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Pharos University EE-385

Pharos University EE-385. Electrical Power & Machines “Electrical Engineering Dept” Prepared By: Dr. Sahar Abd El Moneim Moussa. OHTL OVERHEAD TRANSMISSION LINE (Part 1). T.L Main components. An OHTL consists of main components: Conductors Insulators Support structures Shield wires.

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Pharos University EE-385

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  1. Pharos UniversityEE-385 Electrical Power & Machines “Electrical Engineering Dept” Prepared By: Dr. SaharAbd El MoneimMoussa Dr. Sahar Abd El Moneim Moussa

  2. OHTLOVERHEAD TRANSMISSION LINE(Part 1) Dr. Sahar Abd El Moneim Moussa

  3. T.L Main components • An OHTL consists of main components: • Conductors • Insulators • Support structures • Shield wires. Dr. Sahar Abd El Moneim Moussa

  4. Three-phase conductors, which carry the electric current; • Insulators, which support and electrically isolate the conductors; • Tower, which holds the insulators and conductors; • Foundation and grounding; and • Optional shield conductors, which protect against lightning Dr. Sahar Abd El Moneim Moussa

  5. Types of Transmission lines (according to the voltage) • Extra-high-voltage lines • Voltage: 345 kV, 500 kV, 765 kV • Interconnection between systems • High-voltage lines • Voltage: 115 kV, 230 kV • Interconnection between substations, power plants Dr. Sahar Abd El Moneim Moussa

  6. Sub-transmission lines • Voltage: 46 kV, 69 kV • Interconnection between substations and large industrial customers • Distribution lines • Voltage: 2.4 kV to 46 kV, with 15 kV being the most commonly used • Supplies residential and commercial customers • High-voltage DC lines • Voltage: ±120 kV to ±600 kV • Interconnection between regions (e.g., Oregon-California) Dr. Sahar Abd El Moneim Moussa

  7. 500 kV Overhead T.L Dr. Sahar Abd El Moneim Moussa

  8. 275 kV double-circuit steel tower Dr. Sahar Abd El Moneim Moussa

  9. 11 kV Concrete line post Two-units of Suspension type Line Insulator 11-kV Line Conductor Post-type Distribution Transformer Concrete Line Post Dr. Sahar Abd El Moneim Moussa

  10. O.H.T.L. Conductors • Overhead transmission lines are composed of aluminiumconductors that,even in modest capacities,are stranded in spiral fashion for flexibility. These are primarily classified as: • AAC: all-aluminum conductors • AAAC: all-aluminum alloy conductors • ACSR: aluminum conductors, steel reinforced • ACAR: aluminum conductors, alloy reinforced • For purposes of heat dissipation, O.HT.L. conductors are bare(no insulating cover) Dr. Sahar Abd El Moneim Moussa

  11. ACSR Conductors of O.H.T.L Steel reinforcing strands For purposes of heat dissipation, O.HT.L. conductors are bare (no insulating cover) Dr. Sahar Abd El Moneim Moussa

  12. O.H.T.L. Insulators • PIN-TYPE LINE INSULATORS Dr. Sahar Abd El Moneim Moussa

  13. SUSPENSION-TYPE LINE INSULATORS Cap of Ceramics Ball of Malleable Iron Dr. Sahar Abd El Moneim Moussa

  14. Insulate and support • Made of porcelain, glass or polymeric material • Higher voltages need more insulator units Dr. Sahar Abd El Moneim Moussa

  15. POST-TYPE LINE INSULATORS Dr. Sahar Abd El Moneim Moussa

  16. OHTL PARAMETERS • Transmission line parameter • Resistance /m • Inductance H/m • Capacitance F/m Dr. Sahar Abd El Moneim Moussa

  17. Line Resistance Rdc = ρl/A, …………(1) where R : is the resistance in ohms, ρ : is the resistivity in ohm meter L : is the length of conductor in m A : is the cross sectional area of the conductor in m2 Dr. Sahar Abd El Moneim Moussa

