Stringing of transmission lines

5.1 HANDLING OF CONDUCTOR AND EARTHWIRE

  1. Handling and transporting of the conductor and accessories shall be carried out in such a manner as to minimize the possibility of damages from abrasion through rough handling or dirt and grit and getting into the reel of the conductor by touching or rubbing against ground or objects, causing injury to the conductor etc. Particular care shall be taken at all tines to ensure that the conductor do not become kinked twisted or abraded in any manner. If the conductor is damaged, the section affected shall be replaced or repaired by putting joint or using repair sleeves or polishing with emery cloth, so as to give satisfactory performance.

  2. At all stage of construction proper care shall be taken so that the conductor surface is smooth enough to be given satisfactory corona and radio interference performance. All equipment used in handling or transporting the conductor such as grips, pulleys slings, cable care etc. shall be so designed and maintained that the surface which may contact the conductor, are kept free of foreign or defects.

  3. Care shall be taken while running out the conductors such that the conductors do not touch or rub against the ground or objects which could scratches or damage to the strands. The conductor shall not be over strained during erection. The conductor shall be run out of the drums from the top in order to avoid damage due to chaffing. Drum battens shall not be removed until conductor drums are properly mounted at the drum station on the line, and battens shall be immediately refitted on the drum if any surplus conductor is left thereon. Drums will be transported and positioned on station with the least possible amount of rolling, immediately after running out, the conductor shall be raised at the supports to the level of the clamps and placed into the running blocks. The grooves of the running blocks shall be of a design that the seat is semi-circular and larger that the diameter of conductor/earth wire and it does not slip over or rubs against the sides. The grooves shall be lined with hard rubber or neoprene to avoid damage to conductor and shall be lined with hard rubber or neoprene to avoid damage to conductor and shall be mounted on well oiled bearings. At all stages of construction proper care shall be taken so that the conductor surface is smooth enough to give satisfactory corona and radio interference performance.

  4. The running blocks shall be suspended in a manner to suit design of the cross-arm. All running blocks specially those at the tensioning end, will be fitted on the cross arms with jute cloth wrapped over the steel works and under the sling to avoid damage to the slings as well as the protective surface finish of the steel work. In case, section towers are used even for temporary terminations, if this be unavoidable, they shall be well guyed and steps shall be taken to avoid damage. The drums shall be provided with a suitable breaking device to avoid damage, loose running out and kinking of the conductor. The conductor shall be continuously observed for loose or broken strands or any other damage. When approaching end of a drum length at least three coils shall be left when the stringing operations are stopped. These coils shall be removed carefully and if another and if another length is required to be run out a joint shall be made as per the recommendations of the conductor manufacturers. normally, the joints shall be so made that these are not required to pass through running out of blocks, the joints will be protected with suitable joint protector sleeves.

  5. The conductors, joint and clamps shall be erected in such a manner that no bird-caging over-tensioning of individual wires or layers or other deformation or damage to the conductors shall occur. Clamps or hauling devices shall, under erection conditions, allow no relative movement of strands or layers of the conductors.

  6. Repairs to conductors, in the event of damage being caused to isolated strands of a conductor during the course of erection, if necessary, shall be carried out during the running out operations with repair sleeves. Repairing of conductor surface shall be done only in case of minor damage, scuff marks etc, keeping in view both electrical and mechanical safety requirements. The final conductor surface shall be clean, smooth and shall be without any projections, sharp points, cuts, abrasions etc.

  7. Repair sleeves may be used when the damage is limited to the outermost layers of the conductor and is equivalent to the severance of not more than one third of the strands of the outermost layer. No repair sleeves shall be fitted within 30 m of tension or suspension clamp or fittings, nor shall more than one repair sleeve per conductor be normally used in any single span.

  8. Conductor splices shall be so made that they do not crack or get damaged in the stringing operation. Only such equipment/methods during conductor stringing which ensure complete compliance in this regard shall be used.

  9. Derricks shall be used where roads, rivers, channels, telecommunication or overhead power lines, railways, fences or walls have to be crossed during stringing operation. it shall be seen that normal services are not interrupted or damage caused to property. Shut down shall be obtained when working at crossing of overhead power lines.

  10. The proposed transmission lines may run parallel for certain distance with the existing 400 KV, 220 KV & 132 KV lines which will remain energised during the stringing period. As a result, there is a possibility of dangerous voltage build up due to electromagnetic and electrostatic coupling in the pulling cables, conductors and earthwires which though comparatively small during normal operations can be severe during switching and ground fault conditions on the energised lines.

  11. The sequence of running out shall be from to down-wards i.e. the earth wire shall be run first followed by power conductors.
    After running out the top conductor the conductor at the opposite side at the bottom level should be run out. After wards the remaining phase conductor shall be run out. imbalance of loads or tower shall be avoidable as far as possible.

