What is Skin effect?
When direct current flows in the conductor, the current is uniformly distributed across the section of the conductor whereas flow of alternating current is non-uniform, with the outer filaments of the conductor carrying more current than the filaments closer to the centre. This results in a higher resistance to alternating current than to direct current and is commonly known as skin effect.
This effect is more, the more is the frequency of supply and the size of the conductor. A conductor could be considered as composed of very thin filaments. The inner filaments carrying currents give rise to flux which links the inner filaments only where 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. Hence the inductance/impedance of the inner strands is greater than those of outer strands which results in more current in the outer strands as compared to the inner strands. This non uniformity of flux linkage is the main cause of skin effect.
What is Proximity effect?
The alternating magnetic flux in a conductor caused by the current flowing in a neighbouring conductor gives rise to circulating currents which cause an apparent increase in the resistance of a conductor. This phenomenon is called proximity effect.
In a two-wire system more lines of flux link elements farther apart than the elements nearest each other. Therefore, the inductance of the elements farther apart is more as compared to the elements near each other and the current density is less in the elements farther apart than the current density in the elements near each other. The effective resistance is, therefore, increased due to non uniform distribution of current. The proximity effect is pronounced in case of cables where the distance between the conductors is small whereas for overhead lines with usual spacing the proximity effect is negligibly small.
