Note: Descriptions are shown in the official language in which they were submitted.
36CA 3522-A
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A~ EL,ECTRICAL CAPACITOR HAVI~G A POLYGYLCOL
DIELECTRIC FLUID
Background of the Invention
This application is a division of Canadian
Serial No. ~02,495 filed May 7, 1982.
This invention relates to polyglycol
dielectric fluids Eor use in electrical devices, and
more particularly to polypropylene glycol ~luids for
use as a dielectric fluid in metallized electrical
capacitors.
Description of the Prior Art
Liquid impregnants for electrical capacitors
should have a high dielectric constant, maintain a low
dissipation factor, and be compatible with the other
materials in the capacitor structure~ At the same
time, the impregnants must withstand elevated and
fluctuating temperature, pressure, and voltage stress
conditions with excellent electrical characteristics
for a long operative life of the capacitor. Each of
processing, impregnating and other such physical
characteristics are also much desired.
There are a yreat number of different kinds
of dielectric liquid impregnated capacitors which have
been developed over the years to meet specific
application requirements. Broadly speaking, among th~
larger capacitors are found high voltage (above 660
36CA 3522-A
-- 2 --
volts AC), and low voltage power capaeitors, which
also may be denoted as energy storage capacitors,
induction heating capaeitors, and power faetor
eorreetion capacitors. Small eapaeitors are usually
found in application catagories as motor start and run
capacitors and lighting capacitors.
In some instances different capacitors use
different fluids although there are common desirable
fluid characteristics as well as common capacitor
operatiny environmental charaeteristics such as vacuum
dried fluid filled and sealed units. With the advent
of small capaeitors where the solid dieleetrie is
under very high eleetrical stresses, and partieularly
where the solid dielectric is a plastie film eoated
with an evaporated metal layer, i.e., a metallized
eapaeitor, the prior fluids used have been found not
to be optimum. For example, in some eapaeitors it is
either not necessary or desirable for the fluid to
swell the plastie, or the penetration of the fluid
into the plastic dielectric is undesirable. In yet
other metallized eapacitors there is a deleterious
relationship between the fluid and the metallized
layer whieh eontributes to defieieneies such as
eleetrode degradation and eorrosion, and a resultant
eapaeitanee loss in the eapaeitor. These problems in
metallized eapaeitors have led to a seareh for new and
improved dieleetrie fluids and speeialty dieleetrie
fluids for metallized eapaeitors.
Summary of the Invention
It has been discovered that a certain elass
of fluids known as the polyglyeols have eertain
\
36CA 3522-A
-- 3
desirable characteristics which, when properly treated
and used, will mitigate the foregoing electrode
problems. Electrical capacitors utilizing metallied
polypropylene films, when impregnated with
polypropylene glycol demonstrate superior performance
in resistance to elec-trode corrosion and electrode
clearing ef-fectsO
The Drawings
This invention will be better understood
when taken in connection with the following
description and drawings in which -
FIG. 1 is an illustration of a metallized
polypropylene capactor roll most adaptable for
utilizing the gylcol fluid of this invention.
FIG. 2 is an illustration of the capacitor
roll section of FIG. 1 assembled in a casing.
FIG. 3 is an illustration of a different
metallized capacitor which is adaptable for the use of
the fluid of the invention~
FIG. 4 is an illustration off an all-film
dielectric power capacitor adapted to utilize the
fluids of this invention.
FIG. 5 is an illustration of an assembled
capacitor which uses the rolls of FIG. 4 and utilizes
the fluids of this invention.
Description of Preferred embodiments
The polyglycol components of this invention
are essentially linear polymers having the following
generalized formula wher R, R' and R" can be hydrogen
and/or an alkyl group.
R0- CH2-CH-0 -R"
R'
36CA 3522-A
(See Synthetic Lubricants, Edited by
Gunderson & Hart, Reinhold Publishing Corporation,
New York, New York, 1962, Chapter 3, Polyglycols,
Pages 61-102, and Encyclopedia of Polymer Science and
Technology, Vol~ 6, 1967, John Wiley & Sons, Library
of Congress Card ~6422188.)
