Update |Iberian blackout
investigating the root causes of the Iberian blackout
It’s complicated:
On 15 July, the Expert Panel* set up by ENTSO-E (European Network of Transmission System Operators for Electricity) to investigate the 28 April Iberian blackout updated its findings, and on 18 July participated in a dedicated workshop that provided “an opportunity for the Expert Panel to engage with European stakeholders”
Despite initial challenges in collecting data from several DSOs and generation companies in Spain, the Expert Panel was able to confirm that all parties have now provided data. Based on the data available so far, the Expert Panel said it was able to offer additional information on a number of issues, such as a more detailed description of the forced oscillations (ie, oscillations arising from an external force) observed around 12:05 pm in southern Spain and of the counter-measures taken to mitigate them. The Expert Panel also offered new preliminary information related to the generation trips that took place just before the blackout.
The Expert Panel is looking particularly at the cascading series of generation disconnections and voltage increases as the most probable trigger for the blackout. Such cascading voltage increases have never before been linked to a blackout in any part of the European power system, the Expert Panel notes. If confirmed, this high-voltage blackout mode will require a thorough analysis and investigation by all power system experts of the ENTSO-E community, the Panel asserts.
The exceptional character of the incident highlights the need to improve the resilience of the power system in case of cascading voltage increases, the Expert Panel argues, and ENTSO-E sees at least two important lines to consider: enhancement of voltage control management procedures and capabilities of all active actors of the electricity system to prevent such major voltage-related incidents in the future; assessment of how the system defence plans
440 430 420 410 400 390 380
Upper voltage limit, Spain
Upper voltage limit, Portugal
12:00:00 12:05:00
12:10:00
12:15:00
12:20:00
12:25:00
12:30:00
Voltage evolution from 12:00 to 12:30 CEST in the main 400 kV transmission substations in Spain and Portugal. Source of data: Red Eléctrica and REN telemetry. Image: ENTSO-E
can better protect the European power system against this new type of phenomenon.
System conditions prior to the blackout
During the night of 27 to 28 April, the Iberian power system operated normally, without notable variations in the voltage profile. From approximately 09:00 CEST, the variability of the voltage in Spain started increasing but without significant variations until 10:30 CEST. From 10:30 CEST, more significant voltage variations started to occur. But the voltage in the transmission system remained below the upper operational limits.** During the half hour preceding the blackout (12:00 to 12:30 CEST), two main periods of oscillations (power, voltage and frequency swings) were observed in the Continental Europe Synchronous Area (CE SA).
The first one took place from 12:03 to 12:07 CEST.
A preliminary analysis of the available information, says the Expert Panel, indicates that this was a local, forced oscillation (ie, induced by an external source, namely a power plant), with a dominant frequency of 0.64 Hz, primarily affecting the Spanish and Portuguese power systems. The forced oscillation also excited an inter-area mode East–Centre–West oscillation (0.21 Hz) at small amplitude.
Mode shapes (a technique applied to WAMS measurements that is able to show the “direction” of an oscillatory mode) applied to this first oscillation suggest that this was a local mode between two generation clusters in the Iberian Peninsula system, one in the north-east part of Spain and another in the south-west of Spain and Portugal.
*The joint Expert Panel was set up two weeks after the 28 April Iberian blackout. It was set up in line with Article 15(5) of Commission Regulation (EU) 2017/1485 methodology is the framework for reporting and classifying incidents in the power system and for organising the investigation of such incidents. The Expert Panel consists of representatives from TSOs, the Agency for the Cooperation of Energy Regulators (ACER), National Regulatory Authorities (NRAs), and Regional Coordination Centres (RCCs).
Operational Procedure 1.4: “Voltages at the nodes - under normal operating conditions, the voltage at the 400 kV level at the connection nodes will be between 390 and 420 kV. At the 220 kV level, the voltage will be between 205 and 245 kV. Eventually, maximum values of up to 435 kV and minimum values of up to 375 kV may occur at the 400 kV level. At the 220 kV level, voltages may eventually drop to 200 kV. Any installation directly connected to the transmission network must be able to withstand these values without damage or disconnection.”
8 | July/August 2025|
www.modernpowersystems.com
kV
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