Around midday on 28 April 2025, mainland Spain and Portugal experienced a catastrophic grid failure that disconnected 31 GW of load in just 90 seconds.
The Iberian Peninsula blackout affected approximately 50 million people, with eight people suspected to have died as a result of the disruption, making it one of Europe’s most significant power failures.
The first known blackout caused by overvoltage, it plunged the region into a day of chaos with transport, communications and emergency systems all severely affected.
In the hours before the blackout, the grid had been plagued by frequency oscillations, a warning sign that grid operators struggled to address. A photovoltaic solar plant in Badajoz was forcing a persistent 0.6 Hz frequency oscillation while feeding 250 MW into the network. These oscillations indicated underlying instability in the system’s ability to maintain the frequency required for stable operation.
The failure of a substation in Granada triggered a domino effect: grid infrastructure in Badajoz and Sevilla rapidly disconnected as protection systems responded to the frequency deviation. Critically, the 2.8 GW interconnector to France – the peninsula’s primary link to the European grid – automatically decoupled to protect the continental network from the disturbance. Within seconds, the Iberian Peninsula was an electrical island cut off from support.
The failure
According to electrical engineer Phil Kreveld: “The salient feature of the breakdown appears to be a simultaneous occurrence of transmission voltage oscillation and lightly loaded transmission lines, and the operator actions taken as a consequence.”
In an attempt to stabilise the system, operators paired transmission lines, which lowered impedance to damp the oscillations. However, this action also increased the reactive load requiring absorption.
“What appeared to be the correct operational procedure in damping oscillation exacerbated voltage rise in transmission lines, ultimately resulting in over-voltage tripping of generators and a cascading blackout,” Kreveld said.
The operators’ actions, though technically sound in isolation, created worse conditions in an already stressed system.
Reactive power
At the heart of the failure lay a critical issue: insufficient reactive power management.
“In Spain renewable sources work on a fixed power factor,” Kreveld said. “If they withdraw from service because of no or little energy demand, reactive power is also not available.”
The problem was compounded by operational decisions made earlier that day.
“To provide voltage support, some synchronous generators were absorbing reactive power from lightly loaded transmission lines. This is generally not an advisable procedure as under-excitement can cause instability because of reduced synchronising torque.”
As generators were selectively withdrawn from service to manage the light load conditions, transmission line voltages rose, causing further automatic shutdowns. This was a vicious cycle that operators couldn’t break.
Lessons for Australia
The Iberian failure offers urgent lessons for Australia’s rapidly evolving grid. Firstly, single points of failure can potentially be mitigated via meshed grids, which Spain has, but Australia’s are radial and subject to N-1 failure.
“In the South-East Australian grid, lightly loaded transmission lines occur during high distributed energy resources energy production in distribution grids during the middle of the day.”
The Australian Energy Market Operator (AEMO) is already grappling with similar reactive power challenges.
“AEMO has consequently asked some large energy storage battery systems to have low levels of charge at the start of the day to ensure sufficient power flow to prevent voltage rise. It also has a regime imposed on distribution grids to switch off solar inverters so as to build up energy demand.
“However, the bulk of solar inverters do not comply, as the regulation for inverters to be equipped with remote switch-off dates in October 2024.”
The technology solutions being deployed differ between regions.
“In Australia, AEMO and transmission line operators are recommending and specifying synchronous condensers,” Kreveld said. “Mainland Spain uses VAR compensators which can respond very quickly.”
While both technologies address reactive power management, synchronous condensers provide the additional benefit of system inertia – a critical advantage as renewable penetration increases.
For Kreveld, Australia’s reactive power requirements are a broader policy concern.
“The capacity investment scheme is focused on active power, and although specified in MVA, the tacit assumption is that capacity will be supplied at unity power factor.”
This assumption could prove dangerous as the grid transitions away from synchronous generation.
“The regulators appear to foresee voltage control problems through lack of reactive power by specifying volt-var control, and ultimately volt-watt control. Notwithstanding this, synchronous condensers will be required unless grid forming inverters can provide voltage stability. There are trials underway but transmission line operators prefer synchronous condensers.”
Grid control
The blackout exposed fundamental questions about grid control philosophy and the interactions with automated systems. Traditional grid operation has relied on human judgment supported by the natural time buffers that inertia provides, providing time to make adjustments in a crisis. But as renewable penetration increases and inertia decreases, those precious seconds evaporate.
The Iberian operators followed established procedures when pairing transmission lines to damp oscillations. But in a low-inertia, high-renewable grid, those procedures proved catastrophic. Reports into the disaster have already tasked engineers with rethinking energy grids for an environment where conditions change in milliseconds rather than seconds.
As Australian engineers manage our own energy transition, the Iberian blackout highlights the “unknown knowns” of hyper-complex infrastructure systems. The incident serves as a reminder that the energy transition isn’t simply about adding green sources – it’s about ensuring resilience and stability, and that renewable integration and grid reliability advance along this path together.
The risk of a complete system shutdown is greater than ever. This webinar explores the causes and impacts of total grid failure.