These materia.ls have, at various times, also
been called glycols, polyethers, polyalkylene glycols,
or pc~lyoxyalkylene glycols. This- extensive
nomenclature has given rise to the generally accepted
term polyglycol whether the actual produc-t is a diol,
triol, etc., a monoether, a diether, or an ester. While
various polyglycol fluids are applicable to the
practice of this invention it has been discovered
that the polyglycols having a significant polypropylen
component and preferably tr.iol tvpe polvproPYlene glycols
derived from a glycerine base, have provided excellent
results in the practice of this invention~
A descriptive formula for a triol type
polypropylene glycol fluid is this invention is shown as
follows:
_.
H, H H C,113
H2 C~O C~C~O C,--C--O ~H
H H =X ' =H H _~,
H H H CH3
Hl C~O C ~C~O C~C--O -- H
.1
H H =X H H =y
H 11 . H C,H3
H2 C~O C--C~O C--C--O ~ H
~ Y
36CA 3522-A
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where X = 0 or _Y_ is 3-5. The first bracket represents
ethylene oxide and the second bracket propylene
oxide components.
Amony the presently commercially available
Eluids which have been utilized in this invention are
the following:
1. Union Carbide NIAX 16-46 - ethylene oxide/
polypropylene oxide triol type with a glycerine
starter and available from Union Carbide Company.
2. Union Carbide NIAX LG-56 - polypropylene
oxide triol type with a glycerine starter and
available from Union Carbide Company.
3. Dow P2000 - polypropylene oxide diol
type and avaiable from Dow Chemical Company.
~. Dow 112-2 ethylene oxide/propylene
oxide block copolymer and triol type with a glycerine
starter and available from Dow Chemical Company.
The mol~cular weight of the polyglycol
fluids of this invention, are above about 1000 and
prefera~ly in the range of 3000-4000 and above~ Other
desirable characteristics are dielectric constants in
the range of about 4. 5 to 5.5 measured at 100C,
ease of refining to reduce water content below about
75 parts per million (PPM) and a power factor of
less than about 10% measured at 100 and 100 Hz.
It has been unexpectedly discovered that
polypropylene glycol has unique electrically related
properties as a dielectric fluid when used in electrical
capacitors where the electrodes are metallized layers
on polypropylene film. Also polypropylene glycol
has been found to be advantageous as a capacitor
fluid in metallized capacitors because its chemical
and physical properties are quite adaptable and
compatible with polypropylene films. For example, it
is a relatively non-spreading, non-penetrating,
viscous fluid with low swelling characteristics in
36CA 3522-A
polypropylene film. These properties are particularly
advantageous in very tightly wound, hard roll,
metallized capacitors of the kind disclosed and
described in U.S. Patent 3,987,348 - Flanagan et al -
assigned to the same assignee as the present inventionand illustrated in ~IGS. 1 and 2 of this invention.
Referring now to FIG. 1 there is disclosed
one preferred embodiment of this invention as a
capacitor roll section 10. Roll section 10 comprises
a pair of dielectric material strips 11 and 12 of
polypropylene which have been metallized as illustrated
by aluminum metallizing surfaces or coatings 13 and 14.
As is the usual practice, the strips 11 and 12 are
metallized in a manner which leaves metal free margins
15 and along opposite edges of roll 10. In the
winding process the roll 10 is wound on a core member 17,
and the strips 11 and 12 are laterally offset with
respect to each other in order that each roll edge or
end will display ~n offset with its metalliæed
coatings exposed ~t the edge of the strip. Thereafter,
suitable electrical leads 18 and 19 may be attached
to the exposed metal coating through utilization of
the well-known schooping process to provide a metal
layer 20, and the roll 10 is then placed in a can
or casing as illustrated in FIG. 2. In FIG. 2,
the capacitor 21 includes a casing or can 22 which
contains a single roll 10, and the leads 18 and 19
from the roll connect to the terminals 23 and 24.
