Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
This invention relates ger.erally to c_~acitors
and, more par~icularl~, to power capacitors w~ti- improve~
electrical stress ca~ability.
It has been the widespread com~ercial V)ractice to
use aluminum foil electrodes of equal widths in ~igh voltage
capacitors used for AC power factor correction o~ DC appli-
cations such as energy storage and discharge or filtering.
~ecause of width tolerances in rolls of aluminum foil and
manufacturing tolerances in winding the aluminum foil
electrodes, with dielèctric spacers, into a convolute cap-
acitor section, the ~oil electrode edges are not per~ectly
aligned from one end to the other. It is not considered
practical to exercise the degree of care that would be
necessary to assure perfect alignment. This results in an
electrode system in which the foil edges are offset to some
extent. Normally at one edge of the capacitor a first foil
extends laterally outward less far than the second foil. At
the other side of the capacitor the second foil extends
laterally outward less far than the first. In operation,
the voltage stress is relatively high at the edge of the
recessed foil. Thus, there are points of relatively high
stress at each edge of the capacitor. The higher voltage
stress results in lower breakdown voltage and lower partial
discharge inception voltage than would be achieved if the
foils were perfectly aligned.
Two significant developments in power capacitor
structures are those disclosed in Yagitani Patent 3,857,073,
30 December 24, 1974, and Yagitani et al. published Japanese
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~6, 512
Patent Applica~ions 2~516/71~ and 31~141/74, both of March 11,
197~. Said published Japanese applications were laid open
on September 25, 1975.
In the Yagitani U.S. Patent is disclosed a capa-
citior comprising a layer of capacitor grade paper and a
layer of fi~m, particularly polypropylene, as a composite
dielectric between two electrode foils of which the foil
immediately adjacent the paper is narrower at both edges
than the foil immediately adjacent the film. Such struc-
tures, impregnated with a dielectric fluidJ have been found
to compare very favorably in over-voltage tests with other-
wise like capacitor structures in which the foil electrodes
are like dimensioned and intended to be aligned but subject
to normal manufacturing variance from perfect alignment.
The virtue of this arrangement is that the narrower electrode
is next to the paper layer at both edges which eases the~
stress problem compared with the case where the electrodes
are of the same width and intended to be aligned, but not
actually aligned due to manufacturing variances.
In the pending application of Yagatani et al. a
further improved electrode foil arrangement is disclosed.
Namely, one in which one foil ~s not only offset from the
other at each of its edges, but that narrower foil also has
its edges made rounded ~nd smooth such as by ~olding over
the lateral extreme portions of the foil~ It is found
impro~emen~ in over-voltage characteristics results as
compared to otherwise llke oapacitor~ in which the narrower
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foil has straight cut edges.
Capacitor dielectrics include various capacitor
grade papers and various synthetic plastics in the form of
films of polymeric hydrocarbon materials predominantly
including members of the polyolefin family of which poly-
propylene is most widely used in power capacitors. All
paper, all film, and combinations of paper and film have
been used. As a dielectric liquid impregnant, polychlor-
inated biphenyls (PCB's) have been most commonly used. Such
fluids are today being discontinued because they are con-
sidered to be environmental pollutants. Various alternate
fluids are now being used and others are under considera-
tion. Some of these appear promising as general replace-
ments for PCB's. However, their electrical properties are
not identical to PCB's and the extent of their use and
testing has been so limited as compared with that of PCB's
that complete assurance of satisfactory performance over the
long term is not conclusively available. Therefore, capa-
citor designers and manufacturers are faced with a basic
change in one element, the dielectric liquid, in a tried and
proven system which poses a desire for further improvements
in other aspects of capacitor design such as can provide the
greatest margin for safe, reliable operation, with good
over-voltage handling capability, with long life.
The present invention results in large part from a
fuller understanding of the mechanism of over-voltage break-
down in relation to electrode arrangement, constituents of
the solid dielectric material, and characteristics of the
fluid impregnant which results in novel capacitor structures
and gives the capacitor designer greater ability to make
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sound choices for the various elements o~ the system as will
assure good reliable operation.
SUMMARY OF THE INVENTION
In accordance with the present invention, there
are provided power capacitors with a wide electrode and a
narrow electrode and a composite dielectric including a
first portion or sheet adjacent the narrower electrode that
is of higher dielectric constant and/or lower resistivity
(preferably both) than the portion or sheet adJacent the
wider electrode. The first portion may be a porous material
whose impregnation with a dielectric fluid results in the
desired electrical properties. In such structures the
narrower electrode preferably has rounded and smooth edges
such as by folding. The wide and narrow electrodes may each
comprise one of the externally connected capacitor electrodes.
