Systems, methods and apparatus for transmission line re-conductoring

A method and associated equipment for replacing existing overhead transmission line conductors with new ones while the transmission line remains in service and carrying power. The invention uses the old conductor to pull the new conductor through a series of sheaves installed at the bottom of each tower insulator. Conventional tension-stringing principles are used to assure that both the old and new conductors maintain a safe distance from ground. To achieve this, both the pulling and tensioning (braking) equipment, as well as supply and take-up reels or drums may be elevated to power line voltage and the current is transferred between new conductor and old, while in transit to the line, by means of a brush system and a conducting wheel which may or may not be integral with pulling and tensioning equipment.

FIELD OF THE INVENTION

This invention pertains to the installation of electrical cables or conductors onto towers of high voltage electric power lines. More specifically, the invention is directed to the installation of new conductors by using the existing or old conductor to draw the replacement or new conductor into place. The invention also permits the replacement operation to be carried out while the electric power line remains in service and energized at high voltage. The invention draws on methods and equipment already in use and well defined in the prior art for working on high voltage lines while the lines are energized, referred to in the trade as “live-line maintenance.”

BACKGROUND OF THE INVENTION

In many electric power networks, the growth in generating capacity has outstripped the growth in construction of new transmission lines. This has caused power system design specialists to seek ways to allow existing transmission lines to carry more power. One means of doing so is the replacement of old conductors with new conductors of higher current-carrying capacity. The new conductors may simply be larger if the towers are capable of carrying the extra weight and wind loading. Where the towers are not capable of doing so, special conductors capable of carrying more current with the same or less elongation are now available. These special conductors allow operation at a much higher temperature without posing a safety hazard by exceeding sag limits. Unfortunately, the lines which are the most urgent candidates for re-conductoring are also those which are the most difficult to remove from service, a problem which this invention addresses.

DESCRIPTION OF THE PRIOR ART

The field of prior art deals largely with methods and equipment for putting transmission line conductors into place (stringing) before a line has been commissioned and energized with voltage. The prior art has evolved to the point where it is common to transfer the conductor directly from the conductor supply reel to its overhead position by means of a transportable “tensioner” or brake at the conductor supply end and a transportable “puller” or winch at the conductor pulling end. The tensioner provides sufficient resistance to the pulling force to assure that the conductor does not touch the ground, thus avoiding nicks and scratches that are sources of electrical discharges or corona once the conductor is energized. Tension stringing equipment is supplied by a number of commercial enterprises, e.g., TSE International of Shreveport, La. (www.tse-international.com). Procedures are well established and documented, as for example in the referenceIEEE Guide to the Installation of Overhead Transmission Line Conductors, Product No: SH95170.

FIG. 1illustrates the context of a conventional prior art conductor stringing operation which, in this case is presumed to proceed from right to left. It presumes that tower100and all towers to the right of tower100have been strung and attached to the bottom of insulator strings10with permanent clamps13. It presumes that a section of line between towers101and200is now to be strung and that tower201and those to the left of tower201will be strung in a subsequent stringing operation. Towers100and101are adjacent to one another, as are towers200and201. Stringing blocks14have been affixed to the bottom of insulator strings10on towers101through200, and a lead cable11has been threaded through them in preparation for stringing. Stringing blocks14have two sheaves per conductor at the terminating towers,101and200, one sheave per conductor on intermediate towers (not shown).

InFIG. 2, the conductor4, already installed on the line section to the right of tower100been temporarily tied to anchor12to sustain tension. New conductor4is now being fed from the supply reel61of tensioner60, pulled into place onto tower101and the line section between tower101and200by means of lead cable11which is being pulled from the tower200position, not shown in this figure. The tensioner60must maintain sufficient tension to prevent the new conductor4from sagging to the ground between adjacent spans of the line section101to200. The new conductor4is connected to lead cable11by means of prior art coupler7. The exact length of the pull represented by section101to200will depend on the amount of conductor on supply reel61but is typically 5,000 to 15,000 feet. In the process of replacing stringing blocks14by permanent clamps13, the exact sag of each span between towers has been adjusted to its design value.

