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UNDERGROUND CABLES

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UNDERGROUND CABLES Introduction p.399 Generally electric Cables consists of Conductors :Stranded copper or aluminum conductors (as illustrated in OHTL) Insulation: to ... – PowerPoint PPT presentation

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Title: UNDERGROUND CABLES


1
UNDERGROUND CABLES
2
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

3
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).

4
  • 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.

5
  • 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.

6
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.

7
  • 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).

8
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9
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10
22kv Medium Voltage Underground XLPE Power Cable
11
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
12
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13
500 Kv High Voltage XLPE Cable (YJLW02/ YJLW03)
14
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

15
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

16
  • 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.

17
  • 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.

18
Types of Cables p.466
  • 1- Number of Cores
  • Single- Core Cables.
  • Multi-Core Cables

19
  • 2- According to Insulating Material
  • Paper Cables
  • Polymer Cables
  • PVC XLPE
  • Rubber Cables
  • EPR - PR

20
  • 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.

21
  • 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

22
Electrical Characteristics of Cables p. 408
23
  • 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.

24

25
  • 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.

26
  • 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

27
  • 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)

28
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.

29
  • 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
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