When carrying our detailed appraisal we considered the following issues:
Alternative technologies: We considered different types of technologies which could be used to reinforce the network. Each technology has different features which affect how, when and where it should or could be used.
Geographic scope of strategic options: Building shorter reinforcements generally represent the most efficient development; they are usually associated with lower levels of environmental effects, capital and lifetime costs, and are, in general terms, more compliant with relevant policy guidance.
Reinforcement solutions: We considered a series of connection reinforcement solutions - each of which included a number of different elements and geographically dispersed developments.
Further details of how we undertook the appraisal can be viewed below.
Our assessment concluded that the appropriate technologies to take forward for further assessment were:
onshore connection made up of Alternating Current (AC) overhead lines and underground cables (e.g. through nationally designated areas)
offshore High Voltage Direct Current (HVDC) cables
onshore HVDC cables.
In addition, we concluded we would consider opportunities to upgrade existing transmission infrastructure to 400 kV if currently operating at lower voltages.
Our starting assumption for building new transmission infrastructure is to use overhead lines. This is in line with existing and emerging Government policy set out in National Policy Statements.
National Grid typically uses steel lattice pylons to support overhead lines. The size, height and spacing of pylons are determined by safety, topographical, operational and environmental considerations. A typical 400 kV pylon is 45-50 metres tall.
The main impact of overhead lines is generally considered to be visual, with effects on landscape and views.
We may propose to use underground cables in sensitive protected areas, such as National Parks or Areas of Outstanding Natural Beauty (AONB), to reduce visual impact.
For installing cables underground a large cable swathe is required. This is typically between 65 and 100 metres wide depending on the number and size of cables to be installed, with additional working areas beyond this. Once the cables have been installed, the construction swathe will be reinstated and normal agricultural practices can be resumed. The East Anglia GREEN reinforcement will comprise up to 18 cables.
We need to build joint bays at intervals of approximately 500 metres to 1,000 metres to allow for the individual sections of cable to be joined together. In these areas a wider corridor swathe may be needed. The work required to bury cables is likely to affect archaeology, vegetation and wildlife along the construction corridor.
Cable sealing end compounds
We need to build cable sealing end compounds where the underground cables join to the overhead lines. These sealing end compounds are generally around 30 x 80 metres.
The existing network configuration and the complexity of the system operating conditions mean that building a single connection between two points on the network would not address all the identified constraints on the network in East Anglia.
To address this, we considered a series of connection reinforcement solutions - each of which included a number of different elements and geographically dispersed developments.
Eastern - to transport power from the north of East Anglia into the south-east England area connecting with existing substations such as Tilbury and Grain.
Northern - creating additional capacity by connecting into the north of the existing London area network and then into the south-east England area connecting with existing substations such as Wymondley, Pelham, and Waltham Cross.
Western - creating additional capacity by transporting power westward around London and down into the south-east England area connecting with existing substations such as East Claydon and West Weybridge along with additional reinforcement to the south of London.
We identified a total of 23 reinforcement solution options. The options collectively tested various combinations of both onshore and offshore developments, the use of AC and DC technologies, and some uprating of existing lines to 400 kV.
We carried out further appraisals for each of the options. The factors considered included:
deliverability of the option
system benefit which each option would provide
cost benefit analysis.
Taking all the above factors into account, we concluded that the reinforcement solution which provides the highest overall value to consumers combines both offshore and onshore connections with three distinct elements:
offshore reinforcement between the south coastand East Anglia (Sea Link)
onshore reinforcement between Tilbury and Grain
onshore reinforcement between Norwich andTilbury (East Anglia GREEN).
We have only focussed on the development of the onshore reinforcement between Norwich and Tilbury, referred to as East Anglia GREEN. The other elements described above are being progressed as separate schemes due to the general geographic separation of potential effects.
Our strategic proposal for East Anglia GREEN
Our strategic proposal is for an onshore 400kV line. The connection points are Norwich Main, Bramford and Tilbury substations, with a new connection substation in the Tendring Peninsula. Following our appraisals, we have confirmed our strategic proposal to take forward to the next stage of assessment as an onshore reinforcement made up of:
approximately 60km of new 400kV transmission line between Norwich Main and Bramford substations
approximately 120km of new 400kV transmission line between Bramford and Tilbury substations via a new connection substation to be located in the Tendring district.
We expect the majority of the new reinforcement to be made up of steel lattice pylons supporting overhead line, with the use of underground cables through the Dedham Vale AONB.
We expect some associated work to be needed at substations and elsewhere to connect the reinforcements to the existing network and to ensure the safe construction and operation of the reinforcement.
Following the confirmation of the strategic proposal, our next steps were to consider options for detailed routeing and siting for the proposed reinforcement.