Pharos University EE-385 Electrical Power & Machines “Electrical Engineering Dept” Prepared By: Dr. Sahar Abd El Moneim Moussa Dr. Sahar Abd El Moneim Moussa

OHTL OVERHEAD TRANSMISSION LINE (Part 1) Dr. Sahar Abd El Moneim Moussa

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

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

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

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

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

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

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

O.H.T.L. Conductors Overhead transmission lines are composed of aluminium conductors 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

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

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

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

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

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

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

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

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

ρT1 and ρT2 are resistivity's at T1 and T2 °C respectively. 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

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

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

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

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

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

This will cause unequal voltage drops across line conductors, 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

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

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

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-smell due to ionization of air. Dr. Sahar Abd El Moneim Moussa

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

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

DSL = 3 𝐷𝑠 𝑑 2 For Three-conductor Bundling For three conductors: DSL = 3 𝐷𝑠 𝑑 2 For Three-conductor Bundling a b C D d Dr. Sahar Abd El Moneim Moussa

DSL = 1.091 4 𝐷𝑠 𝑑 3 For Four -conductor Bundling For four conductors: DSL = 1.091 4 𝐷𝑠 𝑑 3 For Four -conductor Bundling a b C D d Dr. Sahar Abd El Moneim Moussa

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 10-7 ln [D / Ds] H/m L per phase =2 X 10-7 ln [4 / (0.78 x10-2 )] = 1.248 x 10-6 H/m C DD D D a b D Dr. Sahar Abd El Moneim Moussa

( This example will be solved in the lecture) 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

The line-to-neutral capacitance: 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

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

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

V- Bundled Conductors Deq = Dsc = 𝑟 𝑑 for a two- conductor bundle The line-to-neutral caacitance: Cn =2 ᴨ Ɛo / ln [Deq /Dsc] F/m Deq = Dsc = 𝑟 𝑑 for a two- conductor bundle Dsc = 3 𝑟 𝑑 2 for a three- conductor bundle Dsc = 1.091 4 𝑟 𝑑 3 for a four-conductor bundle Dr. Sahar Abd El Moneim Moussa

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]= 2 𝜋 𝑋 8.85 𝑋 10 −12 ln 2.5 10 −2 = 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 a b D Dr. Sahar Abd El Moneim Moussa

( This example will be solved in the lecture) 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

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

L per phase=2 X 10-7 ln [Deq / DSL] H/m Deq = DabDbcDca = 12.5 m Ds= (4.6/2) x 10-2 x0.7788= 1.794 cm DSL = √Ds d = 8.985 cm L per phase=2 X 10-7 ln [Deq / DSL] H/m = 2 X 10-7 ln 1250 8.985 = 9.87 x 10 -7 H/m Dsc = √rd =10.7 cm Cn =2 ᴨ Ɛo / ln [Deq /Dsc] F/m = 2 𝑋 𝜋 𝑋 8.85 𝑋 10 −12 ln 1250 10.7 = 7.807 x 10 -12 F/m Dr. Sahar Abd El Moneim Moussa

( This example will be solved in the lecture) 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