Every solar installation, home battery or commercial battery connected to the Belgian distribution grid has to comply with C10/11, Synergrid's technical prescription. Synergrid is now proposing an amendment under the name Grid Update. The trigger is the major blackout in Spain and Portugal in April 2025: Synergrid wants to make the grid more robust sooner, without waiting for the new European grid code RfG 2.0, for which a full revision of C10/11 is planned.
Still a proposal
This is a draft text in public consultation until 1 December 2026. The figures below may still change, and it is not yet known when the new rules will apply or what will happen to existing installations.
1. Inverters may disconnect less quickly
During a short dip or spike in voltage or frequency, solar panels and batteries have to keep running for longer. In the proposal, the automatic disconnection system only trips after a delay of usually 3 seconds, for example at 115% or 80% of the normal voltage, or at 51.5 Hz or 47.5 Hz.
The reason is simple. If thousands of inverters disconnect at the same time during a small disturbance, a lot of generation suddenly disappears from the grid and the disturbance only gets bigger. That effect played a role in Spain and Portugal.
Islanding detection changes too. A new external disconnection relay must include ROCOF (the rate at which the frequency changes) and a narrower frequency window. The older vector shift method must be disabled in it and remains allowed only for existing installations. In addition, nothing in the installation may systematically disconnect generation as long as voltage and frequency stay within normal limits.
2. Inverters help keep the voltage down
In neighbourhoods with a lot of solar panels, the voltage rises on sunny days. If it gets too high, inverters disconnect and the owner loses yield. The proposal therefore makes Q(U) the default and mandatory on low voltage.
This is how it works. From 105% of the normal voltage (about 241 V), the inverter automatically absorbs a little reactive power. That pushes the voltage down without delivering less solar power. The higher the voltage, the more it absorbs, until the maximum of 33% of the inverter's power is reached at 108% (about 248 V). The reverse, raising the voltage with reactive power, is not allowed.
Only if that is not enough may the inverter reduce its active power (P(U)). That remains optional and is recommended, but it may only start from 108%, so Q(U) always acts first.
3. Clear rules for island and backup operation
More and more batteries can keep powering a home or business when the grid goes down. The proposal makes clear what is what:
- Off-grid: the generation can never, not even briefly, operate in parallel with the grid. In that case the C10/11 rules do not apply to that part of the installation. Note: a separation using semiconductors counts as a connection, so that is not off-grid.
- Backup and island operation: the installation is connected to the grid at certain times. In that case the rules apply in full and the inverter must be certified.
When switching over, the installation must first disconnect from the grid before it supplies power itself (break-before-make), with an interlock that rules out simultaneous connection, for example through an automatic transfer switch (ATS). When returning to the grid, it must synchronise properly. If that happens more than once a day, voltage and current around each synchronisation must be recorded and kept for the grid operator.
4. More room for small installations
Households today combine solar panels, a home battery, a heat pump and an 11 kVA (bidirectional) charger, and quickly exceed the old limit of 10 kVA. The proposal raises that limit:
- Three-phase: up to 11 kVA without an injection limit, or up to 33 kVA if at most 11 kW is injected into the grid.
- Single-phase: up to 5 kVA without an injection limit, or up to 18 kVA with at most 5 kW of injection.
No distinction is made any more between solar, battery or another source. These installations follow the simple procedure with a notification instead of a grid study. Anyone using an injection limit needs a power management system that measures via the P1 port of the digital meter or via an EnFluRi sensor. The imbalance between phases remains limited to 5 kVA, with new ways to monitor it, such as an injection limit per phase.
5. An external disconnection relay is needed less often
An external disconnection relay is only mandatory from 56 kVA instead of 30 kVA, the same threshold as between the digital meter and the AMR meter. Below that, the protection built into the certified inverter is sufficient. That saves cost and cabling on medium-sized installations.
Extra: logs at the manufacturer
After incidents, the grid operator can request logs, the configured protection parameters and measurement data (power, voltage, current, frequency) directly from the inverter manufacturer. The manufacturer must deliver them digitally within one month. If an installation turns out to be non-compliant, the owner bears the cost of the inspection and the installation may lose its permit or revenue.
What does this mean for installers?
- Configure new inverters with the new disconnection parameters and leave Q(U) enabled, as soon as the new text comes into force.
- Check that the energy management systems and apps on your installations do not override the inverter's reactive settings.
- Design backup and island operation with a clear switchover, and know whether your installation is truly off-grid or not.
- Revisit residential projects that ended up just above 10 kVA: with an injection limit, they may fit into the simple procedure.
How the Voltmasters EMS handles this
The Voltmasters EMS controls reactive power according to the grid operator's guidelines. When the grid study imposes a reactive operating point, the EMS holds it at the connection point and adjusts it when the grid voltage runs high, as the guideline prescribes. When the grid operator sends a reactive power instruction via telecontrol, the EMS carries it out.
For microgrids, the EMS follows strict procedures when switching to island operation and when returning to the grid. The site only goes back on the grid once the grid has returned and is stable. When reconnecting via a dead bus, the EMS first takes every source off the bus before the grid breaker closes: break-before-make. When reconnecting synchronously, the EMS matches the island to the grid, and a certified synchrocheck relay (C10/24) decides when the grid breaker may close.
Responses accepted until 1 December
Synergrid only takes into account responses submitted via the web form on the consultation page. On 29 October 2026 Synergrid is hosting a webinar on the proposal; you can register via the same page. The full draft text is available online.
Questions about what this means for your projects? Get in touch, read how the microgrid reconnects to the grid or learn more about grid compliance.
Source: Proposed amendment to C10/11 edition 2.4, Grid Update (Synergrid, consultation from 1 October to 1 December 2026).