also believed that the
polybutene depolymerizes under high electrical stress, which can
give rise to corona discharges and early dielectric failure.
However, higher molecular weight polybutene has found use in th~
past in the manufacture of cables, due to the fact that with cables
it is desirable to have low capacitance, and the stresses on the
8ystem are considerably lower.
Summary of the Invention
, The invention relates to an electrical apparatus, such
as a capacitor, having a dielectric system with improved dielectric
p~operties. The dielectric system is formed of a polymeric film,
6uch as polypropylene or polyethylene, which is impregnated with
low molecular polybutene having an average number molecular weight
o~ 300 to 500
10 46 7 ~ 4
With the use of the low molecular wei~ht polybutene,
the rate of impregnation of the polymeric film is increased over
conventional systems, with the result that the processing time
for fabrication of the capacitor is ~ubstantially reduced.
As a further advantage, the dielectric system utilizing
the polymeric film and the polybutene results in extremely low
dielectric losses. For example, the losses are about one-fourth
of the losses of a paper-film capacitor impregnated with tri-
chlorodiphenyl.
As the polybutene is generally biodegradable it will
decompose into harmless compounds when exposed to the atmosphere
through leakage or rupture of the capacitor casing and there are
no determined adverse effects on the environment.
The invention also offers a lower cost product in that
the cost of polybutene is substantially less than that of the
polychlorinated diphenyls presently used in the art.
Further improvement in the stability of the dielectric
8ystem can be achieved by the addition of 0.1% to 10% by weight
of a cyclohexyl amine, such as dicyclohexylamine, to the poly-
butene. The cyclohexylamine acts as a neutralizing agent or
scavenger, serving to neutralize decomposition products that ar-e
released fro~ or generated by, the liquid dielectric or other
materials in the capacitor during its operation, thereby provid-
ing a substantially longer service life,
A further increase in the rate of impregnation can be
achieved by the addition of trichlorobenzene to the liquid diel-
ectric composition in an amount up to 20% by weight of the com-
position. The trichlorobenzene acts to lower the viscosity of
the liquid dielectric, as well as its pour point, and thereby
further improve the impregnation characteristics.
1~46754
01 In addition, anti-oxidants, such as butylated
02 hydroxy toluene, or di-tert-butyl-p-cresol can be
03 incorporated in the liquid dielectric composition in an
04 amount up to 15% by weight and serve to improve the corona
05 extinction characteristics of the dielectric system.
06 More generally, the invention is an electrical
07 apparatus comprising a pair of electrical conducting
08 elements situated in spaced relation with respect to each
09 other and being adapted to provide an electrical potential
therebetween. A dielectric system is interposed between the
11 elements, the dielectric system comprising a layer of
12 polymeric material and a liquid dielectric impregnated into
13 the polymeric material. The liquid dielectric is comprised
14 of polybutene having an average number molecular weight in
the range of 300 and 500 and containing from 0.01% to 10% by
16 weight of a cyclohexylamine.
17 In another embodiment, the inventive electrical
18 capacitor is comprised of a sealed casing, with a capacitor
19 pack in the casing having a pair of electrically conductive
strips and layers of a polymeric dielectric material wound
21 alternately with the conductive strips to form the capacitor
22 pack and a liquid dielectric impregnated into the polymeric
23 layers. The liquid dielectric is comprised of polybutene
24 having an average number molecular weight in the range of
300 to 500 and containing from 0.01% to 10% by weight of
26 cyclohexylamine.
27 In a further embodiment, the capacitor of the
28 last-noted embodiment utilizes layers of polypropylene as
29 the polymeric dielectric material. However, rather than the
liquid dielectric utilizing cyclohexylamine,
31 dicyclohexylamine is used. The polypropylene layers contain
32 at least 1.0% by weight of the polybutene.
~ - 4 -
1~46754
01 Other objects and advantages will appear in the
02 course of the following description.
03 Description of the Drawings
04 The drawings illustrate the best mode presently
05 contemplated of carrying out the invention.
06 In the drawings:
07 Fig. 1 is a perspective view of a typical
08 capacitor incorporating the dielectric composition of the
09 invention; and
Fig. 2 is a perspective view of a capacitor pack.
11 Description of the Preferred Embodiment
12 Fig. 1 illustrates a typical capacitor comprising
13 an outer casing 1 composed of side walls 2, a bottom wall 3
14 and a cover 4. In service the casing is hermetically sealed
and is provided with a small seal hole 5 through which the
16 dielectric liquid is introduced into the casing during
17 fabrication. In addition, a vacuum line can be connected to
18 the hole 5 for vacuum drying of the capacitor during
19 fabrication. A pair of terminals 6 project through the
cover and are insulated from the cover.
21 A series of capacitor packs 7 are disposed within
22 the casing and each capacitor pack, as illustrated in Fig.