Alternatively, the wider electrode may be electrically
floating and arranged between dielectrics and a second
narrower electrode in a symmetrical arrangement with the two
narrower electrodes serving as the externally connected
capacitor electrodes.
A wide choice of materials is available for the
composite dielectric. The higher dielectric const~nt and/or
lower resistivity layer may be one of several synthetic
films as well as capacitor grade "Kraft" paper. The lower
dielectric constant and/or higher resistivity layer may be
one of several other film materials including but not limited
to polypropylene.
The relation of dielectric constants of the first
and second sheet portions is particularly significant in AC
power factor capacitors. The relation of the resistivity of
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the sheet portions is particularly significant in DC energy
storage capacitors and the like. It is readily possible in
accordance wit~ this invention to choose a combination of
mal;erials having the right characteristics for either type
of application. Mowever, the invention applies as well to
cornbinations satis~ying only one o~ the criteria of electri-
cal properties.
Note, also, that while emphasis is made herein of
the electrical properties of the dielectric sheet materials
adjacent each of the electrodes, those materials may be
immediately adjacent each other or may be in a composite
with additional intervening layers of dielectric whose elec-
trical properties are of less significance.
It is to be noted that the relation of dielectric
constant and resistivity of the dielectric portions is
determined in part by the characteristics of the dielectric
fluid impregnant and the relative porosity of the sheet
materials. In general it is desirable for the higher di-
electric constant layer to be more porous than the other, as
by the use of paper or a porous synthetic.
Thorough impregnation is desired in all of the
disclosed capacitor structures. In those in which the
narrower elecGrodes have rounded and smooth edges, an aid to
impregnation can be obtained by using normal aluminum foil
electrode material and folding each edge substantially to
the center line of the foil with the surface having a mat
finish disposed on the outside of the folds.
In the disclosed structures in which the wider
electrode is electrically floating it may be a separate foil
element or may be a metallized layer deposited on one of the
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ad~acent dielectric sheets. Also the floating conductor may
be a single continuous layer or may be of two bands each of
which encompasses one of the edges of the narrower electrodes.
BRIEF DESCRIPTION OF THE DRAWINGS
_
Figures 1, 2 and 3 are cross-sectional views of
capacitor structures in accordance with the prior art,
Figures 4, 5 and 6 are cross-sectional views of
capacitor structures in accordance with embodiments of the
present invention;
Figure 7 is a cross-sectional view of a capacitor
electrode suitable for use in embodiments of the present
invention; and
Figure 8 is a partial sectional view of a further
embodiment.
DESCRIPTION OF PREFERRED EMBODIMENTS
Referring to the drawing, Figures 1, 2, and 3
which show prior art capacitor structures will be briefly
described by way of background.
In Figure 1, a generally conventional capacitor
- 20 structure is shown in which two electrode foil layers 10 and
12 are of like dimensions and spaced by a dielectric layer
or layers 14 comprising either capacitor grade paper or
j~ polymeric film or composites including one or more layers of
both paper and film. As well as with the other structures
to be described, normal manufacturing practice is to assemble
two foil electrodes with dielectric therebetween and another
' dielectric layer next to one of the foil electrodes as
shown. Upon winding of such an assembled stack into a
convolute winding each electrode has dielectric material 14
on botl~ sldes of ~t. Such st~ucturee, and all of thoee to
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be described, are normally and p~eferably impregnated with a
dielectric fluid. In the past this has normally been a
halopenated diphen~l. The problem with the structure in
accordance with Figure l is that the like dimensioned foil
electrodes are impractical to align perfectly resulting in
lower electrical stress capability at the edges as d~scussed
hereinabove.
In ~igure 2, a structure shown in accordance with
the referred to Yagitani Patent wherein one of the foil
electrodes 11 is intentionally made narrower than the other
12 and the dielectric material includes at least a first
layer of paper 16 ad~acent to the narrower foil èlectrode 11
and a second layer of film 18, particularly polypropylene
film, ad~acent the wlder electrode 12. As is demonstrated
in the Yagitani Patent, such structures provide improved
electrical stress capability compared with otherwise like
structures in which the foil electrodes are of the same
dimensions, but suffer inherent misalignment.
