What is Skin Effect?
The non-uniform distribution of alternating current on the surface of the conductor is called the skin effect. The electric charge is more concentrated near the surface of the conductor as compared to the core of the conductor is called the skin effect due to this effect ohmic resistance of the conductor is increased.
The skin effect is proportional to the frequency of the supply. If the frequency of the supply is increased then the skin effect increased. At a small frequency of less than 50Hz, the current density at the surface of the conductor is less, hence skin effect is small. At a higher frequency more than 50Hz, the current density at the surface of the conductor is more, hence the skin effect is more.
In the case of alternating current, current can not flow through the core but the entire current is concentrated at the surface of the conductor.
The direct current is uniformly distributed over the full cross-section of the conductor and in dc current frequency is zero, hence skin effect is zero.
How skin effect occurs in transmission lines?
The aluminium conductor is made from a number of concentric cylinders. When the alternating current passed through the conductor, the magnetic flux is produced. This magnetic flux is linked to centre cylindrical elements as well as a cylindrical surface element of the conductor.
The flux linking is more at the centre of cylindrical elements and less at the cylindrical surface element of the conductor. The centre cylindrical elements are surrounded by both the internal flux as well as external flux and external cylindrical elements are surrounded by only the external magnetic flux only.
The self-inductance at the centre of cylindrical elements is more, so inductive reactance is greater at the centre as compared to the outer surface. So Electrical current is more concentrated on the surface of the conductor as compared to the centre.
The current density or concentration of current at the surface of the conductor is maximum. The current density of concentration of current at the surface of the conductor is minimum. Due to this effect, the cross-section area of the conductor is reduced and the effective resistance of the conductor is increased.
Factors affecting the skin effect are
1. Frequency of the supply voltage
2. Diameter of the conductor
3. The shape of the conductor
4. Type of the material of the conductor
1. Frequency:
The skin effect is proportional to the frequency. If the frequency increase then the skin effect also increased.
2. Diameter:
The skin effect is also proportional to the diameter of the conductor. The diameter of the conductor is increased than skin effect increased.
3. The shape of the conductor:
The skin effect depends upon the shape of the conductor. In a solid conductor skin effect is more because the surface area of the solid conductor is more and the skin effect in a stranded conductor is less.
4. Type of material:
The skin effect is proportional to the permeability of the material. The skin effect is increased with an increase in permeability of the conductor.
The skin effect is negligible at 50Hz supply frequency and at a diameter of transmission line conductor is less than 1 cm.
In an ACSR conductor (aluminium conductor steel reinforced), the outer layer is made from the aluminium strands and carried the current. The inner steel conductor does not carry any current and provides tensile strength to the conductor. So the concentration of current density near the surface of the ACSR conductor is more.
The skin effect is negligible in short transmission lines. With the increase in length of the transmission line, the skin effect increased. The skin effect is reduced with an increase in the resistivity of the conductor material.
The Electrical current flowing at the surface of the aluminium conductor between the outer surface and centre of the conductor is called the skin depth. The skin depth is inversely proportional to the root of the supply frequency.
Disadvantages of skin effect
1. The skin effect increase the effective resistance of the conductor, so power loss takes place in the transmission line.
2. Due to the skin effect overall resistance of the transmission line conductor is increased.
3. Overall resistance of the conductor is reduced by increasing the cross-section area of the conductor which increases the cost of the conductor.
How to reduce or overcome skin effects in transmission lines
1. The skin effect is reduced by the using the bundled conductors
2. The skin effect is reduced by using the high voltage system, in high voltage current is less, so the skin effect is less.
3. The skin effect is reduced by using a stranded conductor instead of a bundle conductor.
4. The skin effect is reduced by using less permeability conductor material because the skin effect is proportional to the permeability of the material.
5. The skin effect is reduced by reducing the size of the conductor.