OPTICAL CONNECTION IN HV LINE

Abstract: A method of forming a joint in an optical line that extends along a phase conductor (1) of a high voltage overhead power transmission system, comprises: (i) forming a length of optical cable (10) which comprises a plurality of optical fibres; (ii) encapsulating the optical fibres; (iii) providing an intermediate region of the length of optical cable with an outer, non-tracking jacket (12) so that a part of the length of optical cable extends from each end of the outer, non-tracking jacket; (iv) installing the jacket so that it extends between phase potential and any other potential to which the cable will be subjected; and (v) connecting one end of the optical fibres of the optical cable to fibres in the optical line of the phase conductor (1), and connecting the other end of the optical fibres of the optical cable to another optical line. The parts of the length of the optical cable that extend from each end of the outer, non-tracking jacket (12) have a sufficient length that connections can be formed between the optical fibres and the optical lines at points remote from regions in which the cable extends along high electrical field gradients. This has the advantage that the optical fibres of the assembly which, in use, will extend between the potential of the phase conductor and earth can be encapsulated in the factory under controlled conditions, and conventional means may be employed to connect the cable.

Description: OPTICAL CONNECTION IN HV LINE This invention relates to overhead electrical transmission systems that include one or more optical transmission lines, and, in particular, those systems in which the optical transmission line is located in the region of a phase conductor so that it is at, or close to, phase potential. Examples of such systems include those in which the optical transmission line forms part of the phase conductor, for example as described in DE-A-26 04 766, and those in which the optical line extends along the outside of the electrical phase conductor. Optical cables that extend along the outside of the electrical phase conductor may be attached to it by lashing, or may be wrapped around the phase conductor, for example as described in EP-A-0 122 163. In any system in which the optical line extends along the phase conductor, the optical line must, at some stage, be brought down to earth potential in order to be connected to other parts of the optical communication system. Examples of methods of terminating such optical systems are described in EP-A-0 122 163 mentioned above, EP-A-0 067 614, EP-A-0433 565 and DE-C-42 27 410. These methods of terminating the optical lines rely on the provision of an outer, non-tracking jacket that extends between the phase potential and earth potential, and encloses the optical fibres between the two potentials. The purpose of the jacket is to protect the optical fibres from the elements, and, more importantly, from the electrical currents or discharges on the assembly surface caused by the electrical field gradients in this region. The jacket should be non-tracking, and will usually have a shedded or convoluted outer surface. Furthermore, the interior of the jacket in this region may have to be substantially void- free since voids can cause electrical discharge activity in the assembly due to the high electrical field gradient, and so allow damage to the optical fibres. In view of this, the optical fibres have typically been encapsulated in an oil or grease within the jacket. In other circumstances, even if it is not necessary to ensure that the interior of the assembly is substantially void-free, it may be necessary to ensure that the cable is fully water-blocked, the fibres are physically protected from vibration, any heat-shrunk component is correctly applied (e.g. to ensure that any mastic or hot-melt adhesive present is correctly applied). The assemblies that have been proposed in the prior art for transferring optical fibres between phase conductors and earth have, however, a number of problems associated therewith. For example, it can be difficult to form the assembly in the field while, at the same time, ensuring that those parts of the assembly extending between the phase conductor and earth are, in fact, correctly assembled, for example, where required, are free of voids. Furthermore, the nature of the assemblies is such that it is difficult, if not impossible, to inspect the completed assemblies non-destructively for correct assembly. In view of this, it is preferable to form that part of the assembly extending between the phase conductor and earth under factory conditions, and to connect the optical fibres of the assembly to optical fibres of the optical line at both the phase conductor and earth ends thereof, for example as described in DE-C-42 27 410. However, that solution leads to a relatively large and unwieldy unit. According to one aspect, the present invention provides a method of forming a joint in an optical line that extends along a phase conductor of a high voltage overhead power transmission system, which comprises: (i) forming a length of optical cable which comprises a plurality of optical fibres; (ii) encapsulating