WO1998006109A1

WO1998006109A1 - Electrical and optical cable

Info

Publication number: WO1998006109A1
Application number: PCT/GB1997/002091
Authority: WO
Inventors: David Alan Seddon
Original assignee: Bicc Public Limited Company
Priority date: 1996-08-03
Filing date: 1997-08-01
Publication date: 1998-02-12
Also published as: GB9616400D0; AU3778197A

Abstract

A combined overhead electrical power transmission and optical transmission cable, for example OPGW, comprises: (i) a metal tube (1) having a bore extending along its length; (ii) a number of metal wires (2) helically stranded about the metal tube; and (iii) a number of optical elements (6), for example plastics tubes in which optical fibres are loosely housed, which are located in the bore of the tube. The optical elements (6) are stranded with a direction of lay that alternates along their length (SZ stranded) and can move in the circumferential direction within the metal tube so that alternate lay-reversal points of the optical elements can rotate at least to a limited extent in opposite directions when the cable is stretched. Because the elements can move in the circumferential direction within the tube, torsional movement of the lay-reversal points of the elements when the cable is subjected to tension will contribute an additional component to the strain margin of the cable.

Description

ELECTRICALAND OPTICAL CABLE

This invention relates to flexible electrical conductors that include optical elements for data transmission. The invention is especially, but not exclusively, concerned with combined overhead electrical and optical conductors of the type that include a plurality of bare elongate metal elements and which are adapted to be freely supported in long lengths from spaced supports, for example from towers, poles or other structures. The electrical conductor may, for example, be a phase conductor in the transmission system, or it may be a ground wire, normally referred to as OPGW.

The invention is particularly, although not solely, applicable to so-called 'loose tube' conductors, that is to say, conductors in which one or more optical fibres is/are loosely housed within the bore of a tube in the cable so that the fibres are able to move, at least to a limited extent in the tube in an axial and/or radial direction. One general design of such a conductor that is widely employed comprises a hollow metal tube having a bore extending along its length, which tube is surrounded by one or more layers of metal strands helically wound around the tube. The bore of the tube contains a number of optical elements, for example optical fibres, bundles of optical fibres or optical fibre ribbons, that are loosely housed in plastics tubes, the optical elements being helically laid up and enclosed in a plastics sheath. The purpose of the plastics sheath is to limit any temperature rise that the optical fibres experience if the conductor is subject to an overcurrent, for example caused during fault conditions. Examples of this design of conductor are given in British patent specification No. 1 ,598,438.

One issue that is always of concern in the design of any loose-tube conductor is the strain margin of the optical fibres, that is to say, the length by which the conductor as a whole may be extended before the optical fibres are subjected to any tensile strain. Such overhead conductors will be elongated to a degree both during and after installation, for example due to the weight of the conductor in the catenary, due to ice and wind loading and due to thermal expansion of the conductor at the operating temperature. It is not acceptable, however, for the optical fibres to be subjected to any strain after installation of the conductor since any strain on the fibres will cause unacceptable attenuation of the optical signals. Typically, strain margins of 0.6% are required in order to ensure no increase in attenuation due to strain on the fibres. The major part of the strain margin (e.g. about 0.5%) is normally provided by lateral movement of the optical fibres within the plastics tubes toward the axis of the metal tube when the conductor is stretched which arises from for example the helical stranding of the plastics tubes, and the minor part of the strain margin (about 0.05%) is provided by excess length of the optical fibres within the plastics tubes. A current trend in the design of combined optical and electrical transmission systems is to increase the fibre count in the conductors. For example, in the 1980s, a typical ground wire would contain between four and twelve optical fibres, while today conductors are being designed for more than 48 fibres. This increase in fibre count inevitably reduces the ability of the fibres to move laterally within the tubes, and therefore reduces the strain margin of the conductor, unless the overall diameter of the conductor is increased.

One form of stranding that has been employed widely in recent years for electrical and optical cables is one in which the elements are wound around a central member with a lay whose direction of lay alternates along the length of the cable between a right-hand lay and a left-hand lay (often referred to as SZ stranding). Such a form of stranding has the advantage that the speed of manufacture of the cable can be increased, and the cost of manufacture can be reduced, partly because the drums on which the optical elements are wound do not have to be arranged on a stranding carriage that rotates around the core of the cable but can be fixed. This also means that it is practical to use larger drums than can be used in helically wound cables, thereby enabling longer lengths of cable to be manufactured.

