f97.beERIK SCHMIDT’S WORKSHOP
← All terms

Glossary · Energy industry & energy transition

Grid-following Inverter

Grid-following inverter inverter power electronics electricity grid frequency voltage renewable energies grid stability grid connection Grid-forming inverter

A grid-following inverter, in German "netzfolgender Wechselrichter", needs an existing AC grid as a reference in order to operate. It measures the grid voltage, derives frequency and phase angle from it and then feeds in a current that matches this grid signal. The term does not describe an energy source but a control mode of power electronics. An inverter in a photovoltaic system, in a battery storage system, in many wind turbines or in industrial converters can operate in grid-following mode. Voltage and frequency of the grid are regulated by other equipment outside the device.

How it works

Externally, a grid-following inverter behaves like a controlled current source and not like a voltage source. The control usually has several levels. A phase-locked loop (PLL) continuously estimates the phase angle and frequency from the voltage at the connection point. Using this angle as a reference, the control converts the setpoints for active power and reactive power into two current components. A fast current controller sets these currents. In storage systems and photovoltaics, a controller for the voltage in the DC link is usually added. An analogy: the inverter does not set the beat itself but listens closely to the music of the grid and adapts its steps to it.

There is no separate unit of measurement for "grid-following". For specialists, what matters are the rated power in kilowatts or megawatts, the voltage level, the permissible frequency and voltage ranges, the response times of the control and the strength of the grid node. This strength is often described by the short circuit ratio (SCR). It relates the short-circuit power at the connection point to the rated power of the connected plant. The smaller this value, the more strongly the measured voltage fluctuates with the injected current. It then becomes harder for the PLL and current control to maintain a stable and unambiguous reference.

Distinction from grid-forming inverters

The most important neighbouring term is the grid-forming inverter. It controls its output voltage itself in magnitude, frequency and phase and behaves externally like a voltage source. Many definitions require that it does not need a PLL as its main reference. In the technical literature, the grid-following inverter is therefore also described as "voltage-following and current-shaping", the grid-forming inverter as "voltage-shaping and current-following". The decisive factor is thus the reference quantity: if the inverter follows an external voltage, it operates in grid-following mode. If it provides the voltage reference itself, it operates in grid-forming mode.

This results in a hard limit: if the grid voltage disappears completely, the PLL no longer finds a phase angle. A grid-following inverter can then neither establish an island grid nor support a black start. Black-start capability requires grid-forming plants or synchronous machines. To be distinguished from this is the term "grid-connected inverter". It only says that a device is connected to the grid and can be grid-following, grid-forming or a hybrid. "Smart inverter" is also not a technical distinction. The term usually refers to extended communication and support functions and says nothing about the reference quantity of the control.

Why this operating mode worked well for a long time

In the European interconnected grid, large synchronous generators shaped voltage and frequency for decades. Their rotating masses, excitation systems and electromagnetic properties provided a robust reference. In such a strong grid, it is technically obvious and inexpensive for a photovoltaic plant or a storage system to measure the existing signal and align its current with it. Most inverter-based plants operated today work in grid-following mode. According to North American expert bodies, these include the common wind and photovoltaic plants, distributed generators, most battery storage systems and many HVDC and STATCOM installations.

Connection rules have also evolved. Previously, many small plants were supposed to disconnect from the grid in the event of frequency or voltage deviations. Today, grid connection rules, such as Regulation (EU) 2016/631 (Requirements for Generators, RfG) for power-generating facilities, require, depending on the plant type, among other things fault ride-through, frequency-dependent power adjustment and contributions to voltage control.

Limits with a high share of converters

When more and more power is fed in via power electronics and fewer synchronous generators are on the grid, the situation changes. There is then less equipment that sets voltage and frequency independently and with physical inertia. The individual grid-following inverter remains fast and easy to control. But the question shifts to the stability of the common reference that many devices follow at the same time. In the event of a voltage dip, a phase jump or a rapid frequency change, the PLL can briefly lose its reference. Expert bodies in North America have described such cases in disturbance analyses as the cause of unexpected power reductions or disconnections. In weak grids, PLL and current controllers can also interact with the grid impedance and with neighbouring converters and trigger oscillations. This is a topic of small-signal stability.

Another point is the fault current. In a short circuit, a synchronous generator briefly delivers a multiple of its rated current. Semiconductor converters, by contrast, limit their current to protect the components, typically to values close to the rated current. This applies equally to grid-following and grid-forming devices. For protection schemes that rely on high short-circuit currents, this is an important boundary condition.

Typical misunderstandings

  • "Grid-following means unstable": This is imprecise. In strong grids, grid-following inverters operate reliably and have made the integration of large amounts of renewable feed-in possible in the first place. Problems arise mainly when the grid environment no longer matches the assumptions for which the control was designed.

  • "Grid-following means passive": This is wrong. Grid-following devices can limit active power, control reactive power, adjust power in response to frequency deviations and ride through faults. Their limit is not a lack of controllability but the dependence on an external voltage reference.

  • "Fast frequency support replaces inertia": This is only partly true. Grid-following inverters can deliver a fast frequency-dependent power response if energy is available. However, they must first measure the frequency change and therefore react with a certain delay. They do not replicate in the same way the immediately acting instantaneous reserve of a synchronous machine, that is, its contribution to system inertia. Grid-forming controls come closer to this.

  • "A lot of installed capacity means a lot of support": Megawatts alone say little. Control behaviour, fault current contribution, grid strength at the grid connection point and the protection scheme also matter. A megawatt at a strong substation has a different effect on the system than a megawatt on a weak rural feeder.

Regulatory classification in the EU (as of November 2026)

In the EU, Regulation (EU) 2016/631 currently continues to apply to the grid connection of generating plants. It does not contain a general obligation for grid-forming capability. On 8 July 2026, the European Commission published the draft of a successor regulation, the so-called RfG 2.0. It builds on the ACER recommendation of 2023, expressly includes storage and provides for requirements for grid-forming capabilities. The public consultation ran until 25 August 2026. Whether an obligation will come, and with which thresholds, transition periods and plant classes, will only be determined by the finally adopted version. This is followed by implementation in national connection rules, in Germany for example via the technical connection rules of the grid operators. For existing plants and ongoing projects, the previous requirements apply until then.

Significance for grid operators and plant operators

Whether an inverter is operated in grid-following or grid-forming mode is not decided by the device alone. Connection rules, standards, certification and remuneration determine which functions are required and paid for. Transmission system operators and distribution system operators must assess how much grid-following power a grid area can absorb stably, which protection schemes are suitable and where grid-forming equipment or synchronous machines are needed. This becomes particularly clear with battery storage systems. A storage system can often switch between operating modes by software. However, grid-forming operation also requires reserves in current-carrying capacity and energy content. The hardware must therefore be designed appropriately. The grid-following inverter thus remains a key term for a converter-dominated grid: it feeds in very precisely as long as a reliable grid reference is available. The greater its share, the more carefully grid strength, control coordination and the provision of grid-forming functions must be planned.

Sources

Related glossary entries

Edited by eCMS on 12.11.2026 05:05

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