Definition: When conductors carry high alternating voltages, the currents are non - uniformly distributed across the cross - sectional area of the conductor. This phenomenon is known as the proximity effect. The proximity effect causes an increase in the apparent resistance of a conductor because of the presence of other current - carrying conductors in its vicinity.
When two or more conductors are positioned close to one another, their electromagnetic fields interact. As a result of this interaction, the current in each conductor is redistributed. Specifically, a higher current density accumulates in the part of the conductor strand that is furthest from the interfering conductor.
If the conductors carry current in the same direction, the magnetic fields of the adjacent halves of the conductors cancel each other out. Consequently, no current flows through these adjacent half - portions of the conductors, and the current instead crowds into the remote half - portions.
When conductors carry current in opposite directions, the magnetic fields in the closer portions of the conductors reinforce each other, leading to a higher current density in these adjacent regions. Conversely, the magnetic fields in the farther halves of the conductors cancel each other out, resulting in minimal or zero current flow in those remote areas. Consequently, the current becomes concentrated in the nearer parts of the conductors, while the far-off halves exhibit significantly reduced current.
If DC flows through a conductor, the current is uniformly distributed across the conductor's cross-sectional area. As a result, no proximity effect occurs on the conductor's surface.
The proximity effect is only significant for conductor sizes larger than 125 mm². To account for this, correction factors must be applied.
When accounting for the proximity effect, the AC resistance of the conductor becomes:
Notations:
- Rdc: Uncorrected DC resistance of the conductor.
- Ys: Skin effect factor (the fractional increase in resistance due to the skin effect).
- Yp: Proximity effect factor (the fractional increase in resistance due to the proximity effect).
- Re: Effective or corrected ohmic resistance of the conductor.
The DC resistance Rdc can be obtained from stranded conductor tables.
Factors Influencing the Proximity Effect
The proximity effect primarily depends on factors such as the conductor material, diameter, frequency, and structure. These factors are detailed below:
- Frequency – The proximity effect intensifies as the frequency increases.
- Diameter – Larger conductor diameters lead to a more pronounced proximity effect.
- Structure – This effect is more significant in solid conductors compared to stranded conductors (e.g., ASCR). Stranded conductors have a smaller effective surface area than solid conductors, reducing current crowding.
- Material – Conductors made of high-ferromagnetic materials exhibit a stronger proximity effect on their surfaces due to magnetic field interactions.
Methods to Mitigate the Proximity Effect
One effective way to reduce the proximity effect is by using ACSR (Aluminum Conductor Steel Reinforced conductors. In an ACSR conductor:
- Steel is placed at the core to provide mechanical strength.
- Aluminum strands surround the steel core, forming the outer conductive layer.
This design minimizes the surface area exposed to magnetic field interactions. As a result, current primarily flows through the outer aluminum layers, while the steel core carries little to no current. This configuration significantly reduces the proximity effect in the conductor.
