PRE-TENSIONING AN OVERHEAD ELECTRICAL CABLE

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims the priority benefit of U.S. Provisional Patent Application No. 62/460,240, filed February 17, 2017, which is incorporated herein by reference in its entirety.

FIELD

[0002] This disclosure relates to the field of overhead electrical cables, and particularly to methods and devices for the tensioning of the conductor portion of an overhead electrical cable before the electrical cable is strung onto support towers.

BACKGROUND

[0003] Overhead electrical cables are used in power transmission lines for the transmission of electrical power, and form the backbone of the electrical power transmission grid. Such overhead electrical cables include a strength member surrounded by an electrical conductor that is typically fabricated from conductive aluminum, i.e. , one or more layers of helically-wrapped aluminum strands. The strength member provides the mechanical strength (e.g. , tensile strength) that enables the overhead electrical cable to be strung onto utility towers at very high tension (i.e. , high tensile loads), an installation technique referred to as "tension stringing" of the electrical cable. Traditionally, the strength member consists of a bundle of helically-wrapped steel cords, a configuration referred to as an ACSR (Aluminum Conductor, Steel Reinforced) overhead electrical cable.

[0004] As the demand for electrical power continues to increase world-wide, new configurations for overhead electrical cables have been sought to replace traditional ACSR configurations. Configurations have been proposed to increase energy efficiency (e.g. , by reducing line losses), to enable the use of increased operating temperatures (e.g. , greater than 100°C), and/or to reduce line sag as compared to ACSR overhead electrical cables. Line sag is the result of the overhead electrical cable elongating due to heating and/or due to load-related tensile stresses (e.g. , due to ice loading or high winds). Elongation of the overhead cable may cause the cable to sag such that the clearance between the overhead cable and objects on the ground (e.g. , trees or buildings) becomes dangerously low. Overhead electrical cable configurations that safely operate at increased temperatures also enable increased current to be passed through the electrical cable, either continuously or intermittently.

[0005] In one type of overhead electrical cable configuration, the strength member is constructed from a fiber-reinforced composite material. For example, a fiber-reinforced composite strength member may include discontinuous fibers and/or continuous fibers (e.g. , continuous fiber tows) disposed in a binding matrix (e.g. , a resin or a metal). Various types of fibers have been suggested for this configuration, including glass fibers, carbon fibers, aramid fibers, ceramic fibers and the like. One particularly successful example of an overhead electrical cable using a fiber-reinforced composite strength member is the ACCC® overhead electrical cable that is available from CTC Global Corp. of Irvine, CA, USA. This overhead electrical cable configuration generally features a first core section that includes carbon fibers, which is surrounded by a second core section that includes glass fibers. The carbon fiber section provides high strength (e.g. , tensile strength) while the glass fiber section provides flexibility (e.g. , high elastic modulus) and toughness, and also insulates the carbon fibers to prevent galvanic corrosion between the carbon fibers and the outer aluminum conductor. See, for example, U.S. Patent No. 7,21 1 ,319 and U.S. Patent No. 7,368, 162, each of which is incorporated herein by reference in its entirety.

[0006] One advantage of fiber-reinforced composite strength members, particularly the ACCC® strength member, is that the overhead electrical cable can safely carry a higher tensile load as compared to an ACSR electrical cable. For example, an ACSR electrical cable having an outer diameter of 1 .108 inches (a size referred to as "Drake") has a Rated Tensile Strength (RTS) of about 31 ,500 lbs. at 1 % strain, with the aluminum conductor strands carrying about 50% of that load (e.g. , about 15,750 lbs) under ambient conditions. The 1 % strain limit is chosen because it safely approximates the breaking strain of the hard (1350-H19) aluminum conductor strands that are used in ACSR configurations.

[0007] In contrast, the breaking strain of annealed (1350-O) aluminum conductor strands that are used in an ACCC ^® cable configuration is over 20%. Thus, the less pliable fiber-reinforced composite core, which breaks at about 2% strain, determines the RTS of the electrical cable. In this regard, a Drake ACCC ^® cable having an outer diameter of 1 .108 inches has an RTS of about 41 ,000 lbs at 2% strain, with the annealed aluminum conductor strands carrying only about 16% of the load (about 6560 lbs) under ambient conditions. As a result, much higher tensile strain in placed on the aluminum conductors in an ACSR configuration, even when the overall tensile strain on the overhead cable is lower.

