G99: larger systems, DNO approval
Solar Pv · Lesson 22 · Advanced
Purpose
Plan a G99 connection before parallel energisation when the generating plant is outside the applicable G98 route.
Learning objectives
- Identify the governing boundary
- Trace energy and measurement paths
- Apply evidence in the required sequence
- Recognise unsafe assumptions
G99 governs the connection of generation within its scope and uses categories and evidence proportionate to plant capability and network impact. Application, studies, settings, type-test evidence and commissioning obligations depend on the exact project.
Core theory
Apply to the DNO and obtain the required permission before connecting in parallel; never treat successful bench operation as authority to energise the network.
Establish Type A/B/C/D classification, registered capacity, voltage level, protection, fault contribution, reactive-power and grid-support requirements from the current issue.
Record approved settings, interface protection, on-site tests, witness requirements, commissioning declarations and any operating/export constraints.
| Term | Meaning | Symbol | Unit |
|---|---|---|---|
| G99 | ENA recommendation for connecting generation within its stated scope | Not applicable | Not applicable |
| Interface protection | Protection coordinating generator disconnection with network conditions | Not applicable | Not applicable |
| Witness test | Commissioning test observed or accepted under the DNO process | Not applicable | Not applicable |
Assumptions: Values are supplied design inputs, not universal limits; Current DNO and product evidence will be checked.
- Classify: Determine the current G99 category and site requirements.
- Apply: Submit exact plant, protection and study information to the DNO.
- Connect: Parallel energisation occurs only after the required permission and commissioning steps.
No. A type-test record is evidence within the process, not connection permission.
Reasonableness check: The conclusion follows the connection point, aggregate plant and declared equipment rather than a marketing label.
- Treating 3.68 kW as a timeless universal threshold
- Assessing each inverter separately instead of aggregate behavior at the connection point
- Confusing a portal reading with settlement-grade metering
Where this appears in practice
A complete PV installation coordinates mechanical support, DC design, network permission, export control, metering, inspection, testing and handover.
Knowledge check
Does a G99 type-test record by itself authorize parallel operation?
No. The site-specific DNO application, approval and commissioning obligations remain.
Answer: No. The site-specific DNO application, approval and commissioning obligations remain.
Practical exercise
Create a document register for a fictional G99 project from application through final commissioning declaration.
Summary
- Use the current rule and exact product record
- Assess the whole installation at the connection point
- Commissioning evidence is broader than successful generation
Sources and review
- Engineering Recommendation G98: Energy Networks Association; Current issue and amendment must be confirmed; Great Britain
- Engineering Recommendations G99 and G100 / Type Test Register guidance: Energy Networks Association; Current issue and Connect Direct status must be confirmed; Great Britain
- IEC 62446-1:2016: PV systems: documentation, commissioning tests and inspection: IEC; 2016 + AMD1:2018; International
- Health and safety in roof work (HSG33): HSE; Fifth edition; Great Britain
Editorial review date: 2026-08-22. Professional electrical review is pending.