Root causes of Iberian blackout | Update
Regarding conventional synchronous power plants, the analysis shows that the reactive power output of several generators reached Q-reference for less than 75% of measurement samples. Under Operational Procedure 7.4 (PO 7.4), which includes requirements for reactive power provision by synchronous generators, samples of key variables, including reactive power, must be checked every 5 minutes to verify that generators are fulfilling their obligations. Reactive power output was thus not aligned with TSO expectations based on Operational Procedure 7.4.
Operational Procedure 7.4 did not include explicit specifications concerning dynamic behaviour, and there were no financial consequences if the requirements regarding voltage control were not met.
The voltage control design of some local generation networks (extensive grids behind the transmission grid connection point, shared by several generation portfolios) was not aligned with system needs. This also contributed to some of these facilities disconnecting when the voltage at the connection point was within limits. Due to Spain’s wider operating voltage range for the 400 kV grid compared to the rest of Europe, the margin between the allowed operating voltage limit and the voltage at which generators are allowed to disconnect was very small or non-existent.
Together, the reactive power assets in the system were not able to address the sudden voltage rise. The Expert Panel also assessed the voltage fluctuations that occurred on certain days prior to 28 April 2025, in particular on 22 April. On this day, the voltage reached very high values shortly after 19:00, leading to some disconnections but not to a cascade of overvoltage disconnections, as the system conditions and configuration on that day differed from those on 28 April. In particular, the system did not experience any significant episode of oscillation, either local or inter-area, on 22 April.
Oscillations
The two episodes of oscillation that occurred during the half-hour preceding the blackout were also analysed. To analyse these phenomena, the Expert Panel developed a simulation model of the CE power system that can represent the oscillatory behaviour of the system and used available data from a large number of PMUs located in the Iberian Peninsula and in the rest of the CE synchronous area. Regarding the first oscillatory episode, at 0.63 Hz, the data analysis shows that this mode is mainly located in the Iberian Peninsula, rather than in the rest of the European system, with higher activity in a specific area of Spain between Carmona and Almaraz. It also reveals that this mode is not naturally present in the system. The Expert Panel noted that, in the immediate vicinity of the node where the highest amplitude was observed (jointly with the highest power oscillations), an inverter-based power plant is connected, classified as an existing generator under the applicable requirements in Spain. The analysis concludes that this oscillation is a converter-driven forced oscillation. The second oscillatory episode, at 0.2 Hz, was a classic inter-area oscillation of the East–Centre–West mode in the Continental Europe Synchronous Area. Specific conditions prevailing in the Iberian peninsula created the preconditions for the triggering of the oscillations: the high transmission angle (around 80° between Carmona and S. Llogaia); some lines out of service; the absence of power system stabilisers at some large units; and the insufficient damping of devices already installed. The simulations carried out also show that the 0.63 Hz oscillation mode can contribute to exciting the 0.2 Hz inter- area oscillation mode, along with other factors. The Expert Panel also assessed the effectiveness of the measures taken to dampen the oscillations. It concludes that the measures applied during the first oscillatory episode (0.63 Hz), which were not designed to damp oscillations in this frequency range, had a minor, but nevertheless positive impact on
the oscillation. On the other hand, the analysis confirms the effectiveness of the operational measures applied during the inter-area oscillation (0.2 Hz).
The analysis identified several measures to improve the damping of forced and inter- area oscillations, respectively and showed that the measures taken had a positive impact on damping oscillations.
Flows between TSO and DSO grids The Expert Panel conducted analysis specifically to identify the main influencing factors explaining changes in flows between the transmission and distribution grids, particularly after the oscillations and during the critical time period from 12:32:00. Changes in TSO–DSO flows and load result from complex interactions among various factors within the distribution and transmission grids. Changes in distribution grids (eg, activation of distribution-grid-connected units providing balancing services, including automatic frequency restoration reserves (aFRRs) and manual frequency restoration reserves (mFRRs)) can influence TSO–DSO flows, which in turn can impact voltage and regional flows across the transmission system.
There is also feedback from the transmission grid to the distribution grids, eg, voltage-dependent loads or possible disconnections of small embedded generators due to voltage variations on medium- or low- voltage DSO feeders.
The Expert Panel analysis indicates that the changes in the total TSO–DSO exchange can only be slightly linked to aFRR or mFRR activations. Based on the data provided by two PV-inverter manufacturers, the changes in total TSO–DSO flows can be partially linked to disconnections of small PV installations (< 1 MW) connected to low-voltage grids due to activation of overvoltage inverter protection. The data shows a correlation between the proportion of inverter trips and periods when the voltage rises on the transmission grid and when TSO–DSO flows increase.
Figure 3. Voltage and frequency during the 28 April incident. Source: ENTSO-E
www.modernpowersystems.com | April 2026 | 15
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45
