G99 is the Energy Networks Association engineering recommendation that sets the requirements for connecting generation, including battery storage, to operate in parallel with a distribution network in Great Britain. For a storage developer it is the gate between a built site and an energised one. Much of what G99 asks for is proven through the site's protection and controls, so the controls package is on the critical path to energisation.
This article does not replace the recommendation or your distribution network operator's (DNO) own requirements, which you should always read in their current form. It explains where the controls engineer fits, and what to prepare so compliance testing is a formality rather than a fight.
Why G99 is a controls problem
G99 sets requirements on how the site behaves at its connection point: how much it exports and imports, how it responds to voltage and frequency changes, how it disconnects under faults, and how it can be controlled or limited. Those behaviours are delivered by inverters, the power plant controller (PPC) and the protection relays working together. If the settings in each are not coordinated, the site fails its tests even when every product is compliant on paper.
| Area | Delivered by | Controls engineering task |
|---|---|---|
| Export and import limits | PPC with metering at connection point | Limit logic, fail-safe behaviour on comms loss |
| Interface protection | Protection relay tripping the main breaker | Interlocks, status signals, test support |
| Voltage and reactive power | Inverters under PPC control | Control mode, setpoints, ramp rates |
| Frequency behaviour | Inverters and PPC | Settings coordinated with any frequency services |
| Remote control by DNO | Interface to DNO systems where required | Signal list, testing and records |
Export limitation done properly
Many storage sites connect with an export or import limit below the full rating of the equipment, to fit available network capacity. The limit is enforced by the PPC measuring power at the connection point and curtailing the inverters. The DNO will want to know what happens when things go wrong: meter failure, communications loss to an inverter, PPC restart. The safe answer is usually to fail to a limited state, not to full output. Write the failure behaviour into the design and test every case.
Coordinating protection and control
The interface protection relay is the last line of defence. It must trip the site off for conditions defined in the connection requirements, independent of the PPC. The controls engineer's job is to make sure the PPC, inverters and protection do not fight each other: for example, inverter ride-through settings that keep the site connected while the relay is meant to trip, or a PPC that tries to restart export after a protection trip without the correct conditions.
Interaction with balancing services
Batteries that provide fast frequency services must respond within strict times. NESO's Dynamic Containment page lists full delivery within 1 second for Dynamic Containment. That response must still respect the site's export and import limits and its connection behaviour. Design the PPC so service logic and connection limits are handled in one place with clear priorities, rather than layered in separate products. Our article on Dynamic Containment battery controls covers the service side.
Compliance testing without surprises
- Agree the test programme and witness requirements with the DNO early.
- Test PPC logic against simulated meter and inverter signals before site, including every failure case.
- Pre-check protection relay settings against the approved settings and record the check.
- Run staged power tests on site and record the results at the connection point meter.
- Package the evidence as the DNO expects it: settings, test sheets, logs and drawings.
Most delays at this stage come from paperwork and settings mismatches, not from equipment that cannot comply. A controls engineer who has the full settings register and test logs ready can often close out issues in hours instead of weeks. Keep the test sheets in the same order as the DNO's programme, so the witness can follow without hunting for evidence.
Transmission versus distribution
Larger projects connecting to the transmission system work under the Grid Code instead, which NESO describes as the technical rules for connecting to and using the National Electricity Transmission System. The Grid Code page explains how it is governed. Some distribution-connected sites also have obligations that flow from transmission requirements, so check with the DNO which documents apply.
Timing in a crowded queue
Connection dates have become harder to predict. NESO's connections reform re-ordered the queue to prioritise ready-to-build projects, and Modo Energy estimates that connection delays could cut storage expansion by 2.4 GW by the end of 2029 (Modo Energy). When a connection date moves forward, the controls package must be ready. A template design with settings and tests prepared in advance lets you respond.
Innovation - Grid Connection Simulation Tool
National Energy System Operator
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Watch on YouTubeDocuments to prepare early
- Single-line diagram showing metering, protection and control points.
- Control philosophy for the PPC, including all limits and failure modes.
- Settings register for inverters, PPC and protection.
- Signal list for any DNO interface.
- Test procedures and blank test sheets ready for witnessing.
Hand these over in an editable, version-controlled form. After energisation the same documents support service prequalification, insurance reviews and any later change to the connection agreement.
Next step
Send us the single-line diagram, connection offer conditions and equipment list. We will scope the PPC logic, settings register and test documentation through our energy systems service. For the wider site system, see BESS SCADA in Great Britain, or contact us.
