4. Cable sizing

Main content:
  1. What is cable 
  2. What is cable sizing
  3. What all the components of cable
  4. What is the difference between wire and cable
  5. What is earthed grade unearthed grade
  6. How do we size the cable
  7. PVC vs XLPE
  8. Al Vs copper
  9. Cost fo cable
  10. Tests for cable
  11. QAP of cable
  12. Typical issues with cables
  13. DC cable sizing

What is a cable:

Conductor with an Insulation called cable

What is cable sizing:

Selection of conductor cross section area in mm2 is called cable sizing

Different types of cables:

  • Power 
    • LT
    • HT
    • EHT
  • Control
  • Instrumentation
Different components of cables:

  • Conductor
  • Insulation
  • Screening
  • Inner sheath
  • Armoring
  • Outer sheath
Conductor
    • A body or substance, which offers a low resistance to the passage of an electric current
    • Aluminum / Copper
    • Solid/stranded
Insulation
    • A non-conductive material usually surrounding or separating two or more conductive materials.
    • XLPE
    • PVC
Screening

  • XLPE Cables with rated voltage over 3300V shall be provided with conductor and insulation screening.
  • To avoid the cavities and voids formation in dielectric particularly on bending operation of cable, perfect bonding of insulation and screening is required.
  • Typically semiconducting compound

Innersheath

    • A uniform and continuous covering used to protect the insulation, especially against moisture, or to protect an inner metallic sheath or armour against corrosion
    • Laid up cables are provided with inner sheath with high quality of PVC which acts as bedding for steel wire / strip armouring

Armor
    • A metal covering usually applied in the form of tape or wire, intended to protect a cable from mechanical damage.
Outer Sheath

    • Similar to innerseath
    • Will have various characteristics
    • General purpose
    • Heat resistant
    • FRLS
    • UV resistant

Some pictures:


Cable designations:

          A = Appearing as a first letter denotes Aluminium Conductor.

Y = TROPODUR (PVC) Insulation or TROPODUR (PVC) Sheath depending on the position in which it appears.

2X = TROPOTHEN-X (Cross-linked Polyethylene) Insulation.

W = Round Steel Wire Armoring.

WW = Double Round Steel Wire Armoring.

F = Formed Steel Wire (Strip) Armoring.

FF = Double Formed Steel Wire (Strip) Armoring.

C = Metallic Screening (Usually of Copper).

CE=Metallic Screening (usually of Copper) over each individual core.

Gb = Holding Helix Tape (of Steel)
      Wa = Aluminium Round Wire & Aluminium Formed Wire (Strip)



What is Earthed Grade and unearthed grade cable:


The picture describes: The voltage stress between conductor and Earth, Conductor to conductor
Conductor to earth is called U0
Conductor to conductor is called U

Typicall the cables are rated with U0/U

If the neutral is solidly earthed then U0<U
If neutral is not solidly earthed U0=U




Suppose it is 415V/33KV transformer

As Neutral is come out and it is solidly earthed, we can use 33KV (E) cable

If the neutral is not connected to earth or if it connected to earth through resistor, we call that as un earthed system. Then we have to use 33KV (UE) cable.

(UE) cable will have more insulation than (E) cable

The point to understand, first we should know, our system is earthed grade system or Un Earthed system, then we need to select the appropriate available cable.


Cooper vs Alluminium

Parameter
Aluminum
Copper
ρ (Ω-m) at 20 °C
Resistivity

60% less resistive than Aluminium
Conductivity at 20 deg cent

61% more conductive than aluminium for same size
Weight
70% lighter than copper for same size

Cost


Co efficient of thermal expansion
42% greater than copper

Dissipation of heat transfer
Al dissipates faster than copper

Availability on earth
3 rd most abundantly available
26th place
Fluctuation in cost
More stable
More fluctuating
Note point
If an increased size isn’t a concern and you need a large amount of it, Al might be a better choice
130 amp to carry
50 sqmm as per catalogue
35 sqmm
Cost of
365 Rs per meter
1673 Rs per meter


PVC vs XLPE


  • Maximum working temperature of conductor:
    •  PVC insulated Cable is 70 Centigrade, XLPE insulated Cable is 90 Centigrade.
  • Maximum short (lasting less than 5 seconds) temperature:
    •  PVC insulated Cable is 160 Centigrade, XLPE insulated Cable is 250 Centigrade.
  • Service life: Usually, 
    • the XLPE insulated cable is longer than PVC insulated cable.
  • Price: 
    • Usually, the XLPE insulated cable price is higher than PVC insulated cable.
  • Water Proof Performance in Application Environment:
    •  PVC insulated cable is better than the XLPE insulated cable.
  • Environment Protection aspect: 
    • For XLPE is not containing chlorine which will release toxic gases once burn, so XLPE insulated Cable is environment friendly compared with PVC insulated Cable.

 Conclusion: In a long run, the XLPE insulated Cable is better than PVC insulated Cable, but the price is a little higher, but if we take all factors in to consideration(such as environment protection, service life, safety and so on), XLPE insulated cable has a high cost-performance. 

Tests:



Cable sizing Objective:

          To select correct size and type of the cables.
          Selection should be such that voltage variation in the cable within permissible limit.
          Cable should be so selected that it can withstand the stresses and resulting increase in temperature caused by maximum short circuit current

Factors Determining Selection of Power Cable:

  • System voltage.
  • Current carrying capacity.
  • Mode of installation.
  • Permissible voltage drop.
  • Short circuit rating.
  • Economic consideration

Correction factors:
  • K1: correction factor due to group of cables in ground.
  • K2: correction factor due to depth of cable.
  • K3: correction factor due to ground or air ambient temperature.
  • K4: correction factor due to Earth Thermal Resistivity
  • K5: correction factor due to Metal Screen/Shield.
  • K6: correction factor due to Phase Spacing.
  • KT: Overall Correction Factor

KT = K1*k2*k3*k4*k5*k6

Basic assumptions of Current rating:

The current ratings given the catalog for a particular size are given in certain conditions:
In actual practice these conditions might be different

Basic assumptions are:


  • Maximum permissible temperature is 90 deg for XLPE and 70 deg for PVC Insulation
  • Ground or duct temperature is 30 deg centigrade
  • Thermal resistivity of soil is 150 deg centigrade - centi meter / watt
  • Thermal resistivity of dielectric 650 deg cen - cm /w for PVC and 350 for XLPE
  • Depth of laying for 1.1KV cables - 750mm , 3.3KV to 11KV - 900mm, above 11KV - 1050mm

Rating Factors:

  • Rating factor related to variance in ambient temperature (We will get it catalog)

  • Rating factors related to variance in ground temperature


  • Rating factors related to variation in ground thermal resistivity
  • Rating factors related to variation in depth of laying

  • Rating factors related to grouping of cables
    • Cable laid direct in ground
    • Cable laid direct in open racks
      • Single core cables
      • Multi core cables






Steps for Sizing of Cable:


  1. Full load current
  2. Consider rating factors
  3. Full load current / Rating factor = Ampacity
  4. As per ampaity






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