Casing 22 is filled with a dielectric fluid 25 such
as a polygylcol fluid of this invention and then sealed.
When the capacitor of FIG~ 1 is impregnated
with prior fluids, and operated at high voltage,
corona discharge occurs at the roll end or edges
and also in the first few turns of the roll. It is
in these areas where most corona occurs and from
which most capacitors fall. But for the fact that
36CA 3522-A
-- 7 --
a metallized capacitor has inherent clearing effects
the corona discharge would cause an early failure of
the capacitor. The clearing effect is also known as
self-healing and occurs when there ls some electrical
discharge between metallized electrode layers. The heat
of the electrical arc or discharge vaporizes an
increasingly larger area of the metallized layer, thus
increasing the arc length until it extinguishes.
Alternatively described, should a dielectric fault
and puncture occur, the very thin film of metallizing
will burn back and away from the failure site, isolating
the fault. The self-healing feature will then permit
the capacitor to continue to function following the
clearing, and the dielectric systen can be used at
voltage stress levels higher than conventional unmetallized
capacitors. Clearing is therefore an instrumental
factor in preventing early failure of a metallized
capacitor. ~Iowever, repeiitive clearings, which have
been described, remove some of the electrode area
and accordingly reduce the rated capacitance of the
capacitor. This is a major disadvantage in metallized
capacitors and a limitation on their applications.
It has been discovered that polypropylene
glycol fluids act to suppress clearing in the favorable
sense of localizing the clearing and not contributing
excessive gases and arcing products which would preserve
or extend the clearing action and cause excessive
evaporation of the electrode metal. This ma~or clearing
advantage of polypropylene glycol fluids is evidenced
by its reaction to the many minor electrical clearing
effects which take place in metallized capacitors.
Polypropylene glycol has a hlgh oxygen co~tent which
~s aifavorable factor in the ability of an aluminum
metallized layer to clear without the build up of
conductive elemental carbon. This build up, which is
typical of metallized dielectric systems contributes
~2~
36CA 3522-A
-- 8 --
to maintaining the arc during clearing, with the adverse
result of excessive electrode evaporation, loss of
capacitance and the development of a high resistance
short circuit.
In a liquid filled tigh-tly would capacitor
as described, it is not desirable to have any liquid
penetration to the roll edges where significant corona
may occur. The presence of a high dielectric constant
fluid such as polyglycol is additionally favorable at
the roll edge. In fact, the rolls are very tightly
would and given a prebaking treatment at elevated
temperatures to heat shrink the roll. In these
capacitors, if the liauid penetrates and swells the
resin it will eause the metalliæed layer to lose its
bond to the resin. The layer then rapdily deteriorates
in those areas. Further, the presence of the li~uid
deeper into the ~Ilinteracts in the electrical field to
further cause corrosion of the metal layer. E]ectrode
corrosion is also a significant factor in the loss
of capacitance in a metallized capacitor. The physical
charaeteristics of polyglyeol fluids in these capacitors,
i.e. their high viscosity and low swelling with respect
to polypropylene film, made them highly desirable
dielectrie fluids for metallized capacitors.
The loss of capacitance due to the two
factors of clearing and corrosion is at present the most
serious disadvantage in metalli~ed capacitors and a
limiting factor on their useful life. The following
examples show the benefits, in this respeet, from the
use of polyglygoel fluids of this invention in metallized
capacitors.
EXAMPLE I
In this example six identical aluminum
metallized capacitors generally following the Flanagan
patent diselosure were assembled and impregnated with
different eapacitor fluids as shown. Severe tests were
36CA 3522-A
9~, _
carried out at 410 V~C and 470 VAC (volts alternating
current) at the temperature noted. Ordinarily these
capacitors would be rated at 330 VAC and 90C and 370 VAC,
70C respectively. Measurements were made as the units
were ther~al cycled at 100C and 80C respectively.
