GB2341011A

GB2341011A - Apparatus and method for stringing cables

Info

Publication number: GB2341011A
Application number: GB9818589A
Authority: GB
Inventors: Bryan Mitchell
Original assignee: CBS PRODUCTS Ltd
Current assignee: CBS PRODUCTS Ltd
Priority date: 1998-08-27
Filing date: 1998-08-27
Publication date: 2000-03-01
Anticipated expiration: 2018-08-27
Also published as: GB9818589D0; GB2341011B

Abstract

An apparatus for stringing cables, such as OPGW cables, in a controlled way, includes a let-off device for dispensing new cable and a powered puller, including at least one winding in drum, for receiving a cable to be replaced. The apparatus includes a control mechanism for controlling the speed at which the cable is paid out from the let-off device; and a control device operative on the puller, for controlling the tension at which the puller winds in the cable. Also provided is a let-off device with motor which is load sensing in order to control the speed. There is also method of replacing an old cable using the let-off device and the puller by connecting the old and new cables together, connecting the old cable to the puller, setting the tension on the puller and initiating the let-off device.

Description

2341011 APPARATUSES AND METHODS FOR STRINGING CABLES This invention

relates to apparatuses and methods for stringing cables. In particular, but not exclusively, the invention relates to apparatuses and methods for stringing cables, particularly Optical Path Grounds Wire (OPGW) cables, onto electricity transmission towers.

Electricity generating and transmission companies throughout the world have in recent years recognised the potential use of their networks of high tension cables for the transmission of data and communications signals. In particular, such companies have identified the earth wires supported at the tops of transmission towards as being suitable for dual earthing and communications purposes.

Electricity generating and electricity transmission companies (herein 4&electricity companies") therefore have a need to replace unsuitable cables by those suitable for both earthing and data transmission (herein "earth data cables"). Also there is a need for periodic replacement of the earth/data cables as they degrade or become damaged.

Damage to the earth/data cables is a particular concern to electricity companies. Some damage occurs through attacks by birds on the cables. Of greater concern is damage caused by vandalism. Landowners in rural areas usually receive generous compensation when electricity companies have to access their land in order to re-string cables on transmission towers. It is therefore quite common for landowners to encourage cable vandalism, which most commonly involves individuals shooting at the cables with shotguns.

The cheapest type of earth/data cable that electricity companies use is a traditional design of earth wire having a data cable wrapped around its exterior in a spiral fashion, from one end of the cable to the other. Such cables are highly susceptible to bird and vandal damage.

Stronger and more expensive than such spiral wound earth wires are so called ADSS (All Dielectric Self Supporting) cables supported on the arms of the transmission towers, instead of at their tops. For this reason ADSS cables have the advantage that they can sometimes be added to existing transmission networks without the need for replacement of any earth wires. Nonetheless, ADSS cables are not regarded as the optimum design for transmission of data.

The most favoured earth/data cable is the OPGW cable, that must be strung at the tops of the transmission towers in replacement of existing earth wires.

The design of a per se known OPGW is shown in cross section in Figure 1.

In Figure 1 the OPGW cable 50 comprises an outer series of sheathing layers, indicated schematically by reference numeral 51, that serve to insulate and, to a limited extent, armour the cable 50. Within the cable cross section a series of conventional conductor cables 52 are interspersed with optical fibres 53. Figure 1 shows a typical pattern of the cables 52 and fibres 53, although other patterns may be used instead.

The fibres 53 are typically made from a glass material, such as.silica glass, as is well known. Such materials are susceptible to damage, 2 especially in the form of surface cracks. Often these cracks arise as a result of excessive strain in the fibres. During manufacture of the cable 50 the fibres 53 are fed into the sheathing 51 under negative strain, in order to try and minimise this effect.

During installation onto a series of transmission towers in prior art methods a new OPGW cable such as cable 50 (herein "the new cable") is connected in a per se known manner to the free end of an old earth wire that is to be replaced (herein "the old cable"). The new cable is initially stored on a pair of rotatable drums forming part of a tensioner. This is a known device that controls the speed at which the new cable may be dispensed from the drums, through braking of the drums.

