GB2343061A

GB2343061A - Protective device for use in replacement of overhead cables

Info

Publication number: GB2343061A
Application number: GB9918473A
Authority: GB
Inventors: Robert Ford
Original assignee: AMEC POWER Ltd
Current assignee: AMEC POWER Ltd
Priority date: 1998-08-06
Filing date: 1999-08-06
Publication date: 2000-04-26
Anticipated expiration: 2019-08-06
Also published as: GB9918473D0; GB9817009D0; GB2343061B

Abstract

There is described a protective device (20) for use in the replacement of a length of overhead power transmission conductor suspended between a pair of towers. The device (20) comprises a plurality of hoops (21) capable of being fitted around the conductor, the hoops (21) being connected by flexible linkages (22) such that the device (20) can be expanded from a collapsed condition in which the hoops (21) lie close to each other to an expanded condition in which the hoops (21) are spaced apart. The device (20) is connected to the towers or to other load-bearing structures so as to be suspended between the towers.

Description

Title-Replacement of Overhead Power Lines This invention relates to the replacement of overhead power transmission lines.

Electricity is supplied to consumers by electricity generating companies predominantly through a network of overhead transmission and distribution lines. Such lines comprise overhead conductors suspended from towers, poles, masts or other structures. Such structures are referred to hereinafter simply as"towers".

As well as building new lines, owners ofthe transmission and distribution systems have aftercare responsibilities covering the maintenance, repair and refurbishment of the transmission lines.

This includes the replacement of conductors, which have to be renewed periodically, for example due to corrosion or lightning strikes. The replacement of an old conductor with a new conductor is called"re-stringing".

Within any given system of overhead transmission lines there may be a number of different designs of tower, eg catering for different voltages. However, for present purposes it is only the distinction between a suspension (or intermediate) tower and a tension tower which needs to be made. A tension tower allows the conductor (s) to be tensioned, while a suspension tower serves only to support the conductor (s) vertically at the correct height. At least two tension towers are required, one at either end of the line, and further tension towers will generally be provided in long lines. The length of the spans between successive towers varies according to local conditions.

When a section of conductor is replaced, it is withdrawn from the circuit between two (or more) tension towers and replaced with a new conductor in a single, simultaneous operation. At each end of the section to be removed a large winch (or"puller/tensioner"as it is referred to) is introduced. One winch pulls out the conductor, and the other provides tension to prevent the conductor sagging towards the ground when being pulled out of circuit. At the tensioning end of the operation a new section of conductor is attached to the old conductor. As the old line is winched out, it pulls the new line in behind it, which it draws from a separate reel.

Where the replacement operation is carried out over areas free of so-called"underlying features" (which include roads, railways, buildings, residential areas, low-voltage power cables, etc), the operation is allowed to proceed without any additional safety procedures. However, the presence along the route of one or more such underlying features necessitates that they must be protected against either breakage of the conductor or tension failure which, whilst occurring only rarely, must be catered for. This normally involves the erection on the ground of scaffolding towers and bridges, nets and specialised cradles over the feature (and to a suitable distance beyond it) to catch the conductor.

In the event of a break, which most commonly occurs at the point where the old and new sections of conductor are joined, the two ends recoil from each other and fall to the ground. If a failure of this type takes place over a barrier, the conductor falls groundwards, strikes the scaffolding or netting and slips to the ground. In some cases the conductor comes to rest on the barrier itself.

Tension failure results in the conductor sinking groundwards and coming to rest in a rather more controlled manner on the barrier.

Whilst the use of scaffolding towers and bridges for the protection of underlying features is generally effective, it is environmentally intrusive, as well as being time-consuming and extremely expensive.

There have now been devised improvements to the replacement of overhead power lines which overcome or substantially mitigate these or other disadvantages of the prior art.

According to a first aspect of the invention, there is provided a protective device for use in the replacement of a length of overhead power transmission conductor suspended between a pair of towers, said device comprising a plurality of hoops capable of being fitted around the conductor, the hoops being connected by flexible linkages such that the device can be expanded from a collapsed condition in which the hoops lie close to each other to an expanded condition in which the hoops are spaced apart, and the device being capable of connection to the towers or to other load-bearing structures so as to be suspended between the towers.

According to a second aspect of the invention, there is provided a method for the replacement of a length of overhead power transmission conductor suspended between a pair of towers, said method utilising a protective device comprising a plurality of hoops capable of being fitted around the conductor, the hoops being connected by flexible linkages such that the device can be expanded from a collapsed condition in which the hoops lie close to each other to an expanded condition in which the hoops are spaced apart, and the device being capable of connection to the towers or to other load-bearing structures so as to be suspended between the towers, and said method comprising the steps of a) raising said protective device, in said collapsed condition, to a position on a first of said towers adjacent the conductor, b) fitting the hoops of said protective device around the conductor, c) connecting said protective device to said first of said towers or other load-bearing structures, d) expanding said protective device from the collapsed condition to the expanded condition, e) connecting said protective device to the second of said towers or other load bearing structures such that said protective device is supported between said towers, f) connecting the end of a new conductor to an end of said length of conductor, and g) drawing said conductor from between said towers, thereby drawing said new conductor into position between said towers.

