FR2778505A1 - Method of replacement of overhead power cables with prevention of dropping of cable - Google Patents

Abstract

The overhead cable is replaced using a support cable (2) above the power cable (1), using pulley blocks (3) to join the two cables at regular intervals. The support cable is pulled tight to support the power cable, which can then be drawn through the support pulleys. The new cable can then be run over the pulleys, and when it is fully fitted, the support cable can be removed

Description

Method for replacing overhead cables and system for

  self-supporting protection for the implementation of the process.

  The present invention relates to a method for replacing overhead cables as well as a self-supporting protection system for implementing this method. It is primarily intended for the field of cables for transporting electrical energy, but could also be used for other overhead cables, that is to say supported by posts or pylons distant from one another by a range can typically reach several hundred meters, for example of the order of

500 m.

  In the preferred field of application of the invention, that is to say the overhead lines for transporting electric current, the permanent improvement in the quality of service leads the electric energy suppliers to a constant restructuring of the transport network. of energy. This restructuring policy consists essentially on the one hand of improving the electrical insulation of structures, and on the other hand of replacing cables

existing by new cables.

  When the lines are in rural areas, replacing cables, without being particularly easy, does not pose any relatively few safety problems. It is quite different when these

  lines overlook industrial or urban areas.

  However, the development of these areas has meant that existing lines, which were found when they were first installed in rural areas, are now integrated into so-called urban or industrialized areas. It is therefore obviously excluded, when replacing cables, from being able to let them rest temporarily on the ground between two pylons. It is just as obviously necessary to ensure the protection of all works or constructions

overlooked by these lines.

  The conventional technique currently used for replacing the cables of an overhead power line consists, - firstly, in ensuring the mechanical protection of all overhung structures, by means of wooden gantries, various scaffolding, rope nets , cranes, crossing machines, etc., and to ensure the protection of all overhung electrified structures, by cutting the currents therein; and - in a second step, to carry out the actual replacement of the cable, which is conventionally done by hooking one end of the new cable to one end of the cable to be replaced, and to use the old cable as the pulling cable. A pulling machine is arranged at the other end of the section of cable to be replaced, and is used to wind it on reels by pulling the new cable. A braking machine is placed at the other end of the new cable and retains the

  rotation of the reel on which the new cable is wound.

  This entire device is used to maintain the required tension in the cable to prevent it from falling

towards the ground between the pylons.

  When replacing the cables of these lines, it must be taken into account that these cables are heavy, that they are subjected to a high voltage, and that a failure of the positioning machines cannot be totally excluded, hence the need to install protections for all works, constructions, other lines,

  etc. overlooked by the line being renovated.

  The present invention aims to overcome as much as possible, if not completely, these protection constraints, without compromising security. It also aims to reduce the discomfort that may be caused to residents or users of tracks or land near the lines, by reducing the extent of the intervention zones for the necessary equipment and also by reducing the duration of these interventions. It also aims to reduce costs, by restricting the use of heavy protective equipment, and to ensure better environmental protection, by avoiding for example the pruning of wooded areas or the deterioration of buildings. With these objectives in view, the invention relates to a method for replacing an overhead cable supported between two pylons, of the type according to which a new cable is put in place by pulling the old cable at one end at the other end. of which the new cable is connected, this process being characterized in that: - before the draw, a support cable is connected in parallel with the old cable, connected to the old cable by slides distributed over the length of the cable between the two pylons , the slides being fixed on the cable and surrounding the free-sliding cable thereon, the installation of the cable being carried out by pulling it along the old cable, from one pylon to the other, and the cable being supported by the old cable, under the latter, by means of said slides, - the cable is stretched between the two pylons,