However, the use of SZ stranding has the significant disadvantage that the mean lay length of the optical elements in the cable is increased as compared with helical stranding (or that part of the SZ stranded cable in which the optical elements have a helical lay), due to the transition regions in the cable in which the lay of the optical elements changes from right-handed to left-handed and vice versa. As will be appreciated, in these regions, the lay length of the elements increases until, at the lay- reversal point when the elements are oriented parallel with the cable axis, the instantaneous lay length is infinite, whereupon the lay length reduces as the elements are wound around the core in the opposite sense. This increase in average lay length of the optical elements due to changeover in the direction of lay reduces the strain margin of the cable significantly, and can easily reduce the strain margin to a level below the anticipated strain that the cable will see in use, thereby causing the optical fibres in the cable to be put under strain. The strain margin could be increased by reducing the minimum lay length (i.e. the lay length of the helically laid part of the stranded elements), but this would cause the bend diameter of the optical fibres in the cable to be unacceptably reduced. Thus, while the minimum lay length will depend on a number of factors in the cable design such as the tube diameter and the number of tubes, it should not be reduced significantly below 60mm since otherwise the signal strength of the traffic in the fibres can be lost due to macrobending of the fibres.

The increase in average lay length of the elements caused by SZ stranding can, of course, be reduced by increasing the number of turns in the stranding between lay- reversal points, so that the relative proportion of the length of the cable in which the stranding is helical is increased and the relative proportion of the length of the cable occupied by the transition region is reduced. However, the use of a large number of turns between lay-reversal points is associated with other problems: First, the length of the manufacturing plant is increased, generally in proportion to the number of turns between lay-reversal points. For example, while the stranding equipment will typically be six metres long for twelve turns, it will be increased to twelve metres for 24 turns. Secondly, and more importantly, stranding of the optical elements requires the application of tension to the elements, for example in order to overcome friction as the elements are wrapped around any accumulator tube that encloses the core, and, whatever equipment is employed, the amount of tension applied to the elements generally increases with the number of turns of the elements between lay-reversal points. This increase in tension can easily cause the plastics tubes which house the optical fibres in the optical elements to be stretched by 0.1 % or more. Although such a degree of stretching is not large, it is sufficient to remove a major part of the excess length of optical fibre in the tubes, and with it, the contribution to the strain margin due to it.

According to the present invention, there is provided a combined overhead electrical power transmission and optical transmission cable, which comprises:

(i) a metal tube having a bore extending along its length;

(ii) a plurality of metal wires helically stranded about the metal tube;

(iii) a plurality of optical elements located in the bore of the tube;

wherein the optical elements are stranded with a direction of lay that alternates along their length and can move in the circumferential direction within the metal tube so that alternate lay-reversal points of the optical elements can rotate at least to a limited extent in opposite directions when the cable is stretched.

