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Glossary · Energy industry & energy transition

Grid Forming Inverter (grid-forming inverter)

Grid-forming inverter grid-forming inverter grid-forming inverters inverter frequency voltage system stability converter Battery storage ancillary services

A grid-forming inverter (in German: netzbildender Wechselrichter) is a power-electronic converter. At its grid connection point, it can provide an electrical voltage with a defined frequency, amplitude and phase angle. It therefore does not merely follow an existing grid voltage, but itself creates a reference by which other installations can orient themselves. In doing so, it takes on a task that, in conventional power systems, was performed mainly by the synchronous machines of coal, gas, hydro and nuclear power plants.

An inverter converts direct current into alternating current. Photovoltaic systems, battery storage systems and many modern wind turbines are connected to the power grid via such devices of power electronics. For system operation, what matters is not only how many kilowatts or megawatts a plant can feed in. It is also important how the converter behaves electrically when the grid frequency fluctuates, the voltage collapses, a short circuit occurs, or a grid section has to be rebuilt after a disturbance.

Distinction from grid-following inverters

The key difference lies in the control. A grid-following inverter measures the existing grid voltage and synchronises with it. For this purpose, it usually uses a phase-locked loop. From the grid signal, this determines the current frequency and the phase angle. The converter then injects a current that matches this given voltage. In simplified terms, it behaves like a controlled current source.

A grid-following inverter therefore presupposes that the grid already has a stable electrical reference. In a strong interconnected grid with many rotating machines, this works well. In a weak grid, in an island grid or with a very high share of converter-coupled installations, this prerequisite may be missing. According to VDE FNN, converter-based customer installations are, according to the current state of the art, predominantly grid-following. They therefore need the instantaneous reserve and short-circuit power of grid-forming installations in order to operate stably. If many devices follow a reference that only a few provide robustly, stability may suffer.

Grid-forming inverters behave more like a controlled voltage source behind an impedance. They actively set voltage and frequency. Their power output adjusts itself according to the grid conditions and is not readjusted only after a measurement. Typical control concepts are droop control, virtual synchronous machine and virtual inertia. These methods determine how a converter responds to power changes, frequency deviations or voltage fluctuations. They are not automatically synonymous with grid-forming, but can be part of a grid-forming control.

In practice, this distinction is important. An ordinary solar inverter is not grid-forming merely because it feeds in alternating current. Nor is a battery storage system automatically grid-forming just because it can quickly absorb or deliver power. The grid-forming capability only results from the interplay of hardware, control, protection concept, grid connection conditions and operational management. This is why regulations require measurement-based tests, simulation-based assessments and certificates.

Frequency, voltage and electrical reference

In an alternating-current grid, voltage and frequency must remain within narrow limits. In the interconnected grid, the frequency indicates whether generation and consumption are currently in balance. If it falls, more power is being consumed than generated. If it rises, more is being generated than consumed. The voltage is shaped more locally. Among other things, it depends on lines, reactive power, transformers and grid structure.

Synchronous machines provide this reference physically, namely through their rotating mass and their electromagnetic coupling to the grid. If load or generation changes, the machine reacts immediately: its rotational speed and its electrical power change. This immediate response is called instantaneous reserve. It dampens the rise or fall of the frequency in the first moments after a disturbance. In the conventional system, it was for a long time a free by-product, as long as enough large power plants were connected to the grid. It is measured in megawatt-seconds (MWs). In the German procurement concept, the quantity of a unit results from its nominal active power multiplied by a start-up time constant.

Converters have no rotating flywheel mass directly coupled to the grid frequency. The energy in their DC link lasts only for a very short time. However, they can be controlled in such a way that they replicate or functionally replace certain properties of a synchronous machine. A grid-forming inverter behaves as a voltage source in this respect. As a result, it reacts to a power imbalance without noticeable delay, that is, without first having to measure a frequency gradient. This distinguishes it from synthetic inertia in a grid-following design, which only reacts to measured values. A prerequisite is always that an energy source stands behind it.

With a battery storage system, this is readily possible because it can move energy in both directions. With a photovoltaic system, it depends on whether enough solar power is currently available. Alternatively, the installation can be deliberately operated below its possible output so that a reserve is available. Without a reserve, a PV system can reduce its feed-in when the frequency rises, but when the frequency falls it cannot deliver more than the modules are currently producing.