  18. Factors affecting the conductor resistance: Conductor resistance depends on the following factors: • Spiraling: For standard conductors, alternate layers of strands are spiraled in opposite directions to hold the strands together. This makes the strands or 2% longer, as a result, the dc resistance of a stranded conductor is 1 or2% larger than that calculated from eq.1. Dr. Sahar Abd El Moneim Moussa

  19. Temperature : Resistivity of conductor metals varies linearly over normal operating temperatures according to: ρT2 = ρT1 (T2+T/T1+T) where ρT1 and ρT2 are resistivity's at T1 and T2°C respectively. T is a temperature constant that depends on the conductor material Dr. Sahar Abd El Moneim Moussa

  20. Frequency(skin effect): The ac resistance or effective resistance of a conductor is Rac = Ploss/ I2 Ω. For dc the current distribution is uniform throughout the conductor cross section . For ac, the current distribution is non uniform . As frequency increases , the current in a solid cylindrical conductor tends to crowd towards the conductor surface with smaller current density at the conductor center . This phenomenon is called skin effect. At power frequency, the ac resistance is at most a few percent higher than the dc resistance. Dr. Sahar Abd El Moneim Moussa

  21. The resistance , 1- Increases by stranding 2- Increases by temperature 3- Increases by skin effect Dr. Sahar Abd El Moneim Moussa

  22. 2-Line Inductance • Inductance of single phase Two Wire line The total circuit inductance : Ltotal=4 X 10-7ln [D / Ds] H/m Where, Ds : GMR: Geometric mean radius = 0.7788 r r r r D Dr. Sahar Abd El Moneim Moussa

  23. C DD -Inductance of 3-Phase 3-WireLine with Equal Spacing The inductance per phase L per phase=2 X 10-7ln [D / Ds] H/m Lab= Lbc= Lca D D b a D Dr. Sahar Abd El Moneim Moussa

  24. C -Inductance of 3-Phase 3-WireLine with Unequal Spacing The average inductance is Laverage=2 X 10-7ln [Deq / Ds] H/m Where, LabLbcLca Dac DD Dbc a Dab b Deq = Dr. Sahar Abd El Moneim Moussa

  25. In case of unequal spacing, the inductances of the three phase conductors are all different. • This will cause unequal voltage drops across line conductors, • Therefore the voltage phasors at receiving-end substation of the line will be unbalanced. • To overcome such unbalance , line transposition is made. Dr. Sahar Abd El Moneim Moussa

  26. IV- Inductance of Transposed lines L per phase =2 X 10-7ln [Deq / Ds] H/m Where, Deq : Geometric mean distance= Dr. Sahar Abd El Moneim Moussa

  27. V- Bundled Conductors • Bundling of conductors means the use of more than one conductor per phase. It is a common practice for EHV lines. • Bundling reduces the electric field strength at the conductor surfaces , which in turn reduces corona and its effects. Dr. Sahar Abd El Moneim Moussa

  28. CORONA EFFECT: • Corona is an electrical discharge caused by ionization of the air surrounding the conductor, which occurs when the strength of the electric field at the conductor surface exceeds the dielectric strength of the air. • Air at 79 cm pressure and 25 C breaks at 30 kV/cm. If the electric field strength exceeds this value Corona occurs. • Corona is observed by violet-colored light around the conductor, a hissing noise, and Ozone-smelldue to ionization of air. Dr. Sahar Abd El Moneim Moussa

  29. L per phase=2 X 10-7ln [Deq / DSL] H/m Dr. Sahar Abd El Moneim Moussa

  30. d Where, • For two conductors: DSL = For two conductor bundle Where, d: is the bundling spacing a C b D Dr. Sahar Abd El Moneim Moussa

  31. For three conductors: DSL = For Three-conductor Bundling a C b D d Dr. Sahar Abd El Moneim Moussa

  32. For four conductors: DSL = 1.091 For Four -conductor Bundling a C b D d Dr. Sahar Abd El Moneim Moussa

  33. Example 3: Find the inductance of a 3-phase overhead transmission line if the conductor diameter is 2 cm and the three conductors are placed at the c0rners of an equilateral triangle of side 4 m Solution:(Equal Spacing) D= 4m ,d=2 cm ,r= 1 cm=10-2 m L per phase =2 X 10-7ln [D / Ds] H/m L per phase =2 X 10-7ln[4 / (0.78 x10-2 )] = 1.248 x 10-6H/m C DD D D b a D Dr. Sahar Abd El Moneim Moussa

