It’s been a year since the power system on the entire Iberian peninsula suffered a voltage collapse that left Spain and Portugal and their near 60 million population without electricity. The last supplies were only restored after around 17 hours with ENTSO-E describing it as “the most severe and unprecedented blackout that has occurred in Europe in the past 20 years, with a major impact on citizens and society”.
It’s worth recalling that, contrary to what some commentators and some ‘armchair engineers’ might have you believe, the collapse was not caused by having too much renewable energy or by a lack of system inertia.
An expert panel convened by ENTSO-E took many months to conduct a detailed review of what happened and what caused it. They published a very detailed final report on March 20th this year. Some of the key conclusions included:
The main overall conclusions were: “the blackout resulted from a combination of many interacting factors, including oscillations, gaps in voltage and reactive power control, differences in voltage regulation practices, rapid output reductions and generator disconnections in Spain, and uneven stabilisation capabilities. These factors led to fast voltage increases and cascading generation disconnections, resulting in the blackout in continental Spain and Portugal.”
The Iberian blackout was a complex event with multiple strands. In short, it was an event triggered by high system voltages complicated by the presence of system oscillations. High voltages are most common when the transmission network is lightly loaded, something to be expected on a sunny day where high levels of solar generation connected at distribution level effectively soak up demand locally and so reduce the amount seen at the transmission level.
The set of voltage controls available to the system operator from the fleet of generators operating at the time were later shown to be inadequate and potentially contributed to some of the challenges seen with system oscillations. Managing voltage perturbations and system oscillations are well established challenges, present in varying forms long before the proliferation of renewables.
It is the role of system operators to put in place the necessary measures and processes to predict and manage such conditions. However, they also need the support of regulators in approving codes and standards that govern the relationships between the system operator and the myriad companies that own plant that is connected to the network. These documents ensure that their equipment has the right capabilities and behaves in ways that contribute to system stability rather than putting it at risk. The Iberian blackout was effectively the result of a failure of some of those systems and processes to manage conditions which were challenging but should not have been unexpected or unmanageable.
What were the key failures?
One of the failure points was allowing renewable generators to connect to the Spanish system without mandating that they control their reactive power output to provide the very voltage support that would have helped manage rising voltages on the day [1]. The renewables are capable of providing that type of system service, they just weren’t required to do it and so weren’t set up to help. The Spanish system operator had identified this shortfall and was awaiting regulatory approval to update the existing requirements – a potential lesson on the danger of slow governance processes.
Another factor was that some of the fossil-fuelled thermal power stations the Spanish system operator had engaged to perform the task of voltage control appear to have underperformed in their duty, perhaps critically so [1]. Closer scrutiny of how well providers perform against the system service contracts they are supposed to deliver was previously highlighted as a learning point for the GB system after the August 2019 system frequency event here that disconnected over 1 million customers for nearly 1 hour [2].
A final part of the puzzle worth touching on is the presence of ‘sub-synchronous oscillations’ of voltages and power. Over years of experience, system operators across Europe have developed certain protocols to help damp out such oscillations on the rare occasions when they arise. During the Iberian event, one of the complications was that pre-defined actions taken to manage the oscillations that were present acted to exacerbate the high voltage issue. This is a further lesson for Great Britain and others to think carefully about any potential unintended consequences of the established operational processes that are in place as the system evolves.
My colleague, Keith Bell, noted in a blog last September that, although some arrangements for managing system voltages are better in Britain than they were in Spain last year, including having some voltage regulation from transmission connected renewables, there is no reason for complacency. As he said, “there tend to be, on average, one or two major power system outages somewhere in the world every year. These affect large regions, or entire countries, impacting systems deriving their energy from all sorts of sources – be it fossil fuels, nuclear power, hydro power or wind and solar. These events all involve a multitude of factors with often very complex interactions.”
He also noted, “it doesn’t matter where the energy comes from, system operators are still supposed to operate the system in a secure and stable manner (and the evidence suggests that, for the most part, they succeed). In other words, regardless of the energy source, you’ve got to get the engineering right. Even when good practice has been followed, experience from around the world over many years shows bad things can and do happen.”
ENTSO-E made a number of recommendations in their report. Thinking primarily about Great Britain, Keith made a number of similar recommendations in a presentation he gave at this year’s Supergen Risk Day, by coincidence the day before ENTSO-E’s report came out. These included:
To sum up, there are engineering challenges associated with decarbonising the electricity system, but there is nothing inherently prohibiting safe and reliable operation of a system dominated by renewable and low carbon energy sources – so long as we ensure the engineering has been done right. We’ve made great strides already in operating the British system with fewer and fewer fossil fuels. At the time of writing the NESO record for maximum penetration of “zero carbon generation”[3] in GB for a single half hour period stands at an impressive 98.8%, set just last week on the 22nd April. Once NESO is confident in their systems and processes, the next challenge is to push that figure out to 100% and end the long standing need to always run some (typically) gas-fired power stations to maintain system stability. They will likely trial this for short periods at first, then, once confidence is established, it will become the norm for a ’Clean Power’ system in 2030 and beyond to be run without the need for any unabated gas at times when there is plentiful clean generation.
[1] Red Electrica – “Blackout in the Spanish peninsular Electrical system on 28th of April 2025”
[3] It should be noted that Biomass generation is treated as zero carbon in this analysis.