High Voltage Cable

Abstract

In a fluid-filled cable for service at 200 kV and upwards a low-viscosity naphthene-free mineral oil is used to impregante a paper/polypropylene/paper laminate. The polypropylene is selected for low solubility in the oil, and the paper has a density of 0.85Mg/m.sup.3 or less, an impermeability of at least 10,000 Gurley seconds, and a thickness, at least in the inner high-stress zone of the dielectric, of 50 micrometers or less, preferably 25 micrometers.

Parent Case Text

This application is a continuation-in-part of our application Ser. No. 82,603 filed Oct. 21, 1970 now abandoned.

Description

Our invention relates to electric cables for service at voltages of 200kV and above. In present commercial practice, cables for service at the highest voltages invariably have dielectrics formed of lapped insulating-paper tapes impregnated with a mobile hydrocarbon oil (ordinarily a selected and refined petroleum oil) maintained under pressure. This type of dielectric has a dissipation factor of approximately 0.25 - 0.5 percent which at voltages up to around 150 kV results in the loss of only a few percent of the MVA rating of a cable. Losses increase rapidly, however, with increased voltage, and it has been recognised that at a voltage of the order of 500kV - 1MV it will become grossly uneconomic if not physically impossible to transmit useful amounts of power by such cables.

Solutions to this difficulty have been sought in the substitution of synthetic polymeric materials having very low dissipation factors for some or all of the paper. This leads, however, to a further difficulty, in that the polymeric materials with the best intrinsic electrical properties tend to swell when exposed to insulating oils, the effect often being so great that the tapes fail mechanically under the resulting pressures. Even when total mechanical failure has been avoided, cables made according to prior-art proposals have developed radial pressure sufficient to prevent the tapes sliding over one another so that it has been impossible to bend the cable without causing damage that would lead to rapid electrical failure.

We have discovered that by selecting certain combinations of materials it is possible to make a cable suitable for service at a voltage of at least 200 kV which will not fail in the manner outlined and which is sufficiently flexible to be wound on a drum and subsequently (even a year or more after manufacture and impregnation) to be unwound and installed.

The invention uses as the tape from which the dielectric is built up a laminate comprising a centre layer of polypropylene and two outer layers of cellulosic paper. The paper layers fulfil three functions:

They make it easy to establish and maintain impregnation of the tapes in situ on the cable; they improve the handling properties of the tapes; and they influence the swelling of the polypropylene layer when impregnated. The third function is very important, and we have found that it is essential to use one specific type of laminate if adequate control is to be obtained, namely an extrusion-bonded laminate formed by extruding a web of polypropylene from a slot die at an appropriate elevated temperature, typically about 300.degree. C, and before it cools trapping it between and bonding it by pressure to two paper webs which are at a much lower temperature (normally ambient temperature). We sometimes refer to this type of laminate as "prestressed" laminate because in the normal working temperature range (and in the absence of impregnant) the paper layers hold the polypropylene layer in an elastically extended condition.

Not all grades of polypropylene are satisfactory: it is important to select a grade that has a very low solubility in the impregnant to be used (which is discussed below). We have obtained good results using a grade of polypropylene available in Great Britain from Imperial Chemical Industries Ltd and designated as grade PXC3391.

By "cellulosic paper" is meant paper consisting substantially entirely of cellulose fibres. The paper layers of the laminate should be much thinner than normal cable papers; in no case should the thickness of a paper layer exceed 80 micrometers and in the inner high-stress zone of the dielectric where the electrical stress is greatest the paper layers should have a thickness less than 50 micrometers, preferably about 25 micrometers. The paper should also be of low density, specifically with a density less than 0.85Mg/m.sup.3 and preferably less than 0.75Mg/m.sup.3. Nevertheless the paper should have an impermeability at least as great as that of normal cable papers, that is 10,000 Gurley seconds or higher.

The preferred paper is an uncalendered electrical grade paper of intermediate fibre length of a density 0.7 g cm.sup.-.sup.3 and a Gurley impermeability greater than 10,000 seconds. Such a paper is Kraft coil winding paper manufactured generally in accordance with BS 698:1956, Class 1A to the very high standard of chemical purity normally associated with capacitor tissue.

The paper layers can be loaded with an active material of the kind described in the Complete Specification of United Kingdom Pat. Specification 1,185,474, that is aluminium oxide or another active metal oxide, hydrated metal oxide, hydroxide, carbonate or basic carbonate that has sorptive powers comparable with that of aluminium oxide, in order to minimise the deterioration in electrical properties due to contamination of the impregnant by residues from the plastics material.

