UK DNO Formal Connection Offer and Grid Constraint Management

Key Takeaways

• All generators in Great Britain connecting to the distribution network must comply with EREC G59 for Loss of Mains (LoM) protection.
• Non-compliant LoM protection, especially vector shift, must be updated to Rate of Change of Frequency (RoCoF) settings.
• The compliance deadline for updated EREC G59 requirements was 1 September 2022.
• Failure to comply can lead to enforcement action, including the de-energisation of your generation assets by the DNO.

Understanding Your DNO Formal Connection Offer in Great Britain

When you plan to connect a solar or other generation system to the electricity network in Great Britain, your Distribution Network Operator (DNO) will issue a formal connection offer. This document outlines the technical requirements, costs, and terms for your connection. It ensures your system meets the necessary safety and operational standards for the grid.

For example, Electricity North West (ENWL) owns, operates, and maintains the North West’s electricity distribution network. They, like other DNOs, provide guidance on the connection process. The offer will detail aspects such as the required engineering works, any necessary upgrades to the network, and the specific technical standards your installation must meet. This includes compliance with Engineering Recommendation (EREC) documents like G98, G99, and G100, which govern how different sizes of generation connect to the grid.

What is Actually Going On: Grid Constraint Management and LoM Protection

The electricity network is a complex system that requires careful management to remain stable and safe. When new generation sources connect, they must not destabilise the grid or create hazards. This is where grid constraint management and specific protection mechanisms like Loss of Mains (LoM) protection become critical.

All generators connected to distribution networks in Great Britain that run in long-term parallel with the grid must have Loss of Mains (LoM) protection. This protection is one of many requirements on the electricity system. Its purpose is to check if a generator is still connected to the supply network. If the main grid supply is lost, LoM protection ensures the generator disconnects, preventing a dangerous situation known as 'islanding', where a section of the network remains energised by the generator without DNO control.

A significant update to EREC G59 requirements changed how LoM protection is implemented. Where LoM was previously provided by vector shift, it must now be removed and/or replaced with Rate of Change of Frequency (RoCoF) protection. This change aims to make generation more reliable and less likely to trip off unexpectedly.

Key Protection Settings and Compliance

The update to EREC G59 introduced specific requirements for RoCoF settings.

The date for compliance with these updated EREC requirements was 1 September 2022. Generators that were not compliant by this date face potential enforcement.

Non-compliance with updated EREC G59 requirements for Loss of Mains (LoM) protection can lead to generator de-energisation. If your generator is not compliant, it is more likely to trip off, leading to inconsistent generation and potential loss of income. By becoming compliant, your generation becomes more reliable.

Any generators who are not compliant and therefore require updates to their protection will be subject to an enforcement programme which could ultimately result in the de-energisation of generation assets.

What You Can Check Yourself, and What You Cannot

As a homeowner or system owner, you can check your system's documentation to confirm its compliance status with EREC G59 and the RoCoF settings. You should verify if your inverter or protection relay is configured with the updated RoCoF setting of 1Hz/s with a definite time delay of 500 milliseconds, especially if your system was installed before September 2022.

You cannot, however, change these protection settings yourself. Grid-protection settings are set by an accredited installer during commissioning and changing them can breach your connection agreement with the DNO. Attempting to modify these settings without proper authorisation and expertise can also compromise the safety and stability of both your system and the wider grid.

If you suspect your system is non-compliant or requires an update, you must contact an accredited installer or your DNO. They can assess your system, perform any necessary updates to the protection equipment, and ensure all work is carried out according to current regulations.

What the Published Sources Do Not Tell You

While DNOs like Electricity North West provide general guidance, specific details that vary by project or are subject to commercial confidentiality are often not publicly available.

For instance, Statutory and engineering regulations do not specify these details. It mentions a "Connections Charging Statement" for further details on Connection Offer Expenses balance payable on acceptance, but this document is not provided.

Similarly, specific "G98 application form table" and "G99 application form table" contents are mentioned but not included. Information regarding the "curtailment index and 'cap'" for flexible connections, which can impact how much power your system can export at certain times, is also not detailed.

Statutory and engineering documentation refers to a "list of recognised contractors" for G59 updates and a "self-serve tool on the Futureproof Your Power website" to check inverter/relay compliance, but these resources are not provided. Specific legal references like the "Distribution Code DGC12" for de-energisation, the "Electricity (Connection Offer Expenses) Regulations 2018," or the "Energy Act 2023" and "Planning and Infrastructure Act 2025" (referenced in the Electricity Act 1989) are mentioned but not supplied. The "Connect Direct" platform for the ENA Type Test Register is also referenced without direct access. These details often require direct engagement with the DNO or access to specific industry portals.