  so that the cable thus tensioned passes over

  above the old cable and in turn supports the said cable by means of the said slides, - the old cable is replaced by a new cable, by drawing, the old cable then the new cable running in the slides distributed between the pylons, - And, when the new cable is definitively in place, we relax the cable which passes under the new cable, being supported by the latter via the slides, and we remove the cable by pulling it from a pylon . As will be better understood below, the method according to the invention makes it possible to ensure with high reliability the protection required when replacing overhead cables, without any protective structure on the ground, since any fall or descent of the cable in progress replacement is prevented by the slides distributed along the line and supported by the cable set up and provisionally stretched. In addition, the said cable and the said slides permanently support the old and new cables during the pulling, which makes it possible to greatly relieve the load borne by the pulling and braking means of the cable, compared with the prior art. . The principle of the invention consists in fact in having below and substantially parallel to the existing cable to be replaced, a cable, for example a galvanized steel cable of length adapted to the span between two pylons, and of section adapted to support the weight of the electric cable to be replaced. Then, by a tension on the said cable, exerted between the two pylons, and taking into account the natural deflection of the cable, passing the cable over the cable so as to support it thanks to the slides distributed over the span. The weight of the cable is then supported by the cable, the cable being maintained substantially in its original configuration, that is to say by substantially retaining the curvature imposed by the arrow between two pylons, but without being subjected to the tension forces caused. by the dead weight of the cable, or at least with considerably reduced forces. Its replacement by printing can thus be carried out easily, in complete safety, and

  with lower tensile forces.

  According to a preferred arrangement, the support cable is placed on the old cable by pulling, from a pylon, one end of said cable, by means of a motorized carriage rolling on the old cable, and the slides are fixed on the cable and placed around the cable as and

measurement of the carriage advance.

  The invention also relates to a self-supporting protection system for replacing cables of an overhead line, characterized in that it comprises: - a cable of sufficient length to connect two successive pylons of the line, - a self-propelled trolley adapted to move by rolling on the cable to be replaced and by driving the cable fixed by one end on said self-propelled carriage, - a plurality of slides adapted to be removably fixed at predetermined spacings on the cable, and to surround the cable sliding freely thereon, - and traction means to maintain the

  cable in tension between the two pylons.

  According to other provisions: - the slides are held on the cable with the possibility of rotation around the cable, - the longitudinal positioning of each slide on the cable is ensured by stops integral with the cable, for example by crimped ends on the cable and between which the slider is fixed, each slider comprises at least one pulley intended to roll on the cable, and an articulated frame allowing the positioning around the cable when the frame is in an open position, and means for locking the armature in the closed position, - the self-propelled carriage comprises rolling pulleys driven in rotation by an electric motor powered by a battery fixed to the carriage, - the system also comprises a separate recovery carriage intended for recovery of the self-propelled trolley in the event of the latter failing, the recovery trolley also comprising means s of rolling on the cable, and a hook that can be automatically snapped onto the self-propelled trolley. Other characteristics and advantages will appear

  in the description which will be made of a system

  in accordance with the invention and its implementation for the

  replacement of an overhead power line cable.

  Reference will be made to the appended drawings in which: FIG. 1 is a view of part of an overhead line, showing a range of the line between two support pylons, when the protection system conforms to the invention, - Figure 2 is a corresponding view, the protection being operational and ready for replacing the electric cable, - Figure 3 is an overall view of the line when replacing the electric cable, - Figure 4 is a perspective view of the self-propelled trolley in place on the cable, - Figure 5 is a side view of this trolley, also showing, in a simplified manner, the recovery trolley, - Figure 6 is a perspective view of a slide, shown in its position when the cable is put in place or removed, - Figure 7 is a front view of the slide, further showing, in dotted lines, the open position of the slide allowing its setting place on the c wheat, - 8 is one of the ways detailed view

  fixing the slide on the cable.

  In the drawing of FIG. 1, part of the overhead power line has been represented. There are two pylons 11, 12 supporting the line, of which only one cable 1 has been shown. The drawing is schematic, the distance between the pylons being in practice generally much greater than that shown, proportionally relative to the height of these pylons. The lines 9 in phantom in the figure symbolize any works, power lines or obstacles, overhung by the overhead line and the areas in the vicinity of these obstacles which must in no case be reached by

  the cables of the line being replaced.