FIG. 3shows the pulling end of the prior art operation illustrated inFIG. 2. InFIG. 3, new conductor4has already been pulled through stringing blocks (not shown) on all towers between101and200, but has not yet been pulled through tower200. Puller70consists of an engine (not shown) that drives one or more bull wheels72and73around which lead cable11makes multiple turns so as to gain the friction needed for the pulling function. Lead cable11is coiled around take-up reel71for reuse on the next pulling section.

Bull wheels62and63, and supply reel61, of tensioner60shown inFIG. 1, are typically mounted on special vehicles, which are prior art with respect to conductor installation practice. For purposes of this illustration they are shown inFIG. 2simply as platforms65and66. Similarly, bull wheels72and73, and take-up reel71, of puller70inFIG. 3are also typically mounted on prior art special vehicles but are shown in the figure as simply platforms75and76. Platforms75and76may be mounted on one and the same vehicle.

In addition to the above prior art, dealing with un-energized transmission lines, there is a field of prior art dealing with “live-line” work; specifically change-out or repair of transmission line insulators and hardware while the line continues to be energized. Live-line maintenance and repair takes advantage of a variety of tools and equipment, including personnel “buckets” which can be elevated to conductor level on insulated booms, in which case the bucket is caused to be at the same potential as the conductor or hardware, allowing maintenance personnel to safely put themselves in direct contact with the conductor or hardware. These techniques and equipment have also been used to adjust the sag of existing lines while they remain energized, as documented, for example, in the reference, R. C. Black and R. S. Throop, “A Live Line Method for Retensioning Transmission Line Conductors” CIGRE paper22-10, 1970.

Only recently have means been proposed to replace the conductors over a long line section while a transmission line is still energized. US Patent application publication 20050133244 suggests doing so by putting in place a spare conductor, paralleling the power line, onto which power can be diverted while a formerly active conductor, no longer under high voltage, can be replaced. This method has the disadvantages of: (a) requiring the installation of temporary towers the full length of the line section being strung; and (b) still requiring safety precautions due to the voltage and current inductively coupled to new conductor as it is being strung.

Economic incentives for increasing the transmission capacity of existing power lines suggest that more efficient and more economical means be devised to replace energized conductors, particularly since the power lines for which the greatest incentive for conductor replacement exists are apt to be those which are hardest to remove from service for conductor replacement.

SUMMARY OF THE INVENTION

The invention comprises systems, methods, and equipment to allow an old or existing transmission line conductor to serve as the lead cable to draw a new or replacement conductor into place in its stead while, in one embodiment, both conductors and the line itself continue to carry full current under full voltage. In this embodiment the supply reel of new conductor and associated tensioner are insulated from ground and maintained at line potential, as is the take-up reel and associated puller.

The invention differs from prior art in that it describes equipment modifications, auxiliary equipment, and methods by which the old conductor can be cut, tied to the new conductor, and used as a lead cable to pull the new conductor into place. In the preferred embodiment, the invention may achieve the replacement while providing a continuous path for current to flow over the line section being replaced, thus causing no interruption of power flow. In another embodiment, the invention may achieve the replacement by restricting the power interruption to a very short duration.

For uninterrupted re-stringing, the transfer of current from one conductor to another, at both the tensioning and pulling ends of the operation, is achieved by causing the conductor to make one or more turns around a conducting wheel from which it receives current. The wheel, which rotates as the conductor is pulled, receives its current from a system of brushes tied, in turn, to the fixed conductor at either end of the re-conductoring section.

The System

The following paragraphs describe a number of operations that may be conducted at line potential. None of these operations are believed to be outside the procedural and/or equipment capability of conventional “live-line maintenance” procedures. These procedures may be carried out from trucks with insulated booms, commonly used in transmission line maintenance and repair, or, in the case of operations in close proximity to the tower, by live-line tools designed for use from a grounded position.

