The distribution of current over the entire cross-section of the conductor is uniform in case of a DC system. But there is unequal distribution of current over the entire cross section of the conductor in case of AC system. Here what Skin Effect arises.

What Is Skin Effect?

When an AC Current flows through a conductor, it is distributed non-uniformly throughout the conductor cross-section. This phenomenon of AC current where it has a tendency to concentrate near the surface of the conductor is called as the skin effect. 
Due to the skin effect, AC current flows between the outer surface of the conductor and the level called skin depth denoted by ẟ as shown in the figure below.

source: wikipedia.com

Skin effect is dependent upon frequency of the current. The effect becomes more and more apparent as the frequency increases. If the frequency of AC current is very high, the current flows to a very thin skin depth layer. Skin effect increases with increase in the frequency.
The main problem with skin effect is that it increases the effective resistance of the conductor for AC. Since, the effective cross-section of the conductor through which the current flows is reduced.
So effective resistance of the conductor at moderate to high frequencies are high as compared to the resistance of the same conductor at direct current (DC) and low AC frequencies.

Factors Affecting Skin Effect

Shape of conductor, type of material, diameter of the conductors and operational frequency are the factors that affects the Skin Effect.

  1. Better conductors and ferromagnetic materials experience higher skin effect.
  2. Skin effect is lesser for stranded conductors than solid conductors
  3. Skin effect increases with increase in the cross-sectional area of the conductor.
  4. Skin effect increases with increase in the frequency.

How to reduce skin effect?

Skin effect can be reduced by using Litz wire or braided wire Litz wire. Another way to reduce skin effect by using stranded conductor.
There is another effect related to alternating current. The effect is known as Proximity effect. Here is the explanation.

What is Proximity Effect?

Suppose two or more conductors carrying AC current are placed near each other. Each conductor is producing its varying magnetic field. When this varying magnetic field is linked with other current carrying conductor then the distribution of current in each conductor is affected due to the varying magnetic field.
The varying magnetic field produced by alternating current induces eddy currents in the adjacent conductors. 
When the nearby conductors have AC current in the same direction then the current will concentrate at the farthest side of the conductors. This is shown below. Dotted are the current.

source: Electro-labs.com
When the nearby conductors have AC current in the opposite direction then the current will concentrate at the nearest side of the conductors. This is shown below. Dotted are the current.

source: Electro-labs.com

This phenomena of distribution of current in the presence of nearby conductors is called as proximity effect. 
The proximity effect increases with increase in the frequency. The effective resistance of the conductor is increased due to the proximity effect.
Excercise
Q1. What due you mean by proximity effect in transmission lines?
Q2. How to reduce skin effect in transmission lines?
Q3. What is skin effect in transmission lines?
Q4. What is skin depth?
Q5. What is the cause of Skin effect in conductors?
Q6. What are the factors that affect skin effects in conductors?
Q7. What is the effects of skin and proximity effects in conductors?

Skin Effect And Proximity Effect In Conductors
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