[0008] The relationship of load-sharing between the aluminum strands and the strength member is dynamic due to thermal conditions and mechanical load history of the cable. Ideally, the aluminum conductor strands should be under substantially no tension to improve resistance to Aeolian vibration and to increase the self-damping characteristics of the electrical cable. As the temperature of an overhead electrical cable increases (i.e., due to resistance heating) the aluminum conductor strands will expand at a rate faster than the expansion rate of the strength member. At a certain temperature, referred to as the thermal knee-point, all of the tensile load on the overhead electrical cable shifts to the strength member and the aluminum conductor strands are fully relaxed (i.e. , are under substantially no tension).

[0009] Therefore, it is desirable to decrease the thermal knee-point of an overhead electrical cable such that the overhead electrical cable is normally operating above the thermal knee-point. One way to lower the thermal knee-point is to reduce the initial stringing tension on the aluminum conductor strands, so that little or no thermal elongation is needed to place them in a tension-free state. In this regard, it has been suggested to pre-tension the overhead electrical cable during installation of the cable on support towers to decrease the initial tensile load on the conductor strands. Pre-tensioning consists of applying a tensile load to the overhead cable that is higher than the desired stringing tension, holding the cable under that increased tension for a period of time (e.g. , 48 hours or less), and then reducing the tension to the desired stringing tension before attaching the supporting hardware to the cable (i.e. , "clipping-in" the cable). For example, it has been found that the ACCC® overhead electrical cable is subject to plastic elongation (i.e. , of the conductor strands) and a minor degree of strand settling which causes the cable tension to drop by approximately 5% to 7% in the first few hours, and by about 10% over a period of 24 to 48 hours after it is initially pulled to a desired "clipping in" tension. To offset this drop and stabilize the electrical cable no matter how long it may have been suspended in the air prior to being clipped in, the conductor can be pre-tensioned by an additional 2% RTS above the desired stringing tension, held for about 30 minutes, and then lowered back to the desired clipping tension and clipped in. Conversely, it can simply be clipped in at the 2% RTS higher tension and allowed to relax over a period of about 24 to 48 hours.

[0010] However, pre-tensioning in this manner must be done with extreme caution. The dead-end and corner structures must be strong enough to accommodate the temporary, higher pulling tension. In this regard, temporary cross arm bracing or guy wires may be required. Using the tensioner itself to pre-tension the conductor may also create excessive down force at the dead end structure equipment. Larger stringing blocks must also be employed. Further, if two overhead electrical cables are clipped in at a different stringing tensions, thermal sag or mechanical sag may cause the cables to come in contact due to different sag characteristics.

SUMMARY

[0011] The methods and devices disclosed herein may enable the installation (e.g. , tension stringing) of an overhead electrical cable onto elevated support structures in a non-shared load state, e.g. , where substantially all of the tensile load is placed on the strength member. As a result, the thermal knee point may be reduced, and/or sagging due to conductor elongation over the lifetime of the cable may be significantly reduced or eliminated. The need for re-sagging the electrical cable once in service may also be reduced or eliminated, barring unforeseen circumstances. The overhead electrical cable may be pre-tensioned prior to arrival at the installation site, or may be pre-tensioned at the installation site, e.g., as the overhead electrical cable is being installed.

[0012] In a first embodiment of this disclosure, a device for pre-tensioning an overhead electrical cable is disclosed. The device includes a first sheave configured to receive an overhead electrical cable within the first sheave and rotate the overhead electrical cable around an axis of the first sheave. A second sheave is operatively disposed in spaced- apart relation from the first sheave and is configured to receive the overhead electrical cable from the first sheave and rotate the overhead electrical cable around an axis of the second sheave. The second sheave has a diameter that is greater than a diameter of the first sheave, causing tension to be applied to the overhead electrical cable. In this manner, when the first and second sheaves are rotated at about the same rotational speeds, the angular velocity of the second sheave will be greater than the angular velocity of the first sheave, and the difference in angular velocities will cause the conductive layer of the overhead electrical cable to elongate (e.g., to stretch) in the space that the cable traverses between the first sheave and the second sheave. In one characterization of this pre-tensioning device, the first sheave is formed in a first bull wheel and the second sheave is formed in a second bull wheel.