the optical fibres; (iii) providing an intermediate region of the length of optical cable with an outer, non-tracking jacket so that a part of the length of optical cable extends from each end of the outer, non-tracking jacket; (iv) installing the jacket so that it extends between phase potential and any other potential to which the cable will be subjected; and (v) connecting one end of the optical fibres of the optical cable to fibres in the optical line of the phase conductor, and connecting the other end of the optical fibres of the optical cable to another optical line; the parts of the length of the optical cable that extend from each end of the outer, non- tracking jacket having a sufficient length that connections can be formed between the optical fibres and the optical lines at points remote from regions in which the cable extends along high electrical field gradients. The method according to the invention has the advantage that the optical fibres of the assembly which, in use, will extend between the potential of the phase conductor and earth potential, can be encapsulated in an appropriate material, in the factory under controlled conditions, thereby to ensure, as far as possible, that the optical fibres are protected from moisture ingress, partial discharges, mechanical vibration and the effects of thermal cycling etc. while, at the same time, enabling the optical fibres to be connected to the optical fibres in the other parts of the optical line (both at the phase potential and at earth potential) by conventional means in conventional enclosures. Preferably the optical fibres are encapsulated in step (ii) with an appropriate material in order to ensure that the length of cable so formed contains substantially no voids. Thus, according to the invention, the optical fibres are encapsulated in step (ii) under factory conditions while step (iii) may be conducted either in the factory or in situ. If step (iii) is conducted in the factory, it may be conducted simultaneously with step (ii) or after step (ii). In this case it is possible for the entire assembly that extends between the phase conductor and earth to be manufactured under factory conditions, and thereby to ensure as far as possible that no voids exist or other assembly defects inside the outer jacket. However, even if the outer, non-tracking jacket is provided on site during installation, the fact that the individual fibres of the assembly are encapsulated in the factory rather than on site means that it is considerably easier to ensure that no defects are present within the jacket since the length of potted cable formed by step (ii) will be a single structure having a generally uniform cylindrical shape of constant cross-section, so that it has a surface area that has a much larger radius of curvature than would be presented by the individual fibres and whose interface with other components or materials is much easier to control. For example, and especially where a large number of fibres are present, the structure will have a surface area that is only a small fraction of the surface area that would be presented by the individual fibres (or other fibre assemblies), and reduces the risk of void formation due to incomplete filling. Any of a number of materials that are known er se may be employed to encapsulate the optical fibres, for example gels, greases, oils, resins, hot- melt adhesives, thermoplastic elastomers etc. although, for ease of handling during manufacture and installation, and simplicity of design, it is preferred for the encapsulant to be solid at ambient temperatures. As stated above, the optical fibres of the optical cable may be connected to the other optical fibres forming part of the optical line by conventional means. The length by which the optical cable extends beyond the ends of the outer, non-tracking jacket will depend on the particular circumstances including the height of the tower, post or other structure from which the overhead line is suspended, and the voltage of the line. In general, it is preferred for the optical cable to extend beyond the ends of the outer, non-tracking jacket by at least the length of the tower or post, at least for medium voltage assemblies ( 1 1 to 66kV), more preferably by at least five metres and especially by at least ten metres. Such a length of optical cable enables the use of techniques for connecting the optical cable to the optical line that are known per se. The optical fibres are preferably connected to fibres in the optical lines not only at points remote from points in which the optical cable extends along high field gradients between the phase conductor and ground, but also at points that are associated with relatively low electric field gradients, for instance by enclosing the connections within a Faraday cage. For example, the optical cable may be connected to the optical line that is suspended by the overhead conductor by means of a metal enclosure which comprises a splice or connection housing that is suspended on the conductor but which can be lowered down to ground level in order to form the splice or other connection. The housing can then be raised up to the conductor, and in so