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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 13 242- 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/ωC Charging current (Ich)= V/Xc = ωC.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: (درع) 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/KMVEouLAhBRW/China-11kv-Copper-Core-and-Shield-Power-Cable-25mm.html
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500 Kv High Voltage XLPE Cable (YJLW02/ YJLW03)
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Types of Cables Insulating materials
Insulator material should have:
- High insulation resistance (MΩ-GΩ).
- High dielectric strength.
- Good mechanical strength.
- High moisture resistance (non-hygroscopic)
- Withstand temperature rise.
- Not affected by chemical
Performance p. 400
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1- Vulcanized Rubber Insulations: Rubber is used in cables with rated voltage 600- 33 kV.Two main groups: General Purpose
Special PurposeFour Main Types: Butyl rubber Silicon rubber Neoprene rubber Styrene rubber
Types p. 400
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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 Type2.2 Polythene (Polyethylene) - XLPE ( التشابكى ايثلين rated (البولى
voltage up to 275 kV.
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3- Paper insulated :3.1 Paper insulator: rated voltage V up to 66 kV3.2 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 kVEHV: V > 230 kV
- Medium Voltage Cables V: 1- 33 kV
- Low Voltage CablesV up to 1 kV.
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4- According to Utilization of Cables- Transmission and Distribution Cables
XLPE Cables- Paper cables- Installation Cables التمديدات
PVC- Submarine Cables البحرية
Rubber cables-Industrial Cables الصناعية المنشآت 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/(2πx)
E = D/ε = q/(2πεx)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.ε: Permittivity (ε = ε0. εr)
εr: relative permittivity or dielectric constant.
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rR
x
V
x
qE
r
RqdxEV
R
r
ln..2
ln2
.
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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/r=e=2.718
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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/r=2.718)
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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 cmThe minimum value of Emax:
= V/r = 24.9/0.92=27.07 kV/cm