Understanding UK DNO G99 Type C and D Commissioning and Testing

Key Takeaways

• G99 is a legal requirement for connecting certain generators to the Great British electricity network.
• Distribution Network Operators (DNOs) have a duty to connect but can refuse if conditions are not met.
• Loss of Mains (LoM) protection settings, specifically Rate of Change of Frequency (RoCoF), are critical for compliance.
• DNOs reserve the right to witness testing of your system and protection settings, which may incur a fee.

What is G99 and Why is it Required for Generation in Great Britain?

G99 is an Energy Networks Association (ENA) standard that outlines the requirements for connecting generators in parallel with public low voltage networks. This standard applies to certain generators connecting on or after 27 April 2019, superseding the previous ENA standard G59 on 17 May 2019. All generators connected to distribution networks in Great Britain must have Loss of Mains (LoM) protection, which is one of the many protection requirements on the electricity system.

Distribution Network Operators (DNOs) in Great Britain have a duty to make a connection between their distribution system and any premises when required, for the purpose of conveying electricity. However, this duty has exceptions. A DNO is not required to make a connection if, for example, doing so would involve a breach of regulations or if it is not reasonable in all circumstances for them to do so. Compliance with G99 ensures that your generation system meets the necessary technical and safety standards for connection to the network.

The Role of Distribution Network Operators (DNOs)

DNOs, such as Electricity North West, own, operate, and maintain the electricity distribution network in their respective regions. They are responsible for ensuring the stability and safety of the grid. When you plan to connect a generator, such as a solar PV system or battery storage, to the network, you must apply to your local DNO.

The application process typically involves submitting details of your proposed generation. For G98 applications, notification forms should be emailed to g98notifications@enwl.co.uk. For general connection applications, completed forms should be sent to connectionapplications@enwl.co.uk. DNOs also offer online application services to get an estimate for the required work.

Understanding Loss of Mains (LoM) Protection and RoCoF Settings

Loss of Mains (LoM) protection is a critical safety feature designed to check if a generator is still connected to the supply network. This protection prevents a generator from inadvertently feeding power into a section of the network that has been disconnected from the main grid, which could pose a significant safety risk to engineers working on the lines.

Where LoM is provided by vector shift, it must be removed and/or replaced with Rate of Change of Frequency (RoCoF). The RoCoF setting requirement has been changed to 1Hz/s with a definite time delay of 500 milliseconds. It is a requirement of G59 (and by extension, G99) that any material change to the G59 protection settings or generating equipment must be communicated to the DNO which supplies the site. Failure to update Loss of Mains protection settings by 1 September 2022, for example, can lead to non-compliance with updated EREC G59 requirements.

What You Can Check Yourself, and What You Cannot

As a homeowner or system owner, you can ensure that your system's documentation is up-to-date and that you understand the basic requirements for connection. You should verify that your installed generation equipment is listed as compliant on the ENA Type Test Register (now migrated to Connect Direct). Devices listed as "Further information required," "Non-compliant," or "Non-compliant for new installations" may not be accepted for connection.

However, you cannot change inverter settings yourself. Grid-protection settings are set by an accredited installer. Changing them can breach your connection agreement with the DNO and compromise grid safety. Work on or adjacent to electrical equipment within a generator site requires maintaining a safe system of work and ensuring generators are disconnected and fully isolated.

DNO Witness Testing and Compliance

DNOs reserve the right to witness the testing of your system and its protection settings. This witness testing may be subject to a fee. This is part of their responsibility to ensure that all connected generation operates safely and in accordance with G99 standards.

Non-compliance with updated EREC G59 requirements, which includes G99, can lead to enforcement action, including the de-energisation of your generation assets. In some rare cases, following the loss of network in-feed to a site, a generator can continue to inadvertently generate until protection settings have been exceeded. This risk is marginally increased where protection settings are relaxed, particularly for synchronous generating units with evenly matched local demand and generation. This highlights the importance of correct protection settings and DNO oversight.

What the Published Sources Do Not Tell You

The available information provides a framework for G99 compliance and the DNO's role, but specific details regarding G99 Type C and D commissioning processes are not extensively detailed. Statutory and engineering documentation does not provide:

• Specific details on the commissioning process steps for G99 Type C and D.
• Detailed requirements for witness testing beyond the DNO's right to witness.
• Specific forms or application procedures for G99 Type C and D connections.
• Information on the different categories of G99 (Type A, B, C, D) and their respective power thresholds or characteristics.
• The exact definition or criteria for "Type C" and "Type D" generation as per ENA G99.
• Information on typical timelines for G99 connection applications and commissioning.
• Details on the specific tests required during commissioning for G99 Type C and D.
• Information on penalties or specific enforcement actions for non-compliance with G99, beyond de-energisation.
• Guidance on how to prepare for a DNO witness test.
• Information on the costs associated with G99 connection applications and commissioning, beyond the general "Connection Offer Expenses."

For these specific details, you would need to consult directly with your local DNO or an accredited installer familiar with the latest G99 requirements. This information is general and not tax or legal advice.