An end of the old cable, at the end of the series of transmission towers remote from the tensioner, is connected to one of a series of drums forming part of a motorised device known as a puller. The puller pulls the old cable off the series of towers, and hence draws the new cable onto the towers in replacement of the old cable. The tensioner includes controls that permit an operator to brake its drums and thereby control the degree of sag in the old/new cable combination. In the prior art the tensioner control is an hydraulic circuit that is a simple linear (nonfeedback) control operating passively on the tensioner drums. The only feedback in the control of the tensioner derives from the tensioner operator observing the degree of sag in the old/new cable combination, and adjusting the degree of braking to achieve a desired sag.

This technique suffers a number of disadvantages when the new cable is, as discussed, an OPGW cable.

3 The transmission towers each include one or more sheaves over which the old/new cable combination passes as the puller pulls it along the series of towers. The diameter of the sheaves often is barely large enough to allow the OPGW cable to pass over it without inducing the aforesaid surface cracking. The risk of cracking is greater if the sag in the new cable is too great, as may result from the non-feedback control of the prior ar-t tensioner.

Also, when the puller is first started the cable usually suffers "stiction" (i.e. breakout friction) that initially causes stress in the new cable to rise unacceptably. This also tends to damage the optical fibres.

Thus there is a need for an apparatus and method that permit a new OPGW cable to be strung onto a series of existing transmission towers without significantly inducing the damage mentioned.

Claims

According to a first aspect of the invention, there is provided an

apparatus according to Claim 1 -

A let-off device, suitable for inclusion in the apparatus of the invention, is defined in Claim 2. A puller similarly suitable is defined in Claim 8.

Optional, preferred features of the apparatus, let-off device and puller are defined in dependent claims 3 to 7 and 9 to 14.

According to a further aspect of the invention, there is provided a method as defined in Claim 15.

Any of the features of the invention defined or disclosed independently 4 may be employed on its own, or in combination with one or more other features defted or disclosed herein.

There now follows a description of preferred embodiments of the 5 invention, by way of non-limiting example, with reference being made to the accompanying drawings in which:

Figure I is a cross sectional view of a per se known OPGW cable; Figure 2 shows apparatus, according to the invention, for stringing of an OPGW cable such as shown in Figure 1, the apparatus including a let-off device and a puller according to the invention; Figures 3a and 3b show the let-off device of Figure 2 respectively in plan, elevation and perspective view; Figures 4a and 4b are similar views of the puller of the invention; Figure 5 shows the hydraulic circuit employed in the let-off device; and Figure 6 shows the hydraulic circuit of the puller.

Referring to the drawings, there is shown an apparatus 10 for stringing of cables such as OPGW cables. Figure 1 shows a new OPGW cable 11 being strung onto a series 12a, 12b, 12c of transmission towers. The new cable 11 is shown being strung in replacement of an old earth wire 13.

Although three transmission towers are shown, the invention is applicable when a greater or lesser number of towers 12 are present in a series.

Each tower 12a, 12b etc. includes a conventional sheave block 14, adjacent its top, over which the old and new cables pass.

The new cable 11 is dispensed from a rotatable storage drum 16, via a letoff device 17 according to the invention. The free end 16a of new cable 11 is connected, by means of a per se known swivel joint 18, to the free end 13a of the old cable.

A length of each cable 11, 13 adjacent their mutual connection is protected against damage by a per se known stocking 19. An anti-twist tail 21, that hangs downwardly to inhibit the tendency of the cables to twist by virtue of the bias in their windings, is connected to new cable 11.

Adjacent the last 12c of the series of towers the old cable 13 is taken up by a puller 22 according to the invention. Puller 22 feeds the old cable drawn from the last tower 12c to a rotatable storage drum 23.

The rotatable drum 16 for storing and dispensing of the new cable 11 is releasably mounted in a vertical orientation as shown, in a stand 20 (Figure 3b). The releasable mounting allows virtually any amount of new cable to be strung through periodic replacement of the drum 16, when empty, with a full drum.

