On Monday, there was a complete power blackout across Spain and Portugal. Hospitals, water pumps and banks were left without energy, and the event will noticeably reduce Spain’s GDP in 2025.

Why did this happen, and were renewables to blame?

What caused the power cut?

The power cut was likely started by a solar farm suddenly tripping offline.

The cause is still being determined, but it’s mostly irrelevant: power station trips occur all the time, from gas to wind to nuclear. What matters is how the system reacts.

How power grids deal with power losses

All power grids are designed to be resilient to a sudden loss of generation from a single source.

Ordinarily the grid operates at a frequency of 50 Hz (the alternating current changes direction 50 times per second).

Under normal operation, the grid is kept tightly between 49.8 Hz and 50.2 Hz. But when there’s a sharp drop in power generation, the frequency of the power grid decreases, causing several response mechanisms to kick in.

#1. Frequency Response

Frequency Response is fast-acting correction that’s automatically applied to bring the frequency back to its nominal value (50Hz). It’s a normal part of everyday grid operation.

Frequency Response is most often supplied by fossil fuel power plants, and batteries.

Batteries are obscenely good at providing Frequency Response, because they can ramp their output up and down in milliseconds. When Tesla installed a big battery in Australia, it ate 55% of the Frequency Response market, and lowered the cost of frequency response by 90%, despite representing only 2% of total installed capacity.

(At Axle, we use a fleet of home batteries across the UK to provide Frequency Response, at a lower cost to consumers than other alternatives - read more here)

#2. Low Frequency Demand Disconnection

Frequency Response does its best to contain frequency between 49.8 and 50.2 Hz, but for big power losses, it doesn’t always succeed.

When there’s a major frequency drop, Demand Disconnection will start occurring automatically.

Substations will automatically detect a drop in frequency and begin disconnecting customers. This typically happens within hundreds of milliseconds. Power demand is reduced, which causes the frequency to increase until it reaches normal levels again.

Whilst this means local blackouts, most of the country remains uninterrupted.

Here’s an example disconnection table for a distribution zone in the UK.

Inertia

Underpinning both of the above mechanisms is inertia.

Inertia measures how quickly a power system responds to imbalances in supply and demand.

In a system with low inertia, the frequency plunges quickly before automatic demand disconnection is able to occur. The grid goes into emergency mode before it even notices what has happened.

If the system frequency drops low enough, it will cause other power plants to trip offline. Typically, this would occur at around 47 Hz.

Thus: low inertia hinders demand disconnection and frequency response, because it gives them less time to respond.

Renewables reduce inertia

Thermal power plants (nuclear and fossil fuels) have high inertia because they contain a big spinning lump of metal that’s synced up to the grid. If the grid frequency drops, the momentum of the spinning metal resists the change, providing inertia.

Most currently installed solar and wind generation has effectively zero inertia, because they use solid state inverters that are not programmed to provide inertia (more on this later).

Measuring Spain’s inertia over time

Inertia can be measured in seconds: how long could the kinetic energy in the system sustain the power grid?

Spain’s inertia has been decreasing as it adds more renewables. Here is the historical trend of Spain’s inertia from national electricity production*

And here is how it varies throughout a high-solