LAYING OF EHV CABLE



Cross Section of 66 KV UG Cable:





Why EHV cables are preferred?
  • Protracted land acquisition procedures
  • Litigation Problems
  • Increasing ecological restrictions
  • Soaring cost of real estate


Advantages of UG EHV Cables :

  • Comparatively Higher Voltage Regulation and reliability
  • Lower Dielectric losses
  • Maintenance freedom when laid in ducts 
  • Independence of route profile and terrain
  • No environmental hazard
  • UG Cable System has comparatively better voltage regulation.
  • Safety to Public – better outlook to the city.
  • Submarine crossing.



Cable Laying And Installations:


  • Selection of the Route
  • Users Requirement Route of the transmission and distribution
  • Side of the street
  • Least obstacles in the route
  • Future load growth
  • Distributors laid nearest to the buildings
  • Check road conditions – Transporting cable – Soil
  • Load bearing capacity of the bridges and culverts
  • Footpath – away from parallel running gas, water line, telephoning communication.
  • Joint bays and end terminations.

Minimum Permissible Bending Radii



Single Core – 20 D Multi Core – 15 D

Maximum Permissible Tensile Strength for Cables:
  • Cables Pulled with stocking
  • Cables Pulled by Pulling Eye
  • Cables Pulling by Winch



Methods of Cable Laying




  • Laying direct in ground
  • Drawing in ducts
  • Laying on racks in air
  • Laying on racks inside a cable tunnel
  • Laying along building or structure


Soil Thermal Resistivity/Soil Resistivity

  • TR - 200M
  • taken different depth
  • Laboratory Analysis and Testing
  • 120 to 1500C cm/w


Laying Direct in Ground :



  • 75mm – riddled soil
  • Sand at bottom trench, riddled soil or sand 75mm
  • Protection cover slab



Clearances :



  • Power cable to power cable:Clearance not necessary; however,larger the clearance,better would be current carrying capacity.
  • Power cable to control cables:0.2m
  •  
  • Power cable to communication cable:0.3m
  •   
  • Power cable to gas/water main:0.3m
  •  


Packing, Transport and Storage:


  • Both ends of the cable most be protected from moisture Lead caps, sealed plastic caps.
  • Ramp or Crane for unloading the cable drums.
  • Storage – weld drained – concrete surface
  • Sufficient space for air circulation
  • Protected from direct rays of sun – sun shielding

Cable Laying by Hand: -


  • - Cable Rollers 2/3 mtrs
  • - Angle Rollers
  • - Speed 600 to 1000 mtrs per hour
  • - Tension Indicator
  • - Cable Ends Seals Checked


LAYING BY HAND:


  • Check that the cable trench is dry, (not water logged) clean and dressed, and there are no sharp stones or hard particles that may damage the cable outer sheath. If required pumps should be used to drain out water from the trench.
  • Check that the rollers are placed properly in the trench at a distance of 2-3 meters. Also check that at bends special corner rollers are placed so that the cable does not touch the edge of the trench at the bends.
  • Check that the drum is mounted on the cable jacks properly anchored and rotating freely. Keep a man stationed near the drum with a plank wedged against the flange so that overrunning of the drum could be prevented if pulling stops.
  • Ensure that drum is rolled against the direction of marked “ROLL THIS WAY” on the drum, and the cable is laid out from the top of the drum.
  • Lay a strong rope from the other end of trench to the cable drum on the route from where the cable has to pass. Attach the end of the pulling rope to the leading end of the cable using a cable stocking or pulling eye to pull the cable. One man should be stationed at each roller and a gang of men should be deployed at the pulling rope attached to pulling eye/cable stocking. All the persons should be facing towards the drum site. The total numbers of persons required for pulling should be decided depending upon weight per meter of cable and route conditions and complexity.
  • The gang man should be positioned near the drum. He should make evenly timed calls for pulling or stopping the pulling operation. If required, and local body allows, two or three loud speakers could be used by the gang man for making the call. There should be signal men carrying red and green flags placed at ever 50 meters. These signal men should show green flag when the pulling of cable has to be done and the red flag when the pulling the cable has to be stopped, ensure that when the gang men use call for pulling the cable, all the men grip and pull the cable simultaneously.
  • If possible, three to four foreman/supervisor equipped with walkie-talkie sets should be placed at cable laying area. One of them should be placed near the drum and one should walk along with the cable pulling eye/cable stockings when the cable laying is being done, while others at vulnerable bends, roads crossings etc to ensure that cable pulled is not getting damaged or bent at shorter radius at bends.
  • For heavy cable especially single core cables and for complicated route, apply graphite grease on the cable at the end of the ramp and before cable goes into the trench. If the site conditions requires it, the graphite grease should be applied by the man at rollers during the course of pulling.
  • During laying if any outer sheath damage is observed, the portion of cable which is damaged should be marked with PVC tape. After laying of the cable and is still on the rollers, visual check for any outer sheath damage should be done. The bottom portion of the cable should be checked with the help of a mirror. The damage sheath should be checked with the help of a mirror. The damaged sheath should be repaired before outer sheath testing and backfilling etc.,
  • Remove rollers and other pulling equipment from site.
  • Dress the cable after providing a proper bedding of minimum 100 mm sand. Provide proper spacing if there is more than one cable.
  • During back filling, supervisor should be present at site, to ensure that no unwanted rejected backfill with boulders and stones are not dropped in the trench.
  • Before undertaking the operation of cable laying, a rehearsal of the process to be followed by each employee should be made known and all the employees shall made to understand their part of the work.


CABLE INSTALLATION PLAN:



  1. Type of Cable
  2. Cross section area
  3. Rated voltage
  4. Details of construction
  5. Cable Number
  6. Drum Number
  7. Year & Month of laying
  8. Location of Cable & Joints, plotted in drawing with giving all references.
  9. Name of the Jointer
  10. Date of Making Joint
  11. Results of original Electrical Measurements
  12. Route Map



FIELD TESTS FOR EHV CABLES (XLPE) ABOVE 33 KV:



  • Sheath Insulation resistance of each cable length after laying
  • Continuity of lead sheath
  • After laying of each section, each cable length shall withstand a voltage of (2.5 e + 5) KV DC between metallic sheath and external conducting surface for 1 minute as per IEC 229 where ‘e’ is the guaranteed nominal thickness of the outer sheath.




PRE-COMMISSIONING TESTS



  • Electrostatic Capacitance of the cable
  • Resistance of the cable conductor
  • Resistance of the Metallic Sheath


Maintenance of Cables:


  • Route Maintenance
  • Patrolling the Cable Route
  • Checking Anti-Corrosive Sheath
  • Checking of the Current Loading
  • Discharge the EHV Cable for LC Works


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