Are underground electric power lines less efficient than above-ground lines?

Answer 1

Efficiency is a relative term.

From a line loss perspective, the underground circuit is more efficient. There are fewer line losses on an underground circuit due to the tight spacing of the conductors. On an overhead line, the bare wires are spaced far apart. This spacing combined with the sinusoidal AC power creates extra losses know as reactance. (referred to as X) The wider the conductor spacing the higher the X. The conductor resistance (R) and the reactance are combined into an overall impedance.

Underground cables need a larger conductor to handle the same amperage as a smaller overhead conductor. This is due to the difficulty of dissipating heat to the earth. Larger conductors means higher cost.

The cost of construction for an underground transmission line is 5 to 10 times that of an equivalent overhead circuit.

Overhead distribution circuits are much easier to modify to serve customers or make change other changes. A simple set of fuses on an overhead circuit might cost $200, yet the underground equivalent costs over $10,000.

Underground cables are manufactured with a specific voltage rating. Overhead wire has no voltage rating. That is determined by the insulators and ratings of all the other equipment in the circuit. Over the years many distribution systems have “grown” from 2.4 kV up to 12.47 kV (or higher) all the while using the same wire.

Overhead circuits can often be worked on while they are still energized. Nearly all work on underground circuits is performed while things are de-energized and grounded.


Hope this helps.

Answer 2

Underground cables would tend to be more efficient.

The reason is, that cables with an insulation and sheathing are required to produce less heat than an overhead cable otherwise the insulation would be destroyed. Its heat being produced that causes any line to be inefficient.

Overhead cables are often aluminium for lightness which, since it has lower conductivity, makes it less efficient. They can also be allowed to produce more heat by using less material because there is no insulation to destroy and the cooling effect from the air is greater.

The downside of underground is that they are much more expensive so this out-ways the efficiency.

Note.

I'm bemused how the impedance is higher on an overhead line by wider spacing between conductors..............................hmmmmm.

Answer 3

Correct. More electical line losses are experienced by the underground electric power lines. In power lines that are above ground, the electrical fields around the powerlines are less affected.

But, there are reasons to bury powerlines even if they are less efficient.

1. Safety. No one has ever smashed their car into an underground power pole. No one has ever leaned an aluminum ladder against an underground power line while trying to climb to the roof.
2. Aesthetics. Neighborhoods with underground power lines look neater.

Answer 4

Thomas C covered things very well. Re: Dave's comments about safety, many people HAVE driven into padmounted transformers and switchgear for underground systems.

Many more have dug into the underground lines.

Areas subject to flooding have had outages when the water fills underground vaults and shorts out the cables.

Underground cables also deteriorate much faster than overhead lines. So they are not only more expensive initially, but need to be replaced sooner at additional expense.

Answer 5

Underground cables are more efficient than above ground cables that's why most of the high voltage cables are burried.

Answer 6

yes, There is not much losses and also very good in the safety point of view. But it is costlier than overhead cables