GB2064579A - Electrical structure having an oil impregnated synthetic paper insulation - Google Patents

Abstract

An electrical structure e.g. an electric cable, comprises a metal body to receive high voltages and insulated with oil impregnated synthetic paper in which the insulating oil has a dielectric loss factor of 0.5 . 10<-3> or less and is constituted by an organic compound containing at least carbon atoms and fluorine atoms. The oil can also contain oxygen atoms. Trifluoromethyl-perfluorodecalin, perfluorodecalin, perfluorotoluene, perfluoro- 1-3 dimethylcicloesane and perfluorinated polyether having a molecular weight between 200 and 5000 are particularly suitable. With such oils, the synthetic paper does not swell excessively.

Description

SPECIFICATION Electrical structure having an impregnated synthetic paper insulation The present invention relates to an electrical structure comprising at least one metal body to which, in use of the structure, high voltage is applied, and which is provided with an insulation comprising synthetic paper impregnated with an insulating oil.

The present invention is generally applicable to any such electrical structure for example a transformer or a condenser, but the invention is particularly applicable to oil-filled electric cables and particularly to those for carrying very high voltages (750-1000 kV) which, for technical and economic reasons, currently attract greater and greater demand.

It is known for the metal bodies of the above electrical structures to be insulated with cellulose paper wound around them and impregnated with oil. Appropriate oils generally include hydrocarbon compounds (alkylbenzenes, mineral oils, polybutenes, etc.) and also chlorinated hydrocarbons and silicone derivatives. For metal bodies carrying alternating current, such insulation is not suitable at very high voltages. In fact, since the dielectric losses rapidly increase with voltage, the temperature of the electrical structure can rise to excessive levels. This happens, for example, in very high voltage electric cables (750-1000 kV) which lack an adequate cooling system.

In such electric cables, the insulation (paper impregnated with oil) must generally have a dielectric loss factor (tan ~) lower than 1 . 10-3, an A.C. breakdown strength of at least 60-70 kV/mm, and a D.C. breakdown strength and an impulse breakdown strength of at least 150~160 kV/mm.

Cellulose paper has a rather high value of dielectric loss factor (tan ~) and even with the purest cellulose papers it is not possible to obtain, for the insulation formed by such papers impregnated with oil, a value of tan a lower than 1.5 . 10-3. Therefore in order to obtain the required dielectric properties of the insulation it is necessary to modify the combination of paper and impregnating oil.

A solution is to use synthetic paper (i.e. sheet material comprising synthetic plastics material) in place of cellulose paper. It is known that synthetic plastics materials generally used as insulations (polyethylene, polypropylene, etc.) have dielectric loss factor usually as most 1/1 0 that of cellulose paper. Such plastics have also, theoretically, a very high dielectric strength when in small thicknesses.

Extruded insulation of cables is also made from said plastics materials and such cables may be used for carrying high voltage levels (1 50-230 kV), but not for voltages of the order of 750-1000 kV.

This is because defects unavoidably form in the plastics material both during construction and use of the cable.

However, the same plastics materials can be modified and formed into sheets (synthetic paper) to be wound, generally in the form of tapes cut from the sheets, around the metal body of an electrical structure, the insulation thus formed being impregnated with insulating oil. Such synthetic paper may comprise a calendered assemblage of short fibers or solid layer (or film).

There are several known types of synthetic plastics material used for synthetic paper, for example high density and high crystallinity polyethylene, stretched and biaxially oriented polyethylene; polyethylene fibers compacted by mechanical action, thermal action, etc, and isotactic polypropylene.

The insulation then obtained by impregnating with conventional oils has, generally, great improvements in its dielectric properties with respect to equivalent insulation comprising cellulose paper.

However, insulation comprising impregnated synthetic paper is liable to swelling caused by the absorption of impregnating oil into intermolecular interstices of the plastics material. The swelling increases with increases in the applied voltage and resultant increases in temperature.

The swelling of the synthetic paper, and consequently of the overall insulation, can cause considerable damage to the electrical structure itself. Thus, the swelling generates internal mechanical stresses which can cause changes in shape of the insulation and therefore cause irreversible deformations of the electrical structure itself.

An attempt to overcome this is to deliberately cause swelling of the synthetic paper by impregnating it with oil before winding it around the metal body to be insulated: however, this procedure is technically complicated and often the preliminary treatment of the synthetic paper leads to undesirable decays in the mechanical properties of the synthetic paper itself.

Instead, considerable improvements can be achieved by utilizing, for the synthetic paper, composite sheet material of very small thickness. Such composite sheet material may comprise a lamination of a plastics sheet (polypropylene, fluorinated ethylene-propylene copolymer, etc.) and a thin layer of cellulose paper or of a plastics sheet inserted between two thin layers of cellulose paper (alone or reinforced with synthetic materials).

The use of such composite sheet materials or synthetic papers in combinations with conventional impregnating oils improves substantially the behaviour to swelling. But there are still other problems related to the dielectric properties and to the formation of the composite synthetic papers. Thus, the presence of one or two thin layers of cellulose paper prevents full advantage being taken of the dielectric properties of the plastics material. Also, it is found in practice that it is not technically easy to combine perfectly a plurality of layers of different materials to form a thin sheet material and may not be economic.

We have now found, however, that good insulation for high voltage metal conducting can be obtained using synthetic plastics materials impregnated with certain insulating oils.