The function of the paper in controlling swelling is of greater importance in outer parts of the dielectric, since inner parts will be restrained also by the overlying tapes of the outer parts. On the other hand the presence of a high proportion of plastics material is most advantageous in the part of the dielectric adjacent to the conductor where the electrical stress is greatest. In most circumstances it will therefore be advantageous to form the dielectric from a number of different composite tapes such that the proportion of the dielectric that is constituted by plastics material decreases with increasing distance from the cable conductor. The number of steps desirable will increase with the dielectric wall thickness and therefore with the working voltage of the cable. For example, using polypropylene as the plastics material and a normal low viscosity cable oil as the impregnant, where the dielectric wall thickness is 10 mm there will suitably be two steps and where the dielectric wall thickness is 25 mm three steps may be desirable.

Surprisingly it has been found that the dielectric loss angle of the complete dielectric varies with the thickness of the individual tapes even though the proportion of plastics material remains constant. Thus in the case of a dielectric made up of tapes each comprising a polypropylene film having bonded to each of its faces a paper of half its thickness and impregnated with a conventional oil-filled cable oil (having a viscosity in the range 12.5 - 15 centistokes at 20.degree. C) the power factor at 85.degree. C averages 0.0007 if the total thickness of each tape is 100 .mu.m but only 0.0005 of it is 160 .mu.m. It may therefore be desirable to use the thickest composite tapes that mechanical and other electrical considerations permit.

Although it will usually be preferable at least at the lower voltages for the whole dielectric to be built up from the composite tapes specified, it may be advantageous for part only of the dielectric to be formed of such tapes, the remainder in such cases preferably being formed of paper tapes. Thus in one example, the dielectric may throughout its length comprise an inner part of the composite tape and an outer part of paper; and in another example the composite tape may be utilised only in restoring the dielectric at joints and terminations. It will usually be preferable to use the composite tape in joints or terminations whenever all or part of the original dielectric is of the composite tape, but the optimal thicknesses of the plastics and paper layers of the composite tape used in the joints and terminations may differ from the optimal thicknesses of the corresponding layers in the cable, owing to the different stress distribution.

In combination with the laminate described, our invention requires the use as impregnant of a selected and refined mineral oil that is substantially free of naphthenes.

Contrary to expectation it has been found that normal amounts of aromatics cause only a small increase in swelling compared with pure parafinnic oils but that any appreciable naphthenic content produces unacceptable swelling. Paraffinic oils can be used, but mixed paraffinic/aromatic mineral oils have better low-temperature properties. The viscosity of the oil should be less than 57 centistokes at 20.degree. C and less than 11 centistokes at 60.degree. C. Preferably the viscosity is less than 25 centistokes at 25.degree. C.

As a further refinement the laminate is preferably preswollen with oil before lapping to form the cable dielectric, in accordance with a proposed application of the second-named applicant corresponding to British applications nos. 8379 and 8380/71.

In accordance with normal practice, the dielectric will normally be bounded at its inner and outer surfaces by a conductor screen and a dielectric screen respectively. These are preferably formed from single or multiple layers of laminated tape similar to that used to form the whole or part of the dielectric suitably metallised and/or loaded with carbon or other conductive material. Preferably all three layers of the laminate are loaded with conductive material but in some circumstances, for example for the outer layer of a conductor screen or the inner layer of a dielectric screen, a triple laminate with one paper layer not loaded may be used, that is the paper layers contiguous with the dielectric. The paper not loaded with conductive material is preferably loaded with aluminium oxide or other active material of the kind referred to in an application of the first-named Applicant divided from Ser. No. 20670 filed Mar. 18, 1970.

If the central load-carrying conductor of the cable is of stranded construction, there is a tendency for the conductor screen to be forced down into the interstices between the wires by the pressure induced by swelling. To avoid this possibility the conductor is preferably lapped with metal tape before the conductor screen is applied. Phosphor bronze tapes with a thickness of around 0.1 mm (5 mil) have been found satisfactory for this purpose.

The invention will be further illustrated by the following examples of single core oil-filled cables, which are illustrated by the accompanying drawings in which

FIG. 1 is a cut-away diagram,

FIG. 2 is a transverse cross-section of the cable, and

FIG. 3 is an enlarged cross-section of a small portion of the cable dielectric.

In the drawings 1 is a steel tape helix defining a central oil duct within the metallic conductor 2. Over the conductor is applied a conductor screen 3, dielectric 4 more fully described below, and dielectric screen 5, the screens 3 and 5 being formed of metallised tapes or tapes loaded with conductive material as already discussed. The cable is completed by a lead sheath 6, bronze tape or other pressure-resisting reinforcement 7 and a plastics oversheath 8.

