The Iberian Peninsula blackout of April 2025, which affected both Spain and Portugal, was a significant event that exposed critical vulnerabilities in modern power systems dominated by inverter-based resources (IBRs). Preliminary reports from Red Eléctrica and the Spanish government, along with independent analysis, reveal a complex interplay of grid conditions, operational decisions, and system limitations that led to a full regional blackout.

Key Drivers and Sequence of Events

The blackout was preceded by elevated voltage conditions and inter-area oscillations, exacerbated by the fixed power factor operation of IBRs and delayed implementation of voltage control protocols. On the day of the event, multiple 400 kV transmission lines were offline for maintenance, and synchronous generators were scheduled for reactive power support. However, one critical generator became unavailable and was not replaced, weakening the system’s reactive power reserves.

Oscillations intensified throughout the morning, culminating in a severe 0.6 Hz event that triggered voltage swings of up to 8%. Mitigation efforts (including switching transmission lines, adjusting HVDC operations, and curtailing market flows) were insufficient to stabilize the grid. Rapid voltage changes propagated through the distribution network, likely causing DERs to trip and increasing system load unexpectedly.

Cascading Failures and System Collapse

Initial plant tripping began around 12:32 PM, resulting in the loss of over 2 GW of generation and substantial reactive power. The synchronous generation fleet was unable to compensate for the reactive power deficit, leading to further voltage elevation and additional plant outages. System defense mechanisms, including under-frequency load shedding and pump storage tripping, failed to arrest the frequency decline. Weak interconnection with France led to a loss of synchronism, and the Spain-France AC lines tripped, isolating the Iberian grid and accelerating the collapse.

Restoration and Recovery

Despite the scale of the blackout, restoration efforts were swift. Portugal restored its grid within 12 hours using hydro and gas resources, while Spain achieved full recovery in under 23 hours with support from France, Morocco, and internal hydro generation.

Lessons Learned and Strategic Implications

The event underscores the urgent need for enhanced voltage control strategies, including dynamic reactive power planning and better utilization of IBR capabilities. It also highlights gaps in protection coordination, particularly between transmission and distribution systems, and the limitations of current oscillation management protocols under stressed conditions.

From a planning and operations perspective, the blackout reveals the importance of:

• Real-time sensitivity analysis of voltage impacts from asset tripping
• Coordinated system defense plans that address both under- and over-voltage scenarios
• Improved training and communication protocols for operators during emergencies
• Integration of market dynamics into reliability assessments, especially under high IBR penetration