Looking for more information about the 9 August power cut?

See our interim technical report and other ESO resources.

Transmission constraint management

Constraint management is required where the electricity transmission system is unable to transmit power to the location of demand, due to congestion at one or more parts of the transmission network. Here you can find out how to provide this service.

This service is geographically defined, so only certain providers will have the required technical capability to deliver the service.

If the system is unable to flow electricity in the way required, we will take actions in the market to increase and decrease the amount of electricity at different locations on the network.

16 April 2019 - We have also published a Request for Information for the provision of long-term reactive power service in Mersey starting from 2021/22. The deadline for submissions is 24th May. More context on this RFI can be found on our Network Development Roadmap page.

March 2019 - Long Term Reactive Power Needs out to 2031.
We are currently undertaking pathfinding projects looking at how we can tender for commercial solutions to future reactive power needs alongside considering asset options.
Find our more here

The next steps for Request for Information for the provision of short-term Reactive Power Service out to 2021 in South Wales and Mersey are published. Thank you to all providers who responded to the RFI, the information provided helped to shape the next steps of the RFI. The next steps can be found here for South Wales and Mersey

We have published a Request for Information for the provision of short-term Reactive Power Service out to 2021 in both South Wales and Mersey areas. The Request for Information forms part of our commitment in the Product Roadmap for Reactive Power. Interested reactive power providers are requested to submit information with an outline of their capability by the 5th November as detailed in the Request for Information. Please use the South Wales Proforma or Mersey Proforma in response to the RFI. 


Ahead of time contracts

We can enter into contracts to agree output with a service provider ahead of time.

  • Constraint management contracts enable us to agree in advance technical parameters with connected parties to facilitate the management of a constraint.
  • The most common style of contract is to agree either a cap or collar on the output of a power station.
    While other commercial tools are available to manage the output of a power station, constraint management contracts are considered where the cost of the action is more economic than other commercial tools.

There are multiple different styles of constraint management service that we may use to resolve a specific requirement.

Situations that can cause a transmission constraint


The energy demand cannot be met by localised generation and the flow on the circuits into that area is limited by the capacity of the circuits; or into that area is limited by the capacity of the circuits


The generation in the area is not offset by the localised demand and the flow on the circuits out of the area is limited by the capacity of the circuits

These constraints can be further broken down:


Where the amount of energy that would flow naturally from one region to another exceeds the capacity of the circuits connecting the two regions.


Where generation is needed in a particular area to support the local voltage.


Where particular circuit configurations have to be avoided because of the risk of oscillations in voltage or current.