Grid-Forming vs. Grid-Following Inverters

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Historically the inputs to an electricity grid have been large rotating machines that inherently have a large inertia. This means that even with a large change in demand in failure of a supply element the frequency is maintained. A small change in frequency is often an indicator that a change in supply is required. The inertia in machines is large enough that there are a few seconds to react. The move to renewable sources, solar and wind, means that there is less inertia in the system. Changes in supply or load can have a faster and more significant impact on the grid frequency. In the UK the statutory limit is 49.5 Hz to 50.5 Hz. However, operationally the grid frequency is operated from 49.8 Hz to 50.2 Hz. The Spanish grid, like most global power systems, still predominantly uses grid-following inverters because they were cheaper, simpler, and “good enough” when renewables made up a small share of total generation.

Grid-Following Inverter Grid-following inverters (GFL) were designed for a strong grid: GFL inverters rely on an existing voltage waveform to synchronize. They inject power (P, Q) in response to commands but do not shape the grid frequency or voltage. They are fast, cheap, and efficient — great as long as there’s a stable grid to follow. Historically, grids were dominated by rotating machines (coal, gas, nuclear) which set the frequency and inertia. So GFL inverters just “plugged in and played.”

Grid-Forming Inverter Grid-forming inverters (GFM) offer a solution — but come with challenges: GFM inverters can create their own voltage waveform, acting like a “virtual synchronous machine.” They can set frequency, provide inertia, and stabilize weak grids — exactly what’s missing in high-renewables systems. But: They are more complex and costly (for now). Require careful tuning and may need grid-wide coordination. Still not mandated by most grid codes (though this is changing).

As renewables grew, the grid got “weaker” More GFL inverters + fewer synchronous machines = less inertia, weaker frequency control, and less damping. GFL inverters can cause control instability if they rely on distorted signals, especially during faults or oscillations. Spain’s blackout is a textbook example: There are only 2 reasons for blackouts.

1. Natural Disasters including Extreme Weather Events (EWEs). 2. Human/Operator error, including lack of maintenance.

Blackouts occur when a grid is damaged or mismanaged. Reason 1, 2, or a combination. They are the only reasons the Iberian Blackout and every other grid blackout has happened or will ever happen. We already know the reasons for the Iberian Blackout [4] 👉 A grid-following PV plant “latched onto” a distorted signal and amplified it, rather than stabilizing the grid. Why Spain (and others) didn’t move earlier: Spain was early to adopt solar and wind, starting in the 2000s–2010s. At the time, GFL inverters were standard, and the grid was strong enough. Retrofitting to GFM is expensive and not yet legally required for older plants. Today, grid codes in places like Australia, Ireland, Germany, and the UK are shifting: They mandate ride-through Require grid support functions Some are now requiring or incentivizing GFM behaviour Spain is likely to follow after this event. Summary Reason Why Grid-Following Dominated: 🧱 Simplicity: Plug-and-play design for strong grids 💰 Cost: Cheaper and easier to commission 🔙 Legacy: Installed before inverter instability was an issue ⚖️ Policy lag: Grid codes didn’t require GFM 🧠 Mindset: GFM was seen as niche or futuristic References: Blackout in Spanish Peninsular Electrical System the 28th of April 2025, Spanish TSO (Red Eléctrica) Report Reporte del Comité para el análisis de la crisis eléctrica del 28A, Spanish Government Report Inertia and the Power Grid: A Guide Without the Spin, NREL We already know the reasons for the Iberian Blackout, John Noonan, LinkedIn Whatsapp reddit Email Copy Share Post Share Threads Bluesky Pinterest

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Grid-Forming vs. Grid-Following Inverters

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