[0013] In a first refinement of this embodiment of a pre-tensioning device, the pre- tensioning device includes a third sheave operatively disposed in spaced-apart relation from the second sheave and configured to receive the overhead electrical cable from the second sheave and rotate the overhead electrical cable around an axis of the third sheave. In one characterization, the third sheave has a diameter that is greater than the diameter of the second sheave. In another characterization, the pre-tensioning device further includes a fourth sheave operatively disposed in spaced-apart relation from the third sheave and configured to receive the overhead electrical cable from the third sheave and rotate the overhead electrical cable around an axis of the fourth sheave, wherein the fourth sheave has a diameter that is greater than the diameter of the third sheave, causing tension to be applied to the overhead electrical cable.

[0014] In another refinement of the pre-tensioning device, the first sheave is formed in a first bull wheel and the second sheave is formed in a second bull wheel, where the first bull wheel is disposed in spaced-apart relation from the second bull wheel. In the refinement where the device includes a third sheave, the first sheave and the third sheave may be formed in a first bull wheel and the second sheave may be formed in a second bull wheel, where the first bull wheel is disposed in spaced-apart relation from the second bull wheel. In the refinement where the device includes a fourth sheave, the first sheave and the third sheave may be formed in a first bull wheel and the second sheave and the fourth sheave may be formed in a second bull wheel, where the first bull wheel is disposed in spaced-apart relation from the second bull wheel. For example, the first bull wheel and the second bull wheel may each include at least three sheaves, each of the three sheaves in each bull wheel having a different diameter than the other sheaves in the same bull wheel.

[0015] In another refinement of the pre-tensioning device that includes bull wheels, the first bull wheel and the second bull wheel are maintained in spaced-apart relation by at least a first linear actuator that is configured to resist movement of the first bull wheel and the second bull wheel toward each other, e.g., in a direction transverse to the axes of the bull wheels. In one characterization, a first end of the linear actuator is operatively attached to the first bull wheel and a second end of the linear actuator is operatively attached to the second bull wheel. In a further characterization, the pre-tensioning device includes a load cell that is configured to measure a force between the first bull wheel and the second bull wheel when the overhead electrical cable is passed through the pre- tensioning device. For example, the load cell may be in operative communication with the linear actuator to maintain a substantially constant state of tension on the overhead electrical cable as the overhead electrical cable is passed through the pre-tensioning device. [0016] In another refinement of the pre-tensioning device, the device is deployable to an installation site, e.g., by being attached to a wheeled trailer. For example, the device may include a spool bracket assembly that is configured to hold a spool in a manner that enables the overhead electrical cable to be fed from the spool to the pre-tensioning device.

[0017] In another embodiment, a method for pre-tensioning an overhead electrical cable is disclosed. The method includes the steps of rotating the overhead electrical cable through at least a first sheave at a first linear velocity, moving the overhead electrical cable from the first sheave to a second sheave, and rotating the overhead electrical cable through the second sheave at a second linear velocity that is greater than the first linear velocity, causing tension to be applied to the overhead electrical cable.

[0018] In one refinement, during the rotating steps, the angular velocity of the first sheave and the angular velocity of the second sheave are substantially the same. In one characterization, the second sheave has a diameter that is greater than a diameter of the first sheave. In another refinement, during the rotating steps, the angular velocity of the second sheave is greater than the angular velocity of the first sheave. In one characterization, the second sheave has a diameter that is substantially the same as a diameter of the first sheave.

[0019] In another refinement, the method includes, after the step of rotating the overhead electrical cable through the second sheave, moving the overhead electrical cable from the second sheave to a third sheave, and rotating the overhead electrical cable through the third sheave at a third linear velocity that is greater than the second linear velocity, causing tension to be applied to the overhead electrical cable.

[0020] In another refinement, the first sheave is formed in a first bull wheel and the second sheave is formed in a second bull wheel.

[0021] In yet another refinement, the overhead electrical cable includes an outer conductive layer comprising aluminum. In one characterization, the outer conductive layer comprises a plurality of conductive aluminum strands. In yet a further characterization, the aluminum strands are fabricated from annealed aluminum. In another characterization, the aluminum strands have a polygonal (e.g. , non-circular) cross-section. In another characterization, the overhead electrical cable includes a fiber- reinforced composite strength member and wherein the outer conductive layer surrounds the strength member.

[0022] In a further refinement, the method results in the outer conductive layer being strained past the yield point of the conductive material, e.g., of the aluminum. In another characterization, the outer conductive layer is strained by at least about 0.5%.

[0023] In a further embodiment, another method for pre-tensioning an overhead electrical cable is disclosed. The method includes pre-tensioning an overhead electrical cable that includes a central strength member and a conductive outer layer. The method includes the steps of passing the overhead electrical cable through at least a first sheave at a neutral length (original length), and passing the overhead electrical cable through at least a second sheave at a second length that is greater than the original length to apply a tensile force to at least the conductive outer layer of the overhead electrical cable.

