Skin Effect in Transmission Lines

The distribution of current throughout the cross section of conductor is uniform only when the steady current(D.C.) is passing through it. However, an alternating current flowing through the conductor does not distribute normally, rather it has the tendency to concentrate near the surface of the conductor as shown in figure below.

What is skin effect

The tendency of alternating current to concentrate near the surface of a conductor is known as skin effect. 

This results in higher resistance to alternating current that to direct current and is more pronounced as frequency is increased. This is known as skin effect. It causes a larger power loss for a given rms ac than the loss when the same value of dc is flowing through the conductor. Consequently, a qualitative explanation of the phenomenon is given below.

" A conductor could be considered as composed of very thin filaments. The inner filaments carrying current gives rise to flux which links the inner filaments only when as the flux due to current carrying outer filaments enclose both the inner as well as the outer filaments. The flux linkages per ampere to inner strands is more as compared to outer strands. The inductance* of each strand will vary according to its position. Thus the strands near the center are surrounded by greater magnetic flux and hence have larger inductance than that near the surface. The high reactance of inner strands causes the alternating current to flow near the surface of conductor. This crowding of current near the conductor surface is the skin effect. "
With the increase of the frequency the non-uniformity of inductive reactance of the filaments becomes more pronounced, so also the non-uniformity of current distribution. For large solid conductors the skin effect is quite significant even at 50Hz. The analytical study of skin effect requires the use of Bessel's functions.
The skin effect depends upon the following factors:-

1. Nature of material
2. Diameter of wire- It increases with increase in diameter of wire
3. Frequency- It increase with increase in frequency.
4. Shape of wire- It is less for stranded conductor than solid conductor.

It may be noted that skin effect is negligible when the supply frequency is low (≤ 50Hz) and conductor diameter is small (< 1 cm).