On April 28, 2025, the Iberian Peninsula experienced Europe's largest recorded power blackout, plunging millions in Spain and Portugal into darkness after a rapid cascade of grid failures. The incident, which began shortly after noon, saw significant grid oscillations escalate over minutes, leading to widespread system collapse. Initial investigations point to challenges in reactive power management and the behavior of grid-following solar photovoltaic (PV) inverters as primary contributors to the destabilization.

Grid operators scrambled to restore power, which gradually returned over approximately ten hours. Subsequent official reports, including a detailed analysis from Spain’s Transmission System Operator (TSO) Red Eléctrica (REE) and early findings from Germany’s Friedrich-Alexander-Universität (FAU), have shed light on the complex sequence of events that led to the unprecedented collapse. The incident began with a 0.6 Hz oscillation repeatedly forced by a 250 MW PV solar plant in Badajoz province, followed by a 0.2 Hz oscillation after the first was stabilized. These oscillations left the grid vulnerable to low-voltage conditions.

The critical turning point occurred just after 12:32 PM when a substation in Granada tripped, potentially due to incorrect transformer tap settings unable to keep pace with rapidly changing grid conditions. This triggered a cascade, as more substations, solar, and wind farms went offline, primarily due to a loss of reactive power absorption. Conventional generators also tripped as the failure outpaced isolation attempts, ultimately leaving nearly the entire peninsula without power until HVDC and AC transmission lines at the Spain-France border disconnected.

Reactive power is essential for maintaining grid voltage stability. Unlike traditional synchronous generators that inherently provide reactive power and inertia, grid-following inverters, common in modern solar PV installations, rapidly synchronize with the grid voltage and frequency. This design means they do not inherently dampen oscillations or absorb reactive power, and can even amplify fluctuations if they over-correct. At the time of the blackout, PV solar generated nearly 60% of Spain's grid power, exacerbating the reactive power deficit and making oscillation damping exceedingly difficult.

This event is not isolated; similar grid oscillations and reactive power deficiencies contributed to the 1996 Western North America blackouts. The increasing penetration of variable renewable energy (VRE) generators, which often lack grid-stabilizing features, has steadily reduced reactive power absorption capacity in the Spanish grid. To mitigate such risks, the European Network of Transmission System Operators for Electricity (ENTSO-E) has recommended the adoption of grid-forming inverters (GFCs). These advanced inverters can actively support grid stability by providing inertia and reactive power, but they are newer and currently less economically viable to retrofit or implement broadly, especially without grid-level storage.

While the engineering solution is known, strong economic incentives currently deter the widespread adoption of GFCs. The responsibility for grid stability often falls disproportionately on TSOs, potentially delaying the necessary rollout of modern grid-forming inverters across the solar industry. As Spain and Portugal continue their transition to a renewable-heavy power mix, addressing these technical and economic hurdles is paramount to preventing future grid collapses.