  34. Example 4: The three conductors of a 3 phase transmission line are arranged in a horizontal plane and are 4 m apart. The diameter of each conductor is 2 cm. calculate the inductance per phase per m of the line ( This example will be solved in the lecture) Dr. Sahar Abd El Moneim Moussa

  35. 2- Line Capacitance • Capacitance of single phase Two Wire line The line-to-neutral capacitance: Cn =2 ᴨ Ɛo / ln [D /r] F/m Where, Ɛo: is the permittivity of air = 8.85 x 10-12 F/m r r r D Dr. Sahar Abd El Moneim Moussa

  36. C DD -Capacitanceof 3-Phase 3-WireLine with Equal Spacing The line-to-neutral capacitance: Cn =2 ᴨ Ɛo / ln [D /r] F/m D D b a D Dr. Sahar Abd El Moneim Moussa

  37. C - Capacitance of 3-Phase 3-WireLine with Unequal Spacing The line-to-neutral capacitance: Cn =2 ᴨ Ɛo / ln [Deq /r] F/m Where, Dac DD Dbc a Dab b Deq = Dr. Sahar Abd El Moneim Moussa

  38. The line-to-neutral caacitance: Cn =2 ᴨ Ɛo / ln [Deq /Dsc] F/m V- Bundled Conductors Deq = Dsc = for a two- conductor bundle Dsc = for a three- conductor bundle Dsc = 1.091 for a four-conductor bundle Dr. Sahar Abd El Moneim Moussa

  39. Example 5: A 200 km, 3-phase transmission line has its conductors placed at the corners of an equilateral triangle of 2.5 m side. The radius of each conductor is 1 cm. calculate the line to neutral capacitance of the line. Solution: (Equal Spacing) D= 2.5 m ,L=200 km ,r= 1 cm=10-2 m Cn =2 ᴨ Ɛo / ln [D /r] F/m Cn =2 ᴨ Ɛo / ln [D /r]= = 1.007 x 10-11 F/m  C = 1.007 x 10-12 x 200 x 1000 = 2 x 10 -6 F C DD D D b a D Dr. Sahar Abd El Moneim Moussa

  40. Example 6: The three conductors of a 3 phase transmission line are arranged in a horizontal plane and are 4 m apart. The diameter of each conductor is 2 cm. calculate the line to neutral capacitance per m of the line ( This example will be solved in the lecture) Dr. Sahar Abd El Moneim Moussa

  41. Example 7: The 3 phases of a TL are placed at the corners of an equilateral triangle of side 12.5 m. Each phase consists of 2 bundled conductors, the conductor diameter is 4.6 cm and the bundle spacing is 45 cm. Find L and C of the TL. Solution: Dab = 12.5 m D bc = 12.5 m D ca = 12.5 m D conductor=4.6 cm d= 45 cm C d a b D Dr. Sahar Abd El Moneim Moussa

  42. Deq = = 12.5 m Ds= (4.6/2) x 10-2x0.7788= 1.794 cm DSL = √Dsd = 8.985 cm L per phase=2 X 10-7ln [Deq / DSL] H/m =2 X 10-7ln = 9.87 x 10 -7 H/m Dsc= √rd=10.7 cm Cn =2 ᴨ Ɛo / ln [Deq /Dsc] F/m = = 7.807 x 10 -12 F/m Dr. Sahar Abd El Moneim Moussa

  43. Example 8: The three conductors of a 3 phase transmission line are arranged in a horizontal plane and are 4 m apart. The diameter of each conductor is 2 cm. calculate the line to neutral capacitance per m of the line ( This example will be solved in the lecture) Dr. Sahar Abd El Moneim Moussa

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