MULTISENSING CABLE FOR OVERHEAD POWER TRANSMISSION LINES

Abstract

It is disclosed a cable for an overhead power transmission line, the cable comprising: a first optical unit comprising a first metal tube housing at least one an optical fiber suitable for sensing strain, the at least one optical fiber in the first metal tube being a tight buffered optical fiber fixed to an inner surface of the first metal tube; a second optical unit comprising a second metal tube comprising one or more loose optical fibers suitable for sensing temperature; and an armor comprising one or more layers of metal wires. The first optical unit is surrounded by at least one layer of semi-conductive or conductive material electrically contacting the outer surface of the first metal tube.

Claims

1. A cable for an overhead power transmission line, the cable comprising: a first optical unit comprising a first metal tube housing at least one optical fiber suitable for sensing strain, said at least one optical fiber in the first metal tube being a tight buffered optical fiber fixed to an inner surface of the first metal tube; a second optical unit comprising a second metal tube comprising at least one loose optical fiber suitable for sensing temperature; and an armor comprising at least one layer of metal wires, wherein the first optical unit is surrounded by at least one layer of semi-conductive or conductive material electrically in contact with an outer surface of the first metal tube.

2. The cable according to claim 1, wherein the first metal tube of the first optical unit is in electrical contact with the armor.

3. The cable according to claim 1, wherein the first metal tube and the second metal tube are made of stainless steel.

4. The cable according to claim 1, wherein the tight buffered optical fiber is fixed to the inner surface of the first metal tube by means of an adhesive compound.

5. The cable according to claim 1, wherein the first optical unit is individually surrounded by an aluminum tube.

6. The cable according to claim 1, wherein the first optical unit is individually surrounded by a sheath of semi-conductive polyethylene or semi-conductive cross-linked polyethylene.

7. The cable according to claim 1, wherein the first optical unit is located substantially at a center of the cable.

8. The cable according to claim 1, wherein said first optical unit is located within the armor in place of a metal wire.

9. The cable according to claim 1, wherein said first optical unit and said second optical unit are surrounded by a same semi-conductive cover embedding the first optical unit and the second optical unit.

10. The cable according to claim 9, wherein said semi-conductive cover is of semi-conductive polyethylene or semi-conductive cross-linked polyethylene.

11. The cable according to claim 9, further comprising a third tube surrounding the semi-conductive cover, the third metal tube being an aluminum tube.

12. The cable according to claim 1, wherein said first optical unit and said second optical unit are surrounded by a same third metal tube and wherein a semi-conductive material is interposed among said first optical unit, said second optical unit and said third metal tube, said semi-conductive material in contact with outer surfaces of the first tube and the second tube and in contact with an inner surface of the third tube.

13. The cable according to claim 12, wherein said semi-conductive material is a semi-conductive adhesive compound.

14. The cable according to claim 12, further comprising a fourth metal tube surrounding the third metal tube, the fourth metal tube being an aluminum tube.

15. A measurement system for measuring strain along an overhead power transmission line, the measurement system comprising a measuring unit and a cable according to claim 1.

16. A cable for an overhead power transmission line, the cable comprising: a first optical unit including a first metal tube housing at least one optical fiber suitable for sensing strain, the at least one optical fiber in the first metal tube being a tight buffered optical fiber fixed to an inner surface of the first metal tube; a second optical unit including a second metal tube comprising at least one loose optical fiber suitable for sensing temperature; and an armor comprising at least one layer of metal wires, wherein the first metal tube of the first optical unit is in electrical contact with the armor and the second metal tube of the second optical unit is in electrical contact with the armor.

17. The cable according to claim 16, wherein the tight buffered optical fiber is fixed to the inner surface of the first metal tube by means of an adhesive compound.

18. The cable according to claim 16, wherein the first optical unit is located substantially at a center of the cable.

19. The cable according to claim 16, wherein the first optical unit is located within the armor in place of a metal wire.

