Table of Contents
Currents are not evenly distributed across the conductor’s cross-section when it is carrying a high alternating voltage. This effect is called proximity effect. The proximity effect results in the increment of the apparent resistance of the conductor due to the presence of the other conductors carrying current in its vicinity.
The electromagnetic fields of two or more conductors interact when they are placed close together. Due to their interaction, the current in each of them is redistributed in such a way that the greater current density is concentrated in the area of the strand farthest from the interfering conductor.
If the conductors carry the current in the same direction, then the magnetic field of the halves of the conductors which are close to each other is cancelling each other and hence no current flow through that halves portion of the conductor. The remote half of the conductor is overflowing with current.
The closer portion of the conductor carries more current when the conductors are carrying current in the opposite direction, cancelling out the magnetic field of the conductor’s far-off half. Thus, the current is zero in the remote half of the conductor and crowded at the nearer part of the conductor.
When direct current (DC) flows across a conductor’s surface, the current is evenly dispersed throughout the conductor’s cross section. As a result, there is no proximity effects on the conductor’s surface.
The proximity effect is important only for conductor sizes greater than 125 mm2.Correction factors are to be applied to take this fact into account.
If Rdc – uncorrected DC level of the core
Ys – skin effect factor, i.e., the fractional increment in resistance to allowing for skin effect.
yp – proximity effects factor, i.e., the fractional increment in resistance to allowing for skin effect.
Re – effective or corrected ohmic resistance of the core.
The allowance for proximity effect is made, the AC resistance of the conductor becomes
The resistance Rdc is known from stranded tables.
Factors Affecting the Proximity Effect
The conductors’ material, diameter, frequency, and structure are the main determinants of the proximity effects. The elements are described in more detail below.
- Frequency – The proximity increases with the increases in the frequency.
- Diameter – The proximity effect increases with the increase in the conductor.
- Structure – This effect is more on the solid conductor as compared to the stranded conductor (i.e., ASCR). Because the surface area of the stranded conductor is smaller than the solid conductor.
- Material – If the material is made up of high ferromagnetic material then the proximity effect is more on their surface.
How to reduce Proximity Effect?
Using the ACSR (Aluminium Core Steel Reinforced) conductor will lessen the proximity effect. In an ACSR conductor, the aluminium conductor is positioned around the steel wire, which is positioned in the middle of the conductor.
The steel increased the strength of the conductor but reduced the surface area of the conductor. As a result, the conductor’s outer layer experiences the majority of the current flow. While the conductor’s centre experiences none. reduced the conductor’s proximity effects as a result.