Both skin effect and proximity effect increase losses in any conductor with time varying current.
When DC is passing through the conductor, the distribution of current throughout the cross-section of the conductor is uniform. This happens only with the direct current (DC). When alternating current (AC) is flowing through the conductor, the distribution of current throughout the cross-section of conductor is non-uniform. In this condition, the current density is higher at the surface of the conductor compared to the current density at its center. As the frequency is increased, this effect becomes more pronounced. This effect is called the “skin effect”.
If we compare the power loss in a conductor with given RMS AC with the same value of DC, the power loss is high in case of AC. This happens because of the effective conductor resistance is more for AC than the DC.
Similar to skin effect, one another effect which causes the non-uniform current distribution in the conductor. This effect is known as the “proximity effect”.
When the high alternating voltage is passing from the conductor, the current distribution in the conductor is nonuniform. This effect known as the proximity effect.
Let’s take an example, in the transmission system, two or more conductors placed near to each other. In this condition, their electromagnetic fields interact with each other. Because of this interaction, the current density is not the same in the entire cross-section area. The current density is least at both far ends of the conductor. So, in this area, more amount of current will flow. (this happens when the direction of the current is the same in both conductors.)
Now, consider the two conductors placed as shown in the below figure. As shown in the figure, each conductor divided into sections of equal cross-section area. Here, we divide both conductors into three sections. These three sections make three parallel loops, AA’, BB’, and CC’. The flux linking in the loop AA’ is least and it increases for further loop BB’ and CC’. The current density of the conductor is highest in the inner loop AA’ and lowest in the outer loop CC’. If the distance between two conductors is less, in this condition proximity effect becomes severe. In the case of the overhead transmission line, the distance between two conductors is more. So, here we can neglect the proximity effect. But in case of the underground transmission line, the distance between two conductors is less. So, the proximity effect is severe in case of the underground transmission line.
Both the skin effect and proximity effects, depend upon the size of conductor, frequency, distance between the conductors and permeability of conductor material.