  12. Adequate steps to prevent clashing of sub conductors after paying out of conductor before spacers/spacer dampers are installed. Care shall be taken that all the two sub conductors of the bundle are taken from same conductor supplier and preferable from the same batch  so that creep behavior of these remains identical during sagging, care shall be taken to eliminate differential sag as far as possible.

5.2 TREE CUTTING

During paying out of conductor and earthwire and also during stringing in forest area, it shall be ensured that minimum trees are cut and that too to the extent permitted by the forest authorities.

5.3 PULLING IN OPERATION

  1. Before the commencement of the stringing, initial and final stringing charts for the conductor and earth wire showing the sags and tension for various temperatures and spans along with equivalent spans in the lines should be available. The stringing shall be done as per approved stringing charts.

  2. The earth wire shall be strung and securely clamped to the towers before the conductors are drawn up in the order of the top conductors first, then the conductor on the opposite side and in the end the lower conductor at the same side. it shall be ensured that all the conductors of one section should have identical tension time history.

  3. The pulling of the conductor into the travellers comprising of serial and ground rollers shall be carried out in such a manner that the conductor is not damaged or contaminated with any foreign substance and that it may not be rubbed with the rough ground surface, or it damages the standing crops. For this the height of ground rollers shall be fixed in such a manner that the conductor/earthwire does by douching. The travellers shall have size and shops most suitable for the type of stringing and size of conductor involved and as recommended by the conductor manufacturer. These shall be approved by the supervision Engineer before use. The travellers surface in contact with aluminum surface of conductor shall be of such a material that conductor is not damaged, e.g. neoprene rubber or stainless steel. These shall be equipped with high quality ball or roller bearings for minimum friction. They should be inspected daily for free and easy movement in blocks during stinging and sagging.

  4. During pulling out operation the tension in each conductor and earthwire shall not exceed the design working tension of the conductor at the actual prevailing temperature. After being pulled the conductor and earthwire shall not be allowed to hang in the stringing blocks for more than 96 hours before being pulled to the specified sag. During the time the conductors and earthwire are on the stringing block before sagging in, It shall be ensured that the conductors and earthwire are not damaged due to clashing vibration or other causes.

  5. The tensioning and sagging shall be done in accordance with the approved stringing charts before the conductor and earthwire are finally attached to the towers through the earthwire clamps for the earthwire and insulator stings for the conductor. Dynamometer shall be employed for measuring tension in the conductor and earthwire. The dynamometers employed shall be periodically checked and calibrated with a standard dynamometer.
    For 400 KV lines the following additional precautions shall be taken

    1. The stringing of the conductor shall be done by controlled tension method by means of tension stringing equipments. The earthwire may also be strung by the same method. The equipment shall be capable of maintaining a continuous tension of not less than 10,000 kg per bundle which shall be such that the sag for each sub conductor is maintained about 20% greater than the sags specified in the stringing sag tables.

    2. Controlled stringing method suitable for simultaneous stringing of the sub conductors shall be used. All the power conductors making one phase bundle shall be pulled in and paid out simultaneously. The two power conductors of the bundle shall be of matched length and of the same manufacturer and batch to ensure that the two conductor making one phase have identical tension characteristics. After being pulled the conductor/earthwire shall not be allowed to hang in the stringing blocks for more than 96 hours before being pulled to the specified sag.

    3. Necessary arrangements should also be made for paying out the pilot wire for stringing of conductors and earthwire by helicopter in line route where dense forests are involved and paying out of pilot wire by conventional method is not possible.

    4. The complete details of the stringing method which are proposed to be followed should be planned and decided. Before the commencement of the stringing the stringing charts for the conductor and earthwire showing the initial and final sags and tension for various temperatures and spans along with equivalent spans in the lines should be got prepared, checked and approved.

    5. During paying out of conductor and earthwire and also during stringing in forest area, it shall be ensured that minimum trees are cut. If required 4 meters extension to towers may be used to obtain proper clearance. If necessary, earthwire shall also be strung by tension stringing equipment to avoid cutting of trees. When paying out of conductor & earthwire by conventional method is not possible without cutting of large number of trees, paying out may be done through helicopter as detailed.
       

    1. The conductor and earth wire shall be sagged in accordance with the approved stringing charts before they are finally attached to the towers through the earth wire clamps for earthwire and insulator strings for the conductors.

    2. The conductors shall be pulled up to desired sag and left in travellers for at least one hour after which the sag shall be rechecked and adjusted, if necessary, before transferring the conductors from the travellers to the suspension clamps. The conductors shall be clamped within 36 hours of sagging in. The adjustment in sag for creep age of conductor should also be made before finally clamping,

    3. The sag will be checked in the first and last span of the section in case of sections upto eight spans and in one intermediate span also for section with more than eight spans. Any damage to the towers or the conductors through over stressing during stringing shall be awaited.
      Dynamometers shall be used in checking the tension in the conductors and earthwire and these instruments shall be periodically checked with a standard dynamometer. The sags shall also be checked when the conductors have been drawn up and transferred from aerial blocks to the insulator clamps.