23 2, includes wound layers of metal foil 8, separated by
24 dielectric layer 9. Electrodes 10 are connected to the foil
layers 8 and the electrodes of the various packs are
26 connected together in series for final connection to the
27 terminals 6.
28 Foil layers 8 may be formed of any desired
29 electrically
- 4a -
~;~3~l
1046754
conductive material, generally a metallic materiaL such as
aluminum, copper, and the like. The layers 8 may be in the form
of 1at sheets, or the layers can be provided with a series of
surface deformations formed by indentations on one side of the
foil and corresponding elevations on theother side, as disclosed
in United States patent 3,746,953.
The dielectric layers 9 are formed of sheets of poly-
meric film, such as polypropylene, polyethylene, polyester, or
,.,~.
polycarbonate. In addition the dielectric layers may take the
form of thin polymeric strips, such as polypropylene, having a
layer of fine polymeric fibers adhering to one or both surfaces
of the strip, as disclosed in United States patent 3,772,578. The
term "all film" as used in the description means that the' dielectric
layers 9 are formed of all polymeric materials including sheet
~S and fibrous types, although it is possible that other components
of the capacitor can be formed of paper or non-polymeric materials,
which would also be impregnated with electric dielectric composi-
tions,
According to the invention, the dielectric layers 9
are impregnated with a liquid dielectric composi'tion which consists
of low molecular weight polybutene having an average number
molecular weight in'the range of 300 to 500. The poIybutene hav-
~ng a molecular weight in this range has a viscosity at 100F in
D the range of ioo to 9ooo ssu, and has a viscosity at 210F in
the range of 3~ to 150 ssu.
In the past, higher molecular weight polybutene having
an average number of molecular weight of about 800 to 1400'has
been used in the production of cables. However, the polybutene
has not been successully applied to the production of capacitors.
Xn high stress, power factor capacitors, it is desirable to have
10 4 6 7 5 4
a reasonably matched system with the dielectric constants of both
the solid and liquid dielectrics being similar Paper has a
tielectric constant of about 6.7, whlle in the case of high
molecular weight polybutene, the dielectric constant is about 2.1,
and thus no balanced dielectric system can be obtained with paper
and polybutene. Utilizing high molecular weight polybutene with
8 paper dielectric results in poor performance at high stresses
due to the fact that the polybutene depolymerizes, resulting in a
lowering of the viscosity of the liquid and the eventual formation
of gas pockets. The gas pockets ionize under stress and produce
corona discharges and dielectric failure. Because of this, the
h~gh molecular weight polybutene has been used in combination with
all-paper or paper-film systems only in cables which are normally
subjected to low stresses
The higher molecular weight polybutene, as has been used
ln the past with all-paper or paper-film systems in cables, has
: not been employed in an all-film system in a capacitor due to the
fact that adequate impregnation of the film could not be achieved.
~owever, in accordance with the invention, it has been
found that when using a lower molecular weight polybutene, having
an average number molecular weight in the range of 300 to 500,
- catisfactory impregnation of an all-film solid dielectric can be
- achieved and the resulting dielectric system has substantially
improved dielectric properties over all-film systems impregnated
with conventional liquid dielectrics, such as polychlorinated
diphenyl.
The polymeric film is impregnated by the polybutene by -
means of diffusion. When the polybutene contacts the film, poly-
butene moleculcs enter the film. ~nce in the film, the liquid
molecules migrate from regions o~ high concentrations to regions
o low concentration until equilibrium is achieved. It has been
i046754
recognized that at least 4% or more hy weight of the liquid
dielectric must penetrate the fLlm to provide adequate impregna-
tion. Using the low molecular weight polybutene in combination
with polypropylene film at a temperature o~ 110C, it has been
found that 18% by weight of the polybutene impregnated the film
by d~ffusion. The low molecular weight polybutene will more
readily impregnate an all-film system than conventional impreg-
nants. This increase in the rate of impregnation is related to
the surface energy of the polybutene and is also dependent at
least in part on the relatively low viscosity of the polybutene.
An increase in the rate of impregnation can result in a substan-
tial time saving in the processing or fabrication of the capacitor.
The combination of the low molecular weight polybutene
with the polymeric film provides corona starting voltage (CSV)
comparable ~ that of a system utilizing trichlorodiphenyl and
polymeric film. However,the polybutene-polymeric film system has
substantially lower dielectric losses than an all-film system
i~pregnated with a trichlorodiphenyl.
. The polybutene is substantially fully biodegradable mean-
ing that if it should be exposed to the environment because of
leakage or rupture of the casing, or through discard of obsolete
capacitors, the polybutene will decompose into a harmless compound
and there will be no significant adverse environmental effect.
Thus, no pollution of the environment has been found to exist
through use of the polybutene as the liquid dielectric.