doing, the two lengths of optical cable that extend from the conductor to ground on either side of the splice or other connection are coiled up inside the housing. Such assemblies have the advantage that relatively bulky items such as the optical fibre connections can be physically supported at points removed from the region spanned by the outer non-tracking jacket, for example the high voltage joint can be supported by the phase conductor and the low voltage joint can be buried in the ground or supported on the pole, thereby enabling conventional connection housings to be employed. The optical cable will normally contain at least six, preferably at least twelve, and especially at least 24 optical fibres which may be arranged individually within the cable or in the form of one or more ribbons. The cable will usually have a circular cross-section of diameter in the range of from 3 to 25 mm preferably 6 to 15 mm whose external surface is formed from the resin used for potting the fibres. If the external surface is smooth, the possibility of any voids remaining between the cable and the outer jacket can be reduced as far as possible. The way in which the number of voids are reduced will depend on whether the cable is provided with the jacket in the factory or in the field. One convenient way of enclosing the cable in the jacket is to coat the outer surface of the cable or the inner surface of the jacket with a grease, oil or mastic, to slide over the cable a dimensionally heat-recoverable jacket, and then to heat the jacket to cause it to recover and express excess grease etc. from the ends thereof. A hot-melt adhesive could be used in place of the grease, oil or mastic, especially if it has a sufficiently low melt viscosity. Such an assembly is novel per se, and so, according to another aspect, the invention provides an assembly for forming a joint in an optical line that extends along a phase conductor of a high voltage overhead power transmission system, which comprises: (i) a length of optical cable which comprises a plurality of optical fibres that are encapsulated so that the length of optical cable contains substantially no voids; and (ii) an outer, non-tracking jacket that extends over an intermediate region of the length of optical cable so that a part of the length of optical cable extends from each end of the outer, non-tracking j acket; the parts of the length of the optical cable that extend from each end of the outer, non- tracking jacket having a sufficient length that connections can be formed between the optical fibres and the optical lines at points remote from regions of high electrical field gradients. Those parts of the optical cable that extend beyond the ends of the outer jacket can, and usually will, be provided with a sheath. The sheaths employed for the two parts of the cable are not necessarily the same. For example, the sheath used for that part of the optical cable that is located on the overhead conductor advantageously is at least partially conductive, preferably having a conductivity that will give the assembly a linear resistance of less than 100 kΩ m " in order to reduce the deleterious effect of any partial discharge from from the proximity of the overhead conductor. The sheath that is used for the other part of the optical cable may be chosen for other reasons, and may, for instance, be electrically insulating. If desired, other elements may be incorporated in the optical cable provided they are not electrically conductive. For example, one or more strength members may be present, for example formed from aramid fibres. The use of an assembly according to this aspect of the invention has the further advantage that it enables sheds on the outer, non-tracking jacket to be formed in the factory by in situ moulding techniques. If necessary, devices may be employed at one or both ends of the outer non- tracking jacket in order to prevent the concentration of electrical stress around the ends of the jacket. For example, one or both ends (but normally just the high voltage end) of the jacket may be provided with a length of electrically resistive material having non-linear electrical characteristics, a stress cone, grading ring or the like. The method and assembly according to the invention will normally be employed to terminate one end of an overhead electrical an optical line where, for example, the optical cable of the overhead line is connected to an underground optical line, or access to a building is required. However, it is possible to employ the method and assembly to form a joint between two overhead lines, for example, at the end of one length of optical cable, or where different methods are used to support the optical cable. For instance, the assembly could be used to connect an optical line located in optical phase wire (OPPW) with a line located in optical ground wire (OPG W) where the two lines will be at different potentials, or also if the two overhead conductors are at the same potential but at different phases. In such an arrangement, a pair of separate outer, non-tracking jackets may be provided on the optical cable, the opposite ends of the optical cable are connected to the optical lines of the