In Figure 3 is generally illustrated the further
improvement of the referred to Yagitani et al. application
wherein the narrower foil electrode 13 in a wide-narrow foil
electrode combination has its edges made rounded and smooth
such as by folding. An example disclosed in the Yagitani et
al application shows improved results for the case in which
the dielectric 14' is of three layers of film.
From a better understanding of the relation of the
dielectric material with the electrodes of a w:lde-narrow
foil arrangement, there are provided in accordance with this
invention improved capacitor structures.
As a general proposition, it can be shown that for
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a given dielectric system, offset electrodes produce a
higher voltage stress than aligned electrodes and the volt-
age stress increases up to a limit with increasing amount of
offset. A difference in fo~l dimensions of about lO mils at
each edge permits manufacture without extra care, e.g., a
situation in which one narrow foil edge is about 8 or 9 mils
in from the wide foil edge and the other narrow foil edge is
11 or 12 mils in from the other wide foil edge is satisfac
tory and much better than a similar variance between like
dimensioned foils. The invention recognizes that fact and
utilizes it to provide capacitor structures with a deliber-
ate offset in the electrodes and control of the nature of
the dielectric material ad~acent the narrower electrode.
Controlled voitage stress at the narrower foil edge without
extreme care in assembly is therefore permitted.
Figure 4 shows a structure utilizing the wide-
narrow foil arrangement with straight cut edges while in
Figure 5 the same structure is shown in which the narrower
foil has rounded and smoother edges. In each case the
dielectric is a composite of particularly chosen materials
wherein a first layer 26 ad~acent the narrower electrode 11
or 13 has a higher dielectric constant and preferably also
lower electrical reslstivity than the second layer 28 adja-
cent the wider electrode foil 12. This relation of electri-
cal properties results in a situation such that when a
voltage is applied between the foil electrodes, the voltage
stress will be distributed across the dieleotric sheet
inversely in relation to their dielectric constants. With a
high dielectric constant material 26 ad~acent the narrow
foil, the stress on this high dielectric constant material
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will be relatively low and consequently the stress at the
narrow f'oil edge will also be low.
It happens to be the case that the dielectric
structure presented in the Yagitani Patent (Fig. 2, above)
usually satisfies this desired electrical property relation-
ship. That is, normally a layer 16 of capacitor grade
paper, impregnated with a dielectric f'luid, will provide a
higher dielectric constant and lower electrical resistivity
than a layer 18 of polypropylene film of normal properties.
What was unrecognized in the Yagitani Patent was that the
improved properties of the structure therein disclosed are
essentially related to the relation of electrical properties
of the different parts of the composite dielectric. Now
with the understanding of the importance of this relation of
electrical properties, it is made possible to provide
structures of composite dielectrics other than paper and
polypropylene that gives the improved performance in the
wide-narrow foil electrode arrangement. For example, the
following Table presents examples of combinations of di
electric materials that may be used together and include
combinations in which the polypropylene film is ad~acent to
wide foil electrodes and dielectrics of material other than
capacitor gra~e paper are employed next to the narrowest
foil electrode. Also in combinations wherein capacitor
grade paper is the dielectric next to the narrower foil,
materials other than polypropylene are employed ad~acent to
the wider foil. When impregnated all these structures
prov1de good electrlcal stre:: capability.
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TABLE OF F~PLE DI~E~TRIC ~'l'~XIALS
Layer 26 Layer 28
Aii_Narrower Electrode Diel. Const. AdJ. Wider Electrode Diel. Const.
Polyester film 3.2 with Polypropylene film 2.2
Polyphenylene oxide film 2.6 with Polypropylene film 202
Cyanoethyl cellulose 18.0 with Polypropylene film 2.2
Capacitor grade Kraft paper 6.2 with Polyethylene film 2.2
Capacitor grade Kraft paper 6.2 with Polycarbonate film 3.1
Capacitor ~rade Kraft paper 6.2 with Polyamide film 4.6
Capacitor grade K~aft paper 6.2 with Polyimide film 4.5
Examples of capacitor tests illustrating the improvement
gained by employing the- higher dielectric constant material
adjacent the narrower electrode is shown and compared to the
opposite situation where the higher dielectric constant
material is ad~acent the wider foil electrode. Examples 1
and 2 were impregnated with mineral oil and Examples 3 and 4
were impregnated with polychlorinated biphenyl.
Ad~acent Ad~acent AC
Example Narrow Wide Test
2 o No.Electrode Electrode Voltage Results
o.7 mil polyester 1.0 mil polypropylene 4750 Visible corona
discharge.
