Need for Sea Link

The electricity transmission network is planned and operated under a set of standards designed to ensure there are no widespread electricity supply interruptions, even if two circuits are out of service.

These standards ensure that, for given operational and fault scenarios:

• the electricity system frequency is maintained within statutory limits
• no part of the network is overloaded beyond its capability
• voltage performance stays within acceptable statutory limits, and the system remains electrically stable.

To understand current and future demands on the electricity network, we use the concept of network boundaries. A boundary splits the system into sections and shows where there are high-power flows between parts of the network. When flows across a network boundary are forecast to be above the capability of the network and the security and quality of supply standards, there are two options to manage this:

1. pay electricity generators on one side of the boundary to reduce the energy they produce (and in turn pay generators on the other side of the boundary to compensate for the shortfall). This then reduces the flows of electricity across the boundary. When the Electricity System Operator pays generators to do this, these are called ‘constraint payments’
2. increase the capability of the network to allow more electricity to flow.

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The existing transmission network infrastructure in East Anglia and the south-east was built in the 1960s, and was not designed to accommodate the large volumes of generation capacity that is planned to connect here. It was built to supply regional demand, centred around Norwich and Ipswich. A large loop runs from Walpole in the north of the region to Pelham and Rayleigh/Tilbury in the south, via Norwich and Bramford.

While the network in East Anglia can accommodate the level of generation and demand that there is today, this situation will change with the growth in offshore wind, new interconnectors and nuclear power stations. This means that the amount of power connecting behind the network boundary (referred to as EC5) is far greater than the ability of the network to transport power out of the region.

By the end of this decade, if everything contracted to connect in the region does connect, there will be significantly more generation than the current network is capable of accommodating. This means the existing high voltage electricity network in East Anglia does not have the capability needed to reliably and securely transport all the energy that will be connected, while meeting the security and quality of supply standard.

In the south-east there are multiple network boundaries (LE1, SC1, SC1.5, SC2 and SC3). The LE1 boundary almost exclusively imports power from the north and west of England into the southeast. As more energy is pulled across London and into Kent, power flows across LE1 are set to increase. Demand for electricity is also set to grow here, and interconnectors will exchange more energy with European countries to help balance intermittent sources of power.

The network therefore requires reinforcement in both East Anglia and the south-east. Before we consider building new parts of the network, we first look at how we can upgrade existing network infrastructure. This can involve building new substations and improving the transmission circuits using thicker conductors/wires on existing overhead lines, and adding smart power control devices to control the flow of electricity on parts of the network where power is needed. Over the next decade, this is what we will be doing. However, these upgrades do not adequately address the shortfall in network capacity and several new network reinforcement projects are required, of which Sea Link is one.

Sea Link helps to reinforce the network in multiple ways:

1. by connecting new planned generation behind the network boundary allowing power to be transferred
2. by connecting into the proposed Friston substation, Sea Link reinforces the Bramford-Sizewell radial circuits transferring power between transmission boundaries EC5 and the Sizewell Generation Group (shown in the diagram on as EC6)
3. by connecting into the Kent coast, it increases the amount of power that can be transported to and from the south-east, helping to meet domestic demand as well as imports and exports to Europe via interconnectors.

The total power carrying capability of the Sea Link reinforcement will be 2 gigawatts (GW).

The strategic proposal

The network reinforcement needs identified in both East Anglia and the south-east were reviewed together. A single solution which simultaneously dealt with both constraints was selected.

The identified reinforcement that will help achieve this is an offshore 2 GW high voltage direct current link between Suffolk and Kent. 

If you would like to view the full detailed needs case for Sea Link, you can find this in the Strategic options report in our Document library.