⚡ What Is a Grid-Forming Inverter?

Posted Jul 27, 2025 Updated Jul 28, 2025

By The BESS Toolbox

3 min read

⚡ What Is a Grid-Forming Inverter?

📌 TL;DR

Grid-forming inverters (GFM) operate as voltage sources
They set local voltage and frequency references
Enable islanded operation and often black start capability

Introduction

As power systems transition to high density of inverter-based resources (IBRs), maintaining stability without synchronous machines becomes a core challenge.
Grid-forming inverters (GFMs) are emerging as a key technology to meet this need. This article explains what they are, how they differ from grid-following inverters, and their role in the evolving grid.

What Is a Grid-Forming Inverter?

A grid-forming inverter is a power electronic device that establishes and regulates local voltage and frequency—similar to the behavior of a synchronous generator.
Unlike traditional inverters, it does not rely on an external voltage reference. Instead, it defines grid conditions at its connection point.

GFM inverters are capable of:

Self-establishing voltage and frequency
Stabilizing weak or islanded grids
Enabling black start (when designed accordingly)
Supporting system restoration after faults
Operating independently of synchronous generation

“GFM IBR controls maintain an internal voltage phasor that is constant or nearly constant in the sub-transient to transient time frame.” — UNIFI/NERC definition, as cited by NREL (2024)

Grid-Forming vs. Grid-Following

Characteristic	Grid-Forming (GFM)	Grid-Following (GFL)
Control mode	Voltage source	Current source
Synchronization	Self-synchronizing	PLL-based (follows grid)
Operation in weak grids	✅ Stable	❌ Typically unstable
Islanded operation	✅ Supported	❌ Not possible
Response to disturbances	Fast (voltage control-based)	Slower (PLL tracking)
Black start capability	✅ If designed for it	❌ Not supported
Synthetic Inertia capability	✅ Yes	❌ No

“GFM inverters emulate voltage source behavior and can support fault current, inertia, and frequency stabilization, even in the absence of synchronous machines” — ENTSO-E, 2020 Technical Report

Why Grid-Forming Matters

High penetration of IBRs introduces new system-level challenges:

Low inertia
Limited fault current
Reduced voltage stability

Grid-forming inverters help mitigate these by:

Providing synthetic inertia
Acting as reference sources in low-inertia systems
Enabling islanding, black start, and system restoration
Supporting fault ride-through and reactive power services

“Without grid-forming capabilities, achieving system stability with very high shares of inverter-based resources is not possible.” — ENTSO-E

Real-World Adoption

Kauai, Hawaii operates with up to 90% inverter-based generation, stabilized using grid-forming battery systems
National Grid ESO (GB) has implemented GFM specifications (GC0137) and a Best Practice Guide for new connecting assets
Nordic TSOs are jointly developing GFM requirements to address declining inertia and cross-border system strength

“GFM inverters allow converters to operate synchronously and instantaneously respond to disturbances, supporting the system like traditional synchronous machines.” — National Grid ESO & IEEE Power & Energy Magazine, 2025.

Summary for BESS Professionals

Grid-following inverters inject current in response to an external voltage signal
Grid-forming inverters generate their own voltage and frequency references
For BESS applications, GFM is essential to enable black start, energization, and operation in weak or islanded networks

GFM is more than a control mode. It is a prerequisite for BESS to operate as active grid participants in high-IBR systems.

References

BESStology, Grid-forming Inverter

grid-forming blackstart grid-following IBR

This post is licensed under CC BY 4.0 by the author.