Note: Descriptions are shown in the official language in which they were submitted.
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SYNTHETIC PAPE~, OIL I~PRE5NATED ELECTRICAL APPARATUS
The present invention relates to electrical apparatus
comprising a metallic body which is subjected to high voltages
and which is insulated with oil impregnated, synthetic paper,
and particularly to an electric cable which is insulated by
means of a synthetic paper impregnated with an oil having high
dielectric properties.
In the present application the term "electrical
apparatus" has a very general meaning, since it refers to in-
stallations, devices and apparatuses in which there are metallic
bodies subjected to very high voltages (transformers, condensers,
electric cables, etc.). However, this invention is of special
value when applied to oil filled electric cables (O~F. cables),
and in particular to very high voltage cables (750-1000 kV)
which, for te'chnical and economical reasons, are becoming
greater and greater in demand in the market. For this reason,
in the present application and in the example ref~rence will be
made primarily to electric cables, but this is a preferred,
and not the only application, of the invention.
It is known that several di~erent types of electrical
apparatus have the insulation of the metallic bod~es thereof,
which are subjected to high voltages, cons~ituted by cellulose
paper impregnated with oils, wound around said bodies. Said
oilsare generally, hydrocarbon compounds (alkylhenzene, mineral
oils, polybutene, etc.) and also chlorinated hydrocarbons,
silicone derivates, etc. In the case of alternating current,
such insulation is not su~table when the volta~e reaches very
high values. In fact, since the dielectric losses'rapidly
increase with the voltage, the temperature of the electrical
,~
structures can increase to an unacceptable value. This happens,
for example, for very high voltage electric cables (750-1000 kV)
in which a suitable cooling system has not been provided and
the efficiency of which is affected by the inner heat dis-
sipation.
In the case of the described types of electric cables,
the insulation (paper impregnated with oil) must have at least
the following characteristics: a dielectric loss (tg ~) lower
than about 1 x 10 3, an A.C. break~own strength of about 60 -
70 kV/mm, and a D.C. breakdown strength and an impulse break-
down strength of about 150 - 160 kV/m~.
Cellulose paper has a dielectric loss value (tg ~)
which is rather high, and, also, with the cleanest cellulose
papers it is not possible to obtain, with the insulation formed
by paper impregnated with oi], a tg ~ value lower than about
1.5 x 10 3. Therefore in order to satisfy the required di-
electric properties o~ the insulation, it is necessary to
modify suitably the combination of the paper with the impreg-
nating oil. One solution is that of using so-called "synthetic
paper" in place of cellulose paper. See, for example, articles
in the IE~E Transactions on Power Apparatus and Systems, pages
2019-2029, Vol. PAS-97, No. 6, Nov/Dec 1978 and pages 2083-2088,
Vol. PAS-98, No. 6, Nov/Dec 1979 and the 1980 IEEE technical
paper 80 SM 555-3.
As is known, the plastic materials most generally used
as insulation (polyethylene, polypropylene~ etc.) have a di-
electric loss value (tg ~) which is usually no more than 1/10
oE that of the cellulose paper. Said materials also have,
theoretically, a very high dielectric rigidity, as it is tested
with small thicknesses.
~.l$.~
Cables having a mass-extruded insulation are made with
said materials, and said cables are suitable also Eor high
voltage levels (150 - 230 kV), but not of the order of 750 -
lO00 kV. This is because of the unavoidable formation oE defects
in the plastic mass both during the construction and during the
use of the cable.
~ Iowever, the same plastic materials can be suitably
worked so as to modify the form and the chemical-physical
structure and so as to make them usable as sheets, that is, as
"synthetic paper" to be wound arourld the metallic bodies of the
electrical apparatus and to be impregnated with insulating oils.
Said sheets are constituted by a calendered bundle oE short
fibers or by a film.
There are several known types of synthetic paper, e.g.
high density and high crystallinity polyethylene, stretched and
biaxially orientea polyethylene and polyethylene Eibers com-
pac-ted by mechanical action, thermal action, etc. rrhe insulation
obtained by impregnating the synthetic paper with conventional
oils has, generally speaking, great improvement from a di-
electric point of view, as compared to the same kind of in-
sulation with cellulose paper.
However, any type of synthetic paper prepared according
to the known techniques, even if in different proportion, one
case compared to the other, is not devoid of a significant
drawback, namely, the swelling caused by the absorption of
impregnating oil in the intermolecular interstices of the
lastic material. The higher the operating voltage o~ the
electrical apparatus is, the more intense the swelling becomes,
said swelling increasing in proportion to the temperature.
The swelling of the synthetic paper~ and consequently,
of the insulation as a whole, can cause substantial damage to
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the entire electrical apparatus. In fact, the swelling of thesynthetic paper gives rise to inner mechanical stresses, which
can modify the geometric configuration of the insulation and,
therefore, cause irreversible deformation of the electrlcal
apparatus itself.
One attempt used to reduce the problem is that of
swelling the synthetic paper with the-impregnating oil before
winding it around the metallic bodies of the electrical apparatus.
However, this solution is complicated from a technical point of
view and often, the preliminary treatment of the syn-thetic
paper gives rise to undesirable degradation of the mechanical
properties of the synthetic paper itsel~.
