This paper elaborates on the various methods of cable laying and the advantages and disadvantages of the same. The common way of cable laying: buried method, tunnel method, cable trench method, pipe laying method, vertical shaft method, hangers and bridge method and so on.
Cable route shall be selected to avoid excess bending portions and to keep them as few as possible. It is recommended that the bending radius at corners should be maintained at least a few times the minimum bending radius, preferably more than 5 metres, as larger radius provides lower side wall pressure. Enough space shall be maintained inside the cable trench. Furthermore, it shall be confirmed from available records of existing services and utilities as well as with sufficient number of trial holes along the trench route, especially at joint bay locations, that the trench route has no obstructions. It shall also be confirmed that no chemical agents, that could be harmful to the cables, are present in the surrounding soil and water along the route. Always use a cable pulling winch. When pulling cable mechanically by the pulling winch, the cabling winch rope is coupled to the cable by means of the pulling eye. The pulling tension is thus transferred to the conductor. Pulling tension should not exceed the maximum permissible pulling tension recommended by Ravin in their technical data.
CABLE DUCT SYSTEM
Larger bending radius is generally required in duct systems as compared to direct buried cable. Since coefficient of friction of ducts is larger than that of cable rollers used in direct burial installation, resulting in higher pulling tensions, consequently the maximum section lengths between joint bays is relatively shorter than that for direct burial. Therefore, each section length should be determined after careful calculation of the pulling tension and the sidewall pressure of the corresponding section. Cable expansion from duct due to thermal expansion should also be considered when dimensions of joint bays are determined. Offset arrangements should be envisaged to absorb the thermal behavior of the cable during its service life. For a single cable in a duct, the inner diameter of the duct should be greater than at least two times the outer diameter of the cable being pulled.
Internal surface of the duct including the surface at joints shall be smooth and free from any defects such as protrusions, projections, cracks, scratches, sharp edges etc., which may cause damage to cable outer sheath during pulling. Any sharp edges at both ends of the duct should be rounded off and pipe collars provided at ends. The PVC pipe collar should be installed at both ends of the ducts so as to prevent the duct edges from damage. Immediately prior to the installation of the cable, the ducts shall be thoroughly cleaned using conduit cleaning devices to ensure that any small pebbles, sand or other foreign material is removed before cable is installed. Use mandrels with a diameter 6 mm less than the duct size to check for breaks or sharp edges in the duct that would damage the cable. On completion of inspection, put the guide rope by blowing in to the pipe and connect to the winch pulling rope.
Each duct should be provided with a draw wire and both ends of the duct should be plugged in a proper manner to prevent ingress of any foreign material like soil, sand, concrete, water etc. Only winch pulling shall be applied for cable duct installation. The cabling winch rope is coupled to the cable by means of pulling eye. The pulling tension is thus transferred to the conductor. The pulling tension should not exceed the maximum permissible pulling tension recommended by Ravin in their technical data.
Lubrications of Cable
The use of cable lubricants, as the cables are pulled into ducts lessens the cable tensions and prevents any scratching of cable jackets due to irregularities in ducts. A liberal use of lubricant is recommended for all pulling operations. Only lubricants that are compatible with the outer sheath material (PVC, HDPE, and/or MDPE) shall be used. Petroleum-based lubricant shall never be used for any formation with polyethylene jackets. The lubricant is usually applied to the cable as the cable enters the duct or the feeding tube.
Cable tunnel dimensions shall be so designed as to secure enough space both for settling of cables on supporting structures and for installation and jointing works. The bending corners of the cable tunnel should also be designed to provide the permissible bending radius to the cable with enough margins. When cables are laid on cable racks or cable supports, they shall then be fixed with cable cleats or clamps at adequate intervals to absorb the thermal expansion and contraction with their lateral or vertical movement within the length between adjacent cleats. If cables are laid without any binding, they may move according to the thermal behaviour and may drop from the rack or supports. Further, the metal sheath of the cable may be subjected to excessive strain at bent portions where the thermal movement of the cable is partially concentrated. In order to avoid this movement, snake installation i.e. laying the cable in uniform waveform, is usually applied along the route on racks or supports, to encourage the absorption of the cable movement smoothly. At the end of the cable route, the cable ends should be set straight and fixed with cable clamps before rising to the sealing ends.