[0024] In one refinement, during the passing steps, the distance to travel over the first sheave and the distance to travel over the second sheave become momentarily greater, increasing the length of the conductive outer layer of the overhead electrical cable as it travels over the sheaves. In another refinement, the second sheave has a diameter that is greater than a diameter of the first sheave.

[0025] In another refinement, the method includes, after the step of passing the overhead electrical cable through at least the second sheave, passing the overhead electrical cable through at least a third sheave at a greater operational distance. In another refinement, the first sheave is formed in a first bull wheel and the second sheave is formed in a second bull wheel. In one characterization, the first sheave is formed in a first bull wheel, the second sheave is formed in a second bull wheel, and the third sheave is formed in the second bull wheel. In another refinement, the absolute difference between the original length of the overhead electrical cable and the pre-tensioned length does not exceed the elongation threshold of the strength member. [0026] In yet another refinement, the conductive outer layer comprises a plurality of aluminum strands. In one characterization, the aluminum strands are fabricated from annealed aluminum. In yet another characterization, the aluminum strands have a trapezoidal cross-section.

[0027] In another refinement, the method strains the overhead electrical cable past the yield point of the aluminum. In yet another refinement, the method strains (elongates) the aluminum to a point of permanent mechanical deformation.

[0028] In a further embodiment, a method for the fabrication of an overhead electrical cable is disclosed, where the overhead electrical cable includes a strength member and a conductive outer layer comprising a plurality of individual conductor strands. The method incudes the steps of pre-tensioning the individual conductor strands, and stranding the pre-tensioned conductor strands around the strength member.

[0029] In one refinement, the conductor strands are fabricated from aluminum. In one characterization, the conductor strands are fabricated from annealed aluminum. In another refinement, the strength member comprises a fiber-reinforced composite.

[0030] In another refinement, the pre-tensioning step includes stretching the conductor strands past their yield point. In another refinement, the pre-tensioning step is carried out on a dancer wheel system.

DESCRIPTION OF THE DRAWINGS [0031] FIGS. 1A and 1 B illustrate a bull wheel configuration.

[0032] FIGS. 2A - 2C illustrate a device that is useful for pre-tensioning an overhead electrical cable.

[0033] FIG. 3 illustrates a device that is useful for pre-tensioning an overhead electrical cable. [0034] FIG. 4 illustrates a device that is useful for pre-tensioning a conductor strand before the conductor strand is applied to a strength member to form an overhead electrical cable.

DESCRIPTION OF THE EMBODIMENTS

[0035] Broadly characterized, the present disclosure is directed to methods and devices for the pre-tensioning of an overhead electrical cable, e.g., to stretch the electrically conductive outer layer of the cable beyond its yield point to permanently deform the conductive outer layer. This disclosure may refer to pre-tensioning of the electrical cable, or to pre-tensioning of the conductive outer layer, but in either case it is understood that only the conductive outer layer of the electrical cable is pre-tensioned, and the mechanical properties of the strength member are not substantially affected. The pre-tensioning step may be carried out at any point between the fabrication of the electrical cable (e.g., the winding of conductive strands onto a strength member) and up to the installation (e.g., tension stringing) of the overhead electrical cable. Thus, in one embodiment, an overhead electrical cable may be subjected to the pre-tensioning before being wound upon a spool and transported to the installation site. Alternatively, the overhead electrical cable may be subjected to the pre-tensioning during the installation of the overhead electrical cable. In yet another alternative, the conductive layer (e.g., the conductor strands) may be pre-tensioned before being stranded onto the strength member.

[0036] In certain embodiments, a device for pre-tensioning an overhead electrical cable may include an opposing set of progressively sized bull wheels, i.e., having sheaves of different internal diameters. The bull wheels are progressively sized to effectively stretch the conductive layer between the bull wheels to a tension level that may be predetermined. The overhead electrical cable is strung through the bull wheels starting from the smaller wheels (e.g. , smaller diameter sheaves) and then, depending on the size of the electrical cable and/or the level of desired pre-tensioning, the cable may then leave the device to go into the support structure (e.g., onto the first tower). The device may enable a preset and consistent stretched state of the conductive layer such that the conductive layer is subjected to little or no tension during tension stringing.

