2Introduction 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 earthExternal protection: against ………
3Overhead 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 : 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).
44- 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.
56- Underground cables give greater safety, so it can be used in: Big cities and densely populated area.Submarine crossing.Power stations and substations.Airports.
6Cable Construction1- 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.
74- 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).
14Types of Cables Insulating materials Performance p. 400Insulator 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
15Types p. 4001- 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
162- Polymer Insulations: 2.1 PVC (Poly Vinyl Chloride)rated voltage 3.3 kV.Grades of PVC: General Purpose TypeHard Grade TypeHeat resisting Type2.2 Polythene (Polyethylene)XLPE (البولى ايثلين التشابكى) rated voltage up to275 kV.
173- 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.
18Types of Cables p.466 1- Number of Cores: Single- Core Cables. Multi-Core Cables
20High and Extra High voltage Cables 3- According to Voltage LevelHigh and Extra High voltage CablesH.V: 33 – 230 kVEHV: V > 230 kVMedium Voltage CablesV: kVLow Voltage CablesV up to 1 kV.
214- According to Utilization of Cables Transmission and Distribution CablesXLPE Cables- Paper cablesInstallation Cables التمديداتPVCSubmarine Cables البحريةRubber cables-Industrial Cables المنشآت الصناعية●PVC up to 3.3 kV ● XLPE up to 11 kV
23E = 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.
25r: 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.
26Emax = 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
27Insulation thickness is: R-r = 1 Insulation thickness is: R-r = r Emax = V/r (as: ln(R/r)=1) Where r is the optimum conductor radius that satisfies (R/r=2.718)
28ExampleA 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.
29a- Emax = V/(r. ln(R/r)) = 27. 17 kV/cm Emin = V/(R. ln(R/r)) = 10 a- Emax = V/(r.ln(R/r)) = kV/cm Emin = 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 Emax: = V/r = 24.9/0.92=27.07 kV/cm