20. The cable according to claim 16, wherein the first optical unit and the second optical unit are surrounded by a same semi-conductive cover.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Further characteristics and advantages will become more apparent by reading the following detailed description of embodiments given as examples with reference to the accompanying drawings (which are not on scale), wherein:

[0042] FIG. 1 schematically shows a cross section of an overhead cable according to a first embodiment of the present disclosure;

[0043] FIG. 2 schematically shows a cross section of an overhead cable according to a second embodiment of the present disclosure;

[0044] FIG. 3 schematically shows a cross section of an overhead cable according to a third embodiment of the present disclosure;

[0045] FIG. 4 schematically shows a cross section of an overhead cable according to a fourth embodiment of the present disclosure; and

[0046] FIG. 5 schematically shows a cross section of an overhead cable according to a fifth embodiment of the present disclosure; and

[0047] FIG. 6 schematically shows a cross section of an overhead cable according to a sixth embodiment of the present disclosure; and

[0048] FIG. 7 schematically shows a measurement system according to the present disclosure.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE DISCLOSURE

[0049] In the present description and claims, unless otherwise specified, all the numbers and values should be intended as preceded by the term about. Also, all ranges include any combination of the maximum and minimum points disclosed and include any intermediate ranges therein, which may or may not be specifically enumerated herein.

[0050] In the description herein, all ranges are inclusive ranges where the limits are included along with what lies in between, unless it is expressively indicated that the limits of a range are not included in the range.

[0051] It is to be noticed that any embodiment described herein below relates to a cable suitable to be installed in an overhead power transmission line. For example, any overhead cable according to the embodiments of the present disclosure may be an OPGW (Optical Ground Wire), MASS (Metallic Aereal Self Supported) or OPPC (Optical Phase Conductor) cable. Indeed, each overhead cable presented herein after comprises a central unit possibly comprising optical fibers and an armor radially outer the central unit and helically stranded around it, the armor comprising one or more layers of metal wires configured to withstand the passage of electrical current. In case of an OPCC, the armor is configured to sustain electrical current for power transmission, while in an OPGW the armor is configured to shield the conductors of the overhead power transmission line, creating a zero potential on top of the overhead power transmission line, and/or to permit the passage of electrical current in specific cases, such as lightning strikes, arc discharges, or short-circuits.

[0052] FIG. 1 schematically shows an overhead cable 1 according to a first embodiment of the present disclosure.

[0053] The overhead cable 1 comprises, in a substantially central position, a first optical unit 2 comprising a first tube 2a housing at least one tight buffered optical fiber 3 suitable for sensing strain along the overhead cable. According to this first embodiment, the position of the first optical unit 2 is in-axis, wherein the expression in-axis indicates that the longitudinal axis of the first optical unit 2 substantially coincides with the longitudinal axis of the overhead cable 1. The longitudinal axis of the overhead cable 1 is the axis along the lengthwise direction of the cable passing through its center. It is indicated in FIG. 1 with letter A.

[0054] The first tube 2a is made of a metal, for example, stainless steel (SST). For example, the first tube 2a may be a longitudinally welded hermetically sealed SST tube. In other embodiments, the first tube 2a may be made of longitudinally welded copper or aluminum. The first tube 2a may have an inner diameter comprised between 0.9 mm and 2.0 mm and an outer diameter comprised between 1.2 mm and 2.4 mm. The first tube 2a may have a thickness between 0.1 mm and 0.3 mm. For example, the inner diameter may be equal to 1.5 mm and the outer diameter may be equal to 1.8 mm and hence the thickness may be equal to 0.15 mm.

[0055] In some embodiments, the first tube 2a may be made of two different metals. For example, it can be made by a layer of SST rolled up and welded forming an internal metal tube, surrounded by a layer of a second metal, for example a layer of aluminum rolled up and welded forming an external metal tube.