  FIG. 1 illustrates the installation, in parallel with the cable 1 to be replaced, of the cable 2 and of the pulley slides 3, by means of a self-propelled carriage 4 which pulls the cable 2 from the first pylon 11, also called by the installation pylon suite, up to the second pylon 12, also called reception pylon thereafter. The cable 2 is for example unwound from a reel 22, placed at the foot of the installation pylon 11, and the slides 3 are fixed to the cable 2 and placed around the cable 1 by operators located at the top of the

  pylon 11, as the carriage 4 advances.

  These operations will be described in more detail below. Figure 2 shows the same portion of line after the carriage has arrived at the second pylon 12, has been removed, and the cable 2 has been stretched between the pylons, thus passing over the cable 1 due to an arrow of the cable less pronounced than that of the cable. It is specified now that, as will be understood hereinafter, the installation of the protection system according to the invention is carried span by span, that is to say each time between two successive pylons along the line, but that the replacement of the electric cable is generally done for a long line length, covering several spans. FIG. 3 illustrates, only by way of example, the case of replacing a cable over four spans, after the cables 2, 2 ′, 2 ", etc., with their slides have been put in place on each span but this number of spans is in no way limiting. The length of each span is conventionally of the order of several hundred meters, for example around 400 m. Cable 2, for example a galvanized steel cable of 10 mm in diameter, consequently has a length greater than this range, for example 500 m, so that it can be unwound, and subsequently rewound, on the reel 21 placed at the foot of the installation pylon 11. The section of the cable is obviously determined according to the weight of the cable to be replaced, to support this weight with a sufficient safety coefficient. The slides are placed at regular intervals, for example from 7 to 8 m, which corresponds to about fifty slides. ur la

  range of 400 m indicated above.

  The self-propelled carriage 4 consists of a chassis 41, formed for example by a thick sheet metal plate, which carries in its upper part two grooved pulleys 42, having a diameter for example of approximately 150 mm and the sheave of which has a width of 70 mm for example, adapted to be able to be placed on the cable 1 as can be clearly seen in FIG. 4. The pulleys 42 are mounted between the plate 41 forming the chassis and a flange 43. The pulleys 42 are mounted in rotation in bearings of the plate 41 and of the flange 43. The pulleys, or at least one of them, are rotated by an electric motor 44, only symbolized in FIG. 5, via a transmission of conventional type, for example a chain 45, placed in a transmission casing 46 located on the other side of the plate 41. The motor 44 is located under a cover 47 fixed to the chassis 41, below the pulleys 42. The lower edge of the flange 43 is connected to cover 47 by safety links és easily removable or carabiners 48 or similar, to the right of each pulley, to avoid derailment of the pulleys relative to the cable and prevent any fall of the carriage once put in

place on cable 1.

  The pulleys 42 are for example made of aluminum, and their groove is coated with a synthetic material, for example rubber, to allow good adhesion of the pulleys to the cable, necessary so that the carriage can advance on the cable while ensuring traction of the

  cable, and without, however, damaging said cable.

  The electric motor 44 is for example a two-speed motor with timed clutch. It is powered by batteries placed in a box 49 suspended under the chassis 41, the weight of this box and batteries ensuring better adhesion of the pulleys on the cable.

  and participating in maintaining the balance of the carriage.

  The chassis 4 is provided on one side with a fastener 50 to which the end of the cable 2 can be connected, and on the other side with another fastener 51 intended to allow, as will be seen later , recovery of the self-propelled carriage 4, in the event of a breakdown, by a recovery carriage 6, also shown

schematically in Figure 5.

  The recovery trolley 6 comprises a metal frame 61 which serves to hold two pulleys 62, arranged in line like the pulleys 42 of the self-propelled trolley, and making it possible to move the recovery trolley on the cable 1. A counterweight 63 ensures the stability of the recovery trolley on the cable 1, and safety chains 67 or the like are provided for detachably connecting the two flanges of the chassis by passing under the cable. The chassis 61 carries at one end a pivoting hook 64, in a position suitable for being able to engage automatically with the attachment 51 of the self-propelled carriage, as seen in FIG. 5, when the two carriages come against each other . The hook 64 can be provided with a return spring. At the other end, the chassis 61 carries an anchor point 65 adapted to connect a rope 66 therein allowing the recovery trolley to be towed, and the self-propelled trolley broken down, as will also be

explained later.