In the first example, which is a 132 kV cable, the radial thickness of the dielectric 4 is 5.5 mm, corresponding to a design stress of 16 MV/m. The whole of the dielectric is made up of composite tapes comprising 50 .mu.m of polypropylene sandwiched between two paper layers each 25 .mu.m thick.

In the second example, which is a 220 kV cable, the dielectric comprises three concentric parts each having a radial thickness of approximately 3.5 mm, each part being formed from composite tapes comprising a layer of polypropylene sandwiched between two paper layers. In the tapes of the inner part, the polypropylene layer is 80 .mu.m thick and each paper layer 10 .mu.m thick; in the tapes of the intermediate part, the polypropylene layer is 50 .mu.m thick and each paper layer 25 .mu.m thick; and in the tapes of the outer part, the polypropylene layer is only 20 .mu.m thick and each paper layer is 40 .mu.m thick.

In the third example, which is a 400 kV cable, the dielectric comprises four concentric parts each having a radial thickness of approximately 6.25 mm. The three inner parts are each formed of composite tape in which central polypropylene layers have thicknesses of 60 , 40 and 20 .mu.m respectively beginning with the innermost layer, and each of the paper layers has thicknesses of 20, 40 and 75 .mu.m respectively. The outer part is formed from tapes of ordinary cable insulating paper 250 .mu.m thick.

In each of the three examples, the paper used in the composite tape is the preferred uncalendered electrical grade of paper referred to above and the composite is prestressed by making it in the manner described above. The impregnant in each case is a naphthene-free refined mineral oil having a viscosity in the range 12.5 - 15 centistokes at 20.degree. C.

Claims

What we claim as our invention is:

1. A power cable for a working voltage of at least 200 kV comprising a central load-carrying conductor, a dielectric wall built up from lapped tapes impregnated with insulating fluid and an overall fluid-tight sheath, distinguished by the said dielectric having the following characteristics in combination, namely that

a. said fluid is a refined mineral oil substantially free of naphthenic constituents and having a viscosity of less than 57 centistokes at 20.degree. C and less than 11 centistokes at 60.degree. C and

b. in at least the radially inner part of said dielectric wall said tapes are formed of an extrusion-bonded laminate comprising a centre layer of a polypropylene that is substantially insoluble in said fluid and two outer layers of cellulosic paper, which paper layers have

i. a thickness not greater than 50 micrometers

ii. a density less than 0.85Mg/m.sup.3, and

iii. a Gurley impermeability of at least 10,000 seconds whereby the dielectric wall withstands forces due to swelling of said polypropylene and the cable is sufficiently flexible to be wound on a drum and subsequently unwound for laying.

2. A cable as claimed in claim 1 wherein said fluid is a mixed paraffinic/aromatic mineral oil.

3. A cable as claimed in claim 2 wherein said oil has a viscosity less than 25 centistokes at 20.degree. C.

4. A cable as claimed in claim 3 wherein said viscosity at 20.degree. C is in the range 12.5 - 15 centistokes.

5. A cable as claimed in claim 1 wherein said paper layers have a thickness of 25 micrometers.

6. A cable as claimed in claim 1 wherein a radially outer part of said dielectric wall comprises tapes formed of an extrusion-bonded laminate comprising a centre layer of a polypropylene that is substantially insoluble in said fluid and two outer layers of cellulosic paper which paper layers have

i. a thickness greater than 50 micrometers but not greater than 80 micrometers

ii. a density less than 0.85Mg/m.sup.3, and

iii. a Gurley impermeability of at least 10,000 seconds

7. A cable as claimed in claim 6 wherein a further, radially outermost, part of the dielectric comprises paper tapes.

8. A cable as claimed in claim 1 wherein said paper has a density less than 0.75Mg/m.sup.3.

9. A cable as claimed in claim 1 wherein said paper is an uncalendered electrical grade paper of intermediate fibre length of density 0.7 Mg/m.sup.3.

10. A cable as claimed in claim 1 comprising a conductor screen defining the radially inner surface of said dielectric well and formed from a conductive extrusion-bonded laminate comprising a central layer of said polypropylene and two outer layers of cellulosic paper.

11. A cable as claimed in claim 10 in which said central conductor is a stranded conductor and wherein a lapped metal tape is interposed between said central conductor and said conductor screen.

12. A cable as claimed in claim 1 comprising a dielectric screen defining the radially outer surface of said dielectric wall and formed from a conductive extrusion-bonded laminate comprising a central layer of said polypropylene and two outer layers of cellulosic paper.