Skin Effect of Electrical Power Transmission Line

What is Skin Effect of Power Lines?

The tendency of the alternating current to concentrate near the surface of the conductor is called the skin effect. The direct current distribution in a conductor is the same. But the alternating current distribution in a conductor is not uniform. The current density near the surface is higher than near the center of the conductor which means that the current flow in the surface of the conductor is greater than the current flow in the center of the conductor. It is influenced by the frequency of current. If the current frequency is high then the current distribution is more non-uniform. This effect is called the skin effect. The effective cross sectional area of ​​the conductor through which the current flows is reduced.

Skin effect on power lines – If the conductor carries a steady direct current (dc), this current is evenly distributed across the X-section of the conductor. However, the alternating current flowing through the conductor is not evenly distributed, and tends to concentrate near the conductor surface as shown in Figure 9.3. This is known as the epidermal effect.

The tendency of alternating current to concentrate near the surface of a conductor is known as the skin effect.

The skin effect of power lines reduces the effective cross-sectional area of ​​a conductor that carries current. As a result, the resistance of the conductor increases slightly when carrying an alternating current. The cause of the skin effect on power lines can be easily explained. A solid conductor can be thought of as a number of strands, each carrying a small portion of the electrical current. The inductance of each strand depends on its location. Therefore, the strands near the center are surrounded by a larger magnetic flux and thus have a greater inductance than the one near the surface. Due to the high reactance of the internal strands, an alternating current flows near the conductor surface. This current crowding near the conductor surface is a skin effect on the transmission line.

Skin effect of power line

The skin effect of power lines depends on the following factors:

1. Material properties

2. Wire diameter-increases with wire diameter.

3.Frequency – increases with increasing frequency.

4. Wire shape-twisted wire is less than solid wire.

Note that the skin effect of the transmission line is negligible at low supply frequencies (<50 Hz) and small conductor diameters (<1 cm).

Skin Affecting Factors

• Frequency - The skin effect increases as the frequency increases;
• Diameter - It increases with increase in diameter of conductor;
• Conductor size - The skin effect is greater in solid conductors and less in trapped conductors because the surface area of ​​solid conductors is higher;
• Material Type - The effect of the skin increases with an increase in the permeability of the material (permeability is the ability of the material to support the formation of a magnetic field).

Material effect on skin depth

• In a good conductor, skin depth varies as the inverse square root of the conductivity. This means that better conductors have a reduced skin depth;
• The overall resistance of the better conductor remains lower even with the reduced skin depth;
• Skin depth also varies as the inverse square root of the permeability of the conductor. In the case of iron, its conductivity is about 1/7 that of copper;
• In case of ferromagnetic its permeability is about 10,000 times greater. This reduces the skin depth for iron to about 1/38 that of copper, about 220 micrometres at 60 Hz;
• Iron wire is thus useless for A.C. power lines. The skin effect also reduces the effective thickness of laminations in power transformers, increasing their losses;
• Iron rods work well for (DC) welding but it is impossible to use them at frequencies much higher than 60 Hz.

How to reduce the effect of skin:

1. ACSR bundle conductors are used to minimize skin effects. ACSR conductors are steel that are placed inside the center or center of the conductor and aluminum conductors are positioned around the steel wire. Steel increased the conductor strength but reduced the surface area of ​​the conductor. Thus, current flows in the outer layer of the conductor and no current flows in the center of the conductor. Thus, reducing the proximity effect on the conductor.
2. Use cable material with low magnetic permeability. (This minimizes the effect, but usually comes at the cost of high basic resistance per unit length, so this can be fine if the lines are short.)
3. Reduce the size of the conductor.
4. Increasing the voltage by reducing the current reducing the effect of the skin in the same conductor.

Mitigation of Skin Effect

• Instead of normal conductors/wires A type of cable called litz wire (from the German Litzendraht, braided wire) is used to mitigate the skin effect for frequencies of a few kilohertz to about one megahertz.

• It consists of a number of insulated wire strands woven together in a carefully designed pattern, so that the overall magnetic field acts equally on all the wires and causes the total current to be distributed equally among them.

• With the skin effect having little effect on each of the thin strands, the bundle does not suffer the same increase in AC resistance that a solid conductor of the same cross-sectional area would due to the skin effect.