5750 Failed.
2 l.0 mil polypropylene 0.7 mll polyester4000 Visible corona
discharge.
5000 Failed.
3 3 sheets 0.9 mil 0.7 mil polyethylene 7000 No visible corona
Kraft paper 7500 Failed.
4 -7 mil polyethylene 3 sheets 0.9 mil 5250 No visible corona
Kra~t paper discharge.
5500 Failed.
Examples 1 and 3 are examples of the preferred construction
with the hi~her dielectric constant material situated ad~a-
cent the narrow electrode and show an improvement of approxi-
mately 15-30% in breakdown voltage, thus demonstrating
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significan~ly improved voltage stress handling capability.
The ~elative porosity of the different layers 26
and 28 of the dielectric and the relatlve dlelectric con-
stants of the dielectric fluid used for impre~nation in
relation to the dielectric constant of the sheet materials
can also be of significance. Thus, as a general proposi-
tion, it is preferred that a rather non-porous dielectric
sheet 28 be used adjacent the wide foil 12 and more porous
dielectric sheet 26 material be used ad~acent the narrow
foil 11 or 13 and a relatively high dielectric constant
fluid be used for impregnation whlch increases the dielec-
tric constant of sheet 26 and thereby decreases the voltage
stress on the sheet 26 at the narrow foil edges. Exampleæ
of suitable porous dielectric sheet materials (for layer 26)
are:
Kraft paper;
Synthetic papers of polymer fibers such as
polypropylene;
Synthetic papers made porous or cellulose by
stretching polymer films filled with
inorganic powders suoh as calcium carbonate.
Examples of suitable ma~erials which are a~ailable as non-
porous dielectric sheets (for layer 2~) are~
Dielectric Constant
Polyester film 3.2
Polyphenylene oxide 2.6
Polypropylene 2.2
Polystyrene 2.5
Polyethylene 2.2
; 30 Polycarbonate 3.1
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The dielectric constant of the porous sheet can be increased
by impregnation with a high dielectric constant (at least
about 5) fluid ~uch as:
Dielectric Constant
Polychlorinated biphenyl 5.8
Tolyl xylyl sulfone 20.
Diethyl hexyl pthalate 5.2
The foregoing discussion of the relation of
dielectric constant and electrical resistivity of the layers
is significant in capacitors for both AC and DC application.
It is, however, the case that for AC applications it is the
dielectric constant relationship that is most important
while for DC application it is the electrical resistivity
relationship that is most important. The examples given
above are primarily with reference to the relation of dielec-
tric constants so as to make good AC capacitors. For DC
applications where relative resistivity is more important
the following are examples of suitable materials that may be
employed. Examples of high resistivity (about 1017 to 1019
; ~ 20 ohm cm-) sheet materials:
Polyester film;
Polyphenylene oxide film;
Polycarbonate film; and
Polyolefin (e.g., polypropylene or polyethylene)
film.
Examples of low resistivity (about 101 to 1014 ohm cm.)
sheet materials:
Kraft or synthetic paper impregnated with low
resistivity fluids such às:
3o mineral oil;
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diethyl hexyl pthalate;
~; isopropylated diphenyl; or,
q~ ,//q~,ed
~ latcd napthalene
with, preferably, fluid additives such as
tin tetraphenyl;
cadmium carboxylate;
beta methyl anthraquinone, or,
an expoxide~ -
which additives further lower the fluid resistivity.
The structure of Figure 5 employing the rounded
and smooth edged narrow foil electrode offers advantages
over that of Figure 4, as result from the teachings of the
Yagitani et al. pending application. It is to be recognized
that in instances in which the smooth edges are provided by
folding it is not critical in which direction the fold is
made or which surface of the foil is outside the fold.
Normal aluminum foil available for use in capacitors in- ~
herentl~ has a shiny or polished surface and a dull or mat ~ ;
finished surface of some difference in roughness. It is
found that if the mat finish surface is on the outside of
the fold, there is some improvement over the case in which
the shiny surface is outside the fold. However, in general
the rounding and smoothing of the edges of the narrower foil
may be accomplished in various ways other than folding. ~or
exampIe, assuming straight cut foils as shown in Figure 4,
the narrower one may have its edges treated by coating,
flame treatments, chemical treatments, and electrical treat-
ments such as to cause partial discharges from sharp points
to cause the points to burn away. It is also an advantage
3o to employ thick foil materials. This is not essential but
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46,512
better success is obtained if a relatively thick foil madeof 0~001 inch material is used as the narrower and rounded
edge foil as compared with a 0.00025 inch thick foil.
