The Iberian outage highlighted the vulnerability of modern grids with high proportions of renewable energy and reduced conventional synchronous generation.
The ENTSO-E (European Network of Transmission System Operators for Electricity) expert panel’s factual report was released on 3 October 2025, but the the root cause of the outage has yet to be fully determined and will be the subject of a further report to be published in early 2026.
Lightly loaded transmission lines, high reactive power conditions and insufficient system inertia are all among likely potential contributors that produced a rapid sequence of trips and load shedding.
Restoration required co-ordinated intervention across multiple operators and operators used black start procedures to bring generation centres back online in stages before they could be reconnected to the grid.
For power engineers, the Iberian blackout was a jarring reminder of the structural challenges facing electricity systems undergoing the transition to net zero. By viewing this event through the lens of inertia, reactive power control and grid stability, we can better understand how to anticipate and mitigate future risks.
How the blackout happened
At 12:32:57 CEST on 28 April, a 400/220 kV generation evacuation power transformer in the Granada region of Spain tripped. This transformer transfers power from several wind and solar power plants to the Spanish transmission system.
Where transmission lines are lightly loaded and a large share of generation is inverter-based, the network can experience elevated voltages and stressed reactive power balance.
The voltage on the 220 kV side reached 242.9 kV, activating an over-voltage protection relay. Crucially, this unit, and subsequent ones, were operating in reactive power consumption (absorption) mode to manage the already-high system voltage.
The loss of this reactive power absorption capacity led to an uncompensated surge in system voltage, exceeding 435 kV in some areas. This severe over-voltage then triggered a cascade of further trips. The simultaneous loss of generation and the voltage excursions induced a rapid fall in frequency, activating automatic load-shedding schemes and precipitating wider disconnection and islanding.
Publicly available operational data and technical analyses, as well as the expert panel factual report, show the following pattern. In the hour before the major outage, there were measurable inter-area/low-frequency oscillations. Generators and phasor measurement units recorded those oscillatory modes, but power system stabilisers (PSS) only damped them intermittently.
The official inquiry, in its report of June 2025, found that certain generation facilities did not provide the dynamic voltage support or reactive power absorption capability required by their respective grid codes, exacerbating the voltage excursions and forcing subsequent protection actions.
What grid conditions allowed this failure?
In Spain and Portugal on 28 April, a combination of high variable renewable energy (VRE) output and the network’s operating point created conditions with small voltage regulation margins. There are also reports that some units contracted or expected to provide dynamic voltage support did not respond as required in the crucial seconds, exacerbating the excursions and forcing the subsequent protection actions.
Understanding the broader system context is essential: the Iberian system, particularly in Spain, has a high penetration of inverter-based resources and, at times, relatively limited synchronous short-circuit strength and inertia compared with historic norms.
That reduced pool of synchronous support constricts system behaviour under disturbance. Lower inertia removes counter-balancing capacity against power frequency oscillations, shortening the time available for corrective action after a fault or a sudden trip. It also renders some traditional, generator-level stabilising mechanisms, such as synchronous condensers and static compensators, less effective unless explicitly configured to operate in the new conditions.
In summary, the technical record available today points to a composite failure: low-frequency oscillations and voltage excursions that triggered protection actions, in a system operating with high renewable share and reduced synchronous support.
Are renewables to blame?
What lessons should we take from the Iberian blackout of 2025?
Of course, very few people would argue that we should reverse course on renewable energy sources.
The progress made across Europe to increase the grid’s proportion of renewable electricity is one of the continent’s most impressive achievements in recent decades. The European Union’s Renewable Energy Directive commits to an overall energy mix with 42.5 per cent renewable energy by 2030, having reached 24.6 per cent in 2023.
However, there are structural-, operational- and equipment-level measures, relating to energy sources, inertia, excitation and reactive-power control, monitoring and protection design, that can increase system resilience.
Asynchronous vs synchronous generation
One plain fact is Spain’s reliance on asynchronous, inverter-based renewable generation. Wind and solar plants connect to the grid through power electronics, not direct electromagnetic coupling in synchronous rotating machines.
While inverters can provide some synthetic inertia and fast frequency response, they don’t inherently contribute synchronous inertia or short-circuit strength in the same way as conventional generators. This limits grids’ natural ability to damp oscillations or withstand sudden voltage swings.
Other countries benefit from a different mix. Norway, Austria and Iceland, for instance, have large proportions of synchronous renewable generation, chiefly hydro and geothermal. These resources behave akin to traditional fossil fuel units, inherently providing stabilising inertia and reactive power current. This gives operators wider margins to absorb shocks and time to intervene before disturbances escalate.
Hydro power does feature in the two Iberian peninsula countries’ energy mix: seven per cent and 13 per cent for Portugal and Spain respectively. Additionally, nuclear power represented 20 per cent of Spain’s generation capacity in 2024. However, relative to some countries, hydro resources are more limited and VRE dominates.
The challenge for TSOs and power plant managers is to replicate those stabilising functions by other means. Already, asset owners are trialling grid-forming inverters to deliver synthetic inertia and fast frequency support, and installing synchronous condensers to bolster reactive power and fault strength.
Lessons to learn to prevent future large-scale blackouts
As national and interconnected international electrical grids adopt more asynchronous renewable generation capacity, the resilience to frequency oscillations provided by inertia and short-circuit contribution will decrease. For system operators, this means living up to the same expectations of a stable and healthy grid without some traditional tools.
In this context, real-time monitoring and visibility of system operating conditions becomes even more critical. Robust SCADA monitoring and control enable response within seconds of disturbances arising. They also allow proactive management, adjusting generation dispatch, line flows or protection settings before small deviations cascade into a wider system separation.
As worsening climate change makes weather patterns more unpredictable, wind and PV solar energy generation will also become harder to forecast. Investment in advanced forecasting tools, such as AI-powered weather trend prediction solutions, can help operators bridge this gap and balance demand with supply.
Technologies to support voltage regulation and overall system stability, like the reactive power compensation equipment mentioned earlier, will be non-negotiable upgrades to electrical grids targeting greater renewable penetration as we approach the EU’s 2030 deadline.
From a synchronous machine manager’s perspective, well-maintained and optimised excitation systems that can cope with evolving grid conditions are a priority. AVRs and power-system stabilisers are essential components to dampen fluctuations in voltage and power frequency, thereby contributing to a more
stable grid.
Ultimately, the power transmission and distribution community must recognise the continuing value of synchronous generation. Established electrical grids operate under assumptions and conditions originating from their fossil fired beginnings, that cannot be ignored.
While compensatory technology can help maintain stability, the phenomena of inertia and reactive power haven’t lost their importance. In fact, the future of our networks depend on resilient, well-monitored electrical infrastructure.
