What is Skin Effect?

It is a common misconception that electricity travels down the center of a wire like a hose.  For DC this is mostly true but in our 60hz AC system 98% of the current travels down a conductor in an outside layer of skin only 8.4 mm deep for copper and 10.6 mm deep for aluminum.  This isn’t a problem for smaller wires but as your capacity needs increase, like in transmission, skin effect becomes a major design factor.

What causes Skin Effect?

1. Alternating current creates an alternating magnetic field (blue H)
2. This magnetic field creates loops of electrical current called eddy currents (red - Iw)
3. In the center of the wire the direction of the eddy currents is opposite the direction of the main current flow and so it slows it down.  Notice how in the diagram the red eddy current loops point opposite the current flow in the center of the wire.  Main current (I) ↑ and Eddy current  (Iw) ↓
4. In the outside of the wire the direction of the electric field is the same direction as the current and so it reinforces it
5. Therefore, skin effect is caused by eddy currents which point opposite current flow in the middle and with it towards the surface.
6. Would you rather walk up a stream along the banks or in the middle?  My 3 young boys would rather walk up the stream in the middle but they are not normal electrons.

 
How does frequency change the skin effect?

Skin effect depth is only effected by frequency and material 2.  As frequency increases the skin depth decreases and with it the useful cross sectional area of the wire.  On the other hand, as frequency decreases so does the skin effect.  In AC the current changes directions and therefore so does the polarity of the electromagnetic field it creates.  The dissipation of this field between cycles is not instantiations.   Lower frequency has less of an impedance to current flow because this field is given more time to discharge between cycles.  On the other hand, skin effect is more exaggerated at higher frequencies because the field is given less time to dissipate between cycles.  Direct Current, which has a frequency of 0, uses the entire cross section of a conductor.  This efficiency is one of the benefits of a high voltage direct current transmission system.

​Transmission Engineers hate him, find out why…
 
Skin effect losses become more apparent in transmission lines due to the conductors size and weight.  You have to make the wire bigger and bigger to get ever diminishing amounts of current carrying capacity.  Sure, increasing the wire size slightly increases the useful current carrying surface area near the circumference but it also significantly (quadratically) increases the cross sectional area which in turn drastically increases weight and cost.   A lot of cost for diminishing benefits is the law of diminishing returns.   This is why you see transmission cables have multiple individual cables in a bundle (2-6) in opposed to just using a thicker wire. 3  Some new underground transmission lines have isolated segments insulated from each other.  Similar to an overhead ‘bundle’ this reduces the required size of the wire by providing more “surfaces” for current to travel along.  Why have a middle at all?  Some applications just use tubes like in switchyards or flat bus bars. 4
Last fact… you might think that since each actual conductor has multiple strands of interwoven wires each would have their own skin effect.  However they touch so much that they act similar to a single solid core in terms of skin effect. 5   Most high voltage lines use an aluminum cable with a steal core in the middle of the interwoven wires to reinforce it called ACSR. 6  Since almost no current is traveling down the middle of the wire the higher resistance of steel is an easy tradeoff for its superior tensile strength.
This is such a deep topic, 8.4mm deep!  * I’ll see myself out *

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