Why electrical cables are mostly placed in overhead structures? Why cables are not generally placed underground?
A recent report from CIGRE widely shows us how we can set up an electrical network, using at its best both overhead and underground ways to put electrical cables.
First of all, some statistics:
The study mentioned above describes the differences in electrical network in various countries.
This article (and the CIGRE report indeed) only refers to AC medium-high voltage cables.
In medium-high voltage (50-109 kV) we assist to an average of 6.6% of underground cables, decreasing in higher voltage:
We can have a wide difference among the countries who participated to the report, but it is assured that as the voltage grows up the underground solution is the less applied.
What are the main differences between these two solutions? There are 3 factors we have to analyze:
- For high voltages the overhead solution in generally the best since the towers with concrete bases and the air will provide the necessary insulation.
- Underground cables are less effective for heat dissipation.
- Overhead cables are easier to place, easier for maintenance
But there are cases when the overhead solution is not affordable. Since they are generally cheaper, towers are extremely threatened by weather (snow, wind), a single high voltage line damaged by a storm can lead to a large blackout.
If we must place cables in underground , how can we reduce its cost?
- Using high quality cables. New cross-linked polyethylene cables (XLPE) with reduced thickness allow to increase installation speed, increasing also the insulation factor.
- Using high-quality laying techniques and high-quality equipment for our excavation work.
When excavating our trench for underground cables we have 4 possible techniques:
- Direct laying. This is generally used in rural areas. The trench generally doesn’t need shoring systems. Cables are directly layed and the trench is generally shored, since no worker needs to enter the trench.
- Laying ducts. In urban areas we place plastic or steel pipes, covered with concrete, and then pull the electric cables inside. This also grant an higher heat dissipation.
- Laying in troughs. After a direct laying, it is possible to put u-shaped housing to protect cables.
- Horizontal drilling. Using HDD equipment allows to ignore obstacles as rivers, bridges or other.
What about the laying techniques?
There are 4 main functions to be considered before the blueprint of a cable laying excavation:
- Thermal design: since most of cables have the heat dissipation problem
- Mechanical protection: we need to protect cables from third parties
- Public safety
- Legal requirements for civil works
Laying in ducts:
This is the normal laying technique in urban areas. It grants the minimum disturbance to local traffic and allows better maintenance operations
Usually for this technique three or more ducts are placed in a trench, using the same depth and configuration. On the bottom of the trench a special low-resistivity material is previously placed. Depending on the number and diameter of the cables to put in the ducts we can assist to different formation, such as the placement of two or more layers of ducts. To ensure the perfect space and configuration are utilized special spacers, which allows the concrete to perfectly flows. To ensure further safety to workers and public is often necessary to put earth cover over the duct bank.
This technique is the simplest in our list. Just dig a trench and lay the cables. More precisely, the trench for a direct burial is often 1m (3 ft) deep, depending on the cables formation and voltage level. To avoid the use of trench shoring systems the trench is usually excavated with slopes. This can result in a significant reduction in the costs and times.
This technique can be considered as an evolution of the traditional laying technique. A lot of manufacturer have indeed produced special attachment for trenchers for a simultaneous operation. Vermeer has produced the CL series attachment for its trenches. So, while the machine is excavating the trench, the attachment is boring the cables. The sole limitation consists on the space needed for this technique. It is usually used in rural areas with no obstacles, and the trench need to be always in a straight line.