FIG. 4provides a simplified schematic diagram of the preferred embodiment of the invention, a system in which an old or existing high-voltage conductor is used to pull a new or replacement high-voltage conductor into place in its stead.FIG. 4shows both the first tower101and the last tower200of the line section over which the previous conductor is to be replaced. Double sheave stringing blocks14, have been attached to towers101and200. Similar blocks with one sheave per conductor have been installed on intermediate towers.FIG. 4also shows a tensioner or similar device60that is used to supply the new conductor4, and a puller or similar device70that is used to take up the old conductor8. New conductor4is attached to the old conductor8by the use of a coupler or similar device7. Puller70is used to pull old conductor8which in turn pulls new conductor4into place through stringing blocks14. The high-voltage line may remain energized during the replacement operation and current may continue to be carried as illustrated by the “Current In” and “Current Out” arrows.

The details of the preferred embodiment can best be understood by illustrating preparations for the connection of the tensioner and the puller assemblies and the conductor replacement operation. This will first be done for the pulling end (tower200) and then the tensioning end (tower101), though both may be undertaken simultaneously.

The Pulling End

InFIG. 5an auxiliary insulator15, temporary guy16, and temporary anchor12have been attached to the old conductor8by means of a coupler7and made to draw up tension using a prior art winch or come-along (not shown), pulling conductor8to the right and down through stringing block14.

InFIG. 6a lead cable11has been threaded through a second sheave on stringing block14and attached to the old conductor8somewhat to the right of the point at which the anchor assembly15,16,12are attached. The lead cable11in this case is capable of carrying full line current. The lead cable11inFIG. 6has been pulled to take up slack.

InFIG. 7the section of old conductor indicated as A inFIG. 6has been cut out of place, leaving the lead cable11carrying the full tension of the conductor to the right of tower200. The anchor assembly15,16,12in turn is carrying the full tension of the conductor to the left of tower200.

FIG. 8shows a schematic of the puller70which supplied the lead cable11inFIG. 6andFIG. 7. Note that prior to the operations cited in those figures, a jumper lead20has been connected from the bull wheel73to the old conductor8by means of a coupler7immediately to the left of tower200, thus providing a path for current from the old conductor8to the right of tower200, through the lead cable11, through a portion of the bull wheel73and to the old conductor8on the line section to the left of tower200, not yet a part of the restringing operation.FIG. 9shows that current path.

FIG. 10shows the same configuration as inFIG. 8andFIG. 9, but near the end of the pulling operation. The new conductor4has been pulled through stringing blocks14over the entire length of the pulling section from tower101to tower200. The new conductor4must now be connected to the old conductor8to the left of tower200in order that the pulling equipment can be removed.FIGS. 11-15illustrate an example means by which the old and new sections of conductor can be connected pending set up for a new pulling section.FIG. 11shows a detail of the line connections ofFIG. 10but with two points, X and Y identified; points to which a tensioning device will be attached.

FIG. 12presumes that a live-line tensioning device (prior art and not shown) has drawn points X and Y closer together, thus creating slack in that segment of old conductor8and the new conductor4which pass through double-sheave stringing block14.

FIG. 13shows that the slack segment of the old conductor8, which was formerly tied to the auxiliary insulator15, has been permanently clamped to new conductor4to the right of tower200by coupler7, thus providing a direct path for current and allowing removal of the brush feed cable20.

FIG. 14shows the same condition but with the tensioning device relaxed and removed, leaving the left hand portion of the new conductor4slack and, inFIG. 15, removed. This leaves the section of line to the left of tower200ready for restringing in the same manner.

The Tensioning End

The foregoing paragraphs illustrated an embodiment of the invention from the pulling end, i.e. tower200. A similar procedure may be used at the tensioning end, i.e. at the first tower in the pulling section, tower101.FIG. 16shows the initial preparation corresponding to that ofFIG. 5for the pulling end. It presumes that new conductor4has already been installed on towers100and all those to the right of tower100. Once again the temporary guy assembly15,16,12is made to pull the conductor4to the left and downward through the stringing block14by means of a prior art winch or come-along. InFIG. 17the end of a new reel of new conductor4has been fed from right to left through the stringing block14and attached to the old conductor8by means of a coupler7to the left of the point of attachment of the temporary guy assembly15,16,17.

