**Inductance of a Single Phase Two Wire Line:**

Consider a simple Single Phase Two Wire Line composed of solid round conductors carrying currents I_{1} and 12 as shown in Fig. 2.3. In a Single Phase Two Wire Line,

It is important to note that the effect of earth’s presence on magnetic field geometry is insignificant. This is so because the relative permeability of earth is about the same as that of air and its electrical conductivity is relatively small.

To start with, let us consider the flux linkages of the circuit caused by current in conductor 1 only.

We make three observations in regard to these flux linkages:

**External flux from r**_{1}to (D – r_{2}) links all the current I_{1}in conductor 1.**External flux from (D – r**_{2}) to (D + r_{2}) links a current whose magnitude progressively reduces from I_{1}to zero along this distance, because of the effect of negative current flowing in conductor 2.**Flux beyond (D + r**_{2}) links a net current of zero.

For calculating the total inductance due to current in conductor 1, a simplifying assumption will now be made. If D is much greater than r_{1} and r_{2} (which is normally the case for overhead lines), it can be assumed that the flux from (D – r_{2}) to the centre of conductor 2 links all the current I_{1} and the flux from the centre of conductor 2 to (D + r_{2}) links zero current.

Based on the above assumption, the flux linkages of the circuit caused by current in conductor 1 as per Eq. (2.21a) are

The inductance of the conductor due to current in conductor 1 only is then

Similarly, the inductance of the circuit due to current in conductor 2 is

Using the superposition theorem, the flux linkages and likewise the inductances of the circuit caused by current in each conductor considered separately may be added to obtain the total circuit inductance. Therefore, for the complete circuit

Transmission lines are infinitely long compared to D in practical situations and therefore the end effects in the above derivation have been neglected.