There may be many types of special constructions along the cable routes, such as river crossings, road crossings, cable tunnel and cable bridges. Special care must be taken to lay cables at such locations. It is not possible to list all counter measures to cope with and cover every special construction; however, following are generally recognised as the basic key points that should be taken into account:
a) The thermal environment must be satisfactory.
b) Any thermo-mechanical forces that could be experienced must be adequately constrained or dissipated.
c) Where cables are exposed to air, care must be duly taken of risks due to fire, vandalism, accidental damage by third party. Cables must also be provided adequate protection against direct solar radiation.
d) Exposure to vibration.
e) Specified bending radius must be observed.
Most adequate method should be adopted to pull the cable considering site conditions, so as to avoid any damage to the cable due to excessive mechanical forces experienced during cable installation and to ensure an efficient installation. It must be ensured that the cable to be installed can be pulled without any damage or defect in terms of cable pulling design according to route drawings and site conditions.
CABLE PULLING DESIGN
In order not to damage the cable during pulling, it is recommended to pull the cable keeping both the pulling tension and the sidewall pressure lesser than the permissible figures.
DATA OF CABLE REQUIRED
Following data on the cable is required for cable pulling:
a) Overall cable diameter (mm)
b) Cross sectional area of conductor (mm2)
c) Unit weight of cable (kg/m)
d) Material of conductor (copper / aluminium)
Permissible maximum value of pulling tension, which depends on the method of pulling, is decided by the criteria as detailed under or that as recommended by Ravin:
Fmax = A. dmax
A = Conductor cross sectional are (mm2)
dmax = Maximum permissible stress (N/mm2)
= 50 N/mm2 for single core Copper Conductor
= 30 N/mm2 for single core Aluminium Conductor
Maximum Cable Pulling Tensions will be followed as per below table:
MAX. CABLE PULLING TENSIONS FOR HV CABLES
|Cross Section& Construction||Max. Pulling Tension for Copper||Max. Pulling Tension for Aluminium|
|630 mm² XLPE||3150 kg||1850 kg|
|800 mm² XLPE||4000 kg||2100 kg|
|1000 mm² XLPE||5000 kg||3000 kg|
|1200 mm² XLPE||6000 kg||3600 kg|
|1600 mm² XLPE||8000 kg||4800 kg|
|2000 mm² XLPE||10000 kg||6000 kg|
|7 2500 mm² XLPE||12000 kg||7500 kg|
XLPE insulated HV cables = 5000 N
Cable laying/pulling on rollers = 1500 N
Cable laying/pulling through plastic pipes = 7500 N
COEFFICIENT OF FRICTION
Following are the coefficient of friction between the cable outer sheathing and the rubbing surface during laying process.
Pulling cables over rollers : ? = 0.15 – 0.3
Pulling cables through concrete ducts : ? = 0.4 – 0.6
Pulling cables through concrete ducts:
– Grease Lubrication: ? = 0.15 – 0.25
– Water Lubrication: ? = 0.15 – 0.25
– Grease and Water Lubrication: ? = 0.10 – 0.20
If cables are to be pulled through long plastic pipes, it is essential that a lubricant be used. Otherwise, the heating produced as a result of friction between the cable and the plastic surface could cause the thermoplastic sheathing to stick to pipe walls.
MINIMUM TEMPERATURES FOR CABLE LAYING
High Voltage Cable should not be laid at temperatures below -2°C. In case, the cable must be laid at temperatures below -2°C, the cable must then be heated in special insulated tents at 30°C – 40°C for 24 hours and then must be laid rapidly.
CABLE PULLING FORCE CALCULATIONS
a) Horizontal/Flat Cable Pulling
The pulling force (F) at the end of the cable path is given by: F = W.I.?
