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Glossary · Energy industry & energy transition

Grid-following inverter

Grid-following inverter grid-following inverter inverter converter grid-forming inverters photovoltaics grid stability

A grid-following inverter is a power-electronic device that feeds electrical energy into an existing AC grid. It aligns itself with the voltage, frequency and phase angle that it measures at the grid. The German term is "netzfolgender Umrichter"; the English term is grid-following inverter, abbreviated GFL. The device does not generate its own voltage reference. It measures the grid signal and adjusts its output current accordingly. According to the North American Electric Reliability Corporation (NERC), most grid-connected inverter-based resources today operate on this principle.

How it works: controlled current source with a PLL

Technically, a grid-following inverter consists at its core of two building blocks: a fast current controller and a phase-locked loop (PLL). The PLL continuously determines the phase angle of the grid voltage at the point of connection. It passes this angle on to the current controller. From it, the current controller generates a current positioned relative to the voltage such that the desired active power and reactive power flow. From the grid's perspective, the device therefore behaves like a controlled current source. One way to picture it: the inverter rides the wave that the grid dictates, delivering as much power as it has been set to deliver.

This design has an important consequence. Because the inverter first has to measure the voltage and then track the angle, it reacts to sudden voltage or phase jumps with a certain delay. NERC points out that a grid-following inverter therefore cannot change its active and reactive current instantaneously. If the grid voltage is lost entirely, it no longer has a reference. As a rule, it then disconnects. This is also intended, because grid connection rules usually require disconnection in the event of a grid outage, so that no unintentional islanded grid is formed.

"Grid-following" describes a control method, not an energy source. Photovoltaic systems, wind turbines, battery storage systems and converter stations of high-voltage direct current (HVDC) transmission links can be operated with grid-following inverters. The term says nothing about rated power, voltage level or the quality of the device.

Distinction from inverter and grid-forming operation

An inverter (Wechselrichter) converts direct current into alternating current. "Converter" (Umrichter) is the more general term for power electronics that transform electrical quantities, for example also between different frequencies. "Grid-following" and "grid-forming", by contrast, designate two ways of controlling such devices.

A grid-forming inverter maintains its own voltage at its terminals, in both magnitude and angle. From the grid's perspective, it behaves like a voltage source. It does not necessarily need a PLL for synchronisation. In the event of a disturbance it reacts immediately, similar to a synchronous machine, rather than only after taking a measurement. The Bundesnetzagentur (Federal Network Agency) states that the vast majority of grid-following inverters cannot provide instantaneous reserve. Grid-forming inverters, by contrast, can do so by means of what is known as synthetic inertia (a synthetic flywheel mass).

Two battery storage systems can therefore look identical and have the same power rating, yet behave very differently in the grid. One follows the grid. The other is designed to be grid-forming and, depending on its design, can support voltage and frequency or supply an islanded grid. Whether a device can operate in a grid-forming manner often depends on control software and certification, but also on the current and energy reserves of the hardware.

What grid-following inverters can do

Modern grid-following inverters are not simple feed-in devices. They can provide reactive power for voltage support and reduce their active power in the event of overfrequency. They can also ride through grid faults as part of fault ride through and inject a fast reactive current in doing so. Many grid connection rules now explicitly require such functions. However, these functions rely on measurement and control. They therefore act with a small delay and not as a physically instantaneous reaction like that of a rotating flywheel mass.

In a grid with many synchronous machines and high short-circuit power, this principle works reliably. The early integration of photovoltaics and wind energy was therefore based almost entirely on grid-following devices.

Limits in weak grids and at high inverter shares

The short-circuit power at a grid node is the apparent power that would flow there in the event of a short circuit. It is a measure of how low-impedance, and thus how voltage-stable, the grid is at that location. If it is set in relation to the rated power of the connected installation, the result is the short circuit ratio (SCR). If the ratio is low, this is referred to as a weak grid. In that case, even the injected current noticeably changes the voltage at the point of connection. Typical cases are long connection lines, offshore connections, remote distribution grids and grid situations with few synchronous machines in operation.

In a weak grid, the PLL measures a voltage that the inverter itself already noticeably influences. This creates a feedback loop: the inverter partly follows its own echo. As a result, oscillations between the controllers can arise. In the event of faults, the inverter can also lose synchronisation and disconnect. NERC and the technical literature explicitly name PLL-based synchronisation as a stability risk at high grid impedance. If many inverters depend on the same reference, a single disturbance can affect several installations at the same time.

It does not follow from this that grid-following inverters are unsuitable. What matters is how many such installations are operated in which grid situation and which other equipment shapes the voltage. In a grid with sufficient voltage-forming capability and coordinated controllers, grid-following installations make a large contribution. Where no stable reference is available, purely grid-following operation is not sufficient.

Typical misconceptions

  • "Grid-following means unstable": This is false. In most applications the devices are proven and economical. Problems arise only when their system-level prerequisites are no longer met.

  • "Photovoltaics is inherently grid-following": Grid-following behaviour is a property of the power electronics and their control, not of the solar cell. For the primary source, what matters above all is whether reserves are available, for example a storage system or curtailed operation.

  • "Grid-forming solves everything": Grid-forming inverters also remain bound by their current limits. According to NERC, grid-forming control does not change the limited fault current, which is important for grid protection. Synchronous machines, by contrast, briefly deliver a multiple of their rated current during a short circuit.

  • "Power is power": For grid stability, the control method matters in addition to installed capacity. One gigawatt of grid-following feed-in acts differently in the system than one gigawatt of grid-forming capacity or one gigawatt of synchronous machines.

State of regulation in Germany (as of November 2026)

Grid-following inverters remain permitted in Germany and are the normal case. However, since 2026 there has been a dedicated procurement route for instantaneous reserve. In proceeding BK6-23-010, the Bundesnetzagentur issued a determination on market-based procurement. There it identifies grid-forming inverters as a prerequisite for inverter-based installations to be able to provide instantaneous reserve. The quantity is stated in MWs (megawatt seconds). According to VDE FNN, the instantaneous reserve market started on 22 January 2026. The technical requirements and proofs are set out in the VDE FNN guidance "Technical requirements for grid-forming properties, including the provision of instantaneous reserve" ("Technische Anforderungen an Netzbildende Eigenschaften inklusive der Bereitstellung von Momentanreserve"). Version 2.1 of 23 January 2026 replaced version 2.0. Purely grid-following installations therefore cannot participate in this market.

For the extra-high-voltage grid, the section revision VDE-AR-N 4130/A1 (edition 2026-06) contains requirements for grid-forming properties for the first time. For new battery energy storage systems, grid-forming properties thus become a minimum requirement. This also applies to generating units that wish to offer instantaneous reserve as an ancillary service. This obligation applies only to connections within the scope of VDE-AR-N 4130. It does not follow that grid-following battery storage systems at medium or high voltage would be generally impermissible. There, the respective technical connection rules and the requirements of the grid operator apply.

Significance for system operation and security of supply

When conventional power plants run less often, properties that were previously available as a by-product are lost: short-circuit power, inertia and robust voltage formation. Electricity from renewable sources may suffice in terms of energy balance, while individual stability functions nevertheless become scarce. Grid operators manage this through connection rules, through the procurement of ancillary services and through their own equipment. Grid-following inverters will continue to carry the bulk of feed-in. What remains open above all is what share of grid-forming capacity is needed at which grid nodes so that grid-following units can follow safely.

Sources

Related glossary entries

Edited by eCMS on 12.11.2026 05:05

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