[0037] The device may include tension monitoring and tension trimming capability, e.g., to control the amount of tension applied to the conductive layer. Consistent tension on the conductive layer between the largest bull wheel sheaves just prior to the electrical cable leaving the pre-tensioning device may be achieved, for example, by allowing the distance between the bull wheels mounted in the device (e.g., center-to-center distance) to vary by employing a load cell to monitor the tension load and one or more linear actuators to maintain a constant and uniform tension load based on the feedback from the load cell monitor, regardless of rotational speed and/or stopping and restarting of the bull wheels. Such linear actuators may be mechanical (e.g., screw-driven or cam-driven), hydraulic (e.g., a hydraulic cylinder) or pneumatic linear actuators, for example.

[0038] The progressively sized bull wheels may be powered in sequence with a cable puller during the installation of the cable onto the support structures. Pre-tensioning the conductive layer during installation of the cable in the field may provide a consistent state of "relaxed" conductor. This is unlike pre-tensioning the conductor after tension stringing, which is dangerous and can lead to variances in the "relaxed state" of the conductor from span to span. The methods and devices disclosed herein may be used to automate the installation process and eliminate or greatly reduce the specialized skill set that is required to pre-tension the conductor after installation. A pre-tensioned (relaxed) conductive layer will transfer all of the weight of the conductive layer onto the strength member (inner core), which will provide a more stable overhead electrical cable once installed.

[0039] In one embodiment, a bull wheel configuration is disclosed where the bull wheel comprises a plurality of sheaves (e.g., circular grooves) having different diameters. Referring to FIG. 1A and 1 B, an example of such a bull wheel is schematically illustrated, FIG. 1A being a top view and FIG. 1 B being a front view. The bull wheel 102 includes a plurality of sheaves (e.g., sheave 120). The sheaves each include a notched portion (e.g., notched portion 120) that is sized and shaped to receive and secure an overhead electrical cable, for example an overhead electrical cable having a substantially circular cross-section and an outer diameter of at least about 0.5 inches, such as at least about 0.75 inches, and not greater than about 8 inches, such as not greater than about 4 inches, and typically not greater than about 2 inches. As is known to those skilled in the art, the sheaves may be coated or lined with a material to enhance the friction grip between the sheave and the electrical cable, such as a hard rubber or similar material.

[0040] As illustrated in FIGS. 1A and 1 B, the sheaves each have a diameter (e.g., diameter 126 of the largest sheave). The diameter of each sheave is different than the diameter of every other sheave, and as illustrated in FIG. 1A and 1 B, the adjacent sheaves progressively increase in diameter, e.g., toward the rear of the bull wheel 102 as oriented in FIG. 1A. Thus, in one characterization, the bull wheel 110 comprises a first sheave and a second sheave (e.g., at least two sheaves), where the diameter of the second sheave is greater than the diameter of the first sheave. For example, the first sheave may have a diameter that is at least about 18 inches and is not greater than about 24 inches, while the second sheave has a diameter that is at least about 4 inches greater than the diameter of the first sheave. In another characterization, the second sheave has a diameter that is at least about 8% greater than the diameter of the first sheave.

[0041] Although illustrated as including five separate sheaves of differing diameters, it will be appreciated that a bull wheel may comprise as few as two sheaves of differing diameters and up to as many sheaves as may be necessary for the pre-tensioning of a given electrical cable. Further, although each sheave in the illustrated embodiment has a different diameter than any other sheave, the bull wheel may include some sheaves having the same or very similar diameters. For example, a bull wheel may comprise two adjacent sheaves each having a first diameter and two other adjacent sheaves each having a second diameter, where the second diameter is greater than the first diameter.

[0042] The bull wheel 110 is rotatable about a rotation axis 124, and is mounted on an axle 124 to facilitate rotation of the bull wheel 110, e.g., by a motor (not illustrated).

[0043] FIGS. 2A - 2C illustrate a schematic top view (FIG. 2A) and front view (FIG. 2B) of a device for pre-tensioning an overhead electrical cable, e.g., a bull wheel apparatus, and a schematic top view including an overhead electrical cable being passed through the device (FIG. 2C).