[0056] According to embodiments of the present disclosure, the first tube 2a may contain one, two or three tight buffered optical fibers. The tight buffered fiber(s) contained in the first tube 2a may have an outer diameter equal to 600 ?m or 900 ?m. Each optical fiber 3 inside the first tube 2a may be a single mode optical fiber compliant with ITU-T Recommendation G.652 (November 2016), G.654 (March 2020); G.655 (November 2009) or G.657 (November 2016), or a multimode optical fiber compliant with ITU-T Recommendation G.651.1 (November 2018).

[0057] The at least one tight buffered optical fiber 3 is fixed to the inner surface of the first tube 2a. For example, the at least one tight buffered optical fiber 3 is fixed to the inner surface of the first tube 2a along its whole length. In this way, the at least one tight buffered optical fiber 3 is mechanically congruent with the first tube 2a and then with the whole structure of the overhead cable 1. As already mentioned above, this leads to improving the sensing accuracy of the tight buffered fiber(s). Fixing is in some implementations performed by means of an adhesive compound or glue, for example a silicone adhesive or a two-components epoxy resin, which is applied longitudinally along the whole length of the first tube 2a and substantially fills the interstices between the tight buffered fiber(s) 3 and the inner surface of the first tube 2a.

[0058] The first optical unit 2 is surrounded by a layer 4 of a semi-conductive or conductive material. For example, according to this first embodiment, the layer 4 is a sheath radially outer the first tube 2a. The inner surface of the sheath 4 is in direct contact with the outer surface of the first tube 2a substantially along the whole length of the first tube 2a. According to an embodiment, the sheath 4 is made of aluminum and it is extruded over the first tube 2a. Alternatively or additionally, the sheath 4 may be an aluminum tube which is longitudinally welded and hermetically sealed. The thickness of the sheath 4 may be comprised between 0.1 mm and 2 mm.

[0059] According to some embodiments of the present disclosure, the sheath 4 is made of a semi-conductive material, for instance a semi-conductive polyethylene (PE) (for instance, linear low-density polyethylene, LLDPE, or low-density polyethylene, LDPE, or medium-density polyethylene, MDPE, or high-density polyethylene, HDPE) or semi-conductive cross-linked polyethylene (XLPE). In this case, for example, the thickness of the sheath 4 may be comprised between 0.4 mm and 3 mm.

[0060] According to some embodiments of the present disclosure not shown in the drawings, the first optical unit 2 is surrounded by a third tube radially outer the first tube 2a and substantially concentric with the first tube 2a. The third tube may be of stainless steel. For example, the third tube may be a longitudinally welded hermetically sealed SST tube. The third tube may have an inner diameter comprised between 1.2 mm and 5.4 mm and an outer diameter comprised between 1.8 mm and 6.0 mm. The thickness of the third tube is in some implementations comprised between 0.2 mm and 0.3 mm. According to these embodiments, the third tube may be welded on the first tube 2a. Alternatively or additionally, a conductive glue may be used between the first tube 2a and the third tube to guarantee the electrical contact between the first tube 2a and the third tube.

[0061] According to the first embodiment of the present disclosure as shown in FIG. 1, the overhead cable 1 comprises an armor 5 which is provided around the sheath 4. The armor 5 comprises a plurality of round wires 6. The wires 6 are metal wires. For example, the wires 6 are aluminum clad steel (ACS) wires. Alternatively or additionally, they may be aluminum wires, aluminum alloy wires or galvanized steel wires. The armor may comprise a mix of these four types of wires. The diameter of each wire is comprised, for example, between 1.8 mm and 4.0 mm.

[0062] According to embodiments of the present disclosure, the wires 6 of the armor 5 are circumferentially arranged around the first optical unit 2. For example, the wires 6 of the armor electrically contact the outer surface of the sheath 4 surrounding the first optical unit 2. According to the embodiment shown in FIG. 1, the wires 6 form a first layer of wires 6 radially outer the first optical unit 2 and a second layer of wires 6 radially outer the first layer of wires. The two layers of wires are circumferential layers that are substantially concentric with each other and with the first optical unit 2. According to other embodiments not shown in the Figures, the armor may comprise a single layer of wires around the first optical unit or three or more layers of wires around the first optical unit. The wires 6 are in some implementations helically stranded around the outer surface of the sheath 4. For example, the wires 6 of the first layer may be laid in right direction around the first optical unit (right hand or Z lay) and the wires 6 of the second layer may then be stranded with a reverse lay direction. In case a third layer is present, its wires may be stranded again in right direction, and so on.

