Overhead lines vs underground cables

When we upgrade the grid, we consider various technologies to create suitable options. In terms of cables, this includes:

Underground cables are used in our electricity grid, but they aren't always feasible. There is no “one size fits all” answer when it comes to overhead vs underground decisions. We look for solutions that are best for the area, secure the grid for the future and offer value for money for consumers. All our proposals to upgrade the wider grid network are subject to approval by the Utility Regulator.

Around 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's more challenging to send AC electricity underground. That's why we are very careful about when we use it. 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 disappears into the earth. When a cable carries power it gets hot. Since 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.

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. This extra energy means the cables sometimes can’t be used in a weak part of the grid, or areas of the grid that aren't well connected to other parts of the grid.

Repairing cables

If an underground cable gets damaged or breaks, it's 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.

DC tends to be used when large amounts of power (several hundreds of MW) have to be sent over very long distances. An typical use is across open sea.

DC underground cables have the same heat and fault-finding problems as AC underground cables. However, 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.

DC must be converted back to AC before it can be used on the grid. To do this, we have to build large converter stations. That makes DC much more complex and less reliable. The DC equipment also has more components, meaning it cannot be used for all projects.

You can see that there are many factors that feed into decisions around cabling. Each project is considered on its own merits. We always consult with local people to get their views. Our goal is always to find the right solution which meets the needs of the local area, ensures a reliable supply of electricity and delivers value for money for consumers.

To find out more about the plans for a specific project, visit our projects section of the website.