%f~C is the percent net change in capacitance. F is a
capacitor failure. Geconol is a fluid used in the
commercial production of other capacitors, and comprises
di-ethyl hexyl phthalate ester, an epoxide and Ionol. A
further description of this fluid is found in U.S. patent
3,754,173, issued August 21, 1973 to Eustance, assigned
to the same assignee as the present invention. Ionol
reEers to a commercially available anti-oxidant comprising
2.6 di tert-butyl-p-cresol and its capacitor use is
described in U.S. Patent 4,117,579 issued October 3, 1978
to Shaw et al. Polypropylene glycol (PPG) was commerciallY
available as Union Carbide NIAX 16-46. In this example,
the polypropylene glycol was first column refined with
fuller's earth until the water content was reduced to
below about 100 P~M and the power factor, measured at room
tem~eraturewas below about 10.0~. Test results wele given
ir the following tables:
410 Volt~ AC 470 Volts AC
%~ C A~ter I~ime T.~O~ C A er Time T.
Fluid In ~ours In lours
300 500 ~00 1600 300 50010001500
PPG NIAX 16-4G 0.5 2.6 7.4 2.13.95.7
Cotton Seed Oil
-~0.05% Ionol 1.3 All-F 2.77.011.7
Corn Oil 1.3 All F 2.7 6.0 8.6
~0.05~ 2-F
Geconol 7.4 24.6 All-F 3.310.016.8
5-F 2-F
Fpoxidized
Soybean Oil 0.30 1.3 - 5.2 0.7 2.24.1
Soybean Oil 0.6 2.8 - F 0.6 2.45.9
3-F
Acetylated ~ F 0.6 2.0 5.1
Castor Oil l-F 5-F
36CA 3522-A
-- 10 --
As can be seen from the above table the %~ C
change in capacitance for the polypropylene glycol
impregnated capacitors is consistently and markedly less
than the % ~ C change associated with the other fluids.
Note the average percent ~C for the 410 volt unit
was compared to 1.3 and 7.4 for -the other fluids.
Translated into more practical terms this reduction may
means as much as 2 to 3 times the predicted operating
life of this capacitor, compared to a Geconol impregnated
capacitor.
EXAMPLE II
~n this example a number of similar capacitors
were made up and subjected to an applied voltage of 400 VAC
which is correlated to the capacitor design to produce
immediate and positive clearings and clearing times
were compared. Clearing time is the number of micro
seconds ~ s, over which the electrical discharge from
a clearing extends, PXE is phenyl xylyl ethane and MO is
mineral oil, mJ i~ milli joules.
Applied Voltage 400 VAC
Fluid ~y PPG GEC PXE MO
Clearing time ~ sec. 9 8 10 14 12
Energy mJ 440 380 360 380 360
This table shows that PPG has a shorter
clearing time than the other ~luids. Microscopic
examination of clearing sites showed a clearner clearing
for PPG is a contributing factor to the shorter
clearing times.
EXAMPLE III
Tests were undertaken to compare the physical
properties of polypropylene glycol (PPG) and Di ethylhexyl
phthalate capacitor fluid DEHP, a widely utilized fluid,
with the results showning polypropylene glycol to be
superior. For example, a physical analysis of DEHP
impregnated capacitor rolls and polypropylene glycol
impregnated capacitor rolls showing that fluid penetration
~2~
36CA 3522-A
-- 11 --
with polypropylene glycol was significantly less than
than of DEHP. Note the following comparison characteristics
which support the results of these examinations.
