title: ENA Engineering Recommendation G99 - Generation Connection (Type B/C/D) tags: [generation-connection, DNO, distribution, ENA, technical-standard, RfG, type-b, type-c, type-d, wind, solar, battery-storage, g99] instrument_row: 108 source_file: sources/energy-networks/ena-er-g99.md
ENA ER G99 - Generation Connection (Type B/C/D)
What it is: The GB technical standard governing how commercial-scale generation and electricity storage connects to distribution networks. Issued by the Energy Networks Association (ENA). Effective 27 April 2019. Current version: Issue 2, March 2025.
Full document (free PDF): https://dcode.org.uk/assets/250307ena-erec-g99-issue-2-(2025).pdf
The core problem it addresses
Any generator above roughly 16A per phase that wants to connect to a GB distribution network must comply with G99. Without a common technical standard, each DNO would impose different requirements, making multi-DNO project development harder and potentially creating inconsistencies that threaten grid stability. G99 implements the EU-derived RfG Regulation (2016/631, row 94) in a practical connection process that DNOs, developers, and equipment manufacturers can follow.
Which projects G99 affects
G99 covers all generation and storage not in scope of the smaller EREC G98 (micro-generators up to 16A per phase). In practice this means:
- Solar farms and wind farms connecting at HV (all commercial scale projects)
- Battery storage systems above roughly 50 kW
- Gas peakers, embedded CHP, run-of-river hydro
- Vehicle-to-Grid installations
- Any plant designed for island mode operation, regardless of size
Issue 2 (March 2025) added new mandatory requirements specifically for electricity storage. Those apply from 1 March 2026.
Type A/B/C/D structure
G99 uses the RfG Type classification to scale requirements to generator size.
| Type | Scale | Connection process |
|---|---|---|
| Type A | Small (roughly up to 50 kW) | Installation Document, basic interface protection |
| Type B | Medium (50 kW to 1 MW approx) | PGMD required, system studies, fault ride-through, LFSM-O |
| Type C | Large (1 MW to 50 MW approx) | Full simulation models, reactive power capability, FSM, full testing |
| Type D | Very large (50 MW+ or HV connection) | CUSC contract, Grid Code compliance, NESO involvement |
Requirements accumulate: a Type C generator must also meet all Type A and B requirements.
Key technical requirements
Interface protection (all Types): Loss-of-mains detection using Rate of Change of Frequency (RoCoF). Vector Shift protection was banned by Ofgem (DC0079). The RoCoF threshold is set by NESO.
Frequency response: - LFSM-O: required for all G99 plant - reduce output when frequency exceeds 50.2-50.5 Hz - FSM (full frequency sensitive mode): required from Type C - continuous bidirectional frequency response, sustain for at least 15 minutes. This is the technical basis for frequency response ancillary services contracts.
Fault ride-through: Generators must stay connected through a voltage dip profile specified in the Annexes. Required from Type B; more demanding at Type D.
Fast fault current injection: Non-synchronous plant (inverter-based: wind, solar, batteries) must inject reactive current during voltage dips to support system voltage. Required from Type B.
Reactive power: Type C and above must demonstrate reactive power capability within a Q/Pmax envelope. Type C PPMs (power park modules) must achieve Q/Pmax 0.66; synchronous plant 0.95.
Connection process milestones
- PGMD (Power Generating Module Document): Applicant submits technical data on the plant.
- System analysis: DNO models the proposed connection against G99 and network constraints.
- Connection offer: DNO issues offer specifying G99 compliance requirements for this specific site.
- Type testing or site testing: Plant complies via type test certificates (ENA Type Test register) or site-specific testing.
- Operational Notification: DNO issues FON (Type B/C) or staged EON/ION/LON (Type D) on successful commissioning.
Relationship to other instruments
- RfG Regulation (row 94): The legal basis. G99 cannot be less stringent than RfG minimums. RfG is the instrument to cite if G99 requirements are challenged.
- EREC G98 (row 107): The companion standard for smaller plant (up to 16A per phase). G98 and G99 together cover all GB generation connecting to distribution.
- Grid Code: Type D plant must also comply with the Grid Code Connection Conditions. For very large plant, the Grid Code governs in addition to G99.
- DCUSA: The distribution connection agreement framework. G99 compliance is a condition of receiving a DCUSA connection.
- ER G5/5 (row 106): Harmonic assessment runs alongside the G99 process for most commercial-scale projects.
- ER G100 (row 109): Export limitation, offered as an alternative to network reinforcement for constrained connections.
Version history highlights
| Version | Year | Key change |
|---|---|---|
| Issue 1 | 2018/2019 | Original publication, RoCoF requirements, VS protection banned |
| G99/15 | 2019 | Fast fault current injection requirements for Types B/C/D |
| G99/16 | 2020 | Reactive capability table, simulation model requirements |
| Issue 2 | 2025 | New storage requirements (1 March 2026), Anchor Plant, BEGA provisions |