Origin of the 0.63 Hz forced oscillation in southwestern Iberia

Determine the physical source and location responsible for the 0.63 Hz forced oscillation observed between 12:03 and 12:08 CET on April 28, 2025 in southwestern Iberia, which produced up to 30 kV voltage swings at certain 400 kV substations and excited the 0.21 Hz East–Centre–West inter-area mode; ascertain whether it originated at a photovoltaic plant in Badajoz or elsewhere.

Background

The paper documents that a forced oscillation with a dominant frequency of 0.63 Hz emerged between 12:03 and 12:08 in southwestern Iberia, producing significant voltage swings and exciting an inter-area mode. Although Red Eléctrica de España (REE) suggests a possible origin at a PV plant in Badajoz, the ENTSO-E Expert Panel did not confirm the cause.

This uncertainty matters because these oscillations shaped operator actions (meshing, reactor switching, HVDC mode changes) that affected system voltage control. Establishing the oscillation’s origin would clarify causal pathways and inform mitigation strategies for similar scenarios.

References

The origin of the 0.63 Hz oscillation remains unresolved (at the time of writing in November 2025), REE characterizes it as a forced oscillation possibly originating at a PV plant in Badajoz, whereas the ENTSO-E Expert Panel did not confirm the cause.

— 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 2 (The Iberian Blackout: Event Sequence)