InFIG. 18the section of old conductor8designated as A inFIG. 17has been cut away leaving the section of new conductor4to the left of tower101, the section already restrung, supported in tension solely by the temporary guy assembly15,16,12, and the section to the left of tower101, the section to be restrung, supported in tension solely by the tensioner60a more detailed illustration of which is given inFIG. 19. As the puller, described in previous paragraphs, pulls the old conductor through stringing blocks over the line section, defined here by towers101to200, a reel of new conductor61, being attached to the old conductor8by coupler7, is fed onto that line section. As in prior art conventional tension stringing, the tensioner maintains sufficient tension during the pulling operation to prevent the conductor from sagging to unsafe levels while the restringing is taking place.FIG. 20illustrates the path of current during the operation cited above.

Once the conductor has been pulled over the entire length of the pulling section; i.e. from tower101to tower200, tying off procedures identical to those illustrated inFIGS. 11-15for the pulling end can be used at the tensioning end. In this case the coupler7may differ in design since it will be a permanent part of the restrung line.

Insulated Pulling and Tensioning Equipment

FIGS. 19 and 20show that, in one embodiment of this invention the supply reel61, and bull wheels62and63, of tensioner60, and all associated motors, brakes, and auxiliary equipment, may be at full line voltage. They are shown mounted here on insulated platforms65and66, which are described later. Attached to platforms65and66, which may be combined as one, is rapid grounding switch18that in turn is connected to ground via ground connector19. Rapid grounding switch18may be operated by a line tension sensor (not shown) so as to immediately short-circuit the conductor and cause the line be tripped out of service by circuit breakers (not shown) at either end in the event that the tension drop indicates loss of control of conductor clearance to ground. These breakers will operate in less than one half second, before a broken conductor would hit the ground, thus protecting both the stringing crew and the general public from high voltage contact.

FIGS.8,9and10shows that the take-up reel71, and bull wheels72and73, of puller70, and all associated motors, brakes and auxiliary equipment, may also be at full line voltage. They are shown here mounted on insulated platforms75and76, which are described later. As with the tensioner discussed above, a rapid grounding switch18is attached to platforms75and76, which may be combined as one, to ground by ground connector19. For the sake of personnel safety both the tensioning and pulling platforms60and70and all associated equipment may be surrounded by a metallic fence mounted on a high-conductivity ground mat, well connected to ground. Other safety issues are described in later paragraphs.

Current Transfer Methods

FIG. 21shows an embodiment of the invention in the form of specially designed bull wheels of the general form used in the tensioner60shown inFIG. 19(bull wheels62and63) and in the puller70shown inFIG. 8(bull wheels72and73). While only bull wheels62and63are shown inFIG. 21, the following discussion is similarly applicable to bull wheels72and73.

There are two sections to each bull wheel: a friction section and a conducting section. The friction section of bull wheels62and63(labeled62aand63a), is dedicated to maintaining friction for tensioning. The conducting section of bull wheels62and63(labeled62band63b), is dedicated to gaining good electrical contact between new conductor4and bull wheels62and63.

Grooves in the friction section may typically be lined with plastic material to improve friction and prevent mechanical wear of the wheels. Grooves in the conducting section may typically be lined with a conducting liner. The conducting section62band63bof bull wheels62and63may be made with a slightly larger diameter to provide a degree of “wiping” action to enhance contact. The liner in the conducting sections may typically see considerable wear and should be designed for convenient periodic replacement.

FIG. 21further illustrates a brush system. InFIG. 21, there are two rotating brush plates38, one coupled to each of the upper and lower bull wheels62and63. Rotating brush plates38mechanically and electrically connect to conducting section62band63bof bull wheels62and63. A number of brushes39are pressed against rotating brush plate38by springs (not shown) and serve to transfer current from fixed brush holder assembly40which, in turn, is connected by jumper leads20to the source of current supplied to the conductor. The number, size, and properties of brushes39should be selected to assure acceptable current density at the point of brush contact with the rotating brush plate38. All components of the brush system described in this paragraph are in common application.

The number of grooves devoted to electrical contact and the diameter of the bull wheels themselves must be selected to assure acceptable current density at the conductor surface.

