Saturday, October 11, 2014

Skin Effect Three phase lines; Factors effecting skin effect; Why skin effect not occur on DC?

Skin effect occurs in transmission lines due to unequal distribution of current over the entire cross section of the conductor being used for long distance power transmission. Skin effect usually occur in Alternating current not in Direct current flow. As Direct current is uniformly distributed across the section so skin effect doesn't takes place.But in alternating current current flow is non-uniform, where outer filaments of conductor takes more current than filament closer to the center. This will leads to higher resistance in conductors due to uneven distribution at alternation current then Direct current.

The inner filament carrying currents gives rise to flux which links to inner filaments only where as 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 the outer strands which results in more current in the outer strands as compared to the inner strands.

Lets understand the same more thoroughly :-

Having understood the phenomena of skin effect let us now see why this arises in case of an AC system. To have a clear understanding of that look into the cross sectional view of the conductor during the flow of alternating current given in the diagram below.

Let us initially consider the solid conductor to be split up into a number of annular filaments spaced infinitely small distance apart, such that each filament carries an infinitely small fraction of the total current.

Like if the total current = I

Lets consider the conductor to be split up into n filament carrying current ‘i’ such that I = n i .

Now during the flow of an alternating current, the current carrying filaments lying on the core has a flux linkage with the entire conductor cross section including the filaments of the surface as well as those in the core. Whereas the flux set up by the outer filaments is restricted only to the surface itself and is unable to link with the inner filaments.Thus the flux linkage of the conductor increases as we move closer towards the core and at the same rate increases the inductor as it has a direct proportionality relationship with flux linkage. This results in a larger inductive reactance being induced into the core as compared to the outer sections of the conductor. The high value of reactance in the inner section results in the current being distributed in an un-uniform manner and forcing the bulk of the current to flow through the outer surface or skin giving rise to the phenomena called skin effect in transmission effect

Factors Affecting Skin Effect in Transmission Lines

The skin effect in an ac system depends on a number of factors like:-

1) Shape of conductor.

2) Type of material.

3) Diameter of the conductors.

4) Operational frequency.