21-08-2013, 04:32 PM
Performance of Transmission Lines
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Introduction
The important considerations in the design
and operation of a transmission line are
the determination of voltage drop, line
losses and efficiency of transmission. These
values are greatly influenced by the line constants
R, L and C of the transmission line. For instance,
the voltage drop in the line depends upon the
values of above three line constants. Similarly,
the resistance of transmission line conductors is
the most important cause of power loss in the line
and determines the transmission efficiency. In
this chapter, we shall develop formulas by which
we can calculate voltage regulation, line losses
and efficiency of transmission lines. These
formulas are important for two principal reasons.
Firstly, they provide an opportunity to understand
the effects of the parameters of the line on bus
voltages and the flow of power. Secondly, they
help in developing an overall understanding of
what is occuring on electric power system.
Classification
A transmission line has *three constants R, L and C distributed uniformly along the whole length of
the line. The resistance and inductance form the series impedance. The capacitance existing between
conductors for 1-phase line or from a conductor to neutral for a 3-phase line forms a shunt path
throughout the length of the line. Therefore, capacitance effects introduce complications in transmis-
sion line calculations.
Performance Transmission
As stated earlier, the effects of line capacitance are neglected for a short transmission line. Therefore,
while studying the performance of such a line, only resistance and inductance of the line are taken
into account. The equivalent circuit of a single phase short transmission line is shown in Fig. 10.1 (i).
Here, the total line resistance and inductance are shown as concentrated or lumped instead of being
distributed. The circuit is a simple a.c. series circuit.
Medium Transmission Lines
Transmission
In short transmission line calculations, the effects of the line capacitance are neglected because such
lines have smaller lengths and transmit power at relatively low voltages (< 20 kV). However, as the
length and voltage of the line increase, the capacitance gradually becomes of greater importance.
Since medium transmission lines have sufficient length (50-150 km) and usually operate at voltages
greater than 20 kV, the effects of capacitance cannot be neglected. Therefore, in order to obtain
reasonable accuracy in medium transmission line calculations, the line capacitance must be taken into
consideration.
Long Transmission Lines
Transmission
It is well known that line constants of the transmission line are uniformly distributed over the entire
length of the line. However, reasonable accuracy can be obtained in line calculations for short and
medium lines by considering these constants as lumped. If such an assumption of lumped constants
is applied to long transmission lines (having length excess of about 150 km), it is found that serious
errors are introduced in the performance calculations. Therefore, in order to obtain fair degree of
accuracy in the performance calculations of long lines, the line constants are considered as uniformly
distributed throughout the length of the line. Rigorous mathematical treatment is required for the
solution of such lines.