[0063] In some implementations, when required, an anti-corrosion grease may be provided to fill the interstices between the first optical unit 2 and the armor wires 6 and among the armor wires 6, to avoid galvanic corrosion that may appear between dissimilar materials (aluminum and stainless-steel).

[0064] According to the present disclosure, the overhead cable 1 comprises a second optical unit 7 housing one or more optical fibers suitable for sensing the temperature. The second optical unit 7 comprises a second tube 7a comprising optical fibers 8 loosely arranged therein. The second tube 7a may be filled with a gel to provide protection against longitudinal water propagation and to protect the optical fiber(s) against hydrogen generation. According to the first embodiment of the present disclosure illustrated in FIG. 1, the second optical unit 7 is located within the armor 5 in place of one metal wire. For example, the second optical unit 7 is arranged within one of the layers of the armor 5. According to the embodiment shown in FIG. 1, the second optical unit 7 is arranged within the first layer of wires 6 of the armor 5. In some implementations, the second tube 7a is in electrical contact with the armor 5.

[0065] The second tube 7a of the second optical unit 7 is made of a metal, for example stainless steel (SST). For example, the second tube 7a may be a longitudinally welded hermetically sealed SST tube. In other embodiments, the tube 7a may be made of longitudinally welded copper or aluminum. In some embodiments, the second tube 7a may be made of two different metals. The second tube 7a may have an inner diameter comprised between 1.4 mm and 3.6 mm and an outer diameter comprised between 1.8 mm and 4.0 mm. In some implementations, the second tube 7a may be formed from an internal layer made of SST, for example a welded SST tube having an inner diameter comprised between 1.4 mm and 3.6 mm and an outer diameter comprised between 1.8 and 3.5 mm, and an external layer made of aluminum, for example a welded aluminum tube having a thickness comprised between 0.1 and 0.5 mm.

[0066] The optical fibers 8 inside the second tube 7a may be single mode optical fibers compliant with ITU-T Recommendation G.652 (November 2016), G.654 (March 2020); G.655 (November 2009) or G.657 (November 2016), or multimode optical fibers compliant with ITU-T Recommendation G.651.1 (November 2018). According to embodiments of the present disclosure, the second tube 7a may contain a number of optical fibers comprised between 2 and 96, for instance equal to 24. The optical fibers 8 may have an extra length (EFL) lower than 0.5%.

[0067] The skilled person will appreciate that the presence of a layer of conductive or semi-conductive material around the first tube 2a allows adding at least one layer of semi-conductive or conductive material around the first optical unit 2 and increasing the diameter of the resulting unit to a value which is similar to the value of the diameter of the wires 6 of the armor (and the second optical unit). In this way, the first optical unit 2 may be assembled together with the metal wires 6 within the overhead cable 1. As a matter of fact, the first optical unit 2 is unable to move among the other elements of the overhead cable and is hence mechanically congruent with them. Any strain imparted on the cable similarly affects the first optical unit. This leads to improving the sensing accuracy.

[0068] In some implementations, the material of the layer around the first optical unit 2 (for example, between the first optical unit 2 and the armor 5) is conductive or semi-conductive, so as to guarantee the electrical contact between the first optical unit 2 and the armor 5 and avoid the presence of an electrical current which may be induced in the first optical unit 2.

