UNDERGROUND CABLES

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UNDERGROUND CABLES
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Introduction p.399

• Generally electric Cables consists of
• Conductors Stranded copper or aluminum
  conductors (as illustrated in OHTL)
• Insulation to insulate the conductors from
  direct contact or contact with earth
• External protection against

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Overhead Lines Versus Underground Cables p. 464

• 1- The insulation cost is more in case of cables
  as compared to O.H.T Lines and depends on
  operating voltage of cable.
• kV 0.4 11 33 66 132 220 400
• Cost ratio 2 3 5 7 9 13 24
• 2- The erection cost of O.H.T lines is much less
  than the underground cables.
• 3- Inductive reactance of O.H.T. Lines is more,
  so the voltage regulation is better in case of
  underground cables (Low voltage drop).

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• 4- Capacitance and charging current is high in
  case of underground cables.
• C Xc 1/?C
• Charging current (Ich) V/Xc ?C.V
• For long distance power transmission, the
  charging current is very high results in over
  voltages problems.
• Its not recommended to transfer power for a
  long distance using underground cables.
• 5- Current carrying capacity is more in case of
  O.H.T Lines conductors (better cooling
  conditions) for the same power transmission.
  Therefore, low cross sectional area and cost for
  O.H.T Lines conductors.

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• 6- Underground cables give greater safety, so it
  can be used in
• Big cities and densely populated area.
• Submarine crossing.
• Power stations and substations.
• Airports.

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Cable Construction

• 1- Conductors (Cores)
• ? Stranded aluminum or copper conductors
• ? Conductors with high conductivity and low
  resistance.
• 2- Insulation to insulate the conductors from
  direct contact or contact with earth.
• 3- Screening (Insulator shielding)
• semi-conductor material to uniformly
  distribute the electric field on insulator.

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• 4- filling material.
• 5- Metallic sheath A sheath made of lead or
  aluminum or cupper is applied over the insulation
  to prevent moisture or chemicals from entering
  the insulation.
• 6- Armour (???) Bars of steel to increase the
  mechanical strength of cable.
• 7- Outer cover to protect the metal parts of
  cables ( rubber).

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22kv Medium Voltage Underground XLPE Power Cable
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11kv Copper Core and Shield Power Cable 25mm
http//jpcable99.en.made-in-china.com/product/KMVE
ouLAhBRW/China-11kv-Copper-Core-and-Shield-Power-C
able-25mm.html
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500 Kv High Voltage XLPE Cable (YJLW02/ YJLW03)
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Types of Cables Insulating materials
Performance p. 400

• Insulator material should have
• High insulation resistance (M?-G?).
• High dielectric strength.
• Good mechanical strength.
• High moisture resistance (non-hygroscopic)
• Withstand temperature rise.
• Not affected by chemical

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Types p. 400

• 1- Vulcanized Rubber Insulations
• Rubber is used in cables with rated voltage
• 600- 33 kV.
• Two main groups General Purpose
• Special Purpose
• Four Main Types Butyl rubber
• Silicon rubber
• Neoprene rubber
• Styrene rubber

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• 2- Polymer Insulations
• 2.1 PVC (Poly Vinyl Chloride)
• rated voltage 3.3 kV.
• Grades of PVC General Purpose Type
• Hard Grade Type
• Heat resisting
  Type
• 2.2 Polythene (Polyethylene)
• XLPE (?????? ?????? ????????) rated voltage up
  to
• 275 kV.

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• 3- Paper insulated
• 3.1 Paper insulator rated voltage V up to 66 kV
• 3.2 Oil- impregnated paper is used in solid type
  cables up to 69 kV and in pressure cables (gas
  or oil pressure ) up to 345 kV.

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Types of Cables p.466

• 1- Number of Cores
• Single- Core Cables.
• Multi-Core Cables

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• 2- According to Insulating Material
• Paper Cables
• Polymer Cables
• PVC XLPE
• Rubber Cables
• EPR - PR

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• 3- According to Voltage Level
• High and Extra High voltage Cables
• H.V 33 230 kV
• EHV V gt 230 kV
• Medium Voltage Cables
• V 1- 33 kV
• Low Voltage Cables
• V up to 1 kV.

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• 4- According to Utilization of Cables
• Transmission and Distribution Cables
• XLPE Cables- Paper cables
• Installation Cables ?????????
• PVC
• Submarine Cables ???????
• Rubber cables
• -Industrial Cables ??????? ????????
• ?PVC up to 3.3 kV ? XLPE up to 11 kV

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Electrical Characteristics of Cables p. 408
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• Electric Stress in Single-Core Cables p. 408
• D q/(2px)
• E D/e q/(2pex)
• q Charge on conductor surface (C/m)
• D Electric flux density at a radius x (C/m2)
• E Electric field (potential gradient), or
  electric stress, or dielectric stress.
• e Permittivity (e e0. er)
• er relative permittivity or dielectric constant.

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• r conductor radius.
• R Outside radius of insulation or inside radius
  of sheath.
• V potential difference between conductor and
  sheath (Operating voltage of cable).
• Dielectric Strength Maximum voltage that
  dielectric can withstand before it breakdown.
• Average Stress Is the amount of voltage across
  the insulation material divided by the thickness
  of the insulator.

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• Emax E at x r
• V/(r.lnR/r)
• Emin E at x R
• V/(R.lnR/r)
• For a given V and R, there is a conductor
  radius that gives the minimum stress at the
  conductor surface. In order to get the smallest
  value of Emax
• dEmax/dr 0.0
• ln(R/r)1 R/re2.718

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• Insulation thickness is
• R-r 1.718 r
• Emax V/r (as ln(R/r)1)
• Where r is the optimum conductor radius that
  satisfies (R/r2.718)

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Example

• A single- core conductor cable of 5 km long has
  a conductor diameter of 2cm and an inside
  diameter of sheath 5 cm. The cable is used at
  24.9 kV and 50 Hz. Calculate the following
• a- Maximum and minimum values of electric stress.
• b- Optimum value of conductor radius that results
  in smallest value of maximum stress.

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• a- Emax V/(r.ln(R/r)) 27.17 kV/cm
• Emin V/(R.ln(R/r)) 10.87 kV/cm
• b- Optimum conductor radius r is
• R/r 2.718
• r R/2.718 0.92 cm
• The minimum value of Emax
• V/r 24.9/0.9227.07 kV/cm