Different conditions apply to voltage control and reactive power. An inverter can provide reactive power even at low active power if its power electronics are designed for it. However, the available apparent power limits how much active and reactive power it can deliver at the same time. Grid-forming behaviour is therefore not purely a software question. Semiconductors, DC link, cooling, protection technology and energy storage determine which stabilisation services can actually be called upon.

Why grid-forming inverters are becoming more important

The need for grid-forming properties grows with the share of wind energy, photovoltaics and battery storage. In many hours, these installations displace conventional power plants from electricity generation. As a result, fewer synchronous machines are running that incidentally provide short-circuit power, instantaneous reserve and a robust voltage reference. There may thus be enough electricity while certain stability functions become scarce. According to VDE FNN, the grid development plans for 2035 and 2037 show a significantly rising need for instantaneous reserve.

This difference is often overlooked in debates. Security of supply is then reduced to the question of whether enough energy is generated. For grid operation, however, it also matters whether the power system can electrically cope with disturbances. A grid must withstand load steps, line faults, power plant outages and switching operations. For this, it needs control reserves, protection concepts, voltage support, frequency stability and sufficient short-circuit current so that protection devices can detect and disconnect faults. Particularly critical are system splits, in which sub-grids suddenly have to balance a large power imbalance.

Grid-forming inverters can support several of these tasks:

  • They stabilise the voltage in weak grid areas.

  • They provide an immediate power response to frequency changes.

  • They can operate island grids.

  • With appropriate black start capability, they can help with grid restoration after a large-scale outage.

They are particularly important for battery storage systems, because these can regulate quickly and move energy in both directions. Wind farms, large PV installations and hybrid installations combining generation and storage can also provide grid-forming functions if they are designed, tested and certified accordingly. The VDE FNN guideline also describes how existing grid-following units can be retrofitted to become grid-forming.

However, the benefit does not arise from the individual installation alone. Many grid-forming devices must work together compatibly. If several converters in a grid section influence voltage and frequency, their controls must be coordinated with one another. Otherwise they may excite each other, be insufficiently damped, or disconnect unexpectedly in borderline situations. This results in an institutional task: grid operators, manufacturers, plant operators and regulatory authorities must determine which capabilities are required, tested, remunerated and called upon in operation.

Short-circuit behaviour, protection technology and misinterpretations

A common misunderstanding is that grid-forming inverters are a complete replacement for synchronous machines. Functionally, they can take over many tasks, but physically they behave differently. In a short circuit, a synchronous machine briefly delivers a multiple of its rated current. These high currents help protection systems to detect faults. Converters are limited by their semiconductors. They can deliver overcurrents only within a narrow range and for a short time, typically in the order of magnitude of their rated current up to a small multiple of it. Modern controls can shape the fault current in a targeted way, but the protection technology must match.

A second misunderstanding concerns speed. Converters can react very quickly, but speed alone does not create stability. A fast control with incorrect measured values, unsuitable parameters or contradictory specifications can even amplify instabilities. What is required is therefore not maximum reaction speed, but suitable dynamic behaviour in the respective grid. This includes damping, current limitation, the prioritisation of active and reactive power, and behaviour during voltage dips. Particularly with current limitation, the converter must retain its grid-forming property as far as possible in the event of a fault.

The term "virtual inertia" is also often used too loosely. The inertia of a synchronous machine comes from stored kinetic energy. A converter can replicate a similar frequency response. For this, however, it needs available energy and a control that releases or absorbs this energy at the right moment. A battery can do this as long as the state of charge and the power electronics allow it. This is why the German procurement concept requires that storage systems can actually discharge at least the contractually committed quantity of instantaneous reserve.

Regulatory status in Germany and the EU

Germany: With decision BK6-23-010 of 22 April 2025, the Bundesnetzagentur (Federal Network Agency) determined that the transmission system operators must procure instantaneous reserve on a mandatory, market-based basis. The legal basis is Section 12h EnWG (German Energy Industry Act). At the same time, the authority repealed the exemption from market-based procurement that it had determined in December 2020. The decision contains a procurement concept with a product definition, a contracting system and verification obligations. For the technical requirements, it refers to the VDE FNN guideline on grid-forming properties.