~ further form of improved structure is that shown
in Figure ~. For reference it is to be understood that in
the structures of Figures 1 through 5 the two electrodes
shown are those ultinlately connected in some suitable fash-
ion to the exterior of the capacitor. In the structure of
Figure 6 there are two narrow foils 13 and 13', preferably
with rounded edges, between which there are two groups of
capacitor dieléctric layers, preferably as dlscussed in
connection with Figures 4 and 5, wherein between each of the
two groups there is a wider electrode foil 22. The wider
foil 22 in this combination is intended as a floating con-
ductor that ls not connected externally to any voltage. Its
voltage will be determined by the voltage distribution
within the dielectric system. The floating conductor 22
reduces the thickness between the actual electrodes 13 and -
13' and the floating conductor 22 to one-half of the total
thickness between electrodes 13 and 13'. The ratio between
edge stress and body stress decreases as the thickness
between electrodes decreases. Therefore, the over-voltage
capability per unit increases with the use of the floating
conductor 22. It is in accordance with this invention that
the floating conductor 22 is made wider than the two actual
electrodes 13 and 13~ and that those two electrodes 13 and
131 preferably have rounded edges and that the dielectric
between the floating conductor and the electrode is a com-
posite of layers chosen for electrical properties as pre-
viously described.
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A further refinement is shown in Figure 7, which
is a round edged foil electrode 23 that may be used as the
narrow electrode in any of the previous structures. In
accordance with ~igure 7, the rounded edges are provided by
folding the ~oil over its entire extent so that the cut
edges of the foil are substantially together at 30. Also,
assuming normal aluminum foils with a polished and a mat
surface, in Figure 7 the mat surface is on the exterior of
the folded structure. Therefore, as to each of the ad~acent
dielectric materials the electrode 23 presents its more
rough surface with the ability to further enhance impregna-
tion of the adjacent dielectric with a dielectric fluid.
-~ Figure 8 shows, in a partial view, further refine-
ment generally consistent with Figure 6 whereln the floating
conductor comprises a pair of separate conductive bands 22a
and 22b each located to encompass the edge portion of the
capacitor electrode 13, the latter being substantially
aligned with the other electrode 13'. In both Figures 6 and
8 the floating conductor may be a separate foil layer 22 or
layers 22a and 22b or it may be formed by deposition of
metallic material directly on one of the dlelectric sheets,
that is a metallized layer or layers. Particularly where
metallization is used, the structure of Figure 8 provides
the desired benefits with economy of material.
Among the choices of dielectric materials that may
be used in the structures of Figures 4 through 8 are film
materials with rough surfaces or otherwise made absorbent
and suitable as wicking materials. These materials include
biaxially oriented fllms of polypropolyene, polyethylene or
films of polyester, polycarbonate, polyamide or other plastic
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~ 5 46~512
films. Any of these films may have one side with an altered
or hazy surface or it may be formed by coextrusion with a
thin film of a copolymer which is embossed to provide a
rough surface on one side of the base film, or such a film
base may have applied to one surface an adherent layer of
polypropolyene or other composition fibers to act as a
wicking layer, or a film layer may be used having a filler
material and made by stretch processing to provide a paper-
like internal cellular absorptive structure, which may be
referred to as "synthetic paper."
A thorough impregnation of all voids in the dielec-
tric layers of the capacitor is necessary for use in alter-
nating current or energy storage type capacitors at high
voltage stresses. This can be achieved using any of the
above-mentioned materials by utilizing one of such materials
as a single layer film layer in con~unction with one or more
standard capacitor grade films having lower cost and higher
electrical stress capability by itself. Since some degrada-
tion of electrical stress capability is likely in the case
of such modified films compared to unmodified films 3 it
becomes even more important that the dielectric fluid im-
pregnant be well chosen, consistent with its dielectric
constant and resistivity, and include an additive such as an
epoxide or beta methyl anthraquinone for longer life at high
stresses.
The invention offers designers a wider selection
of capacitor structures with composite dielectric materials
than has been previously available. While it can be expected
that prior known structures where the dielectric sheets are
of the same material tfor example, all paper or all film) or
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at least the same material ad~acent the two electrodes (for
example, a film~paper-film composite), and with like dimen-
sioned or wide-narrow foil electrodes, will continue to find
appl:ication, new options are now made available for a con-
sidered choice of materials, electrodes and their arrange-
ment.
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