Underground vs Overhead

When we upgrade the grid, we consider various technologies to create suitable options. This includes whether an overhead line or an underground cable is right for a particular project. It also includes whether it is suitable to use alternating current (AC) or direct current (DC).

Underground cables are used in the Irish electricity grid, but they are not always feasible for every project. There is no “one size fits all” answer when it comes to overhead vs underground. We look for solutions that are best for the area and that secure Ireland's grid for the future.

In Ireland, and in most of the world, AC is the most common way to move electricity around an electricity grid. AC is what comes into our houses, businesses, hospitals, farms, and communities. All electricity grids use AC, bringing electricity instantly to everyone who needs it.

For the most part, AC is carried on overhead lines. Sometimes underground cables are used, but there are a lot of reasons why it is more challenging for us to send AC electricity underground. That's why we are very careful about when we use it.

AC and Underground Cables

Below are some of the challenges around sending AC electricity underground. 

Damage to Insulation

When you use AC electricity on an underground cable, the cable needs to be electrically insulated from the surrounding ground. Otherwise the power just disappears into the earth. When a cable carries power it gets hot and because it’s buried in the ground the heat finds it hard to get away through the soil. If the heat builds up too much, the insulation will be damaged.  

To make sure the insulation isn’t damaged we limit the amount of power that flows through the cable. To match the power limit of overhead lines we have to get large cables and we need a bigger trench to bury them in. Depending on voltage level and the power limit that is required, the trench width will vary.

Storing Energy

The insulation also stores electrical energy, which helps power to move. AC underground cables store much more of this energy than overhead power lines. The extra energy stored in AC underground cables means that they sometimes can’t be used in a weak part of the grid, or a part of the grid that isn’t well connected to other parts of the grid.
 

In weak parts of the grid the stored energy makes operation of the grid difficult, and in some cases impossible. Cables over very long distances mean there is much more of the stored energy.

Repairing Cables

If an underground cable gets damaged, or breaks, it is often much harder to find and fix the problem than an overhead power line. Underground cables don’t get damaged or break any more often than an overhead power line. But when they do, they are usually out of service for much longer while the problem is found and fixed. This impacts the reliability of the grid.

In general, DC is used where large amounts of power (several hundreds of MW) will have to be sent over very long distances, such as across open sea.

Direct current, or DC, underground cables have the same heat and fault-finding problems as AC underground cables, but they don’t have the same problems with the energy stored in their insulation. That’s why DC is used for long distances and for undersea cables, like the East West Interconnector or the proposed Celtic Interconnector.

However, DC must be converted back to AC before it can be used on the grid. This means we have to build large converter stations - making DC much more complex and less reliable as the DC equipment has more components, meaning it cannot be used for all projects.

That’s why we consider each project on its own merits and always consult with local people to get their views to find the right solution for the area. Our goal is always to secure a reliable supply of electricity that our growing population needs for the future.