[0069] FIG. 2 schematically shows an overhead cable 1 according to a second embodiment of the present disclosure. The overhead cable 1 comprises a first optical unit 2 comprising a first tube with the at least one tight buffered optical fiber arranged therein and surrounded by a layer of a semiconductive or conductive material, and a second optical unit 7 comprising a second tube with one or more loose optical fiber. The overhead cable 1 also comprises an armor 5. The structures of the first optical unit and the second optical unit are the same as already described above. In the overhead cable 1, the first optical unit 2 is off-axis, wherein the expression off-axis indicates that the longitudinal axis of the first optical unit 2 does not coincide with the longitudinal axis of the overhead cable 1. For example, the second optical unit 7 is located in-axis at the center of the cable and the first optical unit 2 is located within the armor (for example, within a first layer of wires 6 radially outer the second tube). The first optical unit 2 is stranded with the same pitch as the wires 6 of the first layer of the armor.

[0070] According to other embodiments of the present disclosure not shown in the Figures, the first optical unit and the second optical unit may be both located off-axis. In these cases, each of the first optical unit and the second optical unit may be located within any one of the layers of wires of the armor and a metal wire may be located at the center of the cable.

[0071] It is to be noticed that when the first optical unit is provided off-axis, the first optical unit may include an extra length, as in this case the first optical unit is stranded around the central unit (for instance, in the embodiment shown in FIG. 2 the central unit is the second optical unit). The extra length may be between 1% and 4%. It is worth to point out here, that, if the first optical unit is located off-axis with an additional extra-length, the length of the cable will not match the length of the first optical unit, so that, for strain measurement, suitable calculations will be conducted to match the two lengths.

[0072] FIG. 3 schematically shows an overhead cable 10 according to a third embodiment of the present disclosure.

[0073] The overhead cable 10 comprises, in a central position, a first optical unit 11 and a second optical unit 12. In some embodiments, the first optical unit 11 and the second optical unit 12 are laid substantially parallel to the longitudinal axis of the overhead cable 10 and located substantially at the center of the cable 10. In other embodiments the first optical unit 11 and the second optical unit 12 are stranded together.

[0074] In some implementations, the first optical unit 11 comprises a first tube 11a comprising at least one tight buffered optical fiber 13, and the second optical unit 12 comprises a second tube 12a and one or more loose optical fibers 14, as already described above.

[0075] The overhead cable 10, in some implementations, comprises also a semi-conductive material collectively surrounding the first optical unit 11 and the second optical unit 12. For example, according to this embodiment, the overhead cable 10 comprises a cover 15 embedding the first optical unit 11 and the second optical unit 12. The material of the cover 15 substantially fills the space between the first optical unit 11 and the second optical unit 12 and surrounds both of them. The cover 15 is made of a semi-conductive material, for instance a semi-conductive polyethylene (PE) (for instance, linear low-density polyethylene, LLDPE, or low-density polyethylene, LDPE, or medium-density polyethylene, MDPE, or high-density polyethylene, HDPE) or semi-conductive cross-linked polyethylene (XLPE). In some implementations, the outer diameter of the cover 15 is comprised between 3 mm and 10 mm.

[0076] The overhead cable 10, in some implementations, comprises a third tube 16 radially outer the cover 15. The third tube 16 is a metal tube, such as, for instance, a tube made of aluminum. For example, the third tube 16 may be extruded over the cover 15 or it may be a longitudinally welded hermetically sealed aluminum tube whose inner surface is in electrical contact with the outer surface of the cover 15. The third tube 16 may have an outer diameter comprised between 6 mm and 14 mm.

[0077] According to an embodiment of the present disclosure, the overhead cable 10 also comprises an armor 17 which is provided around the third tube 16, the armor 17 comprising a plurality of wires 18 as already described above. The armor 17 shown in FIG. 3 comprises a plurality of wires 18 arranged in a single circumferential layer which is substantially concentric with the third tube 16, the wires being in some implementations helically wound around the outer surface of the third tube 16 in electrical contact with it. According to other embodiments, the armor 17 may comprise a plurality of circumferential layers of wires around the third tube 16.

