Unknown trip causes and oscillation triggers during the cascade

Identify the specific causes of multiple generation trip events and determine the triggers of the oscillations that occurred during the April 28, 2025 Iberian incident, resolving which plant protections and system conditions initiated each disconnection and which mechanisms excited the inter-area oscillations observed prior to the collapse.

Background

Although the paper explains many disconnections via collector-side overvoltage behind lagged tap changers, the authors note that several trip causes and oscillation triggers remain unknown and that the incident is under active investigation.

Clarifying these unknowns would improve understanding of the cascade’s initiation and progression, inform protection coordination at collector substations, and refine models for oscillation detection and mitigation in high-IBR conditions.

References

While some of these actions may have decreased the system's ability to absorb reactive power, further raising voltages and narrowing voltage control margins, it remains unclear whether a different sequence of actions would have avoided a blackout, particularly given the low share of synchronous machines and their inadequate voltage regulation, and the fact that several trip causes and oscillation triggers remain unknown and the reports characterize the event as a multifactor overvoltage cascade under active investigation.

— The Iberian Blackout: A Black Swan or a Gray Rhino? A Thorough Power System Analysis  (2511.17433 - Albustami et al., 21 Nov 2025) in Section 6 (Replication Results: Cascade Onset and Progression)