  The slides 3 comprise means 31 for fixing to the cable 2, a metal frame in flat iron 32, and at least one pulley, preferably two pulleys 33, for example made of aluminum alloy,

  superimposed, as seen in Figures 6 and 7.

  The frame 32 is formed of two parts, each having a U shape between the wings of which the pulleys 33a, 33b are mounted. The two parts 32a, 32b are, in use, face to face, being connected on one side by a hinge 34, and on the other by means of holding and mechanical tightening 35, such as for example a stud 35a on which is aimed a wing nut 35b, enclosing a retaining tab 32c extending one of the wings of the part 32a of the frame 32. Thus, the frame 32 can be opened, by pivoting around the hinge 34, as shown in dotted lines in FIG. 7, so that the sliders can be placed on the cable 1, the pulley 33a rolling thereon, as can be seen in FIG. 7. The second pulley 33b situated on the side of the fixing 31 does not normally support no load, and above all serves to ensure better guidance of the cable 1 by preventing it from accidentally rubbing against

the frame 32.

  The fixing means 31 of the slide on the steel cable 2 comprise for example, as shown in FIG. 8, a shell 310 mounted articulated by means of a hinge 311 on a lock body 312, itself fixed by screws 313 on a fastener 36 welded to the frame 32. The shell 310 has a U-shape, to surround the cable 2, and its wing 314 on the side opposite to the hinge 311 extends on the side of the lock body 312 and has a cutout 315 in L shape. A latch 316, slidably mounted in the latch body 312 and pushed by a spring 317, engages with the cutout 315 to keep the shell 310 locked on the body. This system is provided so as to allow easy installation of the slide 3 on the cable 2, without tightening the latter, and therefore allowing free rotation of the slide around the cable. On the other hand, each slide is held longitudinally on the cable by sleeves 21, previously crimped on the cable according to a predetermined interval, the fixing means 31 of each slide being placed between two sleeves. We will now describe the implementation of the equipment which has just been described, to ensure the

  self-supporting protection according to the invention. The description

  which follows concerns the operating mode for a single span, which will obviously be repeated for all the spans covered by the entire cable section

electric to be replaced.

  The required personnel will, for example, be distributed as follows: - at installation pylon 11: three men on the ground whose function will be to prepare and hoist the equipment at the head of the pylon, and two men at the head of the pylon who will have the mission to receive the material

and its implementation.

  - at the reception pylon 12: a man on the ground whose function will be to collect the equipment, and two men at the head of the pylon who will have the mission of receiving the motor trolley 4 and fixing the steel cable 2 to

the reception pylon.

  The equipment will be distributed as follows: At the installation pylon 11: - a coil 22 of steel cable 2 of a length greater than the range considered; the self-propelled carriage 4; - Fifty guide slides 3. This number is given only as an example; it takes into account a span of approximately 400 m and a spacing between two slides of approximately 7 m. These data are determined with precision, on a case-by-case basis, according to the characteristics of the cable and the power line; - a first service pulley and a service cord, for example with a diameter of 16 mm and a length of around 100 m; At the system reception pylon: - a reel of nylon cord with a diameter equal to 10 mm and a length equal to approximately 200m; - the recovery trolley 6; - a second service pulley and another rope

on duty.

  The implementation of this equipment is done as follows: - At the pylon 11 which is used for the installation of the system During their ascent at the head of the pylon, the two men hoist the pulley and the service rope. At the top, they hook the service pulley into the pylon structure, using a sling. The rope, the two ends of which are connected by a knot, is passed through the pulley. This system allows to bring the

material at the head of the support.

  The men on the ground then hook the self-propelled trolley 4 to the service cord and hoist it at the head of the pylon 11. On arrival, it is recovered by the two assemblers who will position it by placing the two pulleys 42 of the trolley on the driver existing 1. The two safety links 48 are put in place, the trolley is temporarily secured to the structure

  metal of the pylon by a rope.