[0044] Broadly characterized, the device includes a first sheave (e.g., sheave 220a) configured to receive and rotate the overhead electrical cable 270 (FIG. 2C) around an axis 224a of the first sheave 220a. A second sheave 220b is operatively disposed in spaced-apart relation from the first sheave 220a and is configured to receive the overhead electrical cable from the first sheave 220a and rotate the electrical cable around an axis 224b of the second sheave 220b. The second sheave 220b has a diameter 226b that is greater than a diameter 226a of the first sheave 220a, causing tension to be applied to the overhead electrical cable 270. That is, when the first bullwheel 210a and the second bullwheel 210b are rotated at the same speed (e.g., same revolutions per minute), the overhead electrical cable 270 experiences a momentary increase in linear velocity due to the increase in linear speed imposed by the second sheave 220b through the second sheave travels a greater distance around the second sheave 220b in the same amount of time as it, thereby causing tension to be placed on the overhead electrical cable 270. The second sheave 220bB has a diameter 226b that is greater than a diameter 226a of the first sheave 220a.

[0045] As illustrated in FIGS. 2A - 2C, the pre-tensioning device 200 also includes a third sheave 220c that is operatively disposed in spaced-apart relation from the second sheave 220b and is configured to receive the overhead electrical cable 270 from the second sheave 220b. The third sheave 220c and the second sheave 220b have the same diameter, and when the bull wheels 210a and 210b are rotated at the same rotational speed, the overhead electrical cable 270 is not subjected to tension when passing from the second sheave 220b to the third sheave 220c.

[0046] The pre-tensioning device 200 also includes a fourth sheave 220d that is operatively disposed in spaced-apart relation from the third sheave 220c and is configured to receive the overhead electrical cable 270 from the third sheave 220c. The fourth sheave 220d has a larger diameter than the third sheave 220c, and when the bul wheels 210a and 210b are rotated at the same rotational speed, the overhead electrical cable 270 is again subjected to a momentary increase in linear velocity when passing from the third sheave 220c to the fourth sheave 220d to place further tension on the overhead electrical cable 270.

[0047] It will be appreciated that the pre-tensioning device may include additional sheaves as may be desirable for a particular pre-tensioning application, and in the embodiment illustrated in FIGS. 2A - 2C, each bullwheel includes a total of five separate sheaves. It will be appreciated that other configurations are possible. For example, each bull wheel may include a single sheave (i.e., where the two sheaves of the device have different diameters), each may include two sheaves, three sheaves, four sheaves, or more.

[0048] Further, it is not necessary that each bull wheel have the same configuration, e.g., with respect to the size of the sheaves, such as the device 200 illustrated in FIGS. 2A-2C where each bull wheel comprises five sheaves having diameters that are the same as the five corresponding sheaves of the other bull wheel. For example, one bull wheel may include a sheave having a diameter that is different than the diameter of any sheave on a second bull wheel. Further, the first bull wheel and the second bull wheel may each comprise a plurality of sheaves, where none of the sheaves have the same diameter. Yet further, the device may include a plurality of bull wheels (e.g., two or more bull wheels) where each bull wheel comprises a single sheave.

[0049] The device 200 illustrated in FIGS. 2A - 2C includes a pair of linear actuators 240a/240b allow some controlled movement of the first bull wheel 210a relative to the second bull wheel 210b when the electrical cable 270 is passed through the bull wheels in the manner described above, i.e. , where the tension applied to the overhead electrical cable 270 results in forces tending to pull the bull wheels 210a/210b toward one another. As illustrated in FIGS. 2A - 2C, the linear actuators 240a/240b comprise hydraulic cylinders, where a first end of each hydraulic cylinder is operatively attached to the first bull wheel 210a and a second end of each hydraulic cylinder is operatively attached to the second bull wheel 210b. Although illustrated as including two such hydraulic cylinders, it is contemplated that it may only be necessary to include a single hydraulic cylinder depending upon, e.g., the size of the bull wheels, the tension being applied to the overhead electrical cable, etc. A load cell (not illustrated) may also be included that is configured to measure a force between the first bull wheel 210a and the second bull wheel 210b when an electrical cable is being pre-tensioned by being passed through the sheaves. The load cell may be in operative communication with the hydraulic cylinders to vary the resistance of the hydraulic cylinders to the force of the bull wheels, and thereby control the amount of pre-tension being applied to the overhead electrical cable as the cable passes through the bull wheels.

