ELECTRICAL POWER SYSTEM - II

SHORT TRANSMISSION LINE

PREPARED BY :- (BATCH B – 3)SHAH MOHIL ARUNBHAI 130280109102SHAH NEEL GIRISHKUMAR 130280109103SUHAGIYA SAUMIL ASHOKBHAI 130280109111

Contents1. Introduction2. Representation of transmission line3. Classification4. Short transmission line5. Phasor diagrams6. References

Introduction

Electrical power is generated in power station. It is necessary to send this power to different areas.

A conductive line which is used for the transmission of the electrical power from one place to another is known as “Transmission Line”.

Representation of transmission line

Classification Transmission lines are generally classified as

follows:

1) Short transmission line

2) Medium transmission line

3) Long transmission line

Short transmission lineTransmission line having length less than 80

km and operating voltage less than 20 kV are generally known as Short transmission line.

Due to small distance and low voltage capacitance effect is negligible. Hence performance of these lines depend upon resistance and inductance only.

Short transmission lineThe equivalent circuit of a short transmission line is

shown in Fig., where Is and IR are the sending and receiving end currents, respectively, and Vs and VR are the sending and receiving end line-to-neutral voltages.

The circuit is solved as a simple series AC circuit. So,

where Z is zl, the total series impedance of the line .

Short transmission line

Short transmission lineThe effect of the variation of the power factor

of the load on the voltage regulation of a line is most easily understood for the short line and therefore will be considered at this time.

Voltage regulation of a transmission line is the rise in voltage at the receiving end, expressed in percent of full-load voltage, when full load at a specified power factor is removed while the sending-end voltage is held constant.

Short transmission lineCorresponding to Eq. we can write,

where | Vnl | is the magnitude of receiving-end voltage at no load and | Vfl | is the magnitude of receiving-end voltage at full load with | Vs | constant .

After the load on a short transmission line , represented by the circuit of Fig., is removed, the voltage at the receiving end is equal to the voltage at the sending end .

Short transmission lineIn Fig., with the load connected , the

receiving-end voltage is designated by VR , and | VR | = | Vfl |.

The sending-end voltage is Vs ; and | Vs | = | Vnl |.

Phasor diagrams

(a) Load p.f.=70% lag (b)Load p.f.=100% (c)Load p.f.=70% lead

Phasor diagramsThe phasor diagrams of Fig. are drawn for the

same magnitudes of the receiving end voltage and current and show that a larger value of the sending-end voltage is required to maintain a given receiving-end voltage when the receiving- end current is lagging the voltage than when the same current and voltage are in phase.

A still smaller sending-end voltage is required to maintain the given receiving-end voltage when the receiving-end current leads the voltage.

Phasor diagramsThe voltage drop is the same in the series impedance of

the line in all cases; because of the different power factors, however, the voltage drop is added to the receiving - end voltage at a different angle in each case.

The regulation is greatest for lagging power factors and least, or even negative, for leading power factors.

The inductive reactance of (transmission line is larger than the resistance, and the principle of regulation illustrated in Fig . is true for any load supplied by a predominantly inductive circuit .

Phasor diagramsThe magnitudes of the voltage drops IrR and

IlXl for a short line have been exaggerated with respect to Vr in drawing the phasor diagrams in order to illustrate the point more clearl y.

The relation between power factor and regulation for longer lines is similar to that for short lines but is not visualized so easily.

ReferencesElements of power system analysis by W. D.

Stevenson

Power system analysis by Grainger & Stevenson

Modern power system analysis by Nagrath & Kothari

Wikipedia