The capacitor, as illustrated in the drawings, can be
fabricated by standard procedures, in w~ich the casing containing
the capacitor pack is initially vacuum dried to remove air and
moisture and the liquid dielectric composi~ion is separ~tely dried.
The liquid dielctric is then introduced into the capacitor and
1~46754
permitted to ~oak under vacuum for an extended period in order to
completely impregnate the dielectric layers 9 with the liquid
tleiectric composition. Following this period of soaking,-the
unit iQ sealed.
S During operat~on of the electrical apparatus or
capacitor, the dielectric system may decompose if subjected for
Iong periods to low stress, to elevated temperatures, or to the
~ction of reactive chemical agents. The decomposition products
deteriorously effect the dielectric system and can lead to prema- 10 ture breakdown and failure of the capacitor. In the past,
neutrslizing agents or scavengers have been used to neutralize
the decomposition products, thereby improving the dielectric
properties and service life of the capacitor. The scavengers
have taken the form of epoxide compounds, such as 1,2,-epoxy-
3-phenoxypropane; bis(3,4-epoxy-6-methylcyclohexylmethyl) adipate;
l-epoxyethyl-3,4-epoxycyclohexane; 3,4-epoxycyclohexylmethyl-3,4,-
epoxycyclohexanecarboxylate; 3,4-epoxy-6-methylcyclohexylmethyl-
3,4,-epoxy-6-methylcyclohexanecarboxylate; and 3,4-epoxy-6- .
methylcyclohexane oxylate; and mixtures thereof.
In accordance with the invention, the dielectric liquid
can include from 0.1% to lOZo by weight of a cyclohexylamine which
acts to prevent the decomposition of the dielctric or acts as a
scavenger which reacts with decomposition producte to insure
greater reliability. The addition of the cyclohexylamine not
only produces greater reliability in a capacitor impregnated with
polybutene, but results in lower dielectric losses and a higher
stress capability.
The preferred cyclohexylamine is dicyclohexylE~ine
which is recognized to have the ollowing formula:
1~46754
~ H2 CH ~ H ~ 2 ~ CH2
CH2 ~ / CH - N - H ~ ~ H2
CH2 - CH2 CH2 - CH2
From the above formula it can be seen that dicyclohexyl-
S amine has a symmetrical structure which i9 particularly desirablein that a non-symmetrical structure will create greater electrical
losses in the system.
Other cyclohexylamines which can be used are:
N-ethylcyclohexylamine; N-isopropylcyclohexylamine; N-(2-hydroxy-
ethyl) cyclohexylamine; N-(2-cyanoethyl~ cyclohexylamine;
N-(3-aminopropyl) cyclohexylamine; N-cyclohexyl-'~- alanine;
2-cyclohexylamino-1-phenylethanol; N,N-dimethylcyclohexylamine;
N-phenylcyclohexylamine; N-cyclohexylpiperidine; N-methyleyclohexyl-
amine; N-methyldicyclohexylamine; polyethoxylated cyclohexylamines,
a ~ omocn-E-2, E-5, E-10, E-15, E-20, and X-25; N-cyclohexyl-
morpholine; and N,N' dicyclohexylthiourea, hereinafter referred
to in the specification and claims as "Group A'.'
The liquid dielectric liquid can also include from about
. 0.3% by weight to 20% by weight of trichlorobenzene which acts
to lower the viscosity of the dielectric liquid and also lower
the pour point. By lowering the viscosity and pour point, the
~mpregnation of the solid dielectric material is improved.
It is preferred to employ 1,2,4-trichlorobenzene which
i8 liquid at low temperatures, but the other two isomers of
~5 trichlorobenzene, which are solid at room temperature, can also
be used as the additive, if desired.
In the operation of a capacitor it is important that
the dielectric system not be subjected to long periods of corona
discharges. Corona discharges are very deterious to the
dielectric system and can result in premature breakdown and
1~467S4
ailure~ of the capacitor~ Whlle a capac~tor is designed so as not
to operate with the presence of corona discharges, switching surges
and the like very often raise the stress on the dielectric into
the corona region. When the switching surge has terminated, it
S i8 necessary that the corona discharges also ext~nguish.
~ To improve the corona discharge characteristics, the
dielectric liquid can contain from 0.05% to 15% by weight and
preferably from 0.10 to 5% by weight of an antioxidant, such as
butylated hydroxytoluene or di-tert-butyl-p-cresol. These
materials act to raise the coronaextinction voltage and thereby
~'decrease the time that the dielectric system is'in corona.