VDE FNN published this guideline as final version 2.0 on 15 May 2025. It sets out requirements and verification for grid-forming units in grid levels 1 to 5, that is, from the extra-high-voltage grid to the transformation to medium voltage. It applies to generation units, combined generation and storage units, storage systems and controllable consumption units. Together with a supplementary sheet to FGW TR8, it forms the basis for certification.

On 22 January 2026, the four German transmission system operators launched market-based procurement. A fixed-price system applies at the start. According to the grid operators, converter-based installations in particular, such as battery storage systems, but also wind and solar installations, are expected to provide instantaneous reserve if they are designed to be grid-forming. Instantaneous reserve in Germany is thus a remunerated ancillary service. As a result, grid-forming capabilities have a direct market value for the first time.

EU: The applicable network code on requirements for grid connection of generators (Regulation (EU) 2016/631, NC RfG) is being revised. According to ENTSO-E, the draft "NC RfG 2.0" contains grid-forming requirements for non-synchronous power generation and storage modules for the first time. These are framed as non-exhaustive requirements that Member States and grid operators specify at national level. In November 2025, ENTSO-E published a non-binding technical report on this (Phase II). It is intended to serve as the basis for a later implementation guidance document.

The European Commission presented the draft of the revised regulation for public consultation on 8 July 2026, which ran until 25 August 2026. As long as the legal act has not been adopted and implemented nationally, EU law does not impose a general obligation to build new installations as grid-forming. Thresholds, installation types and transition periods are thus not yet finally determined.

Market, grid connection and ancillary services

Electricity markets mainly remunerate energy in megawatt-hours and, in part, firm capacity or balancing energy. However, grid stability also rests on properties that do not appear as a quantity of energy: instantaneous reserve, voltage support, black start capability, dynamic stabilisation and fault behaviour. The German instantaneous reserve market now reflects one of these properties. Others, such as short-circuit power or behaviour in parallel with other grid-forming installations, have so far been captured by the market only partly or not at all.

If such capabilities become technically necessary, they must be described concretely in grid connection rules, tenders or markets for ancillary services. Merely demanding "grid-forming behaviour" is not enough. Only concrete questions and parameters can be tested:

  • What voltage can the converter hold?

  • How does it react to a frequency change?

  • What current limitation applies during faults?

  • Can it start without an existing grid?

  • How does it behave in parallel with other grid-forming installations?

  • What communication is necessary, and which functions must remain locally available even without communication?

The responsibilities are not trivial here. Transmission system operators are responsible for the stability of the overall system and for frequency control. This is why they also procure instantaneous reserve, since frequency deviations act system-wide. Distribution system operators have to manage local voltage problems, grid bottlenecks and protection concepts. Plant operators invest in hardware and control, but only have an incentive if the service is required or remunerated. Manufacturers need uniform technical standards so that devices do not have to be redesigned for each grid operator. Conflicts arise where technical possibility, market rule and political responsibility diverge.

Role in a converter-dominated power system

A power system fed predominantly via converters can be operated stably if the properties of the conventional system that were previously tacitly present are explicitly planned for. Grid-forming inverters are a central tool for this, but not a cure-all. They work together with grid expansion, protection technology, control reserves, storage, load flexibility, synchronous generators, rotating phase shifters (synchronous condensers) and suitable operating rules.

The term makes a shift visible: electricity generation and grid stability no longer automatically coincide in the same installation. A wind farm can deliver energy without providing all stability functions. A battery storage system can support stability even if it delivers little net energy over the course of the day. A grid operator may need local voltage control even though there is no energy shortage on the electricity market. Installations are thus assessed less by their feed-in quantity and more by their electrical function in the grid.

A grid-forming inverter is therefore not simply a more powerful inverter. It is a piece of equipment that can take on a reference function in the power system. Its significance lies in actively supporting voltage and frequency under defined conditions, without relying entirely on an existing stable grid voltage. Whether it fulfils this role is decided by concrete design, control, testing and integration into grid operation.

Sources

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Edited by eCMS on 12.11.2026 05:05

© 2026 Erik Schmidt. All rights reserved. Quotations with a link to the source are welcome.