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: (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 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 التشابكى) rated 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

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

High and Extra High voltage Cables 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

E = D/ε = q/(2πεx) Electric Stress in Single-Core Cables p. 408 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: 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 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 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 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