The follo~ing examples illustrate the invention:
EXAMPLE
A series of small sample capacitors were constructed of
two sheets of 0.005 inch polypropylen~ film ~etween sheets of
aluminum foil~ One group of samples was impregnated with poly-
butene having an average number molecular weight of 330, as the
liquid dielectric, while a second group of identical samples
D was impregnated with trichlorodiphenyl containing 0 5 ~/O by weight
of an epoxide additive, bis(3,4-epoxy-6-methylcyclohexylmethyl)
adipate.
The following table lists the representative data of the
two system;after impregnation,and after operation at 1000 hours at
1200 volts and after 1000 hours of operation at 1500 volts.
. TABLE
. . After After 1000 Hrs. After 1000 Hrs .Im~rePnation at 1200 Volts at 1500 Volts
CSV in D F CSV in D.F c~r~ D.F.
Volts in % Volts in % Volts in %
_, . . .
rrlchlorO-
Diphenyl
Plus an 2,370 .270 2,270 .077 2~320 .060
Epoxide
Additive
. .. ... ._ _
Polybutene 2,280 021 2,110 .005 2,370 010
-10- , .
`1~46~754
From the above Table lt can be seen that the corona
starting voltages (CSV) for both sets of samples were comparable
both after impregnation and after operation at both 1200 volts
and 1500 volts for 1000 hours, However, the dielectric losses for
the samples impregnated with polybutene were considerably lower
than the dielectric losses for ~he samples impregnated with the
trichlorodiphenyl. This data clearly Lndicates that the samples
utilizing polybutene do not deteriorate when under electrical
stress, as was recognized in the past when high molecùlar weight0 polybutene was utilized with paper in a capacitor construction.
EXAMPLE 2.
A series of tests were conducted to show the advantage
of incorporating dicyclohexyl amine as a scavenger with the poly-
butene. In this test a series of small capacitor samples were
constructed having a dielectric layer of two sheets of 0,005 inch
high density bi-axially oriented polypropylene film impregnated
with polybutene. A second series of identical samples were pre-
pared except that the dielectric liquid contained 0.2% by weight
of dicyclohexylamine. After 1000 hours of operation at 1800 volts
two of the six.samples containing only polybutene as the liquid
dielectric had failed and a third had a sever increaee in dissipa-
tion factor. Of the six samples containing the dicyclohexylamine
in combination with the polybutene, none of the six samples had
failed during the test period and none of the samples showed any
deterioration in loss characteristics or corona starting voltages,
~hereby indicating an increase in stability achieved through the
use of the dicyclohexylamine,
XAMPLE 3,
A series o small capacitor samples were constructed of
two sheets of 0.0005 polypropylene film between aluminum foil. A
~ "
~046754
group of th~ samples was impregnatcd with trichlorodiphenyl
containing ~,5/~ by weight of an epoxide scavenger, bis(3,4-epoxy-
6-methylcyclohexylmethyl) adipate, while a second group of samples
was impregnated with polybutene ha~ing an average number molecular
weight of 330, and a third group of samples was impregnated wi~'n
polybutene containing 15% by weight of trichlorobenzene andO.075%
by weight of an epoxide scavenger, bis(3,4-epoxy-6-methyl~yclo-
hexylmethyl) adipate. Each group of samples was tested at -45C,
OCC, 30C and 90C, and the result of the tests are shown in the
following table:
~E II
-4~C 0C 30C 1 90C
CSV D.F. CSV D.F ~~ CSV D.F. I
Description in Volts in % in Volts ,in % in Volts in~O in Yolts ~in %
.
Trichloro-
Diphenyl plus
an Epoxide 1,7001,15 2,4000.19 2,300 0.04 2,7000.07
Additive
Polybutene 1,800.024 1,900.008 2,400 .012 ~,200.025
Polybutene
plus 15% Over Over
. trichloro- 2,500.0593 2,700.0082 2,700 .0045 2,700.0187
benzene
. _ ,
As noted from the results shown in ~able II, the CSV o the
samples impregnated with trichlorodiphenyl and polybutene are
comparable throughout the entire operating range, although the
polybutene samples had considerably lower dielectric losses.
The addition of the trichlorobenzene to the polybutene
produced a higher CSV over the entire operating range. The
' ~ dielectric losses are comparable to those of the samples impreg-
nated with the polybutene alone, but are considerably lower than
the samples impregnated with the trichlorodiphenyl.
The above tes~ da~a indicates ~hat the capacitor samples
~0 impregnated with polybutene have a CSV similar to that of a system
'
.
-12-
- ' ' '' ' '~ 'I
104675~
utilizing chlorinated diphenyl but have substantially lower
dielectric losses over the entire operating range, which is of
particular importance when the dielectric liquid is to be used
in a capacitor.
S Furthermore, the liquid dielectric of the invention is
biodegradable so that if it i8 exposed to the atmosphere it will
decompose over a period of time, thereby preventing any permanent
pollu~ion of t e env ronme~
.
,
,
~3 ~
., . .:., , . . ~