    4. The stringing blocks, when suspended on the transmission structure for sagging, shall be so adjusted that the conductor on the travellers will be at the same height as the suspension clamp to which it is secured.

    5. At sharp vertical angles the sags and tension shall be checked on both sides of the angle. The conductor and earthwire shall be checked on the travellers for equality of tension on both sides. The suspension insulator assemblies will normally assume vertical positions, when the conductor is clamped. Sagging operations shall not be carried out under wind, extremely low temperature or other adverse weather conditions which prevent satisfactory sagging.

5.5 CONDUCTOR DAMAGE AND REPAIR

  1. If the conductor is damaged for whatever reason and the damage is not repaired by repair sleeves or emery cloth, the same shall not be used. Repairing of conductor surface shall be done only in case of minor damages, scuff marks etc. which are safe from both electrical and mechanical points of view. The final conductor surface shall be clean, smooth, without any projections, sharp points, cuts or abrasions etc. for giving satisfactory corone and R.I. performance.

5.6 JOINTING

  1. All the joints on the conductor or the earthwire shall be of compression type in accordance with the recommendations of the manufacturers, for which the necessary tools and equipments like, compressors and dies, grease gums presses etc, shall have to be arranged by the contractor. These joint will be made in the best workman like manner, shall be perfectly straight and having maximum strength. Each part of the joints shall be cleaned by wire brush to make it free of rust or dirt etc, and properly greased before the final compression is done with the compressors.

  2. All joints or splices shall be made at least 30 meters away from the structures. No joints or splices shall be made in spans crossing over main roads, railways, small rivers or in tension spans. Not more than one joint shall be allowed in one span. The compression type fittings used shall be of the self centering type or care shall be taken to mark the conductors to indicate when the fitting is centered properly. During compression or splicing operation the conductor shall be handled in such a manner as to prevent lateral or vertical bearing against dies. Care shall be taken to protect the conductor from scratches, abrasions or other damages. After pressing the joint the aluminum sleeve shall have all corners rounded, butts and sharp edges removed and smoothened.

5.7 CUPPING-IN

  1. Clapping of the conductors in position shall be done in accordance with the recommendations of the manufacturer. Conductor shall be fitted with arm our rods where it is made to pass through suspension clamps and with vibration dampers at all the suspension and tension points, as recommended by the manufacturer.

  2. The jumper at the section and angle towers shall be formed to parabolic shape to ensure maximum clearance requirements. Clearance between the conductors and ground and between jumpers and the tower steel works shall be checked during erection and before handing over the line. If pilot suspension string is used with jumpers, the same shall be clamped with jumper.

  3. Fasteners in all fittings and accessories shall be secured in position. The security clip shall be properly opened and sprung into position.

5.8 FIXING OF CONDUCTOR & EARTHWIRE ACCESSORIES

  1. Vibration dampers, arm our rods and other conductor and earth wire accessories shall be installed as per the design requirement and as per the respective manufacturer's instructions. The Vibration dampers, spacers and spacer dampers shall be provided at both ends of each span at suitable distance from the supporting points for each conductor/earthwire as per recommended practice and placement charts of manufacturer. Spacers/Spacer damper shall be fitted within 24 hours of conductor clamping while installing the conductor and earth wire accessories proper care shall be taken to ensure that surface are clean and smooth and no damage shall occur to any part of the accessories.

5.9 FINAL CHECKING, TESTING & COMMISSIONING

  1. After completion of the works, final checking of the line shall be done to ensure that all the foundation works, tower erection and stringing have been done strictly according to the specifications and approved by the supervising Engineer. All the works shall be thoroughly inspected keeping in view the following main points -
    (a) Sufficient backfilled earth is lying over each foundation pit and it is adequately compacted.
    (b) Concrete chimneys and their coping are in good finely shaped conditions.
    (c) All the tower members are correctly used, strictly according to final approved drawing and free of any defect or damage whatsoever.
    (d) All bolts are properly tightened and punched.
    (e) The stringing of the conductors and earthwire has been done as per the approved sag and tension charts and desired clearances are clearly available.
    (f) No damage minor or major to the conductors, earthwire accessories and insulator strings still unattended are noticed and rectified.

  2. After final checking the line shall be tested for insulation in accordance with tests prescribed. All arrangements for such testing or any other test desired shall be done. Any defect found out as a result of such tests, shall be rectified.

  3. In addition to the above it shall be ensured that the total and relative sags of the conductors and earthwire are within the specified tolerances. Such tests shall be carried out at selected points along the route as deemed necessary.

  4. After satisfactory tests on line, the line shall be energised at full operating voltage.

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