GB2196781A - D.C. electric cables having impregnated layered insulation - Google Patents

Abstract

A direct current electric cable comprising at least one conductor, a layered composition-impregnated insulation 3, a metallic sheath 5 and a reinforcing structure 6 formed by at least one tape winding surrounding the sheath, applied under tension so as to reduce the diameter of the sheath and of a tape or tapes exhibiting a closed mechanical deformation hysteresis loop in which the difference between the maximum and the minimum percentage deformation is from 0.3% to 0.5%. The sheath may be of lead and the tape of steel, an aromatic polyamide, fibre glass or polyester. The tape may have undulations or depressions or slots. Semiconductive layer 2,4 may be provided. Since the tape is applied under tension, microcavities are completely eliminated from the impregnated insulation. <IMAGE>

Description

SPECIFICATION D.C. electric cables having impregnated layered insulation This invention is concerned with direct current electric cables which comprise compositionimpregnated layered insulation and in which pressurized gas is not present. More specifically, the invention is concerned with direct current electric cables comprising at least one conductor, a layered composition-impregnated insulation layer around the conductor, a metallic sheath outside the layered insulation, and a reinforcing structure formed by at least one tape winding surrounding the sheath.

Known d.c. cables of this kind are liable to perforation, in use, owing to the presence of microcavities containing low pressure gas in the layered insulation. These micro-cavities are formed during the construction of the cable and during use of the cable, they change their position and their dimensions.

Such micro-cavities are inevitably formed during cable manufacture since it is, in practice, impossible to effect complete impregnation of the layered insulation with the impregnating composition. The maximum impregnation which can be obtained in known cables of this kind is 99%.

The micro-cavities change their positions and their dimensions during use of the cable for the following reason. Use of a cable causes it to be cyclically heated (when current is passed) and cooled (when current is switched off). In the heating cycles, the viscosity of the impregnant composition is reduced and it undergoes a greater thermal expansion than the other components of the cable. The relative increase in the volume of the impregnant composition causes it to fill the microcavities that were left during cable manufacture. However, in the cooling cycle, the volume of the impregnant composition is reduced to its ambient temperature volume and the micro-cavities are reformed, though not necessarily in the same places or with the same dimensions as previously.

It is known that micro-cavities in composition-impregnated d.c. cables are dangerous since they can act as sites for eiectrical discharges which can, in turn, cause perforations in the cable.

This risk is greatest during thermal cooling cycles when micro-cavities are being reformed in the layered insulation.

It has been proposed to reduce or prevent the risk of perforations in cables of this kind by introducing a high pressure gas into the cable, that is gas at a pressure of at least 14 bar.

Whilst this proposal is currently considered the best solution to the perforation problem, it is subject to considerable disadvantages. Thus, the manufacture of the cable and, in particular, the plant required for the purpose, is made more complex by the need for pressurized gas containers and connections between such containers and the cable in order to introduce the pressurized gas into the cable at the appropriate point in its production. Further, the introduction of pressurized gas severely restricts the maximum length of cable that can be made. Thus, known cables with layered insulation which is fully impregnated with composition (which are known to those skilled in the art as "mass impregnated cables") can only be produced in lengths of up to 5 km when pressurized gas is present therein in order to keep the loss of the gas as it moves aiong the cable to an acceptable figure.Such limitations in the length of the cable represent a very serious drawback, for example in that it effectively prevents such cables being used as submarine cables where very long cable lengths are usually required.

The object of the invention is to provide a d.c. cable of the kind described above in which the perforation risk is substantially reduced or avoided and which does not require the use of pressurized gas in the cable so that the cable can be produced in long lengths.

We have now found that the perforation risk in such cables can be reduced or avoided by winding on the outer reinforcing structure under such a tension that the diameter of the sheath is reduced and using a tape to form this structure which exhibits a closed mechanical deformation hysteresis loop in which the difference between the maximum and minimum percentage deformations is between specified limits.

According to the present invention, there is provided a direct current electric cable comprising at least one conductor, a layered composition-impregnated insulation around the conductor, a metallic sheath outside the layered insulation, and a reinforcing structure formed by at least one tape winding surrounding the sheath, in which the reinforcing structure is subjected to tension such as to reduce the diameter of the sheath and is formed of a tape or tapes exhibiting a closed mechanical deformation hysteresis loop in which the difference between the maximum and the minimum percentage deformation is from 0.3% to 0.5%.

In order that the invention may be more fully understood, a preferred embodiment of cable will now be described, by way of example, with reference to the accompanying drawing, in which: Figure 1 is a perspective view of a length of direct current electric cable with parts removed to better show the structure thereof, and Figure 2 is a curve of a mechanical deformation hysteresis loop obtained by plotting applied force a against percentage deformation e.