The height of stand 20 is adjustable in order to ease mounting of a new drum thereon, and accommodate various drum diameters.

From drum 16 the new cable 11 passes several times about the first 26 of two vertically orientated, rotatable drams 26, 27 forming part of the let6 off device 17. From drum 26 new cable 11 passes to, and several times around, the second 27 of the two drums. Drum 27 is aligned with and adjacent to drum 26 to facilitate passage of cable 11 between the two drums during use of the apparatus. This use of two drums 26, 27 in this way eliminates slip of the cable 11.

The drums 26 and 27 are driven to rotate, at a controlled speed.

Figure 5 shows one form of hydraulic circuit for achieving this control.

Other, not necessarily hydraulic, control mechanisms are possible.

In Figure 5 a motor, such as a spark ignition or diesel engine, is shown at 28. In practice motor 28 is mounted in the chassis 17a of let-off device 17.

The output shaft of motor 28 drives an hydraulic swash plate pump 29, of conventional design, connected in a circuit including a tank T1.

The fluid output of pump 29 is applied in parallel to a pair of hydraulic motors 31, 32 whose output shafts respectively drive the drums 27 and 26 to rotate during use of the let-off device 17. Reduction gears indicated schematically at 33 match the motor outputs to the range of drum speeds.

The angle of the swash plate of pump 29 is adjustable in dependence on the loading in the circuit. The output pressure from pump 29 is applied directly to the reduced area side of a piston 35 acting on the swash plate to alter its angle. The pump output pressure is also applied via line 38 to one end of a spring biassed, double acting pilot control valve 34. The pump pressure is also applicable via valve 34, to the larger area side of the 7 piston 35.

The loading in the circuit is fed back to the control valve 34, via line 36.

This pilot pressure acts on the opposite end of control valve 34 to that of line 38. A spring 37 biases valve 34 assists the action of the pilot pressure in line 36, i.e. it biasses valve 34 to the right in Figure 5.

If during use of the let-off device 17 the loading on the circuit increases, the pressure in line 36 increases so that valve 34 moves to the right. This connects the larger area side of piston 35 to tank, while maintaining the direct connection of the pump output to the reduced area side. This drives piston 35 to the right of Figure 5, with the result that the swash plate angle increases. This increases the volume pumped by pump 29, thereby maintaining constant speed of the motors 31, 32 despite any increase in loading that they experience.

Once the circuit reaches equilibrium, the pressure in line 36 also returns to a balance with that in line 38. The result is that the valve 34 reverts to its null position as shown in Figure 5. In this position leakage flow of the pump pressure to tank maintains equilibrium and the swash plate angle does not change further.

If the loading reduces, the pressure in line 36 falls relative to the pilot pressure in line 38. Valve 34 then moves to the left. This applies the full pump pressure to the larger diameter side of piston 35, which overcomes the effect of biassing spring 37 and the pilot pressure in line 36. Piston 35 then moves to the left, reducing the swash plate angle and hence the volume pumped, until a new equilibrium establishes.

8 The circuit includes a lever actuated onoff valve 39 that switches the flow of hydraulic fluid to the motors 31, 32. Valve 39 is spring biassed to its off position, requiring lever force to move it. There is no latch on the lever of valve 39, so it must be held in position while it is desired to operate the let-off device. When the lever is released the flow of fluid to the motors 31, 32 instantaneously ceases.

Downstream of valve 39 a pressure tapping (line 41) operates respective brakes 42 & 43 for the motors 31 & 32. The brakes are spring biassed to an operative position checking rotation of the drums 26, 27. Only when lever valve 39 is opened does the pressure in line 41 move the brakes to their off positions. Thus when the lever is released the brakes automatically re-apply, stopping rotation of the drums 26, 27.

The set point of the circuit, and hence the rotational speed of the drums 26, 27, may be determined in part by e.g. the throttle or governor setting of the engine 28. The throttle or governor is, during operation, set to a desired position. The output of pump 29 is then fed via a balanced control valve 30 which serves to control the flow to motors 31 and 32 in dependence on the set point determined by the throttle/governor setting.

