Characteristics and Performance of Power Transmission Lines

S H O R T A N D M E D I U M T R A N S M I S S I O N L I N E S

Characteristics and Performance of Power Transmission Lines

R. Femi, Lecturer, EEE & I, BITS Pilani, K.K. Birla Goa Campus.

1

The topics covered in this lecture will be

To develop model for transmission lines of various lengths.

To calculate voltage and current in short, medium and long lines.

To understand the concepts of voltage regulation and efficiency in transmission lines


2

At the end of this lecture you should be able to

Model transmission lines for calculating voltages and currents .

Understand the power flow between two ends of the of a transmission line.


3

Transmission lines shall transmit power while satisfying the following requirement

Voltage should remain as constant as possible through the entire line length.

Lines losses should be as low as possible.

Line conductors should not overheat due the I2R losses.


4

transmission lines are classified

Short line whose length is less than 80 km.

Medium line whose length is between 80 km and 250 km.

Long line whose length in more than 250 km.


5

Nomenclature

r = line resistance/phase/unit length z=series impedance/length/phase y=shunt admittance/length/phase L = line inductance /unit length/phase C = line capacitance/ unit length/phase l=length of transmission line Z=zl=total series impedance/phase Y=yl=tottal shunt admittance/phase to neutral VR= phase voltage at the receiving end. IR = receiving current VS= phase voltage at the sending end IS = sending end current SR = three phase apparent power at receiving end SS = three phase apparent power at sending end.


6

Short line Model and Representation

Capacitance of short lines is very small.

It is ignored in short lines without much error

It is represented by a series impedance , Z, which is given by

Z = (r + jωL)l

Where l= line length, ω = frequency

The impedance is written in terms of the total line resistance and line reactance as

Z = (R + jX )


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Short line model


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Short line model


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Two port network


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Voltage Regulation


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Efficiency


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Example


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Solution


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Solution


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S H O R T A N D M E D I U M T R A N S M I S S I O N L I N E S


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R. Femi, Lecturer, EEE & I, BITS Pilani, K. K. Birla Goa Campus.

Short Transmission line-Phasor Diagram17


Short Transmission line-Phasor Diagram18


Short Transmission19








Medium Line Model

Capacitance of medium lines is significant and can not be ignored.

Y = ( jwC )l

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T- model 24


T- model 25


T- model 26


∏- model27


∏- model28


∏- model29


∏- model30


Example-131


Solution32


Solution33


Solution34


Example-235


Solution36


Solution37


Solution38


Solution39


Example-340


Solution π

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Solution42


Solution43


L O N G T R A N S M I S S I O N L I N E S


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Long Transmission Line


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T- model :Long Transmission Line


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∏- model :Long Transmission Line


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Ferranti Effect


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Ferranti Effect


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Ferranti Effect


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Ferranti Effect


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Ferranti Effect


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Ferranti Effect


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Ferranti Effect


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Ferranti Effect


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Tuned Power Lines


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Example-1


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Example-1


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Example-2


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Solution


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Solution


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P O W E R F L O W T H R O U G H T R A N S M I S S I O N L I N E


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Two bus system


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Sending and receiving end currents


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Sending and receiving end VA


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Sending and receiving end 3phase VA


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Real and reactive powers of both sending end and receiving end


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Maximum received power


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For Short transmission line


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Voltage control


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Voltage ratings of the various buses in the power systemwhich includes generating station buses, switchingsubstation buses, receiving substation buses anddistribution substation buses should be within thepermissible limits for satisfactory operation of allelectrical equipments.

The task of voltage control is closely associated withfluctuating load conditions and correspondingrequirements of reactive power compensation.

Therefore several voltage control methods are employedin power system to keep the voltage levels within thedesirable limits.

Voltage Control


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Reactive Power Injection


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Types of VAR Generators


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Static VAR generators Static capacitor bank

Dynamic VAR generators Synchronous condenser

Static VAR generator


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Rotating VAR generator


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Control by transformers


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Example-1


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Example-1


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Example-1


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Example-1


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Example-2


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Characteristics and Performance of Power Transmission Lines