'Undergrounding’ is the term used when we place parts of the electricity transmission system underground instead of using overhead lines and pylons.
The decision to do this could be for a number of reasons, from protecting views and landscapes to working in built-up urban areas where constructing overhead lines would be difficult.
Following a national consultation, in September 2011 we announced our revised approach to routeing high-voltage electricity transmission connections. You can view the Undergrounding Consultation Report for details of how we have used that feedback to further develop our new approach.
For the Hinkley project, we will underground the connection through and either side of the Mendip Hills Area of Outstanding Natural Beauty (AONB). In total we will install just over eight kilometres (around five miles) of underground cable in this location. More details can be found in our Draft Route Overview Report: Autumn 2012 and Hinkley Point C Connection project Connection Options Report (2012).
To connect an overhead line to an underground cable we need to build a cable sealing end compound. These are simply the compounds where overhead lines and underground cables connect to each other. We will build a cable sealing end compound in an area approximately one kilometre to the south of the AONB. To the north of the AONB we will connect the underground cable directly into the new substation near Sandford. More details on this can be found in our Hinkley Point C Connection project Cable Sealing End Siting Study (2012).
The Institution of Engineering and Technology (IET) has published an important independent report that gives an unbiased comparison of the costs of the different high voltage electricity transmission connection options, for example overhead lines, underground cables and subsea high voltage direct current (HVDC).
The report concludes that there are significant cost differences between the options that are broadly in line with estimates provided by National Grid.
The report clearly sets out how much more expensive the alternatives are to overhead lines, so we will continue to focus on where particularly sensitive locations justify the higher cost of undergrounding.
View the IET report
How does the independent report compare to National Grid’s view?