Presentation on theme: "UNDERGROUND CABLES. Introduction p.399 Generally electric Cables consists of Conductors :Stranded copper or aluminum conductors (as illustrated in OHTL)"— Presentation transcript:
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 The insulation cost is more in case of cables as compared to O.H.T Lines and depends on operating voltage of cable. kV : Cost ratio: 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 X c = 1/ωC Charging current (I ch )= V/X c = 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 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 (M-G). -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 kV. Two main groups: General Purpose Special Purpose Four 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 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 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: 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/m 2 ) E: Electric field (potential gradient), or electric stress, or dielectric stress. ε: Permittivity (ε = ε 0. ε r ) ε r : 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.
E max = E at x = r = V/(r.lnR/r) E min = 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 E max : dE max /dr =0.0 ln(R/r)=1 R/r=e=2.718
Insulation thickness is: R-r = r E max = 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- E max = V/(r.ln(R/r)) = kV/cm E min = V/(R.ln(R/r)) = kV/cm b- Optimum conductor radius r is: R/r = r= R/2.718= 0.92 cm The minimum value of E max : = V/r = 24.9/0.92=27.07 kV/cm