A biassed safety valve 30a exhausts the pressure fed to valve 30 if the pump output pressure exceeds a predetermined level.

This load sensing nature of the pump control ensures a substantially or completely constant, predetermined let-off speed of the new cable 11. Since the pump is load sensing, the let-off device does not alter the tension in the old/new cable combination.

9 In Figures 4a and 4b the puller 22 has a similar appearance to that of let off device 17. The cable 13 being wound in from the last transmission tower 12c passes several times around each of a pair 61, 62 of vertically orientated, rotatable drums mounted on the chassis 22a of the puller. The cable 13 passes around each of the drums 61, 62 in a similar manner to the arrangement of cable I I on drums 26, 27, in order to minimise slip.

From drum 62 cable 13 is wound onto storage drum 23 releasably mounted on stand 24, in a manner similar to the mounting of drum 16 in stand 20.

An exemplary control circuit for the puller 22 is shown schematically in Figure 6.

A motor 63, such as a spark ignition or diesel engine, has its output shaft coupled to a drive splitting gearbox 64. As a result the rotational output of the motor 63 is applied in parallel to a pair of variable displacement hydraulic pumps 66, 67.

Pump 66 drives a pair 68, 69 of hydraulic motors whose output shafts are respectively coupled, via gearing 71, to drive the drums 61, 62.

A spring biassed, lever actuated, three position valve 72 controls the supply of oil to the pumps 68,69. The valve 72 permits selective rotation of the drums 61, 62 forwardly (to wind in cable 13); and in reverse (to slacken cable 13 e.g. during setting up and finishing operations). Valve 72 also has a central, null position in which no fluid flows to the motors 61, 62 and their returns are connected to a tank T. Valve 72 is latchable in any of the three positions.

Pressure downstream of the valve 72 is fed back, via line 73, to an adjustable, spring biassed counterbalance valve 74. Normally valve 74 is biassed to close the drain to tank T2 from the motors 68, 69. As pressure in the circuit increases the pilot pressure in line 73 overcomes the bias and opens valve 74, regulating the tension induced in the cable 13 (and hence the old/new cable combination). Since the biassing of valve 74 is adjustable, the set tension is also adjustable. Thus the puller 22 controls the cable tension.

Since the counterbalance valve 74 is self regulating, the speed of rotation of the drums 61, 62 always matches the speed at which cable is paid out from the let-off device 17.

Each motor 68 has associated therewith a brake 76, 77, whose basic functioning is the same as that of brakes 42, 43. However in the circuit of Figure 6 the brakes are not driven to their off positions automatically. Instead, a latchable, lever operated, two position valve 78 is used to switch the brakes on and off. The valve 78 must be latched to the off position before the brakes release.

Oil to pilot operation of the brakes is tapped via a shuttle valve 79 interconnecting the supply and return lines for the motors 68,69. Shuttle valve 79 remains open as long as the supply pressure to the motors exceeds the return pressure to tank T2.

It is necessary for drum 23 to be driven, otherwise the incoming cable 13 would spill between drums 61 and drum 23. Drum 23 is driven directly by hydraulic motor 81, supplied via line 82 from pump 67. Motor 81 is 11 connected in a simple circuit, as shown, including a brake 83. Brake 83 operates in an analogous manner to brakes 76 and 77. Latchable, lever actuated valve 84 permits switching of brake 83 while motor 63 runs.

In use of the apparatus of the invention the operators of the let-off device and puller may be separated by a considerable distance, so they would usually be in radio communication with one another. After initial setting up involving connecting the old and new cables, and feeding the old cable onto drum 23, the puller operator would set the required tension, using counterbalance valve 74, and would release the brakes 76, 77 and 81. He would then signal by radio to the operator of the let-off device to start. As indicated the let-off device operator must hold the control lever to an operative position, otherwise the let-off device will instantaneously stop. Other than this the system operates automatically, with the let-off device controlling the paying out speed, and the puller controlling tension.