Where: W = Cable weight (kg/m)
I = Length of cable route / path (m)
? = Coefficient of friction
b) Sloping Cable Path Pulling
The pulling force (F) will be increased when laying sloping up/uphill and decreased when laying sloping down/downhill and is given by: F = WI (? cos b ± sin b)
Where: b = angle of slope
+ is used for uphill laying
– is used for downhill laying
On slopes up to approximately 20 deg. (36%), the pulling force may be calculated by:
F = (W.I.?) ± (W.h)
Where: h = difference in level (mm)
c) Curved Cable Path
When the cable is pulled around a curve, the pulling force is increased by a factor (f), which is dependent on the coefficient of friction (?) and the angle of the bend (µ)
Fo = Fi x e?µ = Fi x f
Fo = Force at exit of bend
Fi = Force at entry of bend
? = Coefficient of friction
µ = Bend angle in radians
f = e?µ, factor from table
Table for Value of f (e?µ=f):
Bend Angle ?= 0.1 ?= 0.15 ?= 0.2
Degrees Radians (on new roller)
CABLE LAYING WORK
This section describes general instructions to be observed during cable pulling work.
a) Handling of Cable
At the time of acceptance and prior to installation, the cable reel should be thoroughly examined for any outside physical damage or damage to cable end caps. This examination should also include lagging, reel flanges and reel marking, tags and labels. While handling cables, necessary precaution should be taken to prevent damage to the cables.
Correct Handling Cable Drums
Cable reels that are to be transported on trucks must be securely blocked or anchored to prevent rolling. If a reel of cable is to be rolled for any appreciable distance, it shall be rolled in the direction indicated by the arrow on the reel flange or with the outside end of cable trailing, if not marked, so as not to loosen the turns of cable on the reel. When rolling reels on the ground, they shall be carefully eased over any rough surfaces. Care shall be taken to avoid damaging reel lagging in order that outer layer of cable remains protected. In removing lagging from cable reels, care shall be taken not to damage the cable. Remove all nails, staples or bolts from inside flange before unreeling cable. Care is to be taken to prevent cable from dragging.
Before Cable Pulling
1) It shall be confirmed that trench floor or other places where the cable is to be installed are free from any foreign materials such as stones, nails and pieces of concrete, asphalt, brick pieces, glass pieces, sharp-edged objects etc., which may damage the cable. Such materials should be removed at least 30 cm away from the trench. Any water inside the trench should be pumped out completely. The trench wall must be secured appropriately in accordance with specification prescribed by the client. The cable trench curves must be checked thoroughly, to ensure that the minimum bending radius of the cable is attained. Sweet sand (or backfill soil as required by project specifications) shall be evenly distributed on the trench floor with minimum thickness of 150 mm or to a thickness prescribed in the project specifications.
2) Prior to start of cable pulling, Ravin recommends to conduct the DC sheath test on the reel in order to ensure that no damage has taken place during transporting and handling of the cable. Upon completion of the test heat shrinkable end caps, to protect against ingress of water or foreign particles, must immediately seal the exposed area. Whenever a cable is cut, the exposed end shall be sealed to protect from rain, mist or dripping or splashed water by heat shrinkable caps.
The drum should be mounted on transport and laying trailer. For cable pulling, the cable drum shall be set at the opposite direction to the arrow painted on the drum that indicates the direction of rolling during transportation. Care should be taken not to damage the cable by battens when removing them from the cable drum. The direction of pulling should be so chosen so as to require the least pulling strength at the end of the cable path.
3) Reliable communication between the drum, the head of the cable, on curves, entrance to duct, intermediate post and the winch has to be established to effectively coordinate the work process. The equipment to be used should be of proven reliability and should be thoroughly tested before cable installation begins. Ensure that the winch has a pulling recorder and a tension meter, to continuously monitor the cable pulling force. It should not exceed the maximum pulling force of the cable recommended by Ravin.
4) Essential words or signals for cable pulling, at least START and STOP, are fixed and understood by all the associated staffs. The meeting prior to cable pulling is recommended to decide position of watchmen with communicative equipment to guide cable pulling works. Watchmen shall be positioned every 50 metres and at every corner.