Contact Angle of
~luid on Untreated
lO0~ Swelling PolypropyleneViscosity
Of PP Vol. % (Degrees) 25C
PPG ~ 1% 43 + 2 525 cs
DEHP 7.2% 9 + 2 57 cp
The described advantages of the polyglycol
fluids are primarily useable to their best advantage in
metallized synthetic resin capacitors. They may also be
used in metallized capacitors requiring full impregnation,
i.e. double metallized paper electrode polypropylene film
dielectric capacitors, composite dielectrics of paper
and polypropylene film with aluminum foil electrodes,
and all polypropylene fil~ dielectrics with aluminum foil
electrodes. The capacitors are described as follows:
Referring now to FIG. 3 the exemplary capacitor
roll section 26 comprises a core member 27 on which is
a tightly wound s~ries of synthetic resin strips 28 and
metallized paper electrodes 29. The roll section 26
is wound with the electrodes in offset relationship to
each other so that the metallized edges 30 of one
electrode are exposed at one end 31 of the roll section
and the exposed edges 32 of the other metallized
electrode are exposed at the other end 33 of the roll
section. A suitable metal such as aluminum or zinc is
sprayed at each end of the roll section to form a coating
34 and electrode leads 35 and 36 are joined to coating 3~.
The metallized paper electrodes 29 comprise a
thin, high density paper strip on which is a layer or
coating of aluminum. A number of materials may be
employed for the paper including woven and non-woven
polymeric material or other porous and wicking materials
which will permit the ingress of dielectric fluids
36CA 3522-A
- 12 -
thereln and therealong. However, in the practice of
this invention, capacitor tissue is preferred which is
about 1.0 density. Such tissue is commercially
available as Kraft capacitor tissue. The paper strips
are coated with a metal layer on both sides thereof,
a combination referred to as doubly metallized paper.
Preferably the metal is aluminum which is vacuum
deposited on the paper hy well-known vacuum deposition
to provide a uniform high purity metal layerO Such
layers are measured in terms of their ohms resistance
per square of electrode foil and a range for the
present invention is from about 4.0 to about 7.0 ohms/square.
The synthetic resin strips 28 may be single
or multiple strips of one or more of the more common
dielectric resins such as the polyolefins, polycarbonates
and polyamines, etc., and homopolymers and copolymers
thereof. However, a resin comprising electrical
capacitor grade polypropylene is a preferred resin
strip for this invention. Capacitor grade polypropylene
film is a higher purity, smoother, polypropylene film
of enhanced dielectric characteristics.
~ he polypropylene strips 28 and metallized
paper electrode strips 29 are wound together in roll
form as illustrated in FIG. 1, inserted in a round can
similar to can 19 of FIG. 2, subjected to an elevated
temperature and a vacuum drying process to remove
moisture, and vacuum impregnated with ~ suitable
polypropylene glycol fluid 25 of this invention. In the
practice of this invention the capacitor rolls may be
wound or flattened in a somewhat oval section, both
configurations being adaptable to polypropylene glycol
fluids. Because of the use of paper in this capacitor
the polypropylene glycol fluid impregnates the entire
roll structure. The advantageous use of polypropylene
glycol fluid in the FIG. 3 capacitor is illustrated by
the following example.
~L2~
36CA 3522-A
~;- 13 ~
EXAMPLE IV
In this example a number of identical capacitors
were made up following the FIG. 3 structure and impregnated
with a 50/50 by volume blend of polypropylene glycol
and phenyl xylyl ethane known as PXE dielectric fluid.
The capacitors were then subjected to very high AC and
DC voltages to test the breakdown strength of polypropylene
glycols against the known excellent performance of the
50/50 volume PXE/Geconol blend. The following results
were noted:
DC (Kilovolts) AC (Kilovolts)
FluidAverage of 12 Units Average of 13 Units
PPG/PXE 3.0 2.4
Geconol/PXE 2.5 2.0
Geconol 3.6 2.4
PPG 4-4 3.0
The results of the ahove tests indicate that
pol~propylene glycols provides all of the good
characteristics of know commercially used dielectric
~luids w~ile contributing its own compatibility and
clearing advantages.
The polypropylene glycol fluid of this
invention may also be employed in other impregnated
capacitors such as mixed polypropylene/paper dielectric
capacitors as dislcosed and cliamed in U.S. patent
3,363,156- Cox, as well as the all-~ilm dielectric
capacitor of the same patent. In an all-film capacitor
as illustrated in FIG. 4, a longer roll 37 comprises
ilm strip 38, 39, 40 and 41 which are wound together
with separate aluminum foil strips 42 and 43 in a roll
form similar to roll 10 of FIG. 1 or, a composite
dielectric of alternate polypropylene strips and paper
strips are wound together with aluminum foil strips in a
roll ~orm. ~n assembled capacitor utilizng the flattened
and elongated rolls 37 and electrode leads 44 and 45
is shown in FIG. 5.
