WHAT IS MEANT BY SKIN EFFECT?

Skin effect is the tendency of alternating current (AC) to concentrate near a conductor surface rather than flowing in uniform distribution through its entire cross-section. This occurs due to eddy currents induced by the changing magnetic field from the AC, which oppose current flow in the conductor’s interior.

Mechanism

An alternating magnetic field around and within a conductor generates eddy currents that create an opposing magnetic field, repelling electrons toward the skin or outer layer. At higher frequencies, this effect intensifies, drastically reducing current density inside—dropping to about 37% (1/e) of surface value at the skin depth. It raises effective AC resistance compared to DC, as less cross-sectional area carries the current.

Factors Influencing skin depth

• Frequency: Skin depth is inversely proportional to frequency; e.g., for copper, skin depth is ~8.5 mm at 50 Hz, while it is ~0.066 mm at 1 MHz.
• Material: Higher resistivity or magnetic permeability increases skin depth (e.g., worse in iron and low for copper).​
• Conductor size/shape: Stronger in solid, thick round wires; less in stranded or flat.

Material	Frequency (Hz)	Skin Depth (mm) ​
Copper	50	9.3
Copper	60	8.5
Copper	1 kHz	2.1
Aluminum	50	11.7

Frequency Dependence of AC Resistance

At DC (0 Hz) or low AC frequencies (<60 Hz), Rac ≈ Rdc. Above 1 kHz, Rac can be double or triple of Rdc in thick conductors. At 50/60 Hz power frequencies, increase is 5-20% for typical cables. Transformers and motors see higher Rac in windings due to bundled strands.​

Proximity effect in AC current

When bunched conductors carry AC current, like cables or transmission lines, charges passing through them affect those in one another. Charges in conductors then tend to distribute within conductors either towards each other, or away from one another, depending on current direction in cables.

In transmission lines, proximity effect is seen in the tendency of alternating current to concentrate on the side of a conductor nearest to an adjacent conductor, increasing effective resistance and power losses.

Proximity effect is moderate at power frequencies, but becomes more significant as frequency goes up, and can become a nuisance at high frequencies. The effect depends on closeness between conductors and reduces with increasing distance. Large diameter of conductor tends to have more non-uniform current distribution, while small diameters have much lower proximity effect (limited conductor area). This is why solid conductors are avoided for high-frequency applications. Closely packed and parallel conductors show higher proximity effect, while symmetrical and properly placing can reduce the effect.

Lower Frequency or DC

DC transmission eliminates skin effect entirely (f=0), as in HVDC lines for long distances. For AC, lower frequencies (e.g., 16⅔ Hz rail systems) extend skin depth, though practicality limits this.

Method	Application	Reduction Benefit
Stranded/ACSR	Power lines	10-30% lower Rac
Litz Wire	Transformers	Up to 70% at kHz
Hollow Busbars	Substations	Matches solid at 50% material

These align with capacitor/transformer windings and high-voltage systems where Rac control prevents overheating.

Mitigation of skin and proximity effects

Hollow or Tubular Shapes

Hollow conductors or busbars (common in substations for 50/60 Hz) have their inner material removed, since current flows only near surface. They match solid conductor performance at lower weight and cost, and also minimize proximity effects in parallel runs.

Material Selection

Use of non-ferromagnetic materials like copper or aluminium (low μ_r ≈1) over iron (permeability amplifies skin depth reduction). Aluminium suits overhead lines for its balance of conductivity and low skin effect at power frequencies.

Stranded Conductors

Stranded wires, like ACSR in transmission lines, divide current across many thin strands, each with negligible skin effect. Smaller strand diameters keep skin depth larger relative to radius, cutting effective resistance by 20-50% vs. solid equivalents.

In AC transmission lines, cores of conductors carry no current. The core is removed and replaced by steel wires to give mechanical strength to line, thereby reducing sag, and expansion or deformation effects under tensile stress between conductor supports.

Litz Wire Design

Litz wire weaves individually insulated thin strands in patterns that equalize magnetic exposure, and are ideal for transformers and inductors up to 1 MHz. This eliminates intra-strand skin effect, reducing AC losses significantly in high-frequency power electronics like VFDs or CVTs.

Litz wire is a special type of multistrand wire engineered for carrying alternating current (AC) efficiently at high frequencies. It is constructed of individually insulated magnet wires bunched or braided together in uniform pattern so that each strand takes all possible positions in the cross section of overall conductor. Primary benefit of Litz wire is in reducing AC losses in high frequency windings and increasing efficiency by circumventing the skin effect.