Skin Effect and Hollow Conductor
When current flows through a conductor in AC mode, it does not make use of the full cross section of the conductor available. The core of the conductor does not carry any current, rather only the surface and a little portion below it (skin depth) would take part in conduction. Skin depth is the thickness at which the current density is reduced by 63%. Thus a solid conductor in effect acts as a hollow one, increasing the effective resistance offered to the current flow. The amount of conductor cross-section used for current flow is dependent on the frequency. Higher the frequency, lower will be the cross-section used and hence higher resistance for the same conductor. Thus the resistance offered to AC is higher than the DC resistance by some factor. I've seen AC resistance to be taken 1.6 times DC resistance for 50 Hz AC supply (though some sources say it is 1.1 times or higher).
Why don't we use hollow conductors when it will save a lot of material in long distance transmission? Saving material means less cost too. Or is it not feasible to make hollow conductors for such long distances?
Answer:
The concept of using hollow conductors in power transmission to mitigate the skin effect is an interesting one. In fact, in certain applications such as radio frequency transmission lines, hollow conductors (coaxial cables) are indeed used. However, for power transmission lines, several reasons make the use of hollow conductors impractical:
1. Mechanical Strength: Power transmission lines must withstand the forces of wind, ice, and their own weight over long spans. Solid conductors are necessary to provide the mechanical strength to handle these stresses. Hollow conductors, especially with large diameters, would have much less mechanical strength, making them susceptible to bending and collapsing.
2. Fabrication and Installation: Fabricating and installing long, hollow conductors would be a complex task. It would be difficult to maintain the precise shape over long distances, and any imperfections in the shape could lead to areas of increased electrical stress and potential failure points.
3. Cost: While it may seem that hollow conductors would save material, the increased complexity of manufacturing, installing, and maintaining such conductors could result in higher overall costs than using solid conductors.
4. Oxidation and Corrosion: If the hollow conductor's interior surface is exposed to the environment, it could be susceptible to oxidation and corrosion, which could increase resistance and lead to failures.
5. Low-Frequency AC: The skin effect becomes significant only at higher frequencies. Most power transmission is done at relatively low frequencies (50-60 Hz), where the skin effect is less pronounced. Even though it exists, the effect isn't strong enough to warrant the use of hollow conductors for most transmission lines.
6. Dynamic Load Changes: The loading conditions on a power transmission line can vary widely and change rapidly. The skin effect's depth of penetration will vary with the square root of the load current. Therefore, a conductor that is "optimized" for a particular load condition may not be optimal for another.
7. Non-uniformity of current distribution: Even if we consider using hollow conductors, the skin effect results in non-uniform current distribution over the cross-section of the conductor. This still might not result in an optimal solution.
To mitigate the skin effect in power transmission, other solutions are often employed. For instance, using bundled conductors, where several smaller-diameter conductors are used instead of one larger one, is a common practice.
This increases the total surface area for the current to flow, reducing the impact of the skin effect. This is a more practical solution to the problem that doesn't require the use of hollow conductors.
