Skin effect and proximity effect in transmission line - engineeringforu

What is Skin effect:

When a conductor is carrying a steady direct current i.e dc current, this current is uniformly distributed over the whole x-section of the conductor. However, an alternating current i.e. a.c current flowing through the conductor does not distribute uniformly, rather it has the tendency to concentrate near the surface of the conductor. This is known as skin effect.
The tendency of alternating current to concentrate near the surface of a conductor is known as skin effect.
Due to skin effect, the effective area of cross section of the conductor through which current flows is reduced. Consequently, the resistance of the conductor is slightly increased when carrying an alternating current. The cause of skin effect can be easily explained. a solid conductor may be thought to be consisting of a large number of strands, each carrying a small part of the current. The inductance of each strand will vary according to its position. Thus, the strands near the centre are surrounded by a greater magnetic flux and hence have larger inductance than that near the surface. The high reactance of inner strands causes the alternating current to flow near the surface of the conductor. This crowding of current near the conductor surface is the skin effect.

Factors affecting skin effect:

  • Nature of material: Skin effect increases with the increase in the permeability of the material.
  • Diameter of wire - Skin effect increases with the increase in diameter of wire.
  • Frequency - Skin effect increases with increase in frequency.
  • Shape of wire - Skin effect is less for stranded conductor than the solid conductor.
It may be noted that the skin effect is negligible when the supply frequency is low i.e less than 50 Hz and conductor diameter is small i.e less  than 1 cm.

What is Proximity effect:

The proximity effect also results in non uniform distribution of current in x-section of a conductor.
So, like skin effect proximity effect affects the current distribution and results in an increase in the resistance of the conductor and decrease of self reactance. The phenomenon is more pronounced for large conductors, high frequencies and close proximity. The magnitude of the effect, at normal supply frequencies, in the case of the wide spacings of conductors required for overhead transmission lines, is so small that it can be ignored. In case of stranded conductors, the proximity effect is substantially eliminated.

Factors affecting proximity effect:

The proximity affect mainly depends on the conductor material ,frequency, conductor structure and conductor diameter. These factors are explained below.

  1. Conductor material: If the conductor material is made up of high ferro magnetic material then proximity effect is more on their surface.
  2. Conductor diameter: With increase in the conductor diameter, the proximity effect increases.
  3. Frequency: With increase in frequency, proximity effect increases.
  4. Conductor structure: Proximity effect is more on solid conductors than stranded conductors like ACSR as the surface area of stranded conductor is small compared to solid conductor.

How to reduce proximity effect:

Proximity effect is more in solid conductors than stranded conductors. So by using stranded conductors like ACSR ( Aluminium core steel reinforced) proximity effect can be reduced.