Interchanging the positions of conductors at regular intervals along the line so that each conductor occupies another conductor's position at an equal distance is known as Transposition of Transmission Line.
When in case of an overhead transmission line, all the three phases are supposed to be spaced symmetrically keeping the line constants, identical but due to some mechanical considerations it in not so, these are placed irregularly. It disturbs the line constants, as a result the voltage drop in the three lines will be different and unequal line voltage is achieved at the receiving end.
This difficulty is overcome by the use of transposition of conductors. The conductors of the transmission line are kept at a regular distance and (the system adopted is known as transposition of conductors as shown in Fig. 1.
Figure 1: Transposition of Transmission Line
In this case let the sequence by RYB, then after x/3 distance it is made by transposition as BRY and after again x/3 distance as YBR, by inter changing the line conductors, so the arrangement of changing the positions of overhead line conductors of the transmission line at regular distance in order to make line constants symmetrical is called transposition of conductors.
Procedure of Transposition of Transmission Line
As stated, the transposition of line conductors involves changing the positions of line conductors for one-third of the length of the line. Transposition of a transmission line over the length ‘x’ is shown in the figure 1. The phase conductor ‘R’ will take the position 1 for the first 1/3rd of the length of the line, it will take position 2 and finally it occupies position 3 in the remaining 1/Y” of the length of the line. The phase conductors ‘Y’ and ‘B’ also changes their position in a similar manner as shown in the figure.
Need of Transposition of Transmission Line
The transmission line is transposed,
- To maintain constant voltage drop across each conductor.
- To maintain equal voltage at the receiving end of the transmission line for all phases.
- To reduce disturbances to the nearby communication lines.
Advantages of Transposition of Transmission Line
The advantages of transposition of conductors are as follows.
- Equal average inductance is appeared across each conductor/phase of the line.
- Constant voltage drop is maintained across each conductor/phase of the line.
- Equal voltages are appeared at the receiving end of the lines.
- Electrostatically induced voltages are balanced along the balTel.
- Electromagnetically induced e.m.f is reduced on wires.
- The line constants are similar for all the three phases.
- It also prevents the telecommunication or radio interferences from the neighbouring line
