Pharos University EE-385

Pharos University EE-385

Pharos UniversityEE-385Electrical Power & MachinesElectrical Engineering DeptPrepared By:Dr. Sahar Abd El Moneim Moussa

Dr. Sahar Abd El Moneim Moussa

1OHTLOVERHEAD TRANSMISSION LINE(Part 1)Dr. Sahar Abd El Moneim Moussa2T.L Main componentsAn OHTL consists of main components:

ConductorsInsulatorsSupport structuresShield wires.

Dr. Sahar Abd El Moneim Moussa3

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 lightningDr. Sahar Abd El Moneim Moussa4Types of Transmission lines (according to the voltage)

Extra-high-voltage linesVoltage:345 kV, 500 kV, 765 kVInterconnection between systems

High-voltage linesVoltage:115 kV, 230 kVInterconnection between substations, power plants

Dr. Sahar Abd El Moneim Moussa5Sub-transmission linesVoltage:46 kV, 69 kVInterconnection between substations and large industrial customers

Distribution linesVoltage:2.4 kV to 46 kV, with 15 kV being the most commonly usedSupplies residential and commercial customers

High-voltage DC linesVoltage:120 kV to 600 kVInterconnection between regions (e.g., Oregon-California)

Dr. Sahar Abd El Moneim Moussa6500 kV Overhead T.LDr. Sahar Abd El Moneim Moussa7

275 kV double-circuit steel towerDr. Sahar Abd El Moneim Moussa8

Dr. Sahar Abd El Moneim Moussa9

11-kV Line ConductorPost-type Distribution TransformerConcrete Line PostTwo-units of Suspension type Line Insulator11 kV Concrete line postO.H.T.L. ConductorsOverhead 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 conductorsAAAC: all-aluminum alloy conductorsACSR: aluminum conductors, steel reinforcedACAR: aluminum conductors, alloy reinforcedFor purposes of heat dissipation, O.HT.L. conductors are bare(no insulating cover)

Dr. Sahar Abd El Moneim Moussa10Dr. Sahar Abd El Moneim Moussa11

ACSR Conductors of O.H.T.LSteel reinforcing strandsFor purposes of heat dissipation, O.HT.L. conductors are bare (no insulating cover)O.H.T.L. Insulators PIN-TYPE LINE INSULATORSDr. Sahar Abd El Moneim Moussa12

SUSPENSION-TYPE LINE INSULATORSDr. Sahar Abd El Moneim Moussa13

Cap of CeramicsBall of Malleable IronDr. Sahar Abd El Moneim Moussa14

Insulate and supportMade of porcelain, glass or polymeric material Higher voltages need more insulator units

POST-TYPE LINE INSULATORSDr. Sahar Abd El Moneim Moussa15

OHTL PARAMETERSTransmission line parameter

Resistance /mInductance H/m CapacitanceF/m

Dr. Sahar Abd El Moneim Moussa16Line Resistance

Rdc = l/A, (1)where R : is the resistance in ohms, : 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 Moussa17Factors 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 Moussa18Temperature : Resistivity of conductor metals varies linearly over normal operating temperatures according to: T2 = T1 (T2+T/T1+T) where T1 and T2 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 Moussa19Frequency(skin effect):The ac resistance or effective resistance of a conductor is Rac = Ploss/ I2 .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 Moussa20The resistance ,

1- Increases by stranding2- Increases by temperature 3- Increases by skin effectDr. Sahar Abd El Moneim Moussa212- Line Inductance

Inductance of single phase Two Wire line

The total circuit inductance : Ltotal =4 X 10-7 ln [D / Ds] H/mWhere, Ds : GMR: Geometric mean radius = 0.7788 r

Dr. Sahar Abd El Moneim Moussa22rDrr- Inductance of 3-Phase 3-WireLine with Equal Spacing

The inductance per phase L per phase =2 X 10-7 ln [D / Ds] H/m

Dr. Sahar Abd El Moneim Moussa23DDDDDabCLab= Lbc= Lca-Inductance of 3-Phase 3-WireLine with Unequal Spacing

The average inductance is Laverage =2 X 10-7 ln [Deq / Ds] H/mWhere,

Dr. Sahar Abd El Moneim Moussa24DDDbcDacDababCLab Lbc Lca

Deq = 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 Moussa25IV- 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 Moussa26

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 Moussa27CORONA 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 Moussa28L per phase=2 X 10-7 ln [Deq / DSL] H/m

Dr. Sahar Abd El Moneim Moussa29

Dr. Sahar Abd El Moneim Moussa30DdabCDr. Sahar Abd El Moneim Moussa31DdabCDr. Sahar Abd El Moneim Moussa32DdabCExample 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 mL per phase =2 X 10-7 ln [D / Ds] H/mL per phase =2 X 10-7 ln [4 / (0.78 x10-2 )] = 1.248 x 10-6 H/m

Dr. Sahar Abd El Moneim Moussa33DDDDDabCExample 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 Moussa342- Line Capacitance

Capacitance of single phase Two Wire line

The line-to-neutral capacitance:Cn =2 o / ln [D /r] F/mWhere, o : is the permittivity of air = 8.85 x 10-12 F/mDr. Sahar Abd El Moneim Moussa35rDrr- Capacitance of 3-Phase 3-WireLine with Equal Spacing

The line-to-neutral capacitance:Cn =2 o / ln [D /r] F/m

Dr. Sahar Abd El Moneim Moussa36DDDDDabC- Capacitance of 3-Phase 3-WireLine with Unequal Spacing

The line-to-neutral capacitance:Cn =2 o / ln [Deq /r] F/mWhere, Dr. Sahar Abd El Moneim Moussa37DDDbcDacDababC

Deq = V- Bundled Conductors

Dr. Sahar Abd El Moneim Moussa38The line-to-neutral caacitance:Cn =2 o / ln [Deq /Dsc] F/m

Deq = Dr. Sahar Abd El Moneim Moussa39DDDDDabCExample 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 Moussa40Example 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

Dr. Sahar Abd El Moneim Moussa41abDCdDr. Sahar Abd El Moneim Moussa42Example 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 Moussa43Grounding rodFoundationTowerShield conductorsInsulatorBundled conductor(four conductors)

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