Figure 1 shows a cable having a diameter of from 50 to 80 mm. The cable comprises a conductor 1 having a cross-sectional area of from 400 to 1200 mm2 and made up of a plurality of laid up wires formed, for example, of copper and surrounded by a semi-conductive screen 2 comprising at least one winding of a semi-conductive tape. Around the semi-conductive tape layer 2, there is a layered insulation layer 3 which is impregnated with an insulating composition.

The composition may be any of those which are known to be suitable for impregnating cables of this kind, and preferably has a viscosity, at room temperature, 20"C, of from 1000 to 50000 cst.

The layered insulation 3 is surrounded, in turn, by a semi-conductive screen 4 which may have the same structure as the semi-conductive screen 2. The screen is surrounded by a metallic sheath 5 made, for example, of lead or a lead alloy.

The sheath 5 is surrounded by a reinforcing structure 6. In the embodiment illustrated, the structure 6 takes the form of a single helically wound tape 7, but this is not the only form of reinforcing structure that can be used. Thus, the structure may comprise two or more tape windings which are placed adjacent one another or with one on top of another; in the latter case successive tapes can have opposed winding directions.

The characteristics of the reinforcing structure of the cable according to the invention are as follows: Firstly, the tape(s) of which the structure is formed are tensioned so that the diameter of the sheath is reduced and the outer layers of the layered insulation are compressed, thus subjecting the impregnant composition to hydrostatic pressure. This leads to total impregnation of the cable with the complete elimination of micro-cavities without causing any appreciable movement of the composition in the longitudinal direction of the cable.

Secondly, the tape(s) of which the structure is formed exhibit mechanical deformation hysteresis loops similar to the one shown in Figure 2 in which the difference between the maximum percentage deformation a and the minimum percentage deformation b is from 0.3% to 0.5%.

This range of values corresponds to the percentage variation in the outer circumference of the sheath caused by the thermal expansion of the impregnant composition between the minimum and the maximum temperatures to which a d.c. cable of this kind may be subjected.

As a consequence of the second characteristic, the complete impregnation of the cable is maintained during the use of the cable and the formation of microcavities during the thermal cycling of the cable is essentially prevented.

The mechanical deformation hysteresis loops referred to herein are obtained, as will be known to those skilled in the art, by plotting a tractional force a by means of an extensometer, as described in the standard test procedure ASTM E/83, such as an Instron 2630036 extensometer, which is applied to a sample of the tape being tested, against the deformation (extension) e obtained. A progressively increasing force is applied and the force is then progressively reduced. A closed hysteresis loop of the kind shown in Figure 2 is obtained.

It is a requirement of the tape used to form the reinforcing structure that it should exhibit a closed and substantially stable mechanical deformation hysteresis loop, that is repeated applications of the increasing and then decreasing deformation force will cause the loop to be retraced time after time with the maximum point c and minimum point d of each loop remaining substantially fixed.

Suitable tapes for use in making the reinforcing structure include, for example, nickel- chrome steel tapes, aromatic polyamide tapes, fibre glass tapes, and polyester tapes. Other suitable tapes include carbon steel tapes and tapes of other metals having mechanical characteristics comparable with those of carbon steel; such metal tapes are preferably provided with microundulations which are disposed substantially transverse to the tape or with micro-depressions disposed in honeycomb fashion or with rhombus-shaped slots therein. Such microundulations, micro-depressions or slots confer higher elongations under tension than is shown by the unmodified tape.

Experimental tests carried out on cables according to the invention (which are described below) have shown that such cables are essentially free of perforations risk that can be attributed to micro-cavities, without requiring the production of the cables to be complicated by the need to introduce pressurized gas, without the disadvantage of being able to make only limited lengths of cable, and without the need for permanently supplying the cable with pressurized gas.

Two embodiments, A and B, of cables according to the invention were made.

Cable A The cable comprises a multi-stranded copper conductor, having a diameter of 39mm, surrounded by a semiconductive layer and a layered insulation layer having a thickness of 18mm.

The layered insulation was formed by windings of cellulose tape and it was impregnated with an insulating composition having a viscosity, at 20"C, of 18000 cst and consisting, by weight, of 98% of mineral oil and 2% of polyisobutylene.

The layered insulation was surrounded by a semi-conductive layer which layer was, in turn, surrounded by a lead sheath having a thickness of 3.5 mm. A reinforcing structure surrounded the lead sheath. The structure consisted of two overlapped layers obtained by winding nickelchrome steel tapes having a width of 25 mm and a thickness of 0.2 mm. These tapes exhibited closed mechanical deformation hysteresis loops in which the difference between the maximum and minimum deformations was about 0.45%.

