Overhead Lines vs Underground Cables: Key Differences Explained

 

Power plants, where electricity is generated, are usually located far from the load centers. To carry this electrical power from the plant to the load points, two systems of conductors are developed, called the transmission system and the distribution system.

These conductor systems are developed overhead and underground. In this blog, we explore these two power lines and the differences between them.

Overhead Transmission Lines

An overhead line is an infrastructure set up to send electricity or signals through cables suspended in the air. For transmission lines and primary distribution networks, overhead conductors are generally left uninsulated.

However, for low voltage distribution, wires may be insulated. For example, aerial bundled cables.

Poles or steel towers hold these cable lines at a safe height. These lines are common for power distribution and telecommunications.

These transmission lines are made up of conductors, insulators, supporting frameworks, cross-arms and various supplementary components.

• Insulators: Both the wires and the support structures can conduct electricity. Therefore, engineers place insulators between them to prevent current from travelling into the supports and the ground. Otherwise, this can lead to electrical faults and harm people or animals in the surrounding area.

• The cross arm: This arm is a part of the pole or tower, holding and supporting insulators and wires.

• Lightning arrestors: These arrestors offer protection against lightning strikes.

• Anti climbing wires: These wires help prevent people or animals from climbing the pole.

• Stay rods: These rods keep the structure stable.

Underground Cable Lines

Underground cable lines are insulated lines buried underground for the same goal — power transmission and distribution.

Modern underground cables are built with one or more central cores made of stranded conductors, with the number of cores determined by the specific voltage requirements and application.

These cables typically use cross-linked polyethylene (XLPE) or other advanced polymers as insulation.

A metallic sheath, made of lead or an aluminium alloy, is applied to safeguard the insulation from environmental exposure and physical damage. Finally, an insulation cover is provided.

Differences Between Overhead and Underground Lines

In overhead lines, conductor size is relatively smaller, while it is larger in underground lines.

• Overhead cable lines tend to have higher voltage ratings, up to 765 kV. On the other hand, underground cables have lower voltage ratings, typically limited to 66 kV. However, recent advancements allow underground cables to operate at voltages up to 400 kV in special applications.

• The current carrying capacity of overhead lines is comparatively higher, while underground cables have a less current carrying capacity.

• Overhead lines allow easy detecting and solving faults; however, in underground lines, it is extremely difficult.

• Overhead lines are commonly preferred to transmit and distribute power over long distances, whereas underground cable lines are preferred for short to moderate distances.

• While overhead lines are easy to afford and maintain, underground lines involve higher installation and repairing costs.

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