Although the apparatus and method of the invention have been described in relation to the stringing of OPGW cables, they are equally applicable to numerous other cable stringing operations, both within and outside the power transmission industry. Examples include but are not limited to other kinds of power and data cables; and bridge cables.

12 CLAIMS 1. An apparatus for stringing a cable between a series of two or more supports, the apparatus comprising:

a let-off device, including at least one let-off drum supporting a said cable, as for feeding of the cable from the let-off drum to the first of the series of supports; and a puller, including at least one winding in drum receiving a cable dispensed from the last of the series of supports, characterised in that the let-off device includes a control device for controlling the speed at which the cable is paid out from the let-off drum; and in that the puller includes a control device for controlling the tension at which the winding in drum pulls cable from the last of the series of supports.
2. A let-off device of an apparatus according to Claim 1 wherein the let-off drum is rotatable and the let-off device includes a motor for rotating the let-off drum, the motor being load sensing in order to control the speed at which cable is paid out from the let-off drum.
3. A let-off device according to Claim 2, wherein the motor is an hydraulic pump and motor combination, the pump output being automatically adjustable, in dependence on the loading experienced by the motor, whereby to maintain a predetermined speed of letting off of the cable.
4. An apparatus or a let-off device according to any preceding claim wherein the let-off device includes first and second rotatable let-off drums for supporting a length of cable, the cable interconnecting the first and 13 second let-off drums and extending, in use of the apparatus, to the series of supports.
5. An apparatus or a let-off device according to Claim 4 including a respective first and second hydraulic motor for each of the first and second let-off drums, each said hydraulic motor being operatively connected to a pump whose output is automatically adjustable in dependence on the loadings experienced by the motors.
6. An apparatus or a let-off device according to Claim 5 including a common pump for the first and second hydraulic motors, the output of the pump being adjustable in dependence on the aggregate loading on the said hydraulic motors.
7. A let-off device according to any of Claims 2 to 6 wherein the control device includes a manually moveable member for initiating letting off of cable, the or each let-off drum including a brake biassed to prevent letting off of cable from the let-off device unless the manually moveable member is in a predetermined position.
8. A puller of an apparatus according to Claim 1 wherein the puller includes first and second rotatable winding in drums for supporting a length of cable, the cable extending in use of the apparatus, between:

(i) the last support in the series and the first winding in drum; and (ii) the first and second winding in drums.
9. A puller according to Claim 8 including a third, rotatable winding in drum, the cable in use of the apparatus interconnecting the second and 14 third winding in drums for the purpose of storing the cables on the third winding in drum.
10. A puller according to Claim 8 or Claim 9 including respective third 5 and fourth hydraulic motors for rotating each of the first and second winding in drums, each said hydraulic motor being operatively connected to a pump whose output is automatically adjustable, whereby to maintain a predetermined tension in the cable.
11. A puller according to Claim 10 wherein the third and fourth hydraulic motors are operatively connected to a common hydraulic pump.
12. A puller according to Claim 9 or any claim dependent therefrom, including a fifth hydraulic motor for rotating the third winding in drum.
13. A puller according to Claim 12 wherein the fifth hydraulic motor is operatively connected to a further hydraulic pump distinct from the common hydraulic pump.
14. A puller according to Claim 13 wherein the common and further hydraulic pumps are driven by an engine whose output is shared by the common and further hydraulic pumps, by means of a drive splitting gearbox.
15. A method of stringing a cable onto a series of supports, the method comprising the steps of:

connecting a new cable, stored on a let-off device according to Claim 2 or any claim dependent therefrom, to an old cable on the series of supports; connecting the old cable to a puller according to Claim 8 or any claim dependent therefrom; setting the tension determined by the puller; and subsequently initiating operation of the let-off device.
16, Apparatus, a let-off device or a puller generally as herein described, with reference to or as illustrated in the accompanying drawings.
17. A method generally as herein described, with reference to or as illustrated in the accompanying drawings.

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