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.