In accordance with the present invention, there is provided an electrical structure, comprising at least one metal body to which in use a high voltage is applied, an insulation provided around said metal body and comprising synthetic paper impregnated with insulating oil, said insulating oil having a dielectric loss factor of 0.5 . 10-3 or less, and being constituted by an organic compound containing at least carbon atoms and fluorine atoms.

Moreover, said insulating oil can contain also oxygen atoms.

An embodiment of the present invention will now be described, by way of example only, with reference to the accompanying drawing, the single figure of which is a view of an end portion of an oilfilled electric cable.

Referring to the drawing, there is shown an oil-filled electric cable C comprising a conductor 10 formed with a central, longitudinal duct 1 an insulation 12 comprising tapes of synthetic paper wound around the conductor 10, and a tubular metal sheath 13 disposed around the insulation 12. The synthetic paper comprises synthetic plastics material and may comprise a single layer of such material, a calendered assemblage of short fibers of such material, or a composite laminate of at least one layer of such material and at least one layer of cellulose paper.

An insulating oil fills the longtidunal duct 1 1 and impregnates the insulation 12. This oil has a dielectric loss factor (tan 8) of 0.5 . 10-3 or lower, and is constituted by an organic compound containing at least carbon atoms and fluorine atoms. Said oil can contain also oxygen atoms.

The suitability or otherwise of an insulating oil for use in the present invention can be determined by routine trial and experiment.

The oils may consist only of carbon and fluorine, examples of preferred such oils being perfluorodecalin, trifluoromethylperfluorodecalin, perfluorotoluene and perfluoro 1,3dimethylcicloesane.

The formula of trifluoromethylperfluorodecalin is:

The oils may also contain oxygen atoms for example, a preferred such oil being a perfluorinated polyether having a molecular weight of from 200 to 5000. A preferred such polyether has the formula

This is essentially a polyether of hexafluoropropylene.

Table I hereinafter sets out the viscosity, dielectric loss and boiling point for each of the preferred insulating oils.

Table II hereinafter shows the results of testing a layer of high density and high crystallinity polyethylene for swelling, and of testing a film of isotactic polypropylene synthetic paper for swelling, by immersion in different oils at different temperatures for 72 hours uninterruptedly, a measure then being made of the volume variation per cent arising in consequence of the treatment.

From the values indicated in Table II, it is clear that the swelling of both the synthetic papers at a given temperature, is much lower using oils in accordance with the present invention (Examples 1 and 4 with trifluoromethylperfluorodecalin and Examples 2 and 5 with a perfluorinated polyether having a molecular weight between 200 and 5000) then when using a known oil (Examples 3 and 6 with decyibenzene).

In practice the swelling which is noted using oils in accordance with the present invention is not excessive for the usual conditions of use of the electrical structure, and up to temperatures approaching the melting temperatures of the synthetic plastics material of the synthetic paper of the insulation.

The formula of perfluoro 1-3 dimethycicoloesane is:

TABLE I Viscosity in Dielectric Boiling Oil cst at 200C Ioss(xO.001) point (OC) Trifluoromethyl perfluorodecalin 3 0.1 160 Perfluorinated polyether 10 0.2 80~100 (ave. m. wt. 1000) Perfluorotoluene 3 0.3 103 Perfluoro 1,3 dimethylcicloesane 1.3 0.3 102 Perfluorodecalin 5 0.1 140 All the above oils are inert under the conditions of use in the present invention. Apart from the polyether, they all consist of carbon and fluorine atoms only.

TABLE II Swelling tests of a layer of high density and high crystallinity polyethylene (Examples 1 to 3) and a film of isotactic polypropylene synthetic paper (Examples 4 to 6) when impregnated with different oils and at different temperatures.

Example Volumetrical Variation % after 72 hours Number Oil 1000C 1100C 1200C 1300C Trifluoromethylperfluorodecalin +3.8 +4.5 +5.2 +35 2 Perfluorinated polyether (average molecular weight = 1000) +0.5 + 1.8 +3.7 +28 beginning complete 3 Decylbenzene +5.5 +21.0 of dissolution dissolution 4 Trifluoromethylperfluorodecalin +3.2 +4.0 +5.2 +19 5 Perfluorinated polyether (average molecular weight 1000) +0.4 +1.6 +3.4 +25 beginning of 6 Decylbenzene +6 +15 +20 dissolution

Claims (7)

1. An electrical structure, comprising at least one metal body to which in use a high voltage is applied, an insulation provided around said metal body and comprising synthetic paper impregnated with insulating oil, said insulating oil having a dielectric loss factor of 0.5 10-3 or less, and being constituted by an organic compound containing at least carbon atoms and fluorine atoms.

2. An electrical structure according to claim 1, wherein the said organic compound consists only of carbon and fluorine atoms.

3. An electrical structure according to claim 2, wherein the said organic compound is trifluoromethylperfluorodecalin, perfluorodecalin, perfluorotoluene or perfluoro, 1 3-di methylcicloesane.

4. An electrical structure according to claim 1 wherein the said organic ocmpound also contains oxygen atoms.

5. An electrical structure according to claim 4, wherein the said organic compound is a perfluorinated polyether having a molecular weight from 200 to 5000.

6. An oil-filled electric cable comprising an electrical structure as claimed in any preceding claim.

7. An oil-filled electric cable substantially as herein described with reference to the accompanying drawings.