Skin effect in transmission lines

Skin effect is the tendency of alternating current to crowd towards the outer surface of a conductor, so the current density is higher near the surface than at the centre.

Cause

• The current in the central filaments is linked by more magnetic flux (internal plus external) than the current in the surface filaments.
• So the inner filaments have higher inductive reactance, and the current prefers the outer, lower-reactance path.

   DC: uniform        AC: crowded at surface
    .........           ##########
    .........           #        #
    .........           ##########

Effects

• The effective cross-section carrying current is reduced, so the AC resistance is greater than the DC resistance ($R_{ac} > R_{dc}$).
• Power loss ($I^2R$) increases.
• The centre of a thick conductor is poorly used, which is why ACSR conductors have steel (strength) at the core and aluminium outside.

Factors affecting skin effect

It increases with frequency, conductor diameter, conductivity and permeability. It is small at 50 Hz for normal line conductors (about 1–3%), but it is more noticeable for large conductors and at high frequency. It is zero for DC.