27/03/2026
In Great Britain, EREC G99 is the main technical framework for connecting generation equipment in parallel with public distribution networks. It covers the connection pathway for Type A, Type B, Type C, and Type D Power Generating Modules, from application and technical assessment through compliance, commissioning, and operational notification.
For developers, EPCs, OEMs, and plant owners, G99 studies are far more than an administrative requirement. They form the engineering basis for demonstrating that a generating facility can connect safely, operate stably, satisfy the applicable technical requirements, and avoid creating unacceptable impacts on the DNO network. In practice, that means combining network data, plant design data, controller logic, protection settings, and dynamic performance models into one coherent technical package.
When prepared properly, a G99 study package does not just support approval. It also reduces project risk by identifying weak assumptions, controller issues, unrealistic reactive capability claims, or commissioning mismatches before they become expensive site problems. That is why strong G99 work is not simply about “running studies.” It is about translating the regulation into a submission-ready engineering case.
What G99 studies are designed to prove
A proper G99 study package usually serves four practical purposes. First, it confirms that the proposed plant can connect without causing unacceptable issues such as voltage problems, fault level concerns, power quality impacts, or instability. Second, it demonstrates that the plant itself can meet the required performance obligations for its G99 type, including areas such as voltage behaviour, reactive capability, frequency response, and fault ride through where applicable.
Third, it provides the DNO with the modelling evidence needed to assess the project during the connection process. This is especially important for schemes where network behaviour and plant control interactions cannot be judged from static design documents alone. For larger or more complex projects, the quality of the model and the traceability of assumptions can strongly influence how smoothly the review progresses.
Fourth, a well-structured study package creates a reliable basis for commissioning, testing, and final operational notification. If the studies, settings, and plant configuration are aligned from the beginning, the path to energisation is usually more efficient and less exposed to avoidable rework.
How G99 projects are typically split by type
G99 divides projects into Type A, Type B, Type C, and Type D, and the study burden increases as project size, system impact, and compliance obligations grow. The document structure itself reflects this: Type A is addressed through Section 11 and Annex A, Type B through Section 12 and Annex B, and Type C / Type D through Section 13 and Annex C.
In practical engineering terms, Type A projects are the lightest category from a study perspective. They still require sound technical assessment, but the emphasis is generally on correct application, equipment suitability, protection, commissioning requirements, and completion of the relevant compliance forms rather than a large simulation programme. Annex A contains the key compliance verification and site commissioning documentation for this category.
Type B is where formal simulation studies become a standard part of the process. Annex B includes dedicated simulation requirements, and the Generator is expected to submit reports demonstrating compliance using models appropriate to the proposed equipment and settings. This is the point at which the study package becomes more structured, more evidence-driven, and more dependent on proper RMS and load flow analysis.
Type C and Type D projects require a broader and more formal compliance framework. Annex C includes simulation studies for Type C and Type D, while the annex structure also covers functional specifications for fault recording and power quality monitoring, as well as compliance testing requirements. Type D follows the same broad technical framework as Type C but with a more stringent operational and notification process.
How G99 studies should be performed in practice
A strong G99 study should follow a structured workflow rather than treating each requirement as an isolated task. The first step is to define the plant correctly. That includes the Power Generating Facility boundary, the Power Generating Module type, whether the scheme is synchronous or a Power Park Module, whether storage is involved, whether the project is phased, and what registered capacity is being used for classification and compliance. Errors at this stage can distort the entire study scope.
The next step is obtaining the right data. A study package is only as good as its inputs, and that usually means collecting the single line diagram, transformer and cable data, fault level information, DNO assumptions, inverter or generator parameters, controller philosophy, reactive limits, protection settings, and export/import operating modes. For dynamic studies, validated RMS models are especially important.
Once the data is in place, the project team needs a fit-for-purpose model that can assess both steady-state and dynamic behaviour. For Type B, Type C, and Type D projects, G99 expects simulation studies and appropriate modelling information, and for larger projects it expects models that are suitable for compliance demonstration and validation. A study that only looks at the network side, or only looks at the plant side, is incomplete. The submission has to show both that the network can accept the connection and that the plant behaves correctly during disturbances and operating changes.
