What's involved in building a new high-voltage electricity connection
We appreciate that construction activity can be disruptive to communities. We are very aware of this and, along with our contractors, will seek to minimise impacts wherever possible.
We’re building a temporary road the length of the connection to reduce the amount of time construction traffic uses local roads. We’ll also seek to keep the effects of noise, light and dust to a minimum.
These are some of the activities you may see when we are working in your area:
Vegetation clearance and planting
Before we start construction, we need to clear some vegetation along the route. These trees and hedgerows were identified as part of our planning application and we have committed to replacing them at a ratio of 4 to 1 after construction is complete. Wherever possible, replanting will take place in the same location (in-situ planting); where this is not possible, we will plant at an agreed location nearby (ex-situ planting), subject to agreement with landowners.
National Grid will also undertake planting to screen particular parts of the development, for example at Sandford substation and the cable sealing end (CSE) compound south of the Mendip Hills.
In addition, we have committed to extra planting within 3 km of the development, which will reduce the overall impact of the new connection and enhance local landscape and biodiversity.
We need to use local roads to reach construction compounds and access the temporary haul road. We have agreed a Construction Traffic Management Plan (CTMP) with the local authorities that sets out which roads we can use. It includes road signage and mitigation measures we will take to help minimise local disruption. The CTMP can be found here.
All construction vehicles will display signs to the rear and front so that they can be easily identified as working on this project.
Temporary roads and entrances
To reduce the amount of construction traffic on local roads, we will need to build temporary roads and entrances for our construction vehicles. They will be removed at the end of the project.
The type of roads we use will depend on ground conditions, the type of construction work and the length of time they are required. They can be made of metal, stone, or noise reducing tarmac.
Temporary culverts and bridges
Where the road crosses ditches, we will lay temporary concrete tunnels known as culverts that allow water to pass underneath, or build temporary bridges.
To get construction vehicles on site with as little disruption to local roads as possible, we are going to create new, or modify existing entrances. These will be removed at the end of the project.
Where roads cross the local highway, we will create crossing points with temporary entrances at either side. These entrances will be gated and in some cases the crossings will be manned or have traffic signals which will operate when construction traffic is crossing.
All temporary entrances and crossing points will be designed in consultation with the local highway department.
Temporary construction compounds
We need to set up construction compounds at key locations along the route of the new connection. These will be used to support the construction work and day to day operations on site. Larger compounds will include temporary offices, toilets and equipment storage areas. Smaller sites will be used for equipment deliveries and short-term storage near to construction areas. They will be removed at the end of the project.
Compounds will have a hard-standing surface and will be secured by an outer perimeter fence. Portable low noise generators will be required to power the sites and we will manage drainage and disposal of all waste at these locations. At Tarnock, the compound will be powered by mains electricity. These compounds will be monitored out of working hours by a security guard.
We’re using two types of pylons on this project: T-pylons and traditional steel lattice pylons. While they are very different in appearance and size, the construction process is similar.
Building the new overhead line will involve several different types of engineering activity:
Site set up: the construction area and access road will be fenced off to keep the public and livestock out of harm’s way. The area will be secured and monitored by a security guard.
Pylon foundations: we expect that most pylons will require piled foundations. Long concrete pillars are driven deep into the ground and topped with a layer of reinforced concrete. This forms the foundations that will support the pylon. Machinery, pre-mixed concrete and steelwork for the foundations will be delivered to the construction site in Heavy Goods Vehicles (HGVs).
Building pylons: pylons will be assembled in sections; the number of sections will vary according to the size and type of pylon being built. The T-pylon has large pieces that can be part assembled at ground level before being craned into place. Most pylons will be T-pylons. The lattice pylons are made up of many beams and struts which can be erected in different ways; however, it is likely that like the T-pylon, it will be part assembled and then lifted into place.
Attaching the wires: once the pylons are built, we will install the wires that carry the electricity. This is known as ‘stringing’. We’ll string a section of approximately 10 pylons at a time. The conductor is pulled from one end to the other using large machinery. Temporary scaffolding is installed to protect sensitive obstacles such as buildings or publicly accessible areas.
Reinstatement: once the overhead line is constructed, temporary roads and working areas will be removed and the ground reinstated.
More information on how we will build the T-pylons can be found here.
WPD wood pole overhead line: Western Power Distribution will also build a short section of wood pole 132,000 volt overhead line at Sandford. Cranes won’t be needed to install the wood pols and the working area won’t be fence off.
We recognise that our construction activity impacts local communities. We are working with our contractor to limit that as much as possible. Details of all current construction activity can be found here.
Removing existing 132,000 volt overhead lines
We are removing 67.6 km of existing 132,000 volt overhead line between Bridgwater and Avonmouth. This will make way for the new connection, and minimise the impact on the local landscape.
To facilitate the removal of the existing 132,000 overhead lines, the area around each pylon will be cleared, and where appropriate fenced. The wires will be cut into manageable lengths or coiled on to large drums. The pylons will either be dismantled by crane, with sections cut and lowered to the ground, or the pylon will be safely and securely pulled over. This is done by cutting two legs then pulling the pylon over under tension to the ground using a tractor before being dismantled. Foundations will be removed to a depth of approximately 1m and topsoil reinstated.
As part of this project, we will be installing 400,000 volt and 132,000 volt underground cables. The construction process for both is similar, though different in scale:
Site set up: a construction site will be set up and we will need to build a temporary road to allow our vehicles to move safely along the cable route. The working area along the 400,000 volt cable route will be around 100m wide. Where required, we may have to improve drainage to prevent flooding of the cable trenches.
Excavating: we will dig four cable trenches, approximately 2m wide, 1.8m deep, and a few metres apart. The soil will be carefully removed and stored. Each trench will be lined with sand.
Cable laying: the cables will be delivered on large drums. Twelve cables will be buried, with three cables laid into four trenches in lengths of approximately 650m-1000m.
Jointing: once the cables are laid in the trenches, we will need to join the lengths together in a process known as ‘jointing’. At the jointing site, we will install a temporary tent to provide a clean, dry environment where the underground cables can be safely connected together. A permanent small pillar will be installed to provide access for testing and maintenance.
Reinstatement: the installed cables will be covered with cement-bound sand and a protective tile cover. The trenches will be carefully filled with the stored soil from excavation, and the land reinstated.
The construction process for 132,000 volt underground cables is similar to above; however, the working area will be approximately 60m wide in rural locations and much smaller in constrained urban areas. For a 132,000 volt cable route we will install six cables – three cables in two cable trenches, approximately 1.5m wide, 1.5m deep and a few metres apart.
Cable sealing ends (CSEs)
A CSE is needed at the point where underground cables are joined to overhead lines. We will install temporary scaffolding about 25m tall to provide a clean, dry environment when we are joining the cables.
CSEs on the 400,000 volt connection will be constructed within fenced compounds. CSEs on the lower voltage network, owned by Western Power Distribution, are on specially designed pylons.
Substations contain specialist equipment that changes electricity from very high voltages to lower voltages, so that it can then be distributed to businesses and homes.
The ground at the site of a new substation will be levelled to provide a flat and stable surface before the foundations are excavated and concrete is poured in.
The substation structures will be assembled and fixed to the foundations, with the equipment raised off the ground.
Once the substation is complete and operational, surrounding land will be reinstated and planting undertaken to screen the site.