The device and method according to the invention are advantageous primarily in that their use eliminates the need for protection of underlying features by the erection of scaffolding towers and bridges. Instead, the hoops of the protective device surround the conductor as it is replaced by a new conductor. In the event of a breakage or loss of tension in the conductor the hoops of the protective device retain the conductor or else the conductor falls to the ground below the ends of the protective device into a safe zone.

The hoops are most preferably formed in plastics material. Any suitable plastics material may be used, provided it has the requisite strength and other mechanical properties.

The hoops may have any suitable shape, eg circular or oval. However, the hoops are preferably triangular. The hoops are preferably complete or substantially complete hoops, but may be provided with open portions to facilitate fitting of the hoops around the conductor. More preferably, however, the hoops are provided with means by which they can be opened to facilitate such fitting and then closed. For example, the hoops may have sections which are hinged. Alternatively, the hoops may have a break, the two ends being provided with suitable formations to enable them to be clipped together. In a further alternative, the two ends of the hoop may overlap to form a substantially complete hoop, the resilience ofthe material being such that the overlapping ends are separable to allow introduction of the conductor.

The sizes of the hoops, their separation, and the number of hoops will be dependent on the particular application. However, for a typical application, in which the protective device is intended to protect a roadway, the hoops may have major dimensions of approximately lm and a separation when the device is in the expanded condition of approximately lm. In such a case, the number of hoops may be fifty or more.

The flexible linkages may take any suitable form. For example, the linkages may be ropes or bands attached to successive hoops. In such cases, there are preferably two such linkages, at positions which are substantially diametrically opposed. In other embodiments, the linkages may comprise netting or webbing.

The protective device may be connected to the towers by ropes or the like extending from the end hoops to the towers. Such ropes are preferably connected to winches, which may be mounted on the towers or more preferably on the ground By appropriate operation of such winches the protective device can be expanded and positioned at any desired location in the span between the towers. The sag of the protective device may also be adjusted to correspond to the sag of the conductor during the restringing operation.

In the method of the invention, the sequence of certain of the steps may be altered. For example, the protective device may be connected to the first tower prior to fitting of some or all of the hoops around the conductor. Similarly, the protective device may be connected to the first tower (or other load-bearing structure) at any convenient point during the operation. The protective device may be partially expanded before all the hoops have been fitted around the conductor.

Likewise, the end of the new conductor may be connected to the existing conductor at any convenient point, and not necessarily after the protective device has been installed.

The invention will now be described in greater detail, by way of illustration only, with reference to the accompanying drawings, in which Figure 1 is a schematic view of a pair of overhead transmission line towers positioned on either side of a roadway, showing the conventional method of protecting the roadway during a restringing operation; Figure 2 is a perspective view of a restringing protective device according to the invention; Figure 3 is view similar to Figure 1, showing a first stage in the installation of the restringing protective device; Figure 4 shows a further stage in the installation of the restringing protective device; Figure 5 shows the restringing protective device in a fully installed condition; and Figure 6 shows the restringing protective device being de-rigged.

Referring first to Figure 1, in a prior art method of replacing an overhead conductor 1 suspended between two towers 2,3 positioned on either side of a roadway 4 ("restringing"), the roadway 4 must be protected by the erection of scaffolding towers 5,6 and the suspension of a net 7 between them.

A first puller/tensioner winch 8 is positioned adjacent one tower 2 and serves to draw in the old conductor 1. A second puller/tensioner winch 9, positioned adjacent the other tower 3 maintains tension in the conductor 1. A new conductor is supplied from a reel 10 and is joined to the old conductor 1 at the second winch 9. As the old conductor 1 is drawn off, the new conductor is thus pulled into position.

In the present invention, the need for the erection of scaffolding bridges alongside the roadway (or other"underlying feature") is eliminated by the use of a restringing protective device 20 shown in Figure 2. The device 20 comprises a plurality of identical plastics triangles 21. The triangles 21 are connected to a main tension rope 24 at the top of the triangle, as well as a pair of flexible ties 22 connected to the adjacent triangles 21 at the bottom corner positions such that the device 20 may be collapsed in concertina fashion to a collapsed condition, in which the triangles 21 lie substantially adjacent each other, or extended to an operating condition, in which the triangles 21 are separated from each other. Typically, the triangles 21 have approximately 0.8m sides and the triangles are connected to the ties 22 such that the separation of the hoops in the operating condition is also approximately lm.

