The stringing activities are mainly consisting of the following:
- Hoisting of insulators
- Paying of conductor/ground wire
- Rough sagging of conductor
- Final sagging of conductor/ground wire
- Clipping/clamping of conductor and jumpering.
Hoisting of Insulators
Following may be noted for hoisting of insulators.
The insulators used on transmission lines are 11KV antifog type Disc insulators havingcapacity as 70KN, 90KN, 120KN and 160KN.
The suspension insulator strings shall be used on all suspension locations (tangent towers) and tension insulator strings on all tension locations (angle towers having15/30/60deg Deviation and Dead end towers).
The insulator strings shall be assembled on ground.
All the insulators shall be cleaned and examined for hair cracks or any damage.
The strings are then hoisted and fixed to the tower cross-arm along with hardware andArial rollers.
The security clips (R pin/W pin) should be properly placed before hoisting.
Paying out of Conductor/Ground wire
Before commencing of string works, copies of sag tension charts (supplied by the design department) showing initial and final sag should be made available at site to EIC.
Normally ground wire drums are mounted on turntable near tension location.
The ground wire is pulled manually or by tractor along the line. The ground wire running blocks are hoisted on the towers prior to taking up this operation.
The ground wire drums should be opened and laid carefully in such a way to avoid cracks/damages.
It should be ensured that no joint in ground wire is within 30m from the suspension/tension clamp.
Sufficient number of aluminum snatch blocks shall be used for paying out of conductor.
Precautions should be taken to avoid conductor rubbing on the ground by providing adequate number of ground rollers.
Additional rollers shall be provided for crossing thorny hedges, fencing and other obstructions which are likely to cause damage to the conductor.
No joints of conductor shall be allowed within 30m from suspension/tension hardware .
In case of railway/road crossing spans, no joints shall be permitted. Further not more then one joint in a span of each conductor shall be permitted.
Prior to taking up of conductor paying activity, the areal rollers are fitted to the towers along with suspension strings.
The conductor drums should be handled carefully at site to avoid any damage to the outer aluminum layer.
The mid span joints and tension hardware are compressed by using hydraulic compression machine.
All the compression joints shall be carefully made and a record of initial and final lengths of the joints signed by the contractor’s and purchaser’s representative should be maintained.
Rough Sagging of Conductor
On completion of conductor paying work between two tension locations (i.e. is line section), the conductors of all the phases shall be anchored near the tension location.
During the rough sagging, the conductor will be lifted above the ground by about 3 meters.
Thus the tension on the wire will minimum.
Final Sagging of Conductor/Ground wire
The conductor and ground wire shall be made to sag correctly as per the approved stringing charts.
All conductors shall be stressed to their maximum working tension at the time of stringing according to the sag tension charts.
Dynamometers shall be used in tensioning of conductors, check for sag should also be made at intervals corresponding to the ambient temperature shown by the thermometer placedat site.
Extra sag of 150mm should be allowed at all important tension locations of railway crossing / major road crossing/ river crossing spans.
It should be ensured that the conductor/ ground wire after being pulled shall not be kept on stringing sheaves for more than 72 hours before being pulled to the specified sag.
During the time when conductor/ground wire is on the stringing sheave (before final sagging) it shall be ensured no damage of conductor/ground wire occurs due to wind, vibrations, vehicles or any other reasons.
The conductor shall be pulled up to desired sag (final sag) and left in aerial stringing sheaves for at least one hour. After which the sag shall be rechecked and adjusted if required, before clipping work is taken on hand.
During final sagging of conductor, the sag at intermediate spans should be verified with the help of sag board provided at suitable distances (as per the approved stringing chart and the corresponding site temperature) on the tower body of intermediate locations.
The stringing of bundle conductor (two or more conductor) shall be carried out by using tension stringing equipment.
Using this method, the conductors are kept under tension during the stringing process to keep conductor clear of the ground and obstacles which may cause conductor surface damage as well as clear of the energized circuits.
This method is applicable where it is desired to keep the conductor off the ground to minimize surface damaged or in areas where frequent crossings are encountered.
The equipments involved in this method are reel stands, tensioner, pullers, reel winder, pilot line winder, splicing cart and pulling vehicle.
One more important reason to use the tension stringing equipment for stringing of bundle conductor is to release, payout, rough sag and final tensioning of all the sub-conductors of the bundle under equal physical tension.
This will ensures equal sags of each sub-conductor in a span and sub-span.
Clipping/Clamping of Conductor and Jumpering
The clipping/clamping work includes fixing of suspension hard wares, providing of preformed armour rods, removal of aerial sheaves, etc. Following required for clipping / clamping:
Conductor shall be clamped within 24 hours of final sagging.
The sag, before clamping, should be checked in the first and last span of the section having eight spans.
The check should be provided at intermediate span also, if the section is having more than eight spans.
At all the suspension locations, the preformed armour rods shall be wrapped keeping the centre of the preformed armour rods at the centre point of the stringing sheaves.
Care should be taken to see that all the strands of the rods are fixed on the surface of the conductor without any void.
The suspension clamps of the suspension hardware should be fixed (bolted) at the centre of the preformed armour rods.
The vibration dampers should be fitted on tension locations at an appropriate distance from the tension hardware (specified by the design department) during the rough/ final sagging of conductors.
The vibration dampers on suspension locations shall be fitted at an appropriate distance on either side of the suspension hardware as specified.
The jumpering work of conductor includes cutting of proper length of jumper providing of jumper cones on both the ends of jumper, fixing of the jumper to the tension hardware on either side of the tension location etc. Care should be taken to compress the jumper cones with hydraulic compressor machine so as to have firm grip with the conductor.
The tightening of bolts/nuts should be proper, over tightening should be avoided.
The clearance with the tower body, after providing jumpers, shall be checked as per the approved tower drawing.
The ground wire jumpers are also prepared using appropriate length of ground wire having compressed jumper cones on either side.
The prepared jumpers shall be then fitted to the tension clamps on either side of the tension locations.
An additional ground wire piece called sister wire is fitted on the newly strung ground wire at the top of all the locations using 2 to 3 numbers of cross by clips, also known as sister wire clamps, on either side of the locations.
The bolts and nuts of cross by clips should be properly tightened.
The vibration dampers for ground wires shall be provided at appropriate distance on either side of the location as specified.
The earth wire stringing is normally done before the stringing of conductor.
The rough sagging and final sagging is done exactly as per the procedure described for conductor.
The approved stringing charts are used for stringing of earth wire.
After final tensioning of the earth wire, the suspension clamps are provided on the suspension locations and compression type dead end clamps are provided on the tension locations.
Jumpering work is done on the earth wire tension tower.
The earth wire suspension and tension clamps are connected to the main tower body using braided copper earth band.
This earth bond gives direct path to the lightning discharges traveling on the earth wire to the mother earth through towers and safe guards the conductors.
Vibration dampers are also provided on the earth wire to damp the Aeolian vibrations. The clamps are required to be properly tightened to ensure proper connectivity and trouble free service.
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