phase conductors, and that part of the optical cable located between the two outer jackets may be supported by the tower, post etc. at earth potential. Two forms of method and arrangement 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 schematic view of a joint between an overhead optical line and an underground line; Figure 2 is a schematic view of a back-to-back joint between two overhead optical lines; and Figure 3 is a schematic side sectional view of part of the assembly employed in figure 1. Referring to the accompanying drawings, figure 1 shows schematically part of an overhead 1 1 kV power line in which an uninsulated phase conductor 1 is supported by a wooden pole 2 and ceramic insulators 4. Although only one phase conductor 1 is shown, there will, of course, usually be three phase conductors since the overhead line will normally form part of a three phase circuit. An optical line 6 has been wrapped around the phase conductor 1 , and the line is brought to ground at the pole 2, whereupon it is connected to an underground line 8. This is achieved by means of an assembly comprising an optical cable 10, the central part of which is enclosed in an outer, non-tracking jacket 12 the outer surface of which is provided with sheds 14 in order to increase the leakage path. The length of the outer jacket is about 0.8 metres, although its length will depend on the voltage of the hv power line. The optical cable 10 extends out of the jacket, beyond each end thereof for about ten metres. This enables the end of the optical cable extending from the high voltage end of the jacket 12 to be wrapped around the proximal part of the conductor 1 until it reaches a splice enclosure 16 in which it is spliced to the optical fibres of the optical line 6. The splice enclosure may be such as to require access to the overhead conductor in order to form the splice, or alternatively, it may be one having a part that can be lowered to ground in order to enable the splice to be formed, and then raised up to the conductor, the length of optical cable 10 and the optical line 6 located on either side of the splice which extend from the phase conductor 1 to ground being wound up inside the part of the splice enclosure as it is raised to the phase conductor. With such a form of splice enclosure, the length of the optical cable extending from the high voltage end of the outer jacket 12 will need to be sufficient to extend up to the phase conductor 1, along the phase conductor to the splice enclosure 16 and, if necessary, down from the splice enclosure 16 to ground, while leaving sufficient excess fibre looped in the splice enclosure to form any subsequent splices if necessary. The lower end of the outer jacket 12 is supported on the pole 2 by a by a brace 18, and the other end part of the optical cable 10 extends downwardly from the jacket 12 to ground, whereupon it is fed into a conventional pole mounted joint chamber 20 and spliced to the underground line in conventional manner. The assembly will normally be electrically earthed at the lower end of the jacket 12. Part of the assembly is shown in greater detail in figure 3. The optical cable comprises 24 optical fibres that are bunched together and encapsulated in a cured acrylate, silicone or polyurethane resin to form a structure having a circular cross- section with a diameter of about 0.5 cm and a smooth outer surface. The cable may contain one or more aramid strength members (not shown) in order to remove any mechanical stress from the optical fibres. The central part of the optical cable that is to be enclosed in the outer jacket 12 is then coated with a low melt viscosity hot-melt adhesive 44, and the outer jacket 12, in the form of a heat-shrinkable sleeve, is slipped over the optical cable and heated in an oven in order to recover the sleeve and melt the hot-melt adhesive. By careful control of the resin curing stage and the recovery of the outer jacket, it is possible to ensure that substantially no voids exist in the cable or between the cable and the jacket. The optical cable 10 may be provided with a sheath over which the outer jacket 12 is recovered, or separate sheaths may be formed over each end part of the optical cable that extends beyond the ends of the jacket 12, for example by recovering heat- shrinkable tubes thereon. If separate sheaths are used the sheath covering the high voltage part of the optical cable preferably has an electrical conductivity of about 0.01 S cm "1 . Figure 2 shows an alternative form of assembly for forming a joint between two optical lines 21 and 22 each wrapped on high voltage phase conductors 24 and 26 that have different phases and/or voltages. The assembly comprises an optical cable 28 of the same construction as optical cable 10 of figure 1, but provided with two outer non-tracking jackets 30 and 32, spaced one or two metres apart on the cable. The lower ends of each jacket 30 and 32 are supported at approximately ground potential on the wooden pole 34, although the potential may be at any value between ground and the phase conductors. Each end of the optical cable is spliced to its respective optical line 21 and 22 in the same manner as shown in figure 1.

