Note: Descriptions are shown in the official language in which they were submitted.
The present invention relates to a capacitor which uses
as a dielectric material a polypropylene film impregnated with an
insulating oil and which is improved in its efficiency.
Recently, for the preparation of polypropylene film
capacitors impregnated with insulating oils, hydrocarbon insula-
ting oils such as mineral oil, polybutene, alkylbenzene, alkyl-
naphthalene, diallylalkane, etc. have been used in place of
chlorinated biphenyl such as diphenyl trichloride, diphenyl pen-
tachloride, etc. These hydrocarbon insulating oils have small
polarity and hence they are relatively excellent in dielectric
characteristic. However, their dielectric constant, which deter-
mines the dimensions of a capacitor, is small and at most 40 -
50% of the chlorinated diphenyl. Therefore, the composite
dielectric constant of the capacitor also becomes smaller and
correspondingly it requires more constituting materials for the
same capacity than capacitors impregnated with chlorinated
diphenyl.
Furthermore, the dielectric constant of said hydrocarbon
insulating oils is almost the same as that of polypropylene, so
that when a capacitor is constructed using said insulating oils,
approximately equal voltages are applied to the insulating oil
and polypropylene and this is a great burden for the insulating
oil.
In this connection, the dielectric breakdown voltage of
said insulating oils is at most 25 - 50 V/~ and that of polypropy- -
lene is 300 - 400 V/~. Thus dielectric breakdown of the insula-
ting oils predominates in dielectric breakdown of capacitor.
In order to overcome the defects of said insulating
oils as mentioned above, it is desired to impregnate the poly-
propylene film with insulating oils having higher dielectricconstant.
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However, it is difficult to obtain pure hydrocarbon
compounds comprising only carbon and hydrogen such as said insul- -
ating oils which have a high dielectric constant.
Ester type or ether type insulating oils have relatively
high dielectric constant, but these insulating oils contain
oxygen and cause deterioration of polypropylene ~ilm due to oxi-
dation.
That is, deterioration of polypropylene in insulating -
oil naturally depends greatly upon the amount of oxygen in the
insulating oil. -
Furthermore, when such ester or ether insulating oils
are used for a polypropylene film capacitor, local decomposition
of the insulating oils occurs due to electric stress in the
electric field.
Especially in the case of a capacitor such as a self-
healing capacitor which recovers insulation by partial breakdown,
said phenomenon is conspicuous. At this time, oxygen in ester
or ether group possessed by the insulating oils is released to
oxidize polypropylene to cause a reduction in the life of the
capacitor. - ~-
As a result of the inventors' research on prevention
of oxidation of polypropylene in an attempt to utilize the merit
of the high dielectric constant of said oxygen-containing insu- ~
lating oils, it has been found that incorporation of epoxidized `
compound into the insulating oils is very effective.
Thus according to the present invention there is pro-
vided, in a polypropylene film capacitor impregnated with an oxy-
gen-containing insulating oil and having polypropylene film as
a dielectric material, the improvement that an epoxidized com-
pound is incorporated into said insulating oil.
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2 -
Examples of the epoxidized compounds used in the pre-
sent invention are epoxy resins which are combined products of
epichlorohydrin and bisphenol, epo~idized products of fats and
oils, natural fats and oils, alkyl esters of resin acids, epoxy
derivatives of cyclohexane, etc.
Examples of the oxygen-containing insulating oils are
organic esters such as aromatic acid esters, alicyclic acid
esters, aliphatic acid esters, phosphoric acid esters, etc.,
ether oils, ketone type oils, etc.
The amount of the epoxidized compound has no special
limitation, but preferably is about 0.1 - 10~ by weight of the
insulating oil.
The invention will be further understood from the foll-
owing description by way of example of embodiments thereof with
reference to the accompanying drawings. ~n the following the
term "remaining perc~ntage" is used to mean the percentage of
the number of capacitors which were not broken and remained among
those which were subjected to applied voltage. In the drawings:
Figure 1 is a graph which shows the relation between
the amount (volu~e ratio) of oxygen in insulating oil and the
number of days required for complete deterioration of polypropyl-
ene film.
