An underground cable tunnel 


This paper has been put together by Stour Valley Underground and is intended to inform discussion of the possible undergrounding of the Bramford - Twinstead link.


It has become clear to us that very many people throughout the consultation area want to consider an underground cabling option as part of the consultation process. SVU have researched a number of strategies including partial undergrounding and full undergrounding of the proposed and existing power lines. All are flawed and have down sides that are unacceptable to one community or other. This then is what we regard the best strategy we have looked at so far in that it is most cost effective and would be to the detriment of least number of people.

Our intension is to provide for discussion, a grid reinforcement strategy that will:-

in the long term, free the landscape of the consultation area from the blight of pylons.

be implemented the whole distance between Bramford and Twinstead.

does not involve the need for any of the NG identified corridors because it travels underground in a straight line.

restore and improve the Dedham Vale AONB and Stour Valley Project area and is in keeping with the proposed AONB extension to Sudbury and beyond.

remove the threat of the construction of new Pylons in virgin countryside.

provide a "built up" strategy that staggers investment across time.

provide a long term strategy that makes realistic, a commitment by NG to underground

the existing 400kv power lines between Bramford and Twinstead by 2040.

present a strategy which could enjoy the support of most if not all interested groups affected by NGʼs proposals.

The need for “reinforcement” of the grid

We acknowledge the need to upgrade the electricity distribution grid in light of renewable and nuclear generation developments on and off the East Coast and elsewhere.

We find the case for reinforcement of the grid between Bramford and Twinstead has not been adequately made by NG.

We consider the NG proposals for over ground re-enforcement to be short sighted and for the short to medium term, over engineered. So the new power line may never be fully loaded.For the longer term, we are also aware that we (and NG) cannot know exactly what East Coast generator projects might be proposed in the future. What we do know is that if the currently proposed Sizewell nuclear and Round 3 Wind farm projects all came on line, they would produce more energy than the currently proposed "reinforcement" could transmit . Therefore this would mean that there would have to be further power lines in this area if over land strategies were to continue to be used. Taken together this suggests that a strategy that can be built up to provide the capacity that is actually needed, when it is needed would be more appropriate.

We therefore urge NG to consider more economically, socially and environmentally acceptable solutions and move the power from Sizewell and the Round 3 wind farms to the south (London area) using undersea High Voltage Direct Current (HVDC) technology as is covered elsewhere on this website.

An Underground Reinforcement Strategy 

for the Bramford - Twinstead link

A hybrid underground cabling strategy might be :


Construct 2 x 3m diameter tunnels at depth by boring between Bramford and Twinstead


Place 2 x 400kV circuits plus 1 x 132kV circuit in Tunnel 1.


Place 1 x 400kV circuit plus 1 x 132kV circuit in Tunnel 2.


Demolish the existing EDF 132kv pylon line .


Leave the existing 400kV overhead line in place and upgrade power lines as planned but secure a longer term commitment from NG to underground this line by 2040 (perhaps in a third tunnel).


Install the new 400kV circuits in the tunnels only when required - staggering cabling investment.


Capture the heat from the tunnels and put it to good use.

Estimated costs

Initial tunneling and cabling £400m, rising to £600m for full implementation and total undergrounding of all new and existing lines.

Location definitions:

Bramford – means the NG substation at Bramford.

Twinstead – means a point to the west of the Stour valley project area at Twinstead where the 400 and 132kv are closest.


This strategy:-


provides the most generally publicly supportable solution and greatly reduces public opposition to the project.


provides a time scale for total undergrounding.


means that once the tunnels have been bored using the same techniques used by National Grid in the London area, the cabling can be added in response to actual need thus staggering the financial investment in this new infrastructure.


allows for the shutting down and servicing of cables in one tunnel while the cables in the other carry higher than normal current while being intelligently monitored to keep them within safety limits, thus maintaining supplies during servicing work.


accommodates the possibility of more or less generators coming online: the strategy can be adapted and tailored to suit the new energy distribution requirements.


fits in with the obligations of respecting the existing AONB in the Dedham Vale and also the likely extension of the AONB along the Stour Valley.


is in keeping with the need to ensure that energy distribution infrastructure is secure against extreme environmental incident (now more likely due to climate change) or malicious damage as only 2 circuits of 400kV lines remain above ground in the medium term. The grid is specified to survive the loss of 2 circuits but not 4 which would be possible with an all overground strategy.


avoids the land drainage issues associated with direct burial or cut and cover tunneling techniques.


avoids land sterilisation issues associated with direct burial or cut and cover tunneling techniques.


can be implemented without scaring the natural landscape.


places the electromagnetically shielded cables at a depth that removes the health issues associated with close proximity to overground HVAC power lines.


sets a timescale within which the Stour Valley will be returned to the natural landscape that has inspired some of Britainʼs finest landscape painters.


increases the amenity value of the landscape.


makes possible the economic development of the area based on its heritage and cultural facets.


enhances the possibility of developing the local economy through leisure and tourism.


allows capture of heat shed by the cables, allowing it to be put to good use in (say)


public buildings/housing and not simply wasted.


removes the need for a new sub station.

Negative issues

This strategy has some serious drawbacks and it is only reasonable that we should identify some of them.


the cost for this strategy could be regarded as disproportionate in that the total is not far short of the cost for an undersea solution which has far more benefits.


each end of any tunnel based solution would feature a vast and ugly transition installation. This would further blight the Bramford area and would introduce a new eyesore within Twinstead. Any paring back of the underground length of the project would have the effect of moving these eyesores to other locations.


There would be ventilation / access ducts up to the surface capped by a small building every 4kM along the route. While we do not see this as a major drawback it would impact the landscape.


The Crown Estates Round 3 wind farm connection Study

ENSG Our Electricity Transmission Network: A vision for 2020

NG Bramford to Twinstead 400kv OHL Strategic Optioneering Report Oct 2009

NG Power loading analysis

Picture Credit: Cable Tunnel from Elstree - St Johns Wood 400kV Tunnel Project from National Grid Web Site.