Claims: Claims: 1. A method of forming a joint in an optical line that extends along a phase conductor of a high voltage overhead power transmission system, which comprises: (i) forming a length of optical cable which comprises a plurality of optical fibres; (ii) encapsulating the optical fibres; (iii) providing an intermediate region of the length of optical cable with an outer, non-tracking jacket so that a part of the length of optical cable extends from each end of the outer, non-tracking jacket; (iv) installing the jacket so that it extends between phase potential and any other potential to which the cable will be subjected; and (v) connecting one end of the optical fibres of the optical cable to fibres in the optical line of the phase conductor, and connecting the other end of the optical fibres of the optical cable to another optical line; the parts of the length of the optical cable that extend from each end of the outer, non- tracking jacket having a sufficient length that connections can be formed between the optical fibres and the optical lines at points remote from regions in which the cable extends along high electrical field gradients. 2. A method as claimed in claim 1, wherein the part of the optical cable that extends from each end of the outer, non-tracking jacket has a length of at least five metres. 3. A method as claimed in claim 2, wherein the part of the optical cable that extends from each end of the outer, non-tracking jacket has a length of at least ten metres. 4. A method as claimed in any one of claims 1 to 3, wherein the optical fibres are encapsulated in the potting compound in step (ii) under factory conditions. 5. A method as claimed in any one of claims 1 to 4, wherein the optical cable contains at least twelve optical fibres. 6. A method as claimed in any one of claims 1 to 5, which includes enclosing each part of the optical cable that extends beyond the end of the outer, non-tracking j acket with a sheath. 7. A method as claimed in any one of claims 1 to 6, wherein the optical line that extends along the phase conductor has been wrapped around the phase conductor. 8. A method as claimed in claim 7 as appendant to claim 6, wherein the sheath that encloses that part of the optical cable that is located at the phase conductor is at least partly electrically conductive. 9. A method as claimed in any one of claims 1 to 6, wherein the optical line that extends along the phase conductor forms part of a composite electrical and optical conductor. 10. A method as claimed in any one of claims 1 to 9, wherein the other potential to which the cable will be subjected is earth potential. 11. A method as claimed in any one of claims 1 to 10, wherein a pair of separate outer, non-tracking jackets are provided, and each end of the optical fibres is connected to an optical line that extends along a phase conductor to form a joint in the optical line. 12. An assembly for forming a joint in an optical line that extends along a phase conductor of a high voltage overhead power transmission system, which comprises: 5 (i) a length of optical cable which comprises a plurality of optical fibres that are encapsulated in a potting compound so that the length of optical cable contains substantially no voids; and (ii) an outer, non-tracking jacket that extends over an intermediate region of the length of optical cable so that a part of the length of optical cable extends from o each end of the outer, non-tracking j acket; the parts of the length of the optical cable that extend from each end of the outer, non- tracking jacket having a sufficient length that connections can be formed between the optical fibres and the optical lines at points remote from regions of high electrical field gradients. ***********

Inventors: ROWLAND SIMON MARK, TAHA ARIF JAMEEL, WHITESMITH PETER JOHN, ROWLAND, SIMON, MARK, TAHA, ARIF, JAMEEL, WHITESMITH, PETER, JOHN
Applicants: BICC PLC, ROWLAND SIMON MARK, TAHA ARIF JAMEEL, WHITESMITH PETER JOHN, BICC PUBLIC LIMITED COMPANY, ROWLAND, SIMON, MARK, TAHA, ARIF, JAMEEL, WHITESMITH, PETER, JOHN
Application year: 1997
Main classification: G02B6
IPC codes: G02B6