It is clear that use of the invention described herein should be limited to periods where there is no risk of lightning strokes. That precaution notwithstanding, the system should be capable of surviving a transmission line short-circuit, either unanticipated or deliberate in the case of loss of tension, without damage to the conductor or to the stringing equipment. Short circuit current can be temporarily limited by inserting a reactor in series with the phase being re-conductored at the feeding substations. Additionally, a bypass may be installed as shown inFIGS. 22 through 24as a precaution against damage to the brush system due to short circuit currents.

InFIG. 22, an auxiliary reactor48is shown in series with the jumper lead20, the remote end of which is identified as point R.FIGS. 23A and 23Bshow a short circuit current bypass system consisting of a high conductivity flat bypass contact disc49, mechanically and electrically bonded to the outside edge of the conducting portion of bull wheels,62aand62binFIG. 21. In close mechanical contact with bypass contact disc49is a roller or metallic brush500which need not be designed to carry current under normal conditions since the gap or surge arrester51will normally be equivalent to an open circuit. In the event the line is subject to a high short circuit current, the voltage drop caused by that current as it flows through the reactor48and the brushes39will cause the gap or surge arrester51to flash over, thus diverting the current from the brush/brush plate39/38path.

FIG. 24shows an electrical equivalence diagram where the combined voltage drop across the brushes39and auxiliary reactor48causes the gap or surge arrester51to flash over, thus diverting short circuit current from the brush assembly.

Another example embodiment of the invention is shown inFIG. 25where an assembly consisting, in part, of a single contactor wheel21, separate and distinct from a bull wheel assembly, is used for transferring electrical current from or to a conductor. The assembly shown inFIG. 25would be installed between the point of departure of a conductor from a standard puller and tensioner and the point of entry or departure of the conductor to its overhead position allowing standard bull wheel configurations to be used for either pulling or tensioning. In this example embodiment the conductor4is caused to pass over a major portion of the contactor wheel21by directioning idler wheels21A. The contactor wheel21groove may be equipped with a high conductivity metallic liner. In this example however, the conductor4is forced into a tight metallic contact with the contactor wheel21by pressure wheels21B, thus establishing good contact independent of the tension on the conductor itself. Arrows indicate the direction of pressure. The pressure wheels21B are fixed to the same framework as the contactor wheel21and held against it by appropriate spring mechanisms (not shown).

FIGS. 26A and 26Bshow still another possible embodiment in which a series of clamps52are mounted on the contactor wheel21, mechanically and electrically clamping the conductor4to the contactor wheel21but releasing prior to the entry or departure point of the conductor from the contactor wheel21.FIG. 26Bshows one possible clamp device52where pressure on operating arm55forces the upper clamp face54, free to pivot about its mounting frame53which is directly attached to the contactor wheel21, against the conductor4or, when subject to tension, lifts the clamp face54off the conductor4and out of the way. The operating arm55may be caused to push or pull the clamp face54by any number of mechanical linkages that sense the position of rotation of the contactor wheel21.

Another embodiment, shown inFIGS. 27A,27B and27C, uses the conductor tension as a means of assuring good contact between the conductor and a contactor wheel21completely separate from standard pulling and tensioning equipment and, in this example, mounted on a separate platform useable both pulling and later tensioning functions without the need for relocation of equipment for the two purposes. It may also serve from that location each phase position of a three-phase transmission line.

This embodiment shows a separate contactor wheel21rotating about an axle that is mounted on a frame22, that frame22resting on a working platform25that, in turn, is mounted on a second frame23by means of suitable insulators. In this case the contactor wheel21is neither driven nor braked and serves only to transfer current from a clamp24, the function of which is explained later, through a jumper lead20, to a brush assembly17to the contactor wheel21and by way of that wheel21to whatever conductor is wrapped around it.