[0078] FIG. 4 schematically shows an overhead cable 20 according to a fourth embodiment of the present disclosure.

[0079] The overhead cable 20 comprises, in a central position, a first optical unit 21 and a second optical unit 22. In some implementations, the first optical unit 21 and the second optical unit 22 are laid substantially parallel to the longitudinal axis of the overhead cable 20 and located substantially at the center of the cable 20.

[0080] In some implementations, the first optical unit 21 comprises a first tube 21a comprising at least one tight buffered optical fiber 23 as already described above. The second optical unit 22 in some implementations comprises a second tube 22a and one or more loose optical fibers 24 as already described above.

[0081] The overhead cable 20 in some implementations comprises a third tube 25 collectively surrounding both the first optical unit 21 and the second optical unit 22. The third tube 25 is a metal tube, for example, a tube made of stainless steel. The third tube 25 may also be made of copper. For example, the third tube 25 may be a laser welded hermetically sealed SST tube. The third tube 25 may have an outer diameter comprised between 3 mm and 6 mm and a thickness between 0.2 mm and 0.3 mm.

[0082] Moreover, the overhead cable 20, in some implementations, comprises a semi-conductive material interposed between the first optical unit 21, the second optical unit 22 and the third tube 25, for example, between the outer surface of both the first tube 21a and the second tube 22a and the inner surface of the third tube 25 to assure electrical contact between the different metal tubes. For example, according to this embodiment, the overhead cable 20 comprises a semi-conductive adhesive compound 26 such as, for instance, a semi-conductive glue, filling at least partially the interstices between the first optical unit 21 and the second optical unit 22 and between each of the first optical unit 21 and the second optical unit 22 and the inner surface of the third tube 25 to assure electrical contact between the optical units and each optical unit and the third tube. The adhesive compound 26 allows fixing the first optical unit 21 to the inner surface of the third tube 25. In this way, the first optical unit 21 is mechanically congruent with the overhead cable 20.

[0083] According to an embodiment of the present disclosure, the overhead cable 20 comprises a fourth tube 27 surrounding the third tube 25 and electrically contacting the outer surface thereof. The fourth tube 27 is a metal tube, in some implementations, a tube made of aluminum, which is extruded over the third tube 25. Alternatively or additionally, the fourth tube is a longitudinally welded aluminum tube whose inner surface electrically contacts the outer surface of the third tube 25. The fourth tube 27 may have an outer diameter comprised between 6 mm and 14 mm and a thickness between 1 and 3 mm.

[0084] Furthermore, the overhead cable 20 comprises an armor 28 which is provided around the fourth tube 27, the armor 28 comprising a plurality of wires 29 as already described above. The armor 28 shown in FIG. 4 comprises a plurality of wires 29 arranged in a single circumferential layer which is substantially concentric with the fourth tube 27, the wires being in some implementations helically wound around the outer surface of the fourth tube 27 in electrical contact with it. According to other embodiments, the armor may comprise a plurality of circumferential layers of wires around the fourth tube 27.

[0085] FIG. 5 schematically shows an overhead cable 30 according to a fifth embodiment of the present disclosure.

[0086] The overhead cable 30 comprises, in a central position, a first optical unit 31 and a second optical unit 32. In the embodiment shown in FIG. 5, the first optical unit 31 and the second optical unit 32 are laid substantially parallel to the longitudinal axis of the overhead cable 30 and located substantially at the center of the cable 30. In other embodiments the first optical unit 31 and the second optical unit 32 are stranded together.

[0087] The first optical unit 31 comprises a first tube 31a comprising at least one tight buffered optical fiber 33, and the second optical unit 32 comprises a second tube 32a and one or more loose optical fibers 34, as already described above.

[0088] The overhead cable 30, in some implementations, comprises also a semi-conductive material collectively surrounding the first optical unit 31 and the second optical unit 32. For example, according to an embodiment, the overhead cable 30 comprises a cover 35 of a semi-conductive material embedding the first optical unit 31 and the second optical unit 32 as already described above with reference to the third embodiment of the present disclosure.

