Walter Finch
SPAIN’S worst power outage in modern history may have been triggered by an invisible weakness in the peninsula-wide electrical system: a lack of inertia.
According to Red Electrica, Spain’s national grid operator, a sudden and extreme oscillation in grid frequency triggered cascading failures across the country and into Portugal.
El País reports it was the worst frequency deviation in over a decade, with the country losing 15 gigawatts of energy in a matter of moments.
Some experts believe a key reason the system couldn’t stabilise itself was a lack of inertia in the modern electricity mix.
So what is inertia in this context?
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Electricity grids must maintain a constant frequency of 50 hertz (Hz) in Europe, meaning the alternating current changes direction 50 times per second; this synchronised rhythm ensures that all equipment connected to the grid operates safely and efficiently.
When a big power source suddenly fails, that frequency wobbles, destabilising the entire system and leading to widespread outages.
In traditional power systems, inertia is provided by large spinning turbines in gas, coal, hydro or nuclear plants.
Their momentum acts like a stabiliser, smoothing out the bumps and keeping the grid upright.
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But as Spain has shifted rapidly to renewables — especially solar (PV) and wind — it now relies on energy sources that do not naturally provide inertia.
As energy journalist Angelica Oung puts it: “At the time of the blackout, 70% of Spain’s electricity was coming from producers without grid-synchronous inertia.”
Without this invisible stabiliser, the grid becomes fragile. Oung compares it to two kids on bikes: one heavier, one light. Give them a push and the skinny one – in this case renewables – wobbles more.
When the frequency dropped yesterday, the grid couldn’t absorb the shock.
The entire ‘house of cards,’ as experts describe it, came crashing down — leading to what’s known as a ‘black start’ scenario: a complete reboot of the national power system, starting from scratch using special backup stations.
Ironically, the only generation that stayed running in some areas was solar — the same PV systems that can’t offer inertia.
Morocco and France provided emergency electricity to Spain through existing interconnections – including one under the Strait of Gibraltar – which helped stabilise the grid, in turn helping the restoration process.
But to bring the power back, Spain called on its hydroelectric and thermal power plants in the northern regions, such as the Cantabrian Mountains and the Pyrenees, according to US security architect James Cupps.
These facilities are well-suited for black start operations due to their ability to quickly generate power off their own backs without external input.
In the south, fossil fuel power plants – possibly including gas-fired stations in Andalucia – were also brought back online.
The strategy involved creating isolated ‘electrical islands’ – self-sustaining pockets of the grid that could be gradually reconnected to the system.
By energising these islands, Red Electrica aimed to stabilise the grid incrementally, ensuring that each section was secure and providing power to homes before moving on to the next.
By 7am today, 99.95% of the country’s power supply was restored – or 25.7 gigawatts.
No definitive cause has yet been confirmed. But the blackout serves as a warning that the transition to green energy must include safeguards like synthetic inertia — or risk even more frequent, costly failures in future.
