Sundon substation is an existing substation near the village of Chalton and adjacent to the M1 motorway in Bedfordshire.
We will be upgrading the point where the overhead lines connect into the substation, and updating some of the equipment and systems that help keep the substation running safely and reliably.
The 400kV overhead line which runs between the Elstree and Sundon substations is a key part of the local electricity network.
In January 2025, we completed the first phase of refurbishment work to replace the wires along the 35km route. More information on the work that we undertook during this first phase can be found here.
We are now delivering the second phase of work to help ensure long-term energy security on both the high-voltage electricity transmission network, and the local distribution network supplying homes and businesses.
Our work in Hertfordshire is an important part of North West London Upgrade. This includes upgrading the existing 400kV Elstree substation and building a new 132kV substation.
Elstree substation
The 400kV Elstree substation, off Hilfield Lane in Hertsmere, has supplied electricity to the local area since the 1960s.
We are now upgrading the substation as part of the North West London Upgrade so it can continue to support local electricity needs and meet growing demand in the years ahead.
The work includes an extension to the substation, upgrading equipment and new cabling on site.
These upgrades will also support the delivery of the new Letchmore Heath substation, which is being built on land next to the existing Elstree substation.
Letchmore Heath substation
Letchmore Heath is a new 132kV substation being developed on land National Grid owns next to the existing Elstree substation.
As the existing Elstree substation is at capacity, the new Letchmore Heath substation will help meet a growing demand for connections to the grid – including enabling five new data centre customers to connect to the transmission network in an area of national strategic importance.
To help deliver Letchmore Heath, we will also be creating a new access road from Hilfield Lane to serve the new substation.
We have been speaking with local community stakeholders and representatives to discuss our plans for the new Letchmore Heath substation.
As the project develops and ahead of construction commencing in early 2027, we will also be speaking to other local groups and residents. We will continue to update this website with information around how you can get involved.
If you have any questions or would like further information in the meantime, please get in contact with our community relations team at [email protected] or 0800 331 7097.
Community Fund
In March 2025, the UK Government published guidance for delivering community funds in areas where we need to build new onshore electricity transmission infrastructure.
The community around the new Letchmore Heath substation will be eligible for £530,000 to be spent on projects that will improve the social, economic and environmental wellbeing of the community and deliver benefits that endure over the long term.
There will be a public engagement process held to define the eligible area, funding themes and criteria for the investment. Local stakeholder engagement in this process will be essential to ensure local needs, priorities and preferences are reflected. We will be sharing more details on the public engagement process in due course.
The Elstree to St John’s Wood cable tunnel – formerly known as “The London Connection” – is a 20km underground tunnel running between Elstree and St John’s Wood substations.
Constructed in the early 2000s and with an eye on meeting future demand and building resilience in the network, the tunnel was designed to allow for an additional cable circuit.
We are now installing the second cable circuit as well as upgrading the ventilation and control systems.
We will also be undertaking upgrades to the eight headhouses along the route. Headhouses are above-ground structures which provide access into the tunnel and house necessary ventilation systems.
St John’s Wood substation, off Lodge Road, is one of London’s largest transmission substations and plays a key role in powering homes and businesses across the city.
The new cable circuit we are installing in the Elstree to St John’s Wood cable tunnel will be connecting into the St John’s Wood substation. To support this, we need to undertake work at the substation to ensure the new cable can connect without impacting current operations.
Contact us
If you'd like more information or want to speak with a member of our community relations team about the work, please:
Email: [email protected]
Phone: 0800 331 7097
Freepost: RTBU-HAYY-LCUX, 7 Bayley Street, London WC1B 3HB (no stamp needed)