Update | Root causes of Iberian blackout System defence plans
The system defence plans of Spain and Portugal were automatically triggered by the drop in frequency in the final stages leading up to the blackout. They were unable to prevent the system’s collapse, despite the fact that their activation proceeded as designed. Analysis of the system defence plans assessed several areas and factors using models suitable for analysing frequency and voltage stability phenomena, respectively. The frequency stability analysis included: simulation of a perfectly executed load shedding and pumped hydro plant disconnection; the effect of inertia on frequency variation; and the role of HVDC power reversal mode.
In particular, the analysis of the impact of inertia indicates that, even with significantly higher inertia values, the loss of system synchronism would not have been avoided, considering the sequence of events. This can be explained by the rapid reduction in synchronising torque in the Iberian Peninsula (due to cascading generator tripping) that led the system to rapidly reach the point of no return. Furthermore, analysis shows that the rapid intervention on the HVDC interconnection between France and Spain aimed at restoring the Iberian Peninsula’s system balance could have worsened system behaviour due to fast transient phenomena.
After validation of the simulation model, the analysis assessed sensitivity regarding the following aspects: the effect of different parameters (cross-border exchanges, TSO/DSO exchanges, generation changes, connecting transmission lines, connecting/disconnecting shunt reactors) on voltage variations; the role of voltage regulation via conventional generators; the role of voltage regulation by additional synchronous condensers; and the role of the defence system.
The simulations demonstrated that, at the time of the incident, the voltage of the Spanish power system was sensitive to any variations – whether related to exchanges
with Portugal, France or Morocco, TSO- to-DSO flows, or generation – resulting in significant voltage fluctuations. The analyses clearly indicated that the key phenomenon in the incident was the non-effectiveness of voltage control within the Spanish power system. Simulations show that increased reactive power margins could have prevented system collapse, enabling it to operate at lower voltage levels and maintain overall system stability.
The Expert Panel conducted a detailed analysis of the performance of the Low Frequency Demand Disconnection (LFDD) scheme. The goal was to verify the load shed at each step, and to identify the exact instant of the trips, and the frequency value at the moment of pickup and tripping of the load shedding relays. Overall, this analysis concluded that the performance of the LFDD scheme at transmission and distribution level was in line with the applicable requirements and in accordance with the design.
Management of voltage-related alarms Given the numerous voltage fluctuations that preceded the blackout, the Expert Panel considered it useful to analyse the voltage-related alarms recorded by the TSOs (in Spain, Portugal and France), DSOs (in Spain and Portugal) and generation control centres (in Spain) in the half-hour preceding the incident, and how they were handled. The analysis indicates that the distribution of alarms recorded by Red Eléctrica exhibited zonal clustering, with the southern and eastern zones repeatedly experiencing the most undervoltage alarms, while the interior, southern and northern zones recorded the majority of overvoltage alarms. The remedial actions taken within the control rooms (coupling and decoupling of reactors, opening and closing of lines, setting HVDC voltage setpoints) were effective in bringing the voltage back under the alarm threshold within minutes in most cases (typically less than 5 minutes). In the last overvoltage episode that
The role of renewables
“It’s not about renewables, it’s about voltage control… There is nothing in the recommendations that cannot be implemented tomorrow,” said ENTSO-E president Damián Cortinas during presentation of the Expert Panel’s final report.
The report clearly demonstrates the complexity of running a modern electricity grid, said Solar Power Europe, and should be used as an opportunity for learning.
“The ENTSO-E full report into the root causes of the Iberian Blackout provides greater clarity after months of unhelpful speculation and rumour. We caution against the allocation of blame in the aftermath of any future blackout events,” commented a grouping of renewables associations including Solar Power Europe.
“Solar and wind are now the largest source of electricity in the EU and have the technical capabilities to provide voltage stability. On 12 June 2025, Spain updated Operational Procedure 7.4 to enable renewables to contribute to voltage control. The full implementation of this procedure was completed on 17 March 2026 and will allow a more robust system that is better prepared to prevent this type of incident.”
preceded the system collapse, Red Eléctrica’s dispatchers were unable to implement manual remedial actions due to the rapid development of the incident.
Root cause tree
The analyses enabled the Expert Panel to develop a root cause tree of the incident (a simplified version of which is shown on pp 18-19). The purpose of the root cause tree is to explain, in schematic and simplified form, how the incident unfolded as a result of a combination of key factors, as well as the causal relationships between the various elements. From the tree, it can be seen that the fast voltage increase, which led to the cascade of overvoltage disconnections in Spain and ultimately to the blackout, occurred due to the
combination of numerous factors: ● reactive power output of several conventional generators reached Q-reference in less than 75 % of measurement samples (contravening the 75% rule);
● the regulatory framework for conventional generators’ reactive power output did not include specifications for dynamic behaviour and there were no financial consequences if the reactive power output was not aligned with the 75 % rule;
● renewable power plants operated under a fixed power factor regime;
● the design of voltage control of local generation networks (behind connection point) is not aligned with system needs;
● there was no limitation on ramping for generators with fixed power factor;
● shunt reactors are operated manually, requiring decision-making and processing time;
● according to data provided and estimations performed, many overvoltage disconnection protection settings diverged from applicable requirements or were not aligned with system needs;
● two oscillatory episodes occurred in the half-hour preceding the blackout: first, a (forced) oscillation of 0.63 Hz, and second, an (inter-area) oscillation of 0.2 Hz - the power system experienced converter-driven instability that interacted with other generators in the same area;
- there was an absence of power system stabilisers at some large units and insufficient damping by the existing ones;
● a certain proportion of small PV units (< 1 MW) experienced voltage-related disconnections via activation of inverter protection;
● the Spanish 400 kV grid is operated at a wider voltage range than in other EU countries, enabled by specific provisions applicable to Spain;
● the system entered an operating point where, by design, its defence plan was unable to interrupt the cascade of overvoltage disconnections and prevent the total collapse of the Spanish and Portuguese power systems.
16 | April 2026 |
www.modernpowersystems.com
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