FIGS. 28A and 28Bshow the contactor wheel21assembly ofFIGS. 27A,27B, and27C in somewhat more detail and with the same current transfer scheme cited previously. The contactor wheel21with a soft metal surface or liner should have a sufficient length of conductor-to-wheel contact area to transfer current between the two members without generating excess heat. InFIGS. 28A and 28B, a conductive brush contact plate38, mounted on and rotating with the contacting wheel21and well connected to the surface against which the conductor turns 4 are pressed. A series of brushes39are caused to press against the brush contact plate38by conventional brush support assemblies, the brushes39being supported by and electrically fed by a stationary assembly40which, in turn, is connected to either the incoming or outgoing line conductor as shown in previous figures.

FIG. 29shows the transfer device ofFIGS. 28A,28B, and28C located in mid-span with the old conductor8drawn down to the clamp24preparatory to the stringing operation.FIG. 30shows a conducting lead cable11attached to the old conductor8, fed around the contact wheel21, and to the puller. Pulling up slack on the old conductor8to sever it and achieve the configuration shown inFIG. 30can be achieved by methods outlined previously. It should be noted that longitudinal tension on the transfer device will be approximately balanced throughout this transfer. The puller, which may now be comprised of standard equipment and prior art, need only be adapted for operation at line potential as discussed later.

FIG. 31shows the use of the same transfer device, at the same location, in conjunction with a tensioning device for the pulling section immediately to the left of the one completed while the device was used in conjunction with pulling equipment.

The previous figures presume a single three-phase transmission line with one conductor on each phase. Inasmuch as the techniques described by this invention build on methods well developed for stringing new conductors (without voltage) on new towers, there is no reason that the invention disclosed herein may not be embodied into projects and equipment for simultaneously stringing multiple conductors on each phase.

Insulation of Equipment from Ground

There are many ways in which the principles and equipment disclosed in this invention can be implemented.FIGS. 32A and 32B, for example, show a means way by which a tensioner or a puller might be mounted on a dedicated vehicle26. Height could be reduced by recessing the bottom of the insulators into wells inherent in the truck bed design. The engines or brake assemblies could be incorporated onto the insulated platform or placed on an un-insulated trailer28and29, mechanically coupled to the tensioner or puller by means of an insulated shaft27.

FIG. 33shows one simple manner in which tensioning or pulling equipment can be applied, i.e., by erecting a transportable platform30and supporting it by means of a system of insulators32, gaining truck access by a ramp31that is removed once the equipment is in place.

FIGS. 34A and 34Bshow a collapsible platform30with a ground platform34in which the support insulators32are attached to pivoted support assemblies33which allow the platform to go from its collapsed state, as shown inFIG. 34A, to its erected state, as shown inFIG. 34B, for use.

Recognizing that personnel access may be required while the platform and equipment are at line potential,FIGS. 35A and 35Bshow an insulated arm35, on the end of which is a personnel bucket37. The insulated arm35is attached to swiveling platform extension36. The arm35is capable of being rotated outward and downward to allow safe entrance to or egress from the high voltage platform.

Power Lines of Vertical Configuration

The figures previously shown presume a transmission line in which the conductors are presumed to side-by-side in a horizontal configuration. There is no reason the invention cannot be used on transmission towers on which two transmission circuits are arrayed vertically as illustrated inFIG. 36, which shows a double circuit tower41. In this case it will be necessary to both feed and pull the new conductor from a point to the side of rather than below the conductor being replaced.

The laterally oriented tensioning and pulling may, as illustrated inFIG. 36, require use of an auxiliary block44suspended either from an insulated auxiliary support structure or “gin pole”43, or from a vehicle-mounted insulated structure (not shown). The fed or pulled new conductor4would then pass from the above-described tensioner or puller45through the auxiliary block44to the previously described stringing block9longitudinally onto the line.

Pulling or tensioning in the direction shown inFIG. 36will cause a high overturning moment on the tower and may thus require another auxiliary support structure or “gin pole”43to support a guy wire42which in turn is affixed to a temporary anchor12in the ground.