The present invention is partly based on our observation that, provided the optical elements are able to move in the circumferential direction with respect to the etal tube, a significant part of the strain experienced by the conductor can be accommodated by untwisting of the stranded optical elements between those points at which the direction of lay of the elements changes (the lay-reversal points), with the result that the adjacent lay-reversal points will undergo a torsional movement with respect to one another. This movement of the optical elements will make a contribution to the strain margin of the cable in addition to any movement of the fibres within the optical elements. The value of this contribution will depend at least to some extent on the precise form of stranding of the optical elements. For example, if there are a large number of turns in the SZ stranding between lay reversal points of the optical element assembly, the relative proportion of the length of the assembly in which the elements are effectively helically wound increases, and the transitional region (in which the lay of the elements deviates from a helical geometry) becomes relatively small, in which case the advantage of the SZ stranding is reduced. Furthermore, the effect of circumferential movement of the elements will not be uniform along the length of the conductor, with the result that either the observed strain margin will be yet further reduced, or that some longitudinal movement of the fibres within the tubes will be required as the conductor is strained. For this reason, the optical elements preferably undergo not more than twelve turns, and more preferably not more than eight turns between lay reversal points. However, as indicated above, the instantaneous lay length of the optical elements at points along the assembly is not constant, and increases to infinity at the lay reversal points (where the optical elements are parallel to the axis of the conductor), with the result that the mean lay length of the elements is increased as the number of turns between lay reversal points is reduced. As will be appreciated, for very long lay lengths, the increase in the strain margin due to the rotation of the lay reversal points is reduced. For this reason, the elements preferably undergo at least one, more preferably at least two, especially at least three, and most especially at least four turns between lay reversal points. In any instance, the optimum number of turns will depend on a number of factors such as the torsional stiffness of any central strength member or core present and the minimum bend radius requirement of the optical fibres. As indicated above, it is important to the function of the cable according to the invention, that the optical elements are able to move, at least to a limited extent, in the circumferential direction within the tube, thereby to allow torsional movement of the elements between the lay-reversal points. Such movement should not, therefore, be prevented by any core about which the optical elements may be stranded. The ability of the optical elements to move circumferentially may be assisted by appropriate design of the core in a number of ways. For example, the core may have a cylindrical geometry so that the elements may slide over the core. The ability of the elements to slide over the core may be improved further by choice of the core material, for example by employing polytetrafluoroethylene (PTFE) or other low coefficient of friction materials for the core. In addition or alternatively, a lubricant may be incorporated to assist sliding of the elements about the core, e.g. a water-blocking gel or grease may be introduced between the core and the elements during stranding of the elements, or the core may be coated with a water-swellable material.

An alternative method by which the core allows the optical elements to move in the circumferential direction is to ensure that the core is relatively weak in torsion so that it will accommodate itself to movement of the elements. Ways in which the core may have a very low torsional modulus and yet have a relatively high tensile modulus include employing a roving of high strength fibres as the core, for example a roving of glass fibres, aramid fibres or the like. Another way would be to encapsulate a thin strength member such as an aramid member or the like within a sheath of relatively weak material so that the sheath will undergo torsion without significant deformation of the strength member. More than one wire or other strength member could be encapsulated in the sheath provided that all the strength members are sufficiently close to the axis of the sheath that they do not prevent torsional movement of the sheath. Thus, for example, the strength member may comprise a roving of high strength fibres as mentioned above. The sheath could be extruded on the strength member or could, for example, be a loose tube that is not bonded at all to the strength member so that torsional movement of the sheath does not require corresponding torsional movement of the strength member. If the central core is has a low torsional modulus so that it will not hinder movement of the optical elements, it need not have a cylindrical geometry. For example, it may have a geometry that includes longitudinally extending recesses for accommodation of the optical elements, such as a core with a generally cruciform cross-section if four optical elements are employed. Such a core could have the advantage that its resilience could assist return of the optical elements to the unstressed SZ configuration when tensile stress on the cable is removed. However, it is quite possible, and in many cases preferable, for the core to be designed so that it has a low torsional modulus and, at the same time, to allow the optical elements to slide thereover. Indeed, many of the design features of the core mentioned above may be combined in the same conductor.

Another option is simply to dispense with the core altogether, so that the stranded assembly consists only of the optical elements.

In addition to this, it is also preferred for those parts of the construction lying around the assembly of optical elements not to hinder circumferential movement of the elements. As stated above, the optical elements are able to move circumferentially within the metal tube to allow the lay-reversal points of the stranded assembly to twist when the cable is stretched. Thus, the movement should be reversible, i.e. the lay- reversal points should be able to twist back when the tensile stress on the cable is released, in order to prevent gradual straining of parts of the elements when the cable is strain cycled. Such movement is distinguished from merely allowing the elements to move with respect to the sheath in order to allow access to intermediate points of the elements by unwinding the elements at the lay-reversal points. Thus, in one preferred construction, the optical elements may be in direct contact with the interior of the metal tube, i.e. the assembly of optical elements is not enclosed in a plastics sheath that has conventionally been employed. It is believed that dispensing with the plastics sheath around the optical elements may not, in fact, lead to an undue increase in temperature experienced by the optical fibres when the conductor is subject to an overcurrenL In addition or alternatively, the ability of the optical elements to move relative to the tube may be increased by filling the space within the metal tube that is not occupied by the optical elements with a lubricant such as a gel or grease. Such gels are employed in conventional conductors in order to protect the optical fibres from moisture ingress. The ability of the elements to move circumferentially within the metal tube may also be increased by limiting the extent to which they are in contact with the interior surface of the metal tube. For example, the assembly of elements will, in most cases, be surrounded by a plurality of binder tapes that are helically wrapped around the assembly (with opposite directions of lay) to hold the assembly together before it is located within the metal tube. If the binder tapes have a significant thickness (e.g. at least 0.2mm) they can reduce the degree to which the optical elements touch the internal surface of the metal tube. The tapes could also be formed with low friction surfaces, e.g. from PTFE. The binder tapes should not hinder movement of the optical elements when the cable or elements are exposed to high temperatures, for example during manufacture or a short circuit (which can achieve temperatures in the region of 180°C). Thus, the binder tape preferably does not contract when subjected to such temperatures since this would cause them to form indentations in the elements. For this reason the preferred binder tapes are formed from aramid fibres.