36CA 3522-A
In FIG. 5 capacitor 44 comprises a casing
45 containing a plurality of rolls 37 suitably
electrically connected to terminals 46 and 47. Rolls
37 are submerged in polygylcol fluid of this invention.
Such a capacitor is denoted as a power capacitor and
typically may have a rating of 100-300 Kvar and range
upwardly to about 13,000 volts.
Polypropylene glycol may be used alone or
with certain additives whihc have been found beneficial
in a capacitor environment. Polypropylene glycol may
be subject to oxidation both in the handling and
impregnation system or in the capacitor environment.
For this reason it is advisable to use an anti-oxidant
additive, Ionol being one example. Usually an anti-
oxidant is added in amounts ranging from about 0.01% by
volume to about 10.0% by volume.
Epoxides have been found to be beneficial in
a capacitor environment having either chlorinated
fluids or ester fluids present. (See U.S. Patent
3,353,156 - Cox and 3,754,173 - Eustance) for a further
description of epoxide use. A typical epoxide which
may be used effectively in this invention is commercially
available, Unox 221 a dicyclo diepoxy carboxylate.
Epoxides are usually added in the ranye of about 0.1% to
10% by weight of the fluid.
The polypropylene glycols are not considered
as aromatic fluids and accordingly are not good gas
absorbers. In some capacitor applications gas absorption
generally, and often times rapid gas absorption, is
necessary to reduce deleterious corona discharge. In
this instance certain gas absorbing additives may be
used with the polypropylene glycol. One class o~
additives includes the alkenes such as an aliphatic
olefin, tetradecene being a good example. U.S. Patent
4,190,682 - Shaw described the use of aliphatics as gas
absorbers in combination with ester fluids. A diferent
36CA 3522-A
- 15 -
class of materials, the anthraquinones may also be
employed in this invention. Alternatively a polyglycol
may be chemically modified to improve its gas absorbency
by using starting materials which will provide carbon
to carbon double bonds.
Where a preferred fluid would comprise
essentially polypropylene glycol (with additives) this
fluid may be blended with other fluids to provide
special fluids for special needs. The blending of
fluids is described for example in ~an. App. Serial
No. 379,085 filed June 5, 1981 - Grahame. In the
Grahame application certain fluids such as Phenyl
Xylyl Ethane (PXE) and Mono Iso Propyl Biphenyl
(MIPB) are blended with certain esters. For the
present invention the polypropylene glycol of this
invention may be substituted in place of the ester.
It is preferred that the polypropylene glycol be the
major constituent in any blend, e.g., that it comprises
about one-half or more of volume of the mixtureO
Al~ernatively however, the advantages of the polyglycol
may be ~nh~nced or buttressed by chemically modifying
or combining other materials therewith. Suitable
examples are ether and ester linkages. Included also
are copolymers, random or block and polypropylene
glycols containing ethylene oxide subunits.
The fluids of this invention are those that
remain in the capaci-tor through its effective life as
a fluid as opposed to cured and solid compounds which
may contain some combination of polyglycol.
Other fluids tested included Soybean Oil,
Acetylated Castor Oil, Soybean Oil, and Polybutene.
While in some limited tests, good results were obtained,
they were usually obtained at shorter hours of life.
Life tests at hours beyond about 1000 clearly show
the predominance of the polyglycol fluids of this
invention.
36CA 3522-A
- 16 -
While this invention has been disclosed with
respect to particular embodiments thereof, numerous
modifications may be made by those skilled in the art
without departing from its true spirit and scope.
Therefore, it is intended that the appended claims
cover all such modifications and variations which come
within the true spirit and scope of the present
invention.