  Then the men on the ground place the reel 22 of steel cable 2 on a metal frame provided with an axis (unwinder). They unwind from this reel about 100 m of cable, which will be coiled on the ground. The end of the cable forms a loop 23. This loop as well as a return pulley 24 (for example with a force of 6 tonnes) are hooked to the service cord and hoisted at the head of the pylon. The return pulley 24 is hooked into the metal structure of the pylon. The steel cable 2 is passed over this pulley, then its end 23 brought to the self-propelled carriage 4 on which it is fixed, for example by a shackle, to ensure the connection between the end loop 23 of the cable and the anchor point 50

carriage 4.

  The men on the ground then place in a receptacle about ten slides 3 and hoist them at the head of the pylon 11 by means of the service cord. This receptacle is hung in the structure of the pylon. The men on the ground will be responsible for regularly supplying pulleys of

  guiding the fitters placed at the head of the support.

  When all the material is thus prepared, the self-propelled carriage 4 is started; its lashing to the pylon is removed, and it rolls on the existing cable 1 and

tows the steel cable 2.

  As the carriage 4 advances, a

  fitter fixed on the steel cable 2 the slides 3.

  These are placed between the two sleeves 21 provided for this purpose and at regular intervals. Then he opens the frame 32 of the slide, rotates it, positions the pulley 33a, located on the side opposite to the fixing 31, on the cable 1 and closes the frame 32 in the

locking with nut 35b.

  The pulleys 33a of the slides roll on the existing conductive cable 1 and hold the steel cable 2 below and parallel to the cable, as

illustrated in figure 1.

  The second fitter continuously monitors the tension of the steel cable 2. It gradually recovers the cable unwound on the ground, and lets it slide in the return pulley 24 while maintaining a low tension on the section of the steel cable in

unwinding course.

  The personnel on the ground unwinds the reel 22 of the steel cable as the carriage advances, avoiding excessive tension in the cable. In the event of excessive tension exerted on the steel cable, the motor of the carriage 4 automatically disengages and remains disengaged for a period of

  predetermined time delay, for example eight seconds.

  After this lapse of time, the carriage leaves in the direction of the draw. This function makes it possible to conserve the tension of the steel cable and to ensure its maintenance substantially in

parallel with the cable.

  At the reception pylon 12, when they climb to the top of the support, the two men hoist the pulley and

the service cord.

  At the top, they hook the service pulley into the pylon structure, using a sling. The rope, the two ends of which are connected by a knot, is passed through the pulley. This system allows to bring the

material at the head of the support.

  A second return pulley 25 (force 6t) is attached to the service cord and hoisted at the head of the pylon. It is hung at the top of the pylon on the metal structure. A sling 26 is attached, 4 to 5 meters below the head, to a node of the metal structure. When the self-propelled carriage 4 arrives at this pylon 12, the engine is stopped and the carriage is stowed at the

  metal structure of the pylon by a rope.

  The steel cable 2 is unhooked from the carriage, the end loop 23 is introduced into the return pulley 25. Then it is pulled down into the pylon barrel, where it is connected by a shackle to the sling

26 previously asked.

  The two safeties 48 of the carriage 4 are removed and the carriage is released from the existing conductive cable, and

  hung on the service cord then lowered to the ground.

  At this time, the self-supporting protection system is in place along the cable 1: the steel cable 2 is hooked into the barrel of the receiving pylon 12, it passes through the return pulley 25, runs along the cable 1 to which it is connected through the slides 3, passes through the return pulley 24 to the installation pylon 11 and

descends to the ground.

  At the installation pylon 11, the steel cable 2 is then tensioned. For this, the men on the ground place, for example, a wedging jaw on the cable 2, connect it to a chain hoist, symbolized in FIG. 2 by the reference 27, and fix the latter at the foot of the pylon 11. Then to the using the hoist, they stretch the

steel cable 2.

