UNDERGROUND CABLES - موقع الدكتور مجدى

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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 insulate the conductors from
direct contact or contact with earth
External protection: against ………
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- 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.
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.
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.
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: )‫ (درع‬Bars of steel to increase the
mechanical strength of cable.
7- Outer cover to protect the metal parts of
cables ( rubber).
22kv Medium Voltage Underground XLPE Power Cable
11kv Copper Core and Shield Power Cable 25mm
http://jpcable99.en.made-in-china.com/product/KMVEouLAhBRW/China-11kvCopper-Core-and-Shield-Power-Cable-25mm.html
500 Kv High Voltage XLPE Cable (YJLW02/ YJLW03)
Types of Cables Insulating materials
Performance p. 400
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
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
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 (‫ )البولى ايثلين التشابكى‬rated voltage up to
275 kV.
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.
Types of Cables p.466
1- Number of Cores:
- Single- Core Cables.
- Multi-Core Cables
2- According to Insulating Material
- Paper Cables
- Polymer Cables
PVC – XLPE
- Rubber Cables
EPR - PR
3- According to Voltage Level
- High and Extra High voltage Cables
H.V: 33 – 230 kV
EHV: V > 230 kV
- Medium Voltage Cables
V: 1- 33 kV
- Low Voltage Cables
V up to 1 kV.
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 ‫المنشآت الصناعية‬
●PVC up to 3.3 kV
● XLPE up to 11 kV
Electrical Characteristics of Cables p. 408
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.
R
R
V   E.dx 
ln
2

r
r
E
q
q
2 .x

V
R
x. ln
r
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.
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
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)
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.
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 = 24.9/0.92=27.07 kV/cm

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