Home / Articles / 28 September 2026
Theme of the month: Power engineeringGrid-forming inverters: when the battery has to hold the plant on its own
An ordinary inverter follows the grid and stops when it disappears. A grid-forming one creates it. The difference decides whether a battery system is merely a saving on the bill or a standby supply for the whole site — and why that changes the protection.
Almost every inverter installed to date on photovoltaics or a battery is grid-following. It measures the grid’s voltage and frequency with a phase-locked loop and injects current in step with them. It behaves as a current source: the grid sets the voltage, the inverter adds power. When the grid drops, there is nothing to follow and by standard it must switch off within fractions of a second — that is the anti-islanding protection every distribution operator requires.
For a plant with a battery that means something unpleasant: on a supply failure the battery, full of energy, stops together with the grid. It reduces the bill at peak hours, but does not help at the one moment when its energy would be worth the most.
What makes a grid-forming inverter different
It behaves as a voltage source. It does not follow frequency and voltage — it sets them. Its control is built on the model of the synchronous generator: a set frequency that drops slightly as active load grows, and a set voltage that drops slightly with reactive load — the so-called droop characteristic, the same by which parallel generators have shared load for a century. More advanced implementations also emulate inertia — a “virtual synchronous machine” that, for the first few hundred milliseconds after a disturbance, responds the way a spinning rotor would.
The practical consequences are three. The inverter can operate in island mode and supply the site alone. It can start a grid from nothing — black start — without external voltage. And when connected to the grid, it supports it through disturbances instead of switching off at the first flicker.
Why that changes the protection
This is the part most often missed. A synchronous generator delivers five to seven times rated current into a short circuit — enough to operate the overcurrent relays and circuit breakers sized for the grid. An inverter cannot: its semiconductors typically withstand 1.1 to 1.5 times rated current briefly, and that is all. Short-circuit current in island mode is an order of magnitude lower than with grid supply.
- A 100 A breaker with magnetic tripping at 500–1000 A may never be operated by an inverter rated 200 A.
- Protection has to be coordinated for two regimes — grid-connected and islanded — or rely on voltage and frequency criteria, not only current.
- Selectivity set for the grid is a different task in island mode. Sometimes the answer is to supply not the whole site, but a selected part.
This is not a drawback that should stop the decision. But it is a calculation to be done before the order, not after the first outage.
What the design must include
- Point of connection. A breaker or transfer switch that separates the site from the grid on loss of supply and returns it with synchronisation when the grid comes back. The synchronism check — frequency, voltage, phase — is mandatory; connection without it is destructive.
- Interface protection towards the distribution operator. In Europe the requirements are in EN 50549-1 and 50549-2 — voltage and frequency thresholds, behaviour during disturbances. The grid-forming inverter must meet them in grid mode and ignore them in island mode, which is a setting, not a given.
- Sizing for peaks. In island mode the inverter takes the starting currents of motors. A compressor that draws six times rated current at start determines the inverter power, not the average consumption. Short-term overload capability — typically 1.5 to 2 times for seconds — is a key parameter.
- Load management. On transition to island, part of the consumers are shed by priority. That is a signal from the inverter to the site’s control system and has to be designed.
- Black start. If expected, the sequence is described: which circuit is energised first, how simultaneous starting is limited, when the large motors are allowed.
Grid codes are moving in this direction. ENTSO-E and national operators are introducing grid-forming requirements for new large battery systems, and in several countries it is already a condition of connection. An inverter bought today without that capability will be constrained tomorrow.
Who it is for
A site where an outage costs — a continuous process, cold storage, server rooms, anything with product that spoils or equipment that is damaged by a sudden stop. There the battery with a grid-forming inverter replaces the diesel generator for the first minutes or hours, without noise, without fuel and with a transfer time of milliseconds instead of seconds.
For a site that simply wants a lower bill, a grid-following inverter is sufficient and cheaper. The difference is not in the battery — it is the same. The difference is in what you want from it on the worst day.