[0050] The device may also include a mechanism to substantially prevent the bull wheels from rotating backwards, i.e., in a direction that reduces tension on the cable. Referring to FIG. 3, an alternative embodiment of a device 300 for pretensioning an electrical cable is schematically illustrated. The device 300 includes a first bull wheel 310a and a second bull wheel 310b that are configured to pretension an overhead electrical cable 370 passing through the bull wheels 310a/310b, e.g., in a manner similar to that described above with respect to FIGS. 2A-2C. To effect the pretensioning, the first bull wheel 310a has a larger diameter than the second bull wheel 310b, e.g., to move the electrical cable 370 at a faster linear velocity through the first bull wheel 310a and/or to move the electrical cable 370 a greater distance through the first bull wheel 310a as compared to the second bull wheel 310b. Alternatively, or in addition to, a first bull wheel could be rotated at a higher angular velocity than a similarly situated second bull wheel, which would enable the use of two bull wheels having essentially the same diameter.

[0051] The device 300 also includes a linear actuator 340, e.g., to maintain a constant tension between the two bull wheels 310a/310b. As illustrated in FIG. 3, a plurality of sprag clutches (e.g., sprag clutches 332a/332b) are operatively engaged with the electrical cable 370 as the electrical cable 370 passes through the bull wheels 310a/310b. The sprag clutches slightly compress and "grip" the electrical cable 370 as it passes through the device and permit movement of the electrical cable 370 in the desired direction (e.g., clockwise through the bull wheels 310a/310b as illustrated in FIG. 3). In this regard, the sprag clutches are configured to rotate in a counter-clockwise direction (as shown by the arrows in FIG. 3), but are not capable of rotation in the clockwise direction, thereby preventing slippage of the electrical cable 370, e.g., present backwards movement of the cable. It will be appreciated that sprag clutches (or similar devices) may be used in other device configurations, e.g., in the configuration illustrated in FIGS. 2A- 2C

[0052] As noted above, the devices for pre-tensioning an overhead electrical cable in accordance with the foregoing embodiments may be utilized at or near a manufacturing facility, e.g., such that the pre-tensioning can be applied to the conductor after manufacture of the cable (e.g., immediately after stranding the conductor onto the strength member). The devices may also be utilized on a mobile apparatus, e.g., one having wheels to facilitate transportation of the device to a work site where the overhead cable is strung onto support towers under a tensile load. In this embodiment, the apparatus may also include a bracket assembly that is configured to hold a spool of the electrical cable in a manner that enables the electrical cable to be fed from the spool to the pre-tensioning device. Examples of mobile apparatus for the tension stringing of an overhead cable are disclosed in US Patent No. 3,054,572 by Williams et al., US Patent No. 3,203,640 by Garnett, US Patent No. 2,948,483 by Petersen, US Patent No. 2,954,702 by Petersen and US Patent No. 4,343,443 by Grounds, each of which is incorporated herein by reference it its entirety.

[0053] The present disclosure is also directed to methods for pre-tensioning the conductive outer layer of an overhead electrical cable. The methods may be implemented using the devices described above, but may be implemented using other devices that are configured to facilitate the methods.

[0054] In one embodiment, the method includes passing the overhead electrical cable through at least a first sheave at a first linear velocity, and thereafter passing the overhead electrical cable through at least a second sheave at a second linear velocity that is greater than the first linear velocity to apply a tensile force to the conductor of the overhead electrical cable. The difference in the first linear velocity and the second linear velocity causes the conductor to be pre-tensioned, e.g., stretched beyond the yield point of the material constituting the conductive outer layer.

[0055] In another characterization, the method may be described in terms of the distance traveled by the electrical cable. Thus, in one embodiment, the method includes passing the overhead electrical cable through at least a first sheave at its original length, and thereafter passing the overhead electrical cable through at least a second sheave, which causes the cable to travel a greater distance to apply a tensile force to the electrical cable, e.g., to the conductive outer layer. The difference in the original length of the cable and the greater second length traveled by the cable through the device causes the cable to be pre-tensioned, e.g., stretched beyond the yield point of the conductor. In certain characterizations, the absolute difference between the first distance traveled and the altered distance after traveling through the device is at least about 10%, such as at least about 20%, or even at least about 30%.

[0056] Different linear velocities or different lengths of travel may be attained by a difference in the diameter of the first and second sheaves, e.g., where the second sheave has a diameter that is larger than the diameter of the first sheave. In this characterization, for example, the angular velocity of the first sheave and the angular velocity of the second sheave may be substantially the same while the electrical cable is passed through the sheaves - e.g., the angular velocity of bull wheels onto which the sheaves are formed may be substantially the same.

