CA1153797A

CA1153797A - Synthetic paper, oil impregnated electrical apparatus

Info

Publication number: CA1153797A
Application number: CA000365777A
Authority: CA
Inventors: Bernardino Vecellio; Claudio Bosisio
Original assignee: Cavi Pirelli SpA
Current assignee: Pirelli and C SpA
Priority date: 1979-11-30
Filing date: 1980-11-28
Publication date: 1983-09-13
Also published as: SE8008293L; AU545488B2; DE3044782A1; FR2471031B1; ES8202181A1; AR224913A1; AU6403980A; NZ195522A; BR8007810A; JPS5699908A; IT7927733A0; GB2064579B; GB2064579A; ES498197A0; FR2471031A1; IT1163739B

Abstract

ABSTRACT OF THE DISCLOSURE

This application relates to an electrical structure comprising a metallic body maintained under high voltage and insulated with oil impregnated synthetic paper, and discloses, in particular, a very high voltage electric cable which is so insulated and which is intended for operation at 750-1000 kilovolts. The synthetic paper is constituted by a bundle of short fibers of calendered plastic material or by a film of plastic material or by a composite material in the form of a laminate of plastic and one or two thin layers of cellulose paper. To reduce swelling, the insulating oil is constituted by a fluorocarbon compound having a dielectric loss (tg .delta.)lower than 0.5 x 10-3. The oil can also contain oxygen atoms. Tri-fluoromethylperfluorodecalin and a perfluorinated polyether, having a molecular weight comprised between 200 and 5000, are particularly suitable.

Description

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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.

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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 '7~

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.

~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

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.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY
OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. High voltage electrical apparatus comprising a metallic body which is surrounded by insulation and which is subjected to high voltage, said insulation comprising synthetic paper impregnated with an insulating oil and said insulating oil having a dielectric loss factor at least as low as 0.5 x 10 3 and comprising a fluorocarbon.

2. Apparatus as set forth in claim 1 wherein said metallic body is the conductor of an electric cable and wherein said oil consists essentially of a fluorocarbon compound.

3. Apparatus as set forth in claim 1 or 2 wherein said compound includes oxygen atoms.

4. Apparatus as set forth in claim 1 or 2 wherein said compound is trifluoromethylperfluorodecalin.

5, Apparatus as set forth in claim 1 or 2 wherein said compound is a perfluorinated polyether having a molecular weight in the range between 200 and 5000.