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 13 242- 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).

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: (درع) Bars 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-11kv-Copper-Core-and-Shield-Power-Cable-25mm.html

500 Kv High Voltage XLPE Cable (YJLW02/ YJLW03)

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

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

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.

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.

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 kVEHV: V > 230 kV

- Medium Voltage Cables V: 1- 33 kV

- Low Voltage CablesV 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 الصناعية المنشآت 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.

rR

x

V

x

qE

r

RqdxEV

R

r

ln..2

ln2

.

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 cmThe minimum value of Emax:

= V/r = 24.9/0.92=27.07 kV/cm