Considerable improvements can be achieved by utilizing,
as the synthetic paper, composite insulations having very small
thickness. Said composite insulations are constituted by a
laminate of a plastic (polypropylenel fluorinated ethylene-
propylene copolymer, etc.) with a thin layer of cellulose paper
or inserted between two thin layers of cellulose paper (alone or
reinforced with synthetic materials). ~he use o~ composite in-
sulations in combination with conventional impregnating oilsimproves substantially the behavior to swellingl but there are
still other drawbacks related to the dielectric characteristics
and to the realization of the composite insulations.
In fact, the presence of one or two thin layers of
cellulose paper does not permit one to take complete advantage
of the dielectric characteristics of the plastic material. More-
over, it is readily understood and it can he found in practice,
that, from a technical point of view, it is not a-t all easy to
combine perFectly a plurality of layers of different materials
to form a thin film. This also has economic consequences which
are not to be disregarded.
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~Iowever, both ~rom a technical and economic point o~
view, it appears very convenient in the electrical apparatus
to be able to utilize synthetic paper (represented by a
calendered bundle of short fibers or by a film or by a composite
material) and to impregnate the same with insulating oils, pro-
vided that said oils do not compromise the dielectrlc properties
of the synthetic paper and that, unlike the conventional in-
sulating oils, they do not cause the swelling o~ tlle synthetic
paper, or at least, cause so slight a swelling that -they do not
constitute a danger for the electrical apparatus in use.
One object of the present invention is to proviae oil
impregnated insulation which overcomes the drawbacks of the
prior art.
In particular, the object of the present invention is
an electrical apparatus, particularly an oil Eilled elec-tric
cable, comprising at least a metallic body, to be subjected to
high voltage, which is insulated with synthetic paper arranged
around said metallic body and impregnated wi-kh insula-ting oil,
said insulating oil having a dielectric loss (tg ~) lower than
0.5 x 10 3 and being constituted by an organic compound con-
taining a fluorocarbon. Said insulating compound can also
contain oxygen atoms.
The present invention will be better understood by the
description of a particular example which relates to a preferred,
but not exclusive, application of the invention, namely, an oil
filled electric cable (OoF~ cables). However, it is to be
understood, as has already been stressed, that the present in-
vention is suitably applied to all types of electrical apparatus
which comprises at least a metallic body subjected to high
electric voltage and insulated with paper impregnated with oil
(transformers, condensers, etc.).
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In the single figure of the accompanying sheet of
drawing, the oil filled cable C comprises a conduc-tor 10 having
a longitudinally extending channel 11, insulation 12 constituted
by a winding of synthetic paper (a calendered bundle of short
fibers or a film or a composite material) placed around said
conductor 10 and a containinl3 metallic sheath 13 arranged
around said insulation.
An insulating oil is contained inside said longitudinal
channel 11 and impregnates the insulation of synthetic paper 12.
According to the present invention, said oil, which has a
"dielectric loss" (tg ~) no more than 0.5 x 10 3, ls constituted
by an organic compound containing a fluorocarbon. Said compound
can contain also oxygen atoms.
It has been found that the trifluoromethylperfluoro-
decalin and the perfluorinated polyether having a molecular
weight comprised between 200 and 5000, are particularly ad-
vantageous for the purposes of -the present inven-tion. The ~ormer
is a compound containing only carbon a~oms and fluorine atoms
and has the followiny formula:
F~L
F ~ f~
The latter is a polymer containing carbon atoms, fluorine atoms
and oxygen atoms, has a molecular weight comprised between 200
and 5000 and has the following formula:
r~ CF3 ÇF3
l ~ ~ CF - CF2 - 0 - CF - CF ) - 0 - CF J
Its chemical structure is substantially that of a polyether of
hexafluoropropylene.
The advantage deriving from the compounds of the present
invention used as impregnating oils of synthetic papers, with
~ t)
respect to the known insulating oil compounds, are shown by the
data reported in Table I set forth hereinafter. Said data
refers to swelling tests made at different temperatures. Said
swelling tests have been made by maintaining a specimen of
synthetic paper (a film of high density and high crystallinit~
polyethylene) immersed in the oil at a desired temperature for
72 hours uninterruptedly and measuriny the volumetrical
variation per cent which occurs in consequence of said treatment.
T A B L E _I
10 Swelling tests of a film of high density and high crystal-
linity polyethylene made with different oils and at
dif-ferent temperatures.
Exam~le Oil Volume variation P~ after 72 hours
No. O
100C 110C 120 C 130C
1 Trifluorome-thyl~
perfluorodecalin + 3.8 -~ A.5 -~ 5.2 ~ 35
:
2 Perfluorinated
polyether (average
molecular weight =
1000)` -~ 0.5 ~ 1.8 + 3.7 + 28
-
3 Decylbenzene + 5.5 -~ 21.0 ~egln- complete
ning of dis-
dis- solution
solution
. . . _ ~
From the values set forth in Table I, it is clear that
the swelling of the synthetic paper~ at the same temperature,
is very much lower with the oils of the present invention. In
example 1, trifluoromethylperfluorodecalin was the oil, and in
example 2, perfluorinated polyether having a molecular weigh-t
'7
between 200 and 5000 was used. In example 3 decylbenzene, a
prior art oil, was used.
In practice, the swelling, which is noted with the oils
of the present invention, is not dangerous for the usual opera-
ting conditions of the elec~rical apparatus and up to temperatures
near the melting temperature of the synthetic paper.
Although preferred embodiments of the present invention
have been described and illustrated, it will be apparent to those
skilled in the art that various modifications may be made with-
out departing from the principles of the invention.
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