[0089] According to an embodiment of the present disclosure, the overhead cable 30 comprises an armor 36 which is provided around the cover 35, the armor 36 comprising a plurality of wires 37 as already described above. The armor 36 shown in FIG. 5 comprises a plurality of wires 37 arranged in two circumferential layers, the wires 37 being in some implementations helically wound around the outer surface of the cover 35 in electrical contact with it. According to an embodiment, however, the armor may comprise one or more circumferential layers of wires.

[0090] FIG. 6 schematically shows an overhead cable 40 according to a sixth embodiment of the present disclosure.

[0091] The overhead cable 40 comprises, in a central position, a first optical unit 41 and a second optical unit 42. In the embodiment of FIG. 6, the first optical unit 41 and the second optical unit 42 are laid substantially parallel to the longitudinal axis of the overhead cable 40 and located substantially at the center of the cable 40. In other embodiments the first optical unit 41 and the second optical unit 42 are stranded together.

[0092] The first optical unit 41 comprises a first tube 41a comprising at least one tight buffered optical fiber 43 as already described above. The second optical unit 42 comprises a second tube 42a and one or more loose optical fibers 44 as already described above.

[0093] The overhead cable 40 in some implementations comprises a third tube 45 collectively surrounding both the first optical unit 41 and the second optical unit 42, as already described above with reference to the fourth embodiment of the present disclosure.

[0094] The overhead cable 40 also comprises a semi-conductive material interposed between the first optical unit 41, the second optical unit 42 and the third tube 45. For example, the semi-conductive material is interposed the outer surface of both the first tube 41a and the second tube 42a and the inner surface of the third tube 45 to assure electrical contact between the different metal tubes. For example, according to this embodiment, the overhead cable 40 comprises a semi-conductive adhesive compound 46 which fills at least partially the interstices between the first optical unit 41 and the second optical unit 42 and between each of the first optical unit and the second optical unit and the inner surface of the third tube 45 to assure electrical contact between the optical units and each optical unit and the third tube. The adhesive compound 46 allows fixing the first optical unit 41 to the inner surface of the third tube 45. In this way, the first optical unit 41 is mechanically congruent with the overhead cable 40.

[0095] Furthermore, the overhead cable 40 comprises an armor 47 which is provided around the third tube 45, the armor 47 comprising a plurality of wires 48 as already described above. The armor 47 shown in FIG. 6 comprises a plurality of wires 48 arranged in two circumferential layers, the wires being in some implementations helically wound around the outer surface of the third tube 45 in electrically contact with it.

[0096] FIG. 7 schematically shows a measurement system 70 for measuring strain along an overhead power transmission line by means of the distributed fiber optic sensing system of the optical cable described herein above.

[0097] The measurement system comprises a measuring unit 71 connected to an overhead cable 72 according to an embodiment of the present disclosure. The measuring unit 71 comprises a light emitter (e.g., a laser operating at 1550 nm) for sending signals into the at least one thigh buffered optical fiber of the first optical unit and the one or more loose optical fibers of the second optical unit. The measuring unit 71 also comprises a light receiver (e.g., a photodetector) for detecting signals coming from said at least one thigh buffered optical fiber of the first optical unit and said one or more loose optical fibers of the second optical unit and measuring strain and temperature along the cable. The operation of the measurement system 70 may be based, as known, on the Stimulated Brillouin Backscattering (SBS). In this case, the measuring unit 71 may be a known Brillouin Optical Time Domain Analyzer or BOTDA such as, for instance, the VISION Interrogator manufactured by Omnisens SA, Switzerland.

[0098] The various embodiments described above can be combined to provide further embodiments. Aspects of the embodiments can be modified, if necessary to employ concepts of the various embodiments to provide yet further embodiments.

[0099] These and other changes can be made to the embodiments in light of the above-detailed description. In general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled. Accordingly, the claims are not limited by the disclosure.