The device 20 might comprise a large number of triangles 21, typically fifty triangles or so, but for simplicity only four triangles 21 are shown in Figure 2. Each triangle 21 has a section 23 which can be opened to permit the triangle 21 to be fitted about a conductor and then closed again.

Using the device 20, the conductor 1 is replaced as follows (see Figures 3 to 6). A radio controlled motorised tug 31 or spaces chair is hoisted up the tower 3 and fixed to the conductor 1 in readiness for travelling along the span between the towers 2,3 towards the tower 2. The device 20 is then hoisted up to the conductor 1 and the triangles 21 clipped over the conductor 1. The leading triangle 21 is secured to the rear of the tug 31. The tug 31 may then be moved down the conductor 1 to allow space for fitting of the device 20.

To assist in recovery of the device 20, and in the event of triangles 21 becoming parted, a light rope bond 32 with a length somewhat greater than the span between the two towers 2,3 is installed within the device 20.

Once the device 20 has been fully mounted on the conductor 1, two bonds 33 from winches 34 at ground level are secured to the end triangle 21. The tug 31 is then driven along the conductor 1, the light bond 32 and rear end bonds 33 being let out in a controlled manner to allow the device 20 to elongate to its desired length (see Figures 3 and 4).

Towing of the device 20 continues until the tug 31 reaches the far tower 2, whereupon the tug 31 is removed and bonds 35 from further ground mounted winches 36 are secured to the front end of the device 20, in the same manner as those 33 at the rear end. The light bond 32 is secured to the tower 2. The device 20 is now fully installed and by winching of the bonds 33 and 35 the position of the elongated device 20 can be adjusted to protect the underlying feature, in this case the roadway 4 (see Figure 5).

Restringing using conventional industry practices is now carried out. During these operations, varying tensions applied to the conductor 1 result in varying depths of sag of the conductor 1 within the span. To minimise contact between the device 20 and the conductor 1, the bonds 33,35 are let in or out to give the device 20 a form and shape consistent with the catenary of the conductor 1 to allow it to pass as nearly as possible through the centre of the device 20.

Once restringing has been completed, the device 20 is de-rigged. The winch bonds 35 are removed from the device 20 and the light bond 32 is released from the tower 2 and secured to the end triangle 21 of the device 20. The whole assembly is then towed back to the tower 3 using the winch bonds 33 (see Figure 6). On the device 20 reaching the tower 3, the winch bonds 33 are removed and the light bond 32 used to assist in drawing in the device 20. The triangles 21 are removed from the conductor 1 and the device 20 then lowered to the ground.

Claims

Claims 1. A protective device for use in the replacement of a length of overhead power transmission conductor suspended between a pair of towers, said device comprising a plurality of hoops capable of being fitted around the conductor, the hoops being connected by flexible linkages such that the device can be expanded from a collapsed condition in which the hoops lie close to each other to an expanded condition in which the hoops are spaced apart, and the device being capable of connection to the towers or to other load-bearing structures so as to be suspended between the towers.
2. A device as claimed in Claim 1, wherein the hoops are formed in plastics material.
3. A device as claimed in Claim 1 or Claim 2, wherein the hoops are triangular.
4. A device as claimed in any preceding claim, wherein the hoops are provided with means by which they can be opened to facilitate fitting of the hoops around the conductor and then closed.
5. A device as claimed in Claim 4, wherein the hoops have a break, the two ends being provided with suitable formations to enable them to be clipped together.
6. A device as claimed in any preceding claim, wherein the flexible linkages are ropes or bands attached to successive hoops.
7. A device as claimed in Claim 6, wherein there are two flexible linkages between successive hoops.
8. A method for the replacement of a length of overhead power transmission conductor suspended between a pair of towers, said method utilising a protective device as claimed in any preceding claim, said method comprising the steps of a) raising said protective device, in said collapsed condition, to a position on a first of said towers adjacent the conductor, b) fitting the hoops of said protective device around the conductor, c) connecting said protective device to said first of said towers or other load bearing structures, d) expanding said protective device from the collapsed condition to the expanded condition, e) connecting said protective device to the second of said towers or other load bearing structures such that said protective device is supported between said towers, f) connecting the end of a new conductor to an end of said length of conductor, and g) drawing said conductor from between said towers, thereby drawing said new conductor into position between said towers.
9. A method as claimed in Claim 8, wherein the protective device is connected to the towers by ropes or the like extending from the end hoops to the towers.
10. A method as claimed in Claim 9, wherein the ropes are connected to winches mounted on the ground.
11. A protective device substantially as hereinbefore described and as illustrated in Figure 2.
12. A method of replacing a length of overhead power transmission line substantially as hereinbefore described and as illustrated in Figures 3 to 6.