5) It shall be confirmed that cable drum, drum brake, winch, rollers, wire rope, drum for wire rope and any other necessary tools/equipment have been set properly. Swivel shall be installed between wire rope and pulling eye to prevent the cable from twisting. At the entrance to the cable trench, a cable guiding and bearer stand is positioned, over which the cable taken off the cable drum is pulled into the trench. The rollers that are inclined towards the centre ensure that the cable is fed centrally into the trench. The rule for curves in the trench is that under no circumstances should they fall below the minimum prescribed bending radius of the cable (15 x ØD of cable). The cable rollers in straight portions shall be set at approximately 2 metre intervals to avoid the slack of the cable pulling. Whereas the cable rollers in bend portions shall be set as close as possible, so as not to be subjected to the sidewall pressures partially. Sufficient numbers of corner cable rollers are positioned on the curves in the trench, to prevent cable sliding off. In field practice anchor pins have proved ideal for anchoring the corner rollers.
6) Final check of trench shall be carried out so as to eliminate the risks that may damage the cable. And it shall be confirmed that trench will not collapse at any portion along the cable route. Timbering/Shuttering shall be provided securely prior to cable pulling at positions where trench may or is liable to collapse.
7) When the cable is pulled into shaft or inclined tunnel, sufficient capacity of brake facilities shall be applied so that the cable may not slip down due to its weight.
During Cable Pulling
1) Maximum pulling speed shall be 7 m/min. The pulling speed shall be reduced at corners, inlets of duct, etc. where the smooth pulling may be blocked. After confirming that no damage has occurred at these portions, the pulling tension may be increased. Necessary action should be taken to eliminate the cause of damages, in case of its occurrence.
2) Watch the cable on the cable rollers, especially at bend portions and maintain sufficient clearance between existing services and the cable along the whole cable route all the time during cable pulling.
3) The pulling tension and the sidewall pressure should never exceed the permissible values. It is essential to be able to control the speed of pulling by monitoring pulling force speed and the length. These parameters should be recorded continuously on a recorder, which is an indication and proof of the correct laying. When pulling force exceeds the maximum permissible setting, the pulling should stop.
4) The outlook of the cable shall be checked and it shall be confirmed that the cable has not been damaged during cable pulling. The cable pulling should be stopped immediately on finding any damage and should not be resumed until the causes are removed. The damage portion of cable shall be repaired as required.
5) When the inner end of the cable on the cable drum is loosened during cable pulling, it shall be tightened again to the cable drum.
After Cable Pulling
1) The outlook of the cable shall be checked whether the cable has been damaged or not. Any damage shall be repaired in a proper manner after careful examination.
2) Integrity of anti-corrosive sheath shall be confirmed by carrying out DC high voltage test on it before and after backfilling. Immediately after the first phase of backfilling and prior to final backfilling, DC sheath test at 10 kV for 1 minute should be conducted to ensure that no damage has occurred to the cable sheath during cable pulling. Upon completion of the test heat shrinkable end caps to protect against ingress of water or any foreign particles, must immediately seal the exposed area.
3) Necessary protection to the cable shall be provided against mechanical damage, vandalism and any other possibility to damage the cable. It is recommended to backfill the trench as soon as possible and not to leave the cable exposed.
4) The cable end shall be raised up higher than the maximum water level expected. The integrity of sealing at the cable ends shall be confirmed to prevent water or any moisture entering the cable. Repair work shall be carried out if necessary.
5) Power cables and auxiliary cables shall be arranged so as to keep the specified distance between them, in order to maintain the current rating capacity and to limit sheath current/sheath induced voltage of the cable and induced voltage on auxiliary cables as per designed values.
Ravin Group supplies and installs high voltage cables and systems on turnkey packages. We provide our customers with a comprehensive cable service package which encompasses system design, design and selection of cables and compatible accessories, supply of quality materials, installation, testing, commissioning and finally ensuring full safety and reliability of the installation.
Our installation team consists of a highly qualified and experienced team of engineers, who work with clients and manufacturers to identify the quickest and most cost effective power solutions. Our team has been trained and experienced in various countries around the world, and they carry with them over 250 years of cumulative experience in specialty jobs. We have an experience of installation of more than 150 kms of EHV cables and 300 joints and terminations at voltages greater than or equal to 220 kV.
We provide our customers with a comprehensive cable service package which encompasses system design including design and selection of cables and compatible accessories, supply of quality materials, installation, testing, commissioning and finally ensuring full safety and reliability of the installation.