Figure 2 is a cross-sectional view of one embodiment
of a capacitor according to the present invention.
Figures 3 - 6 are graphs which show remaining percentage
of capacitors as shown in Figure 2 upon application of an alter-
nating voltage.
Figure 1 shows the relation between the amount of
oxygen present in insulating oil and the time required for com-
plete deterioration of a polypropylene film in the insulating oilat a temperature of 95 C. It is clear from E'igure 1 that the
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deterioration time decreases with increase in the amount of oxygen.
The following Examples illustrate the present invention.
Example 1
As a representative ester insulating oil, dioctyl phtha-
late was used. One percent by weight of epoxidized soya-bean oil
was incorporated into said insulating oil. This insulating oil
was impregnated in self-healing polypr~pylene film capacitors
having the construction as shown in Figure 2 wherein 1 is a metal-
lized paper which was obtained by vapor-depositing a metal on
both surfaces of an insulating paper and which had the same poten-
tial at both surfaces and 2 is polypropylene film.
Figure 3 shows changes of remaining percentage of
self-healing polypropylene film capacitors a which were impregna-
ted with dioctyl phthalate into which the epoxidized soya-bean `
oil was incorporated and self-healing polypropylene film capaci-
tors b which were impregnated with only dioctyl phthalate under ~ `
continuous application of alternating voltage at 85C. `
As is clear from Figure 3, the remaining percentage
of the capacitors b impregnated with only dioctyl phthalate was
extremely low and complete dielectric breakdown was caused in a
short period.
On the other hand, the capacitors a impregnated with ~ -
dioctyl phthalate containing the epoxidized soya-bean oil showed
no breakdown and exhibited a remaining percentage of 100% after `
1000 hours.
The above results show that the epoxidized soya-bean
oil captured oxygen generated from ester group by application of
voltage to prevent oxidation and deterioration of the polypropyl-
ene film.
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Example 2
As a representative ether insulating oil, alkyldiphenyl
ether was used. Into said ether was incorporated 1% by weight
of epoxidized soya-bean oil. This ~as impregnated in self-healing
polypropylene film capacitors having the construction as shown in
Figure 2.
Figure 4 shows the changes in the remaining percentages
of capacitors a impregnated with alkyldiphenyl ether containing
epoxidized soya-bean oil and capacitors b impregnated with alkyl
diphenyl ether containing no epoxidized soya-bean oil under con-
tinuous application of alternating voltage at 85C.
As is clear from Figure 4, the results are similar to
those in Example 1. That is, deterioration of polypropylene
caused by oxygen freed from ether group by application of voltage
was prevented by the incorporation of the epoxidized soya-bean oil.
In this connection, the same tests as those in Examples
1 and 2 were made on self-healing polypropylene film capacitors
impregnated with mineral oil and alkyl benzene which are pure
hydrocarbon compounds containing no oxygen. The results are shown
in Figures 5 and 6. As is clear from Figures 5 and 6, both capa-
citors a impregnated with insulating oil containing epoxidized
soya-bean oil and capacitors b impregnated with only insulating
oil showed high remaining percentages. It is considered that
these results were obtained because oxygen generated by electric
stress is not present in the pure hydrocarbon compounds such as
alkyl benzene and alkylnaphthalene and deterioration of polypro-
pylene due to oxidation occurs with difficulty.
As explained above, it has been found that incorporation
of an epoxidized compound into polypropylene film capacitors
impregnated with oxygen-containing insulating oil is extremely
effective for prevention of oxidation of the polypropylene.
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In the above Examples, epoxidized soya-bean oil was
used, but other epoxidized compounds may be used to obtain similar
results. Furthermore, any capacitors having polypropylene film
as a dielectric material may be used regardless of whether they
are of the self-healing type or of the non-self-healing type,
such as aluminum foil capacitors.
As explained hereinbefore, the polypropylene film capa-
citor of the present invention is improved in efficiency.
All of the capacitors a and b used in the above Examples
have the constructions as shown in Figure 2.
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