At a temperature of 10CC, the tapes were under a tensile stress of 9.6 kg/mm2 which was equivalent to a compressive force of 9 atmospheres upon the sheath and this served to eliminate any micro-cavities in the cable.

Cable B This cable differed from Cable A only in the nature of the reinforcing structure; all the other parts of the cable were as described for Cable A. In Cable B, the reinforcing structure consisted of four overlapping layers obtained by winding fibre glass tapes having a width of 30 mm and a thickness of 0.20 mm. These tapes exhibited closed mechanical deformation hysteresis loops in which the difference between the maximum and minimum deformations was up to 1%.

At a temperature of 10 C, the tapes were under a tensile stress of 5 kg/mm2 which, for these tapes, was again equivalent to a compressive force of 9 atmospheres upon the sheath.

The experimental tests used were those known as "Loading Cycle and polarity Reversal Tests" proposed by Working Group 21-10, Study Committee No. 21 of CIGRE which have been published in "Electra", Issue No. 72. Following the procedure laid down in these tests, 30 m lengths of each cable under test were subjected to sets of thirty heating and cooling cycles between room temperature and the maximum working temperature, which was 65"C for the cables in question, while maintaining the cable under tension. After each set of thirty cycles, the tension was increased and this was continued until a perforation was observed; the cable tension at which this happened was noted. The temperatures mentioned above were measured at the conductor.

In addition to Cables A and B described above, the following cables, C, D, F and G, which had elements in common with Cables A and B, but which were not in accordance with the invention, were also tested for the purposes of comparison.

Cable C was identical to Cable A except that the tapes of the reinforcing structure were not tensioned sufficiently to compress the sheath. This cable was taken as the control with which the other cables were compared; Cable D was identical to Cable C, except that the impregnated layered insulation was subjected to nitrogen at a pressure of 14 atmospheres (this cable thus corresponds to the prior art pressurized gas cables described above); Cable F was identical to Cable B, except that the reinforcing structure was formed of four overlapping layers of cotton tapes. These tapes did not exhibit a closed mechanical deformation hysteresis loop.The cotton tapes were applied to the sheath at a tension of 5 kg/mm2 and served to subject the sheath to a compressive force of 9 atmospheres; Cable G was identical to Cable B, except that the reinforcing structure was formed of four overlapping layers of copper tape. This tape exhibits a closed mechanical deformation hysteresis loop only when the difference between the maximum and minimum deformations is not greater than 0.12%. The copper tapes were applied over the sheath at a tension of 5 kg/mm2 and served to subject the sheath to a compressive force of 9 atmospheres.

The results obtained are summarised in the following Table.

Increase in the tension at Cable which perforation occurred with respect to Cable C Cable A 80% Cable B 8056 Cable D 48% Cable F 15% Cable G 20% It will be seen from these results that it is necessary for the reinforcing structure of the cable to have both the characteristics specified, the first to ensure that complete impregnation with elimination of micro-cavities is obtained when the cable is made and the second to ensure that this condition is maintained during use of the cable.

It will also be noted that cables in accordance with the invention show an increase of 66% in the tension at which perforation occurred as compared with the pressurized gas cable, Cable D, the latter generally being held at present to be the best solution to the micro-cavity induced perforation problem.

Claims (7)

1. A direct current electric cable comprising at least one conductor, a layered compositionimpregnated insulation around the conductor, a metallic sheath outside the layered insulation, and a reinforcing structure formed by at least one tape winding surrounding the sheath, in which the reinforcing structure is subjected to tension such as to reduce the diameter of the sheath and is formed of a tape or tapes exhibiting a closed mechanical deformation hysteresis loop in which the difference between the maximum and the minimum percentage deformation is from 0.3% to 0.5%.

2. A cable according to claim 1, in which the reinforcing structure tape is formed of nickelchrome steel, aromatic polyamide, fibre glass, or polyester.

3. A cable according to claim 1, in which the reinforcing structure is formed of steel tape having microundulations which are disposed substantially transverse to the tape.

4. A cable according to claim 1, in which the reinforcing structure is formed of steel tape provided with micro-depressions disposed in honeycomb fashion.

5. A cable according to claim 1, in which the reinforcing structure is formed of steel tape having rhombus-shaped slots therein.

6. A cable according to any of claims 1 to 5, in which the composition impregnating the layered insulation has a viscosity, at room temperature, of from 1000 to 50000 cst.

7. A direct current electric cable substantially as herein described with reference to Cable A or B.