  The steel cord being lighter than the cable 1, with a tension of the cord lower than that of the cable, it will therefore be more taut than the latter. The steel cable has been unwound below the conductor cable, and when it is tensioned, it therefore passes over the conductor, as shown in FIG. 2. In doing so, the slides 3 pivot around the steel cable 2 and support the conductive cable 1 over the entire length of the span, which will prevent any fall of the cable to the ground, or curves

excessive of the latter.

  With the self-supporting protection in place, the operation of replacing the conductive cable 1 can begin according to the method schematically illustrated in FIG. 3, which uses the conventional method of pulling the new lb cable by the old cable la, the two cables being connected end to end. in a manner known per se, for example by a connecting sock. The old cable 1a is pulled by a pulling machine 14 and drives, in the direction of the arrows F, the new cable 1b unwound at

  from a reel located on a braking machine 15.

  During the draw, the cables, old and new, are

  supported and guided by the pulleys 33a of the slides.

  We will now describe the system removal operations, after the cable 1 has been replaced: On the pylon 11 which was used to install the system, the men on the ground loosen the steel cable 2 using the hoist 27. The steel cable therefore returns below the cable, the slides pivoting on the

  steel cable and then hanging on the cable 1.

  On the system reception pylon 12, the men on the ground hook the self-propelled cart 4 to the service cord and hoists it at the head of the pylon. When it arrives, it is picked up by the two assemblers, who will position it by placing the two pulleys 42 on the cable 1. The two safety rings 48 are put in place, and the carriage is temporarily stowed at the

  metal structure of the pylon by a rope.

  The installation is removed in the reverse order to the installation. The self-propelled trolley 4 has the function of retaining the steel cable 2 during its winding on the reel 22 and thus of keeping the latter in tension. Also it is positioned on the cable in such a way that its free progression is contrary to the

  pulling direction of the steel cable.

  The steel cable 2 is detached from its anchor point on the sling 26 of the second pylon 12, then it

  is brought back and out of the idler pulley.

  Then its end loop 23 is hooked to the carriage

  self-propelled 4 via a shackle.

  The motor of the carriage 4 is then started. It is recalled that the motor has a timed clutch. then

the lashing is removed.

  At this instant, a fitter, placed at the other end of the span, that is to say at the installation pylon 11, pulls the steel cable 2 backwards towards him. A contrary force being applied to the carriage 4, the motor disengages and the carriage leaves in the direction of

  pull on the cable, towards the first pylon 11.

  At the end of the declutching delay, the carriage starts in the other direction, corresponding to the direction of drive by the motor, putting back the steel cable

2 in tension.

  During the entire return of the carriage 4 to the installation pylon, and to this pylon, as the steel cable is pulled back, a fitter removes the slides 3. For this, he opens the fixing 31 and releases the steel cable 2. Then he opens the frame 32 and releases the whole of the conductor slide. The slides are placed in a receptacle and lowered

on the ground by the service cord.

  The second fitter gradually removes the steel cable 2 and lets it descend into the pylon barrel. As the men on the ground recover

  the steel cable and winds it on the reel 22.

  When the self-propelled carriage 4 arrives at this pylon 11, it is secured to the metal structure of the support

  by a rope, and the engine is stopped.

  The steel cable 2 is then unhooked from the carriage 4. It is brought back, then out of the return pulley 24. Its end loop 26 is hooked to the

  rope service and it went down to the ground.

  The two safeties 48 of the carriage are removed, the carriage is released from the cable 1, then it is hooked to the

  service cord and also lowered to the ground.

  We will now describe the arrangements provided in accordance with the invention in the event of a breakdown of the carriage.

self-propelled 4.

  If a breakdown occurs on the self-propelled truck, it is planned to tow it using the

recovery trolley 6.

  For this, at the pylon 12 for receiving the system, the men on the ground unwind a rope 66 of small diameter, for example 10 mm, and of a length of around m. The end of this rope is hoisted to the top of the pylon 12. An assembler receives it and ties it in the

  structure. The rope hangs along the pylon.

  Secondly, they hang the recovery trolley 6 on the service cord and hoist it to the

top of the pylon.

