Overhead conductor carries the same current as an underground cable

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Before diving into the detailed explanation of why an overhead conductor experiences more power loss than an underground cable, it is essential to understand the fundamental principles that govern power loss in electrical conductors.

Power loss in a conductor is primarily caused by resistance and is calculated using Joule's law:

P = I^2 R

Where:

P = Power loss (in watts)

I = Current flowing through the conductor (in amperes)

R = Resistance of the conductor (in ohms)

This equation shows that power loss is directly proportional to both the square of the current and the resistance of the conductor. However, in reality, several additional factors influence power loss in an overhead conductor, making it higher than that of an underground cable even under similar current-carrying conditions.

Factors Contributing to Higher Power Loss in Overhead Conductors

1. Skin Effect and Increased AC Resistance

One of the primary reasons for increased power loss in overhead conductors is the skin effect. The skin effect is a phenomenon where alternating current (AC) tends to concentrate near the surface of the conductor rather than being evenly distributed across its cross-section. This reduces the effective cross-sectional area available for current flow, increasing the conductor’s effective resistance.

The skin effect is more pronounced in overhead conductors because they are often operated at higher voltages and frequencies, which exacerbate the concentration of current near the surface.

The underground cable, on the other hand, has its current distributed more uniformly, reducing its effective AC resistance and thus lowering power loss.

Mathematically, the skin depth $δ$ is given by:

δ=2ρμωdelta = sqrt{frac{2rho}{mu omega}}

Where:

$ρ$ = Resistivity of the conductor material

$μ$ = Permeability of the conductor material

$ω$ = Angular frequency of the AC current

Since overhead conductors usually carry high-frequency currents, the skin effect becomes more significant, leading to increased power loss.

2. Proximity Effect and Magnetic Interference

The proximity effect is another factor that increases power loss in overhead conductors. It occurs when conductors carrying current in the same direction induce eddy currents in each other, which leads to an uneven distribution of current within the conductors, thereby increasing their resistance.

In overhead power lines, multiple conductors run parallel to each other, often at significant distances, leading to strong electromagnetic interference.

In contrast, underground cables are typically designed with proper insulation and shielding, which helps mitigate the proximity effect and reduces additional power losses.

3. Corona Loss in Overhead Conductors

One of the most significant factors contributing to increased power loss in overhead conductors is corona discharge. Corona loss occurs when the electric field around a high-voltage conductor ionizes the surrounding air, creating a partial discharge.

The ionization of air leads to power loss in the form of heat, light, and sound.

Humidity, conductor surface condition, and voltage levels all influence corona loss, making it more prevalent in overhead transmission lines than in underground cables, where the conductors are insulated and shielded from the surrounding air.

Corona loss can be estimated using the Peek’s formula:

Pc=243.6f(V−Vc)2δrP_c = 243.6 f (V - V_c)^2 sqrt{frac{delta}{r}}

Where:

$P_c$ = Corona power loss per unit length

$f$ = Frequency of the transmission system

$V$ = Operating voltage of the conductor

$V_c$ = Critical disruptive voltage

$δ$ = Air density factor

$r$ = Radius of the conductor

Since underground cables do not interact with air, corona loss is virtually absent in them, making them more efficient in power transmission.

4. Temperature Variations and Conductor Expansion

Temperature plays a crucial role in determining the resistance of a conductor. The resistance of a conductor increases with temperature, and this contributes to greater power loss in overhead conductors for two reasons:

Exposure to Ambient Conditions:
- Overhead conductors are exposed to direct sunlight, atmospheric heating, and seasonal temperature variations.
- High temperatures cause the conductor’s resistance to increase, leading to more I²R losses.
- In contrast, underground cables are buried beneath the surface, where temperature fluctuations are minimal, leading to relatively stable resistance values.

Thermal Expansion and Sagging:
- As overhead conductors heat up due to increased resistance, they expand and sag, increasing their length and effective resistance.
- This sagging further contributes to power loss because the longer conductor length adds more resistive path, thereby increasing overall losses.

5. Wind-Induced Vibrations and Eddy Current Losses

Overhead conductors are subject to environmental forces such as wind, which can cause Aeolian vibrations and conductor oscillations. These oscillations induce eddy currents in the conductor material, leading to additional energy dissipation.

In an underground cable, the rigid insulation structure prevents excessive movement, minimizing eddy current formation and associated power losses.

Overhead conductors, on the other hand, are constantly in motion due to wind, which contributes to additional energy losses in the system.

6. Leakage Currents and Insulation Factors

Leakage currents also play a role in determining power losses in electrical transmission systems.

Overhead conductors rely on air insulation, which is not a perfect insulator and allows small leakage currents to flow, contributing to minor energy losses.

Underground cables are enclosed with high-quality insulation materials, such as cross-linked polyethylene (XLPE), which greatly reduces leakage currents, thereby improving efficiency.

This difference in insulation quality further explains why overhead conductors experience greater power loss despite having better cooling conditions.

Final Verdict: Why Overhead Conductors Lose More Power?

From the detailed discussion above, it is evident that multiple physical and environmental factors contribute to the increased power loss in overhead conductors compared to underground cables. Despite having better cooling conditions, overhead conductors experience:

Higher AC resistance due to the skin effect and proximity effect

Significant corona losses at high voltages

Increased resistance due to temperature variations and thermal expansion

Wind-induced eddy currents and Aeolian vibrations

Leakage currents due to air insulation

All these factors combined lead to greater power loss in overhead conductors, making them less efficient compared to underground cables for transmitting the same amount of current.

This explains why power utilities and grid operators carefully balance the choice between overhead and underground systems based on efficiency, cost, and practical feasibility.

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OVERHEAD CONDUCTOR CARRIES THE SAME CURRENT AS AN UNDERGROUND CABLE

Overhead conductor carries the same current as an underground cable