What You Need for a G99 Connection Application
A G99 connection application is the standard process for connecting generation above 3.68 kW (single-phase) to the UK distribution network. Getting it right the first time saves weeks. Getting it wrong means resubmission, delays, and missed project deadlines.
This checklist covers every document, data point, and form required for a successful G99 application, plus the mistakes that cause the most rejections.
Before You Start: Determine Your Application Type
G99 defines different application routes based on the size and type of your installation. Your route determines the documents you need.
| Installation Type | Capacity | Application Route | Key Requirement |
|---|---|---|---|
| Small generation | 3.68 kW to 1 MW (single-phase up to 17 kW) | G99 Stage 1 | Simplified application |
| Medium generation | 1 MW to 10 MW | G99 Stage 2 | Full technical data + protection settings |
| Large generation | Above 10 MW | G99 Stage 3 | Full studies + SoW (Statement of Works) may apply |
| Storage (BESS) | All sizes | Same stages as above | Additional cycling data required |
If your installation requires a Statement of Works (SoW), the DNO will request NESO assessment of transmission impact. This adds 3 to 6 months to the timeline.
The Complete Document Checklist
Every G99 application needs these core documents. Missing any one causes rejection.
Required for All Applications
- Application form: DNO-specific form (each DNO has its own version). Download from the DNO portal.
- Site plan: Ordnance Survey map showing the site boundary, point of connection, and proposed cable route. Scale 1:2500 or better.
- Single-line diagram: Electrical schematic showing all generation, switchgear, protection, and metering from the point of connection to each generator. Must include CT/VT ratios.
- Equipment datasheets: For every inverter, generator, transformer, and switchgear item. Must include fault ratings, impedance values, and frequency/voltage response curves.
- Protection settings schedule: Proposed settings for G99 interface protection. Must cover frequency, voltage, RoCoF, and vector shift.
- Earthing arrangement: Drawing and calculation showing the proposed earthing system, step and touch voltage compliance.
Additional for Stage 2 and 3
- Fault level contribution data: Three-phase and single-phase fault current contribution from each generator/inverter at the point of connection.
- Reactive power capability chart: P-Q diagram showing the available reactive power range across the operating envelope.
- Frequency response data: Governor droop settings, frequency deadband, and response time constants.
- Fault ride-through (FRT) curves: Voltage versus time curves showing the installation can ride through voltage dips per G99 requirements.
- Power quality assessment: Harmonic current injection data (up to 50th harmonic), flicker coefficients, and voltage step change calculations.
Additional for Stage 3 (Above 10 MW)
- System study results: Load flow, fault level, and voltage step change study results using the DNO's network model or approved equivalent.
- SoW application form: If the DNO determines a Statement of Works is required. This triggers a separate NESO assessment.
- Compliance matrix: Mapping of every G99 technical requirement to the proposed installation design.
DNO-Specific Requirements
Each DNO adds its own requirements on top of the standard G99 documents. These are the most common sources of confusion.
UKPN (South East, East, London)
- Uses the ENA G99 application form with UKPN-specific supplementary questions
- Requires IPSA or PSS/E format network model data for studies above 5 MW
- Earthing report must follow UKPN's specific template for HV connections
- Separate forms for Type A (up to 1 MW) and Type B/C/D installations
NGED (Midlands, South West, South Wales)
- Uses its own NGED application portal with digital form submission
- Requires DigSILENT PowerFactory format for network studies
- Additional noise assessment for generator installations near residential areas
- Provides pre-populated network data through its Connected Data Portal
SSEN (North Scotland, Central Southern)
- Requires paper-based forms for some Scottish regions (digital for Southern)
- Additional winter minimum fault level check for remote Scottish networks
- Longer processing times in northern regions due to limited network capacity
- Requires SHEPD-specific protection relay settings for North Scotland
Common Mistakes That Cause Rejection
These errors account for over 60% of G99 application rejections across UK DNOs.
1. Wrong Protection Settings Format
DNOs expect protection settings in a specific format. Submitting generic inverter datasheet values instead of calculated G99 protection settings is the most common rejection reason.
- RoCoF: Must specify both the setting value (Hz/s) and the measurement window (ms)
- Vector shift: State clearly if enabled or disabled, and the degree setting if enabled
- Voltage stages: Provide all four voltage protection stages (UV1, UV2, OV1, OV2) with time delays
2. Incomplete Single-Line Diagram
The single-line diagram must show every component from the grid connection point to the generators. Common omissions include:
- CT and VT locations and ratios
- Isolator switch positions
- Earthing connections
- Metering locations (generation and export)
3. Mismatched Capacity Values
The capacity stated on the application form must match the equipment datasheets and single-line diagram. Any mismatch triggers a query that delays processing by 2 to 4 weeks.
4. Missing Fault Level Data
Applications above 1 MW require fault current contribution data. Submitting the application without this data means automatic rejection.
5. Incorrect Site Plan
Site plans must use Ordnance Survey mapping. Google Maps screenshots or hand-drawn plans are not accepted. The connection point and cable route must be clearly marked.
How Noda Generates Compliant Document Sets
Noda automates the preparation of G99 application packages.
- Auto-populated forms: Noda fills DNO-specific application forms from your project data, ensuring no fields are missed
- Protection settings calculator: Calculates G99-compliant protection settings from your equipment datasheets
- Single-line diagram generator: Creates compliant diagrams from your equipment list and site configuration
- Compliance checker: Validates every document against G99 requirements and DNO-specific rules before submission
- Multi-DNO support: Same project data, different output formats for UKPN, NGED, SSEN, SPEN, NPG, and ENW
The result is a complete, validated application package that passes first-time review.
Key Takeaways
- G99 applications require 6 to 12 documents depending on the installation size and DNO
- Each DNO adds specific requirements on top of the ENA standard, particularly around study formats and protection settings
- Over 60% of rejections come from five common mistakes: wrong protection format, incomplete single-line diagrams, mismatched capacity, missing fault data, and incorrect site plans
Next Steps
Stop losing weeks to G99 application rejections. Book a demo to see how Noda generates complete, DNO-compliant application packages from your project data.