A good report then maps the study results directly to the relevant G99 section, annex, and evidence requirement. That usually means clearly stating the project classification, assumptions, model basis, data sources, study scenarios, acceptance criteria, results, compliance conclusion, limitations, and any commissioning follow-up actions. This makes the submission easier for the DNO to review and helps reduce avoidable comments and delays.
What engineers should always consider
Regardless of project type, several technical points should always be checked carefully. One of the most common errors is using the wrong connection point or plant boundary in the study basis. That can lead to incorrect assumptions on voltage, reactive power, and fault contribution, which then affects both the simulations and the reported compliance position.
Operating point selection is equally important. Many G99 obligations must be checked at specific conditions such as full active power, minimum or maximum reactive output, or minimum notified fault level. A result that looks acceptable in one mode may not remain acceptable in another. This is especially important for inverter-based plants, where PPC, inverter control, AVR-equivalent functions, power factor control, reactive power control, and voltage control can interact in ways that are not obvious unless the hierarchy is properly tested.
Reactive capability claims also need to be realistic. Capability cannot be based only on equipment nameplate values; it must reflect transformer taps, collector system losses, auxiliary demand, inverter current limits, and voltage-dependent behaviour. For larger projects, model validation is another critical issue. G99 is explicit that compliance models must be validated, which means a black-box model on its own is not enough unless it credibly represents the installed plant for the actual study cases being submitted.
Finally, the studies must stay aligned with future commissioning. If the final installed settings differ materially from the assumptions used in the compliance model, the basis of the study becomes weak. Good G99 work therefore links study assumptions, controller settings, commissioning logic, and operational notification into one consistent thread.
Which studies typically apply to each G99 type
Type A
For Type A projects, the study scope is generally lighter and focused more on connection suitability than on a full dynamic simulation programme. Typical studies and checks may include:
The main objective for Type A is to demonstrate that the proposed equipment, settings, and connection arrangement are appropriate for the relevant G99 requirements.
Type B
For Type B projects, G99 requires formal simulation studies under Annex B.4. The following studies are typically included:
In practical PowerFactory terms, a Type B package usually combines steady-state load flow cases, reactive capability scans, RMS dynamic studies, fault event simulations, and the associated compliance documentation.
Type C&D
For Type C&D projects, the study package becomes broader and more formal under Annex C.7. Typical studies include:
Type C&D projects therefore require a more detailed and coordinated modelling approach, with stronger emphasis on plant controls, dynamic behaviour, and compliance evidence.
Why DIgSILENT PowerFactory is well suited to G99 studies
For modern generation and storage projects, especially inverter-based schemes, the study environment matters. A major advantage of DIgSILENT PowerFactory is that it allows the project team to keep the network model, plant model, and compliance study cases within one coordinated platform. That supports a more consistent workflow across load flow, fault level, reactive capability, RMS dynamics, controller interaction studies, and compliance-focused disturbance analysis.
This matters because many G99 requirements are not independent of one another. The same plant controller assumptions can affect reactive capability, voltage control response, frequency behaviour, and fault ride-through performance. Working in one modelling environment helps keep those assumptions aligned and makes it easier to prepare a report that is technically coherent rather than stitched together from disconnected calculations.
Conclusion
G99 studies should never be treated as a generic template exercise. The correct scope depends on project type, capacity, technology, operating mode, connection voltage, and the actual behaviour of both the plant and the surrounding network. Type A projects usually focus on connection suitability, protection, and compliance forms, while Type B introduces a clear simulation requirement. Type C and Type D projects require a much broader Annex C framework covering reactive capability, voltage control, fault ride through, frequency response, controller validation, and closer integration with monitoring and commissioning obligations.
When carried out properly, these studies do much more than support compliance. They help de-risk the connection process, identify technical issues early, align OEM and DNO expectations, and create a smoother route to energisation and final operational approval.
