Spain blackout probe to focus on reasons for overvoltage: Entso-e

The external investigation into the Iberian blackout in April will focus on why the first-generation units were disconnected from the system and why system defence plans were unable to halt the process that led to the full power loss, European TSO group Entso-e said June 6.

Preliminary findings from the independent report indicate that during the half hour preceding the blackout on April 28, two periods of oscillations (power and frequency swings) were observed in the Continental Europe Synchronous Area (CE SA), it said, providing extra details.

The first oscillation, some 30 minutes before the main incident, was a local one, lasting four minutes and primarily affecting the Spanish and Portuguese power systems, it said.

A second oscillation started about 15 minutes before the incident, lasting six minutes and was an inter-area oscillation corresponding to the well-known East-West Continental mode on the interconnector with France, Entso-e said.

This was "effectively mitigated through countertrading measures, which reduced power flows between France and Spain," and with a "switch from AC emulation to fixed power in the operation mode of the HVDC link and the coupling of the internal power lines in the South of Spain," it said.

The voltage was back within the range of 390–420 kV, before increasing again, but still within the operational voltage range in the transmission network.

At that time, Spain was scheduled to export 1,000 MW to France, 2,000 MW to Portugal and 800 MW to Morocco.

Full final data for April 28 and April 29 is yet to be published by grid operator REE, with the measuring equipment failing due to the power loss.

The main incident at 12:33 CEST (11.33 BST) occurred with three Spanish generation trips within seconds, accounting for 2,200 MW located in southwest Spain, the report confirmed.

"As a result of these events, a voltage increase was observed in Spain, leading to a similar increase, consequently also in Portugal."

Overvoltage triggered a cascade of generation losses, causing the frequency of the Iberian system to drop and lose synchronism with the European System.

"The automatic load shedding and System Defence Plans of Spain and Portugal were activated but were unable to prevent the collapse of the Iberian power system," the report said.

This led to the AC lines between France and Spain being disconnected by protection devices against loss of synchronism and then the HVDC lines between France and Spain stopped transmitting power.

Final report

The external expert panel is scheduled to meet again June 23 and on July 15, with a view to publishing a final report to present to the European Commission and Member States via the Electricity Coordination Group.

The governments of Spain and Portugal are carrying out separate investigations, but have not yet produced further details. The Iberian governments are appealing to France to establish firm timeframes and a binding agreement on additional interconnectors.

Cross-border capacity between France and Spain is currently 2.8 GW.

A new 2-GW Bay of Biscay link has been delayed into 2028 after an initial target date of May 2025.

Meanwhile, Portugal continues to constrain power imports from Spain during peak solar hours, which led to a decoupling of hourly prices between the closely aligned markets.

Spain's grid has come under scrutiny in the aftermath of the blackout, with media reports on June 9 linking the blackout to a previous incident a week earlier on April 22, which saw a loss of power at Spain's national rail operator and also the Cartagena refinery.

Since the April 28 blackout, grid operator Red Electrica has been carefully managing the balance of renewables supplied into the daily market.