[0057] Depending upon the desired level of pre-tensioning, the electrical cable may be subjected to one or more additional passes through additional sheaves, e.g., as implemented with the device of FIGS. 2A-2C. Thus, in one characterization, after the step of passing the overhead electrical cable through at least the second sheave, the overhead electrical cable is passed through at least a third sheave at a third linear velocity that is greater than the second linear velocity. Characterized another way, the overhead electrical cable is passed through at least a third sheave at a distance that is greater than the second distance through the second sheave. As is disclosed above with respect to the figures, the first sheave may be formed in a first bull wheel and the second sheave may formed in a second bull wheel.

[0058] The method is applicable to a wide variety of overhead electrical cable configurations and is not limited to any particular configuration. In certain characterizations, the conductive outer layer is formed from aluminum strands, and in one particular characterization is formed from annealed aluminum strands such as Type 1350- 0 aluminum strands. The strands may have any cross-sectional shape, including circular or non-circular (e.g., polygonal), and in one characterization the strands have a trapezoidal cross-section. The conductive outer layer may include multiple layers of helically wound strands, such as two or three layers of the strands. In addition to or in lieu of aluminum, the electrical cable may include other conductor materials, including but not limited to copper, which is often used for overhead electrical cables in a railed vehicle (e.g., train) system.

[0059] The overhead electrical cable may include virtually any type of strength member and the method is particularly applicable to electrical cables having very high tensile strength members, such as a fiber-reinforced strength member, e.g., including carbon fibers, Kevlar fibers, high-strength polymer fibers, glass fibers, and the like. One example of such a strength member is illustrated in US Patent No. 7,368, 162 by Hiel et al., which is incorporated herein by reference in its entirety. Another type of strength member to which the methods and devices disclosed herein may be applicable are metal matrix composites, such as those discloses in US Patent No. 6,245,425 by McCullough et al., which is incorporated herein by reference in its entirety. However, it is to be understood that the foregoing methods and apparatus may be used on any type of overhead electrical cable, including but not limited to aluminum conductor steel reinforced (ACSR) cables, and aluminum conductor steel supported (ACSS) cables.

[0060] The devices and methods described above are directed to the pretensioning of an overhead electrical cable, e.g., the methods are applied to an electrical cable having a conductor applied to (e.g., wound around) a strength member. In another embodiment, the individual conductor strands that form the conductive outer layer of the cable may be pre-tensioned before the conductor strands are wound around the strength member to form the electrical cable. That is, the conductor strands may be "stretched," e.g., past the yield point of the conductor. Thereafter, the pre-tensioned conductor strands may be applied to a strength member in a stranding operation.

[0061] FIG. 4 schematically illustrates a device for implementing such a method. The device 400 illustrated in FIG. 4 utilizes a dancer system to control the tension that is applied to a single conductor strand 474 as the strand is passed through the device 400, e.g., the dancer system maintains a substantially constant tension on the conductor strand 474. As illustrated in FIG. 4, the conductor strand 474 enters the device and is engaged by a dancer wheel 480 is configured to move in the vertical direction to control the tension that is placed on the conductor strand 474 as the strand is passed through the dancer wheel 480. Before passing around the dancer wheel 480, the conductor strand 474 passes between a first idler pulley 462a and a first sprag clutch 432a. After passing around the dancer wheel 450, the conductor strand 474 passes between a second idler pulley 482b and a second sprag clutch 432b. In each case, the sprag clutch 432a/432b slightly compresses the conductor strand against the respective idler pulley 482a/482b to "grip" the conductor strand 474 to maintain tension and to prevent movement of the conductor strand in the backwards direction. The dancer wheel 480 is operatively supported in the vertical direction by a cord 484 that is operatively connected to a load cell 446. A weight 486 is attached to the opposite end of the cord 460, which runs through two pulley wheels 488a/488b In this manner, the dancer wheel 480 may move in the vertical direction to maintain a constant tension on the conductor strand 474 as it moves through the device. Although the dancer system 400 is illustrated as comprising a weight attached to the end of a cord running through two pulleys, other configurations may be utilized such as a dancer arm to maintain substantially constant tension on the conductor strand.

[0062] As is discussed above, the conductor strand 474 may be formed from aluminum, including annealed aluminum such as Type 1350-O aluminum. The conductor strands may have any cross-sectional shape, including circular or non-circular (e.g., polygonal), and in one characterization the strands have a trapezoidal cross-section.

[0063] While various embodiments of a method and machine for the installation of an overhead cable have been described, it is apparent that modifications and adaptations of those embodiments will occur to those skilled in the art. However, it is to be expressly understood that such modifications and adaptations are within the spirit and scope of the present disclosure.