By means of the above methods, it is possible to increase the strain margin in such a cable by at least 0.02%, and typically by 0.05% or more.

The metal tube in which the assembly of optical elements is located may be formed by any of a number of methods known in the field of combined optical and electrical overhead cables. Thus, the tube may be formed by wrapping a metal strip around the assembly into a substantially "C" shaped section and closing the section by bringing opposed edges of the section together, for example as described in British patent specification No. 1,598,438. If desired, the longitudinal join in the tube may be provided with an adhesive or sealant. Alternatively, if it wished for the tube to be seamless, the longitudinally extending seam may be closed by welding, for example by means of a laser, or the metal tube may be formed by means of the so-called "Conform" process in which metal stock pieces are continuously formed into the tube around the assembly. The minimum number of plastics tubes employed in the cable is three, and four tubes will be typical, although it is possible for up to six tubes to be used. Also, each tube will normally contain from two to sixteen fibres to give a typical fibre count of up to 96 fibres in the cable, although it is anticipated that yet higher fibre counts will be required in future.

As stated above, the conductor is preferably of the loose tube type, in which case the optical elements preferably each comprise a tube in which at least one optical fibre is loosely housed, where the nature of the stranding of the elements will increase the strain margin of the fibres in the conductor. Normally the fibres will be housed in plastics tubes, for example polybutylene terephthalate, ethylene terephthalate or polypropylene, although metal tubes can be employed, for example, the tubes may be formed by forming a relatively thick aluminium or steel transversely corrugated tape into a tube around the fibres having a longitudinal seam. Other forms of metal tube could, in principle, be used. However, the invention is not limited to loose tube designs of conductor, and is applicable also to so-called 'tight-buffered' conductors, in which the optical fibres of the optical element(s) are subject, to some extent, to tensile strains experienced by the conductor as a whole. Thus, the conductor may include a slotted core in which optical elements are tightly located, provided the core is capable of torsional movement. In such a case, the nature of the stranding of the elements will provide a degree of strain relief to the optical fibres. Whichever the form of the cable, the optical fibres may be present in the optical elements in the form of single fibres, fibre bundles, ribbons or stacks of ribbons.

One form of combined optical and electrical conductor according to the present invention will now be described by way of example with reference to the accompanying drawings in which:

Figure 1 is a partially cut-back, schematic isometric view of the conductor; and Figure 2 is a graphical representation showing the instantaneous lay length of an SZ stranded assembly along its length.

Referring to the accompanying drawings, a conductor that is suitable for use as an optical ground wire (OPGW) comprises a central, hollow aluminium tube 1 which is surrounded by a number of aluminium strands 2 for carrying any electrical fault current in the system. An assembly 4 of optical elements 6 is located within the bore of the aluminium tube 1 so as to occupy most of the volume of the bore. The assembly 4 comprises a central strand 8 about which four hollow polybutylene terephthalate tubes 10 of 2.3 mm diameter are stranded in an alternating lay or SZ configuration, and a pair of aramid binding tapes or fibres 12 helically wound around the plastics tubes 10 with opposite lays in order to maintain the assembly together before it is located within the tube 1. Each plastics tube 10 contains eight optical fibres 16 for optical transmission, only some of which are shown for clarity. The interior of the plastics tubes 10 is filled with a water-blocking gel (not shown) as is the interior of the aluminium tube 1.

