This document discusses transposition in transmission lines. Transposition refers to the exchanging of positions of the three conductors in a three-phase transmission line at regular intervals. This is done to equalize the inductance and voltage drop across the conductors since their spacing is unsymmetrical, which would otherwise cause unequal voltage drops along the line. Transposition is important for long transmission lines where voltage unbalance would be significant without it.

1. Transposition in
Transmission Lines
 K-13EL-79
Presentation
ON
Zulfiqar Mangrio

2. Introduction
 When 3-phase line conductors are not
equi-distance, then flux linkages and
inductance of each conductor are not
same.
 A different in inductance in each
phase results in unequal voltage
drops.
 In order that voltage drops equal in all
conductors, we generally interchange
the position of conductors at regular
interval; called transposition.

3. Transposition
 Transposition of Conductors refers to
the exchanging of position of
conductors of a three phase system
along the transmission distance
in such a manner that each
conductors occupies the original
position of every other conductor over
an equal distance.

5. Electrical Transmission lines
 In Single circuit AC transmission line 3
conductors of all the 3 phases are
unsymmetrical placed.
 Due to unsymmetrical spacing of the
conductors Inductance and Capacitance of
each conductors becomes different.

6.  Suppose we assume that resistance is
same for all the three conductors than
due to difference in inductance and
capacitance, all the three conductors
will have difference in Impedance.
 So, due to difference in impedance
Voltage drop is different and
unbalance of voltage is obtained.

7. Use of Transposition
 Transposition of electrical
transmission line is must, when Long
AC transmission line is to be erected.
 Because Voltage unbalance in Long
transmission line becomes remarkable
and cannot be tolerated.
 In case of short transmission line it
does not create much imbalance in
voltage.

8.  So, to make this voltage unbalance
to minimum, the conductors are
transposed such that the relative
position of three phase conductors is
interchanged after certain length of
transmission line by putting Special
Transposition towers.