Skin effect is a phenomenon that occurs in conductors. It mainly occurs while a conductor carries alternating current. Actually, an alternating current always tries to flow through the outer layer of a conductor. As a result, the current distribution across the cross-section of the conductor becomes uneven.
Alternating current flows through the conductor at 50 or 60 Hertz. This effect increases with increasing supply frequency. Since the alternating current tries to flow through the surface, or skin, of the conductor, we refer to this phenomenon as skin effect.
Theory of Skin Effect
Actually, alternating current produces an alternately changing magnetic field. This changing magnetic field induces eddy currents within the conductor itself. This eddy current opposes the main current. Actually, the maximum number of magnetic field lines links the central portion. This is the reason the influence of the alternating magnetic field is maximum at the center. So the opposition of the eddy current is maximum at the center of the conductor.
The number of magnetic flux linkages on the conductor surface is less than that of the central axis. So the induced eddy currents are less at the surface. Therefore, the opposition by the eddy currents is also less on the surface. So the main current does not face opposition on the surface of the conductor. The current now faces a smaller effective cross-sectional area to flow. Therefore, the effective resistance of the conductor increases.
Skin Effect Formula
We use a mathematical formula to determine skin effect. Specifically, the formula for skin effect is\[\delta = \sqrt{\frac{\rho}{\pi f \mu}}\]Here, \(\delta\) represents the skin depth of the conductor. Moreover, it indicates the depth through which significant current flows. We measure \(\delta\) in meters.
Additionally, \(\rho\) denotes the resistivity of the conductor material. We express it in ohm-meters. Furthermore, f represents the frequency. \(\mu\) indicates the magnetic permeability. From this equation, we can observe that when frequency increases, skin depth decreases. Therefore, frequency plays a crucial role.
For example, consider a copper conductor at 60 Hz. In this case, the skin depth equals 8.5 mm. However, when frequency rises to one megahertz, skin depth drops to just 0.066 mm. Consequently, this demonstrates how frequency influences this effect.
Diagram
This diagram illustrates the current distribution due to skin effect. Specifically, it shows the current density at different depths from the conductor surface. Moreover, each curve represents a particular frequency. We can observe sharp changes in current density for curves representing higher frequencies. In contrast, lower frequency curves appear flatter. Therefore, frequency significantly impacts the distribution pattern.
Additionally, in the image below, the dark red color on the outer portion indicates maximum current density. Whereas the white central portion shows lower current density. Hence, the diagram below clearly shows the skin effect phenomenon.
Skin Effect in Transmission Lines
The skin effect increases ohmic loss by increasing effective resistance. Also, underground transmission lines face the same challenge. Specifically, skin effect increases the ohmic loss in the conductor portion of the cable. Obviously, this generates higher heat. This heat causes derating of the cable.
Skin Effect in Power Systems
Skin effect also influences all electrical connections. It increases the effective resistance of the armature alternators. This increases the effective resistance of transformer windings also. Moreover, solid bus bars in indoor systems also experience this problem. By using multiple thin insulated busbars connected in parallel instead of a single thick busbar, we can reduce the skin effect. However, for outdoor systems, we use different solutions. We use either stranded power conductors or hollow aluminum pipes. Both of these designs reduce skin effect.
Disadvantages of Skin Effect
The skin effect increases the ohmic loss by increasing the effective resistance.
Additionally, the central portion of the conductor remains unutilized due to skin effect. Hence, valuable conductor material goes unutilized.
Furthermore, concentrated current at the surface of the conductor produces heat. As a result, the insulation becomes degraded faster.
How to Reduce Skin Effect
Use of Stranded Conductor
We avoid using solid conductors. Instead, we always use stranded conductors. Because stranding increases the effective surface portion of the conductor. As a result, the current gets more paths to flow. Therefore, the overall resistance of the conductor reduces. So, this improves current flow efficiency.
Use of Hollow Conductor Pipes
We avoid using solid conductors in substations. Instead, we always try to use hollow aluminum pipes. Specifically, we use them in bays and bus bars. Hollow pipes have nothing in the inner portion. Therefore, we make efficient use of aluminum or copper.
Milliken Conductor
In the Milliken conductor, we not only use stranded conductors. Moreover, we use multiple groups of stranded conductors. Additionally, these groups are insulated from each other by thin layers of insulating material. Furthermore, this arrangement improves the current distribution through the conductor. As a result, it increases the effective cross-sectional area. Hence, resistance decreases significantly.
