How Skin Effect occurs ?
When AC current flows through the conductor, it produces alternating magnetic field. Change in magnetic flux results in electromotive force or back emf. According to Lenz law, induced emf opposes the cause which produces it. Since the cause is magnetic flux which is higher at the center of conductor. Hence emf is strongest at the center and it opposes the flow of current due to which a very less or no current flows through the center of conductor.
In simple words, the presence of electric field (back emf) at the center of conductor forced the ac currents to flow near the outer surface of conductor. In that case, current density is higher at the outer surface as compared to the interior of conductor. Moreover, this phenomenon restricts the current in the small cross sectional area which increases the resistance of conductor (Since resistance and cross sectional area of conductor is inversely proportional to each other ( R=rho L/A). Overall, we can define the skin effect in transmission line as:
Skin Effect in Transmission Line :
An alternating high frequency electric current concentrate on the outer surface of conductor due to high resistance at the center of conductor. This effect give rise to the phenomenon known as skin effect in transmission lines, cables and wires.
Factors affecting the Skin Effect :
Operational Frequency:
Skin effect increases with the increase in frequency. As the frequency of current increases, the current tend to concentrate around the outer surface of conductor.
Shape of Conductor :
Stranded conductors are preferred in long AC transmission lines
rather than single solid conductor since it reduces the skin effect.
Diameter:
As the diameter of conductor increases, more current flows through the surface which increases the skin effect.
How to reduce skin effect in transmission lines ?
Skin effect can be decreased by increasing the surface area of “conductor skin” where high frequency current is concentrated. For this purpose, stranded conductors are used instead of using single solid conductor. Stranded conductors increases the surface area for the flow of currents which results in reduce diameter of conductor for carrying same amount of current. Hence increasing the surface area by using stranded conductors decreases the skin effect.
Disadvantages of Skin Effect :
Due to skin effect, the resistance of conductor increases which results in power losses during transmission. Hence overall current carrying capacity is reduced.
Why Skin Effect doesn’t occur in DC Current?
Skin effect occurs due to the back emf produced by the alternating magnetic flux in a conductor. In case of dc current, change of magnetic flux is zero and there is no back emf. Hence current is uniformly distributed throughout the cross section of conductor and there is no skin effect.
Conclusion:
From the above discussion, we can say that AC current is distributed in a non-uniform way through the cross-sectional area of the conductor. It is to be noted that skin effect occurs at high AC frequencies of 50 Hz and more. It is negligible when the supply frequency is less than 50 HZ and diameter of conductor is small (< 1cm).
That’s all. Hope this will helps you.
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