Transmission Line Parameteres

BY

NAILA ZAREEN

TRANSMISSION LINES PARAMETERS

Transmission Lines Class 6

Transmission Line Concept

PowerPlant

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Transmission Lines

The purpose of transmission network is to

transfer electric energy from generating units at various locations to the distribution system which ultimately supplies the load.

Electrical Properties

All transmission lines in a power system exhibit the electrical properties of

ResistanceInductanceCapacitanceConductance

Equivalent Circuit for an Incremental Length of Transmission

Transmission Line Resistance

Important in transmission efficiency evaluation and economic studies.

Significant effectGeneration of I2R loss in transmission line. Produces IR-type voltage drop which affect

voltage regulation

Transmission Line Resistance

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Conductor resistance is affected by three factors:- •Frequency (‘skin effect’) • Spiraling •Temperature

Skin Effect

When ac flows in a conductor, the current distribution is not uniform over the conductor cross-sectional area and the current density is greatest at the surface of the conductor.

This causes the ac resistance to be somewhat higher than the dc resistance. The behavior is known as skin effect

Skin Effect

Spiraling

For stranded conductors, alternate layers of strands are spiraled in opposite directions to hold the strands together.

Spiraling makes the strands 1 – 2% longer than the actual conductor length.

DC resistance of a stranded conductor is 1 – 2% larger than the calculated value.

Temperature

The conductor resistance increases as temperature increases. This change can be considered linearover the range of temperature normally encountered.

WhereR1= conductor resistances at t1 in °C R2= conductor resistances at t2 in °C

Structure and EM Fields in Co-ax

The inductance and capacitance are due to the effects of magnetic and electric field around the conductor

Diagram of EM Fields Around Wire Pair

Presence of Electric and Magnetic Fields

V

I

I

E

+

-

+

-

+

-

+

-

V + DV

I + DI

I + DIV

IH

IH V + DV

I + DI

I + DI

INDUCTANCE :

A current-carrying conductor produces a magnetic field around the conductor.

The magnetic flux can be determined by using the right hand rule.

For nonmagnetic material, the inductance L is the ratio of its total magnetic flux linkage to the current I given by

L=λ/IWhere

λ=flux linkages, in Weber turns

Flux Linkage

Flux Linkage

Inductance due to Internal flux linkage

Inductance due to Internal flux linkage

Inductance due to External flux linkage

Inductance due to External flux linkage

Inductance of Single-Phase Lines

Inductance of Single-Phase Lines

Flux Linkage in Terms of Self and Mutual Inductances

Inductance of Three-Phase Transmission Lines

Asymmetrical Spacing

Transpose Line

Transpose Line

Inductance of Composite Conductors