3. 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”.
5. Classification
Transmission lines are generally classified as follows:
1) Short transmission line
2) Medium transmission line
3) Long transmission line
6. Short transmission line
Transmission 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.
7. Short transmission line
The 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 .
9. Short transmission line
The 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.
10. Short transmission line
Corresponding 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 .
11. Short transmission line
In 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 |.
13. Phasor diagrams
The 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.
14. Phasor diagrams
The 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 .
15. Phasor diagrams
The 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.
16. References
Elements of power system analysis by W. D. Stevenson
Power system analysis by Grainger & Stevenson
Modern power system analysis by Nagrath & Kothari
Wikipedia