The tube 1 has been formed from a strip of aluminium that is formed into a "C" shaped section before being wrapped around the assembly of optical elements and closed. When formed in this way, the tube will have a longitudinally extending seam

14 extending along its length. Alternatively, if desired, the tube may be welded, e.g. by means of a laser, in order to form a seamless tube.

The central strand 8 of the assembly of optical elements is formed from a 0.6mm diameter strand of polyethylene terephthalate. Alternative forms of central strand may be employed provided that it's elastic limit exceeds 0.65% in order to allow the assembly 4 of optical elements as a whole to stretch when the conductor is put under tensile load. The plastics tubes 10 are stranded about the central strand with an SZ configuration having an average lay length of 74mm, and in which the lay reverses every 8.5 turns of the optical elements. Figure 2 shows graphically (curve 21) the way in which the instantaneous lay length varies over a region of the cable from a point (point A) mid way between lay reversal points, and one of the lay reversal points (point B). Also shown (curve 22) is a side elevation of one optical element over this range. The instantaneous lay length of the optical element is 57mm in the helical region mid way between the lay-reversal points and increases slowly over the transition zone toward the lay reversal point. As the lay reversal point is approached the instantaneous lay length increases rapidly until it becomes infinite at the lay-reversal point.

The conductor has been found experimentally to have a strain margin of approximately 0.75%. When the conductor was subjected to tensile load giving an elongation of 1 %, the number of turns of the optical elements between lay-reversal points reduced from 8.5 to 8. The length of the optical element was calculated by numerical means both for the unstrained cable and for the cable strained at 1% elongation, and the results are shown in the Table.

From the Table:

Fibre strain = 319.38/318.03 = 1.0042 (0.42%) Strain margin = l% - 0.42% = 0.58%

Repeating the calculation without taking into account the rotation of the elements (which will give the value for a helically stranded cable) give a value of

0.49% for the strain margin (ignoring the contribution due to the excess length of fibre in the plastics tubes). Thus, the relative rotation of the lay-reversal points of the cable contributes approximately 0.09% to the strain margin of the cable.

In addition to this, there will be a contribution to the strain margin in both cases due to the residual excess fibre length in the plastics tubes. TABLE

UNSTRAINED STRAINED

Radial distance of element from cable centre 1.65 1.23

(mm)

Average lay length (mm) 74 79.4

Cable Strain (%) 0 1

Turns between reversal points 8.5 8

Distance between reversal points (mm) 314.5 317.65

Minimum lay length (mm) 57.0 61.0

Total Element length (mm) 318.03 319.38

Claims

Claims:

1. A combined overhead electrical power transmission and optical transmission cable, which comprises:

(i) a metal tube having a bore extending along its length;

(ii) a plurality of metal wires helically stranded about the metal tube;

(iii) a plurality of optical elements located in the bore of the tube;

2. A cable as claimed in claim 1 , which includes a core about which the optical elements are stranded, wherein the optical elements are capable of causing the core to be twisted at least to a limited extent along its length.

3. A cable as claimed in claim 2, wherein the core comprises a strength member in the form of a plurality of fibres.

4. A cable as claimed in claim 3, wherein the core includes a sheath located over the strength member.

5. A cable as claimed in claim 1, which includes a core about which the optical elements are stranded, wherein the optical elements are capable of sliding over the core in a circumferential direction at least to a limited extent.

6. A cable as claimed in claim 1 , wherein the optical elements are stranded in the absence of a core.

7. A cable as claimed in any one of claims 1 to 6, wherein the optical elements are in direct contact with the interior of the metal tube.

8. A cable as claimed in any one of claims 1 to 7, wherein any space inside the metal tube that is not occupied by the optical elements is filled with a gel that lubricates any movement of the optical elements.

9. A cable as claimed in any one of claims 1 to 8, wherein each optical element comprises at least one optical fibre that is loosely housed in a tube.

10. A cable as claimed in claim 9, wherein each optical element comprises a plurality of optical fibres in the form of a fibre bundle, a fibre ribbon or a stack of fibre ribbons.

11. A cable as claimed in any one of claims 1 to 10, wherein the optical elements are capable of moving within the tube in the circumferential direction sufficient to contribute a strain margin of at least 0.05% to the cable.

12. A cable as claimed in any one of claims 1 to 1 1 , wherein the optical elements undergo from one to twelve turns between lay-reversal points.

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