Need for Transposition of Over Head Transmission Lines

 

Need for Transposition of Over Head Transmission Lines

Generally the transmission line conductors of a 3-phase, 3-wire systems are irregularly spaced because it is very difficult to maintain an exact height of the conductor from ground on the poles. Due to these facts the transmission line constants, i.e., the inductance and capacitance values are not the same for the different phase and also there may be interchange of power between the phases which may lead to a difference between the apparent resistance of the conductors. Hence, inorder to equalize these line constants in a 3-phase transmission syatem, the three wires must be transposed such a way that each of three wires occupies a particular position relatively to the other wires for 1/3^rd length of transmission line and is show in fig.

Effect of Transposition

1.      The transposition in transmission line will balance the capacitance of the line, so that the electrico-statically induced voltages balanced in the length of a complete set of transposition.

2.      It also decereses electromagnetic induced emfs on the wires.

3.      By proper coordination of transposition of power and communication lines, the induced voltages can be reduced to very small and also it reduced the noise in the communication lines.

Calculation of inductance in a transposed line

Conductors are transposed in 3 phase systems, when they are irregularly spaced. Then for purpose of calculations, the may be treated as thorough the conductors were spaced symmetrically at an equivalent delta spacing of D =       where D1, D2, and D3, are the spacing between the conductors as shows in fig````

Capacitance of a single phase line

Let A and B represent two round parallel conductors of radius “r” each and are displaced at a distance “d” m apart.

If +q is the charge per unit length of conductor A and –q is the charge per unit lenth of conductor B induced by A as shown in fig. 1.20.

Then intensity at point P due to A=