  The two fitters at the top of the pylon receive the recovery trolley and place it on the cable 1 and temporarily secure it to the structure of the pylon. Safety chains 67 fitted to the trolley

  recovery 6 are implemented.

  An assembler grasps the rope 66 and passes the end of the latter through a service pulley and connects it to the attachment means 65 of the recovery trolley. The carriage is then launched on the cable. By its weight and the curvature due to the deflection of the conductive cable, the recovery trolley will gain speed and

  strike the self-propelled carriage 4 which has broken down.

  At this instant, the hook 64 of the recovery carriage 6 lifts and staples the anchor point 51

of the self-propelled trolley 4.

  When the recovery trolley 6 is anchored to the self-propelled trolley 4, the assembler withdraws the line 66 towards him until the set of two carriages attached to each other arrives at the receiving pylon 12. During this time , to the pylon 11 used for the installation of the system, and as the carriages advance, an assembler continues to fix the slides 3 on the steel cable 2, as indicated above. The invention is not limited to the use of the means which have been described above solely by way of example. In particular, the carriages and slides can be modified in their technological implementation without departing from the scope of the invention. The operating mode described above in detail can also be adapted while retaining the functionality of the

basic principle of the invention.

Claims (9)

  1. Method for replacing an overhead cable (1) supported between two pylons (11, 12), according to which a new cable is put in place by pulling the old cable to which it is connected by one end, characterized in that: - a support cable (2) is placed in parallel with the old cable, connected to the old cable by slides (3) distributed over the length of the cable between the two pylons, the slides being fixed on the cable (2) and surrounding the cable (1) sliding freely thereon, the installation of the cable being carried out by pulling it along the old cable, from one pylon to the other, and the cable being supported by the cable old, under the latter, by means of the said slides, - the cable (2) is stretched between the two pylons (11, 12), so that the tensioned cable (2) passes over the cable (1) and in turn supports said cable by means of said sliders (3), - we proceed to the ramp lacing of the old cable by a new cable, by pulling, the old cable then the new cable running in the slides distributed between the pylons, - and, when the new cable is definitively in place, the cable is relaxed which goes back under the new cable , being supported by it via the slides, and the cable is removed by pulling from a pylon. 2. Method according to claim 1, characterized in that the support cable (2) is placed on the old cable (1) by pulling, from a pylon (11), one end of said cable, by means of a motorized trolley (4) rolling on the old cable, and the sliders (3) are fixed on the cable (2) and put in place around the cable (1) as it advances of the carriage.   3. Self-supporting protection system for replacing cables (1) of an overhead line, characterized in that it comprises: - a cable (2) of sufficient length to connect two successive pylons (11, 12) of the line , - a self-propelled trolley (4) adapted to move by rolling on the cable (1) to be replaced and by driving the cable (2) fixed by one end (23) on said self-propelled trolley, - a plurality of sliders (3 ) adapted to be removably fixed at predetermined spacings on the cable (2), and to surround the cable (1) in free sliding thereon, - and traction means (27) to maintain the   cable (2) in tension between the two pylons.   4. System according to claim 3, characterized in that the slides (3) are held on the cable (2) with a possibility of rotation around the cable.   5. System according to claim 4, characterized in that the longitudinal positioning of each slide (3) on the cable (2) is ensured by   stops (21) integral with the cable.   6. System according to claim 5, characterized in that said stops are formed by end caps (21) crimped on the cable (2) and between which the slide is fixed.   7. System according to claim 3, characterized in that each slide (3) comprises at least one pulley (33a) intended to roll on the cable, and an armature articulated in two parts (32a, 32b) allowing the establishment around cable when the armature is in an open position, and locking means (35) for   keep the armature in the closed position.   8. System according to claim 3, characterized in that the self-propelled carriage (4) comprises rolling pulleys (42) driven in rotation by an electric motor powered by a battery (49) fixed on the carriage. 9. System according to claim 3, characterized in that it further comprises a separate recovery trolley (6) intended for the recovery of the self-propelled trolley (4) in the event of the latter failing, the recovery trolley also comprising rolling means (62) on the cable, and a snap hook (64)   automatically on the self-propelled cart.