In Sync or In Trouble? Lessons from the Iberian Blackout

At midday on Monday, the Iberian Peninsula experienced one of the most significant electricity blackouts in recent memory. Both Spain and Portugal were affected by a sudden, widespread power outage that, at its peak, left an estimated 14 gigawatts of demand unmet - roughly the equivalent of 14 Belgian nuclear reactors going offline at once. Some reports suggest the actual shortfall may have been even greater.

Figure 1: Electrical energy demand on the Iberian Peninsula (GW). Data source: REE

While the precise cause is still under investigation (a process that could take weeks) early indicators point to a familiar vulnerability: the lack of sufficient synchronous capacity in the grid at a critical moment. This shortfall may have allowed a brief and still-unexplained frequency deviation to spiral into a large-scale blackout.

To understand why synchronous capacity is so important, imagine the European electricity grid as a giant tandem bicycle. Every generator; be it a gas turbine, coal plant, or nuclear reactor, is pedaling together, turning the same chain exactly 50 times per second. On the other end, all of us (the consumers) are applying resistance to that chain by drawing power. When demand rises suddenly, like a tandem crew hitting a steep hill, the generators need to push harder to maintain speed. If they don’t react quickly enough, the whole system slows down and the frequency drops. On the other hand, a sudden drop in demand causes the pedals to spin faster, increasing the frequency. Either way, stability depends on keeping that pedaling pace steady. What makes traditional generators so valuable is their rotating mass; large spinning turbines that store kinetic energy like flywheels. This inertia acts as a buffer against sudden mismatches between supply and demand. If there’s a hiccup in the balance, the system’s momentum can absorb the shock, buying time for automatic systems or operators to respond. It’s like having a heavyweight cyclist on your tandem team who can push through small bumps without slowing down.

This stabilizing feature is mostly absent in renewable sources like solar or wind. Solar panels generate direct current, which needs to be converted to alternating current to match the grid’s frequency. This is done using inverters that simulate the 50 Hz signal electronically. But because this frequency is not the product of a spinning mass, there’s no inertia behind it. It’s stable under normal conditions, but offers little resistance when things go wrong. This doesn’t mean renewables caused the blackout, but they do change the game. On Tuesday, solar production was high across the region. If a disturbance occurred, the grid may have lacked enough inertia to prevent that disturbance from escalating. Without a buffer, the frequency may have deviated too quickly for control systems to catch up, triggering protective shutdowns and amplifying the impact.

The European grid is designed with safeguards like the N-1 principle, which ensures that any single component like a power plant or transmission line can fail without causing a blackout. So when a major failure does occur, it typically means several layers of protection were breached. That appears to be the case on Monday. Grid operators across Europe are now working to reconstruct the event, determine what went wrong, and figure out how to prevent it from happening again.