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.