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Showing posts with label Cable. Show all posts
Showing posts with label Cable. Show all posts

Wednesday, August 21, 2013

Cable

Quality of Supply


  • Quality of supp;y is measured in term of accept able value of Voltage frequency
  • Permissible Voltage Variations(Standard of Performance,MARC)


Permissible Frequency Variations(Standard of Performance, MERC)
Rated Frequency=50hz

Permissible Variations=+/-3%(48 to 51.5)
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TYPES OF CABLE
  • Paper Insulated Lead Cover(PILC)
  • Polly Vinyl Chloride Insulated (PVC)
  • Cross Link Poly Ethylene Insulated (XLPE)
  • LIMITATIONS OF PILC CABLES
  • Lead sheath required for protection from water penetration
  • Lead adds to cost of the cable
  • Handling
  • Lead a heave metal and adds to weight of the cable during handling. Paper id brittle and PILC cables require a large bedbug radius ti prevent cracking of paper insulation and subsequent failure by puncture.
  • Power transmission
  • Conductor termp. limited to 70C to prevent migration of impregnant creates words resulting in insulation puncture. Hence limited power transmission.

PVS CABLES
  • Advantages of plastic cables.
  • Good electric strength
  • Non hygroscopic:Require no lead sheath
  • Light in Weight and easy to install
  • Limitations of PE Cables.
  • Thermo plastic PE is cross linked by a chemical process, PE is converted into thermoset material which do not melt and remains stable up to 95c
  • This discovery leads to development to XLPE Cables.

XLPE CABLES
  • Effect of moisture in XLPE Cables.
  • Though XLPE is highly moisture resistant, moisture can still cause electrical failure under the influence of high electrical stress.
  • Water tree are formed in the XLPE insulation which subsequently develop in to electrical stress.
  • Entry of Moisture:
  • Radial:Moisture permeates through PVC Sheath
  • Longitudinal: Water enters during installation due to absence of cap on cable end.
  • Protection from Moisture:
  • Moisture Swellable Tapes used under PVC Sheath.
  • These are made from swellable layer of water blocking powder and water soluble binder.
  • Entry of water is blocked due to release of powder by the binder which swells and fills the voids
  • Tree Retardant: Effective ingredients are added to XLPE to limit the growth of water tree
  • Life expectancy of XLPE
  • Cable with swellable Tapes----- >20-30 Years
  • TR-Xlpe Cables-            >30-35 Years
  • Cables with TRXLPE and water swellable Tapes--->35-40 Years
  • Cables with Metal Moisture Barrier(Lead Sheath-->60 Years

Loading of cables
  • Assumptions
  • 1) Maximum Conductor temp. for continuous operation PVC/PILC vables-70dC
  • XLPE Cables-90 degreeC
  • Ambient Air Temp-40 degreeC
  • Thermal resistivity of soil-150oc/cm/watt
  • Multi core cables laid single OR with 2X diameter center to centre apart
  • Depth of cable laying as per IS
  • De rating factor
  • Rise in temp. to higher than permissible level-
  • 1)PILC Cables(>65 Degree C) Causes the impregnate in the paper tapes to migrate and creation of voids in paper tape leading to insulation failure.
  • 2) XLPE cables (>90 degree c) causes loss of insulation properties leading to its failure.