Description of the Preferred Short Interruption Embodiment

The procedures and equipment cited above for uninterrupted embodiment can be achieved with conventional equipment operating at ground potential if, while the line remains energized prior art hot-line methods are used to:a. Replace permanent clamps7with stringing blocks14on all towers in the restringing line section, i.e. from tower101to tower200inclusively.b. The procedures illustrated inFIGS. 5 through 6are used at both ends of the pulling section except that an insulator assembly15is used to separate the puller or tensioner from the live conductor to which it is attached.FIG. 37shows such a configuration for the pulling end at tower200.
Once the configuration shown inFIG. 37is achieved at both towers100and200, the line may be de-energized, the insulator assembly15mechanically bypassed and removed, and conventional prior art pulling operation undertaken.

Upon completion the tie-off procedures illustrated ifFIGS. 11-15may be used, after which the line may be re-energized. Following energization, stringing blocks14can be replaced by permanent clamps13, and preparations for the next section undertaken. Thus the transmission line is taken out of service during a period of light system loading for several hours while the old conductor is used to pull in the new conductor in the manner described in the no-interruption case above except with both puller and tensioner at ground potential and with no provisions for transfer of current from one conductor to another.

The foregoing has the advantage of using conventional, prior art, equipment but the disadvantage of (a) requiring line interruption during the actual pulling operation, (b) limiting the length of line which can be pulled per day with a given equipment set-up and (c) creating a risk that the transmission line will be unavailable for service if delays or difficulties are encountered in the pulling in of the new conductor.

Safety Precautions

The safety of personnel is a primary concern in any live line maintenance or construction work. The following features are therefore included in the above invention:1. As shown inFIG. 36, elevated platforms and their immediate surrounding work area may be enclosed by a high, well grounded fence46, and the access gate may be interlocked to prevent ingress or egress while pulling is in progress.2. As further shown inFIG. 36, a well grounded ground mat47may extend over the entire area enclosed within the fence cited above to prevent danger from step potential in the event of a short circuit.3. A means of safely entering and leaving the insulated platform while the major equipment is at line potential may be provided.4. Low tension, high tension, and rapid tension change sensors capable of actuating high speed grounding switches at both tensioning and pulling ends of the section being strung may be provided.
There is also a risk inherent in the prospect that, during stringing, a sheave in a particular stringing block will jam, e.g. because of a sheave fracture or a stuck clamp or coupler, thus transferring the full stringing tension, intended to draw a conductor into place, to the tower on which the jam occurs. A jam of that kind can result in damage to the cross arm or even cause the tower itself to topple. This is especially important with live-line stringing, both from the standpoint of a safety and the outage cost of a line which, having justified the extra cost of live-line stringing, is obviously critical to the system.

A system to prevent such damage is illustrated inFIG. 38.FIG. 38Aillustrates a special jam-sensing system consisting of a stringing block56preparing to rotate and to transport the new conductor4into position.FIG. 38Bshows that once the pull has begun, some change in the suspension angle, α, of the insulator/block assembly will result from the pull. Reasonable and safe limits to a can be predicted. InFIG. 38B, the sheave of the stringing block56is rotating normally and the conductor4is moving.

Should the sheave jam either due to a fracture, sticking, or a stuck coupler, rotation of the sheave will stop, thus causing the suspension angle to increase to β as shown inFIG. 38C. This dangerous condition can be detected either by (a) increase of the suspension angle by some pre-calculated amount or (b) increase in the suspension angle of any amount and a failure of the jam-sensing sheave56to rotate. Both suspension angle and sheave rotation can be measured by a variety of existing prior art sensor systems.

Sensors to make such measurements, plus a microwave transmitter to send them to both pulling and tensioning positions, are shown schematically inFIG. 38as57. Since each pulling operation takes relatively little time, the on-board jam sensor can be battery powered. The signal from each block in a stringing section may, within certain severity limits, be cause for alarms and, within greater limits, automatic cessation of the pull.

The jam-sensing sheave system56can easily include measurement of sheave bearing temperature. A high temperature and/or a high rate of rise of temperature may give an early warning that a jam may be pending. The system may also be made to read conductor speed relative to the stringing block, adding to the logic of jam detection. All of the above sensing systems, taken individually, constitute prior art. Their collective use to detect and transmit a sheave jam or impending jam constitutes a part of this invention.

A variety of modifications to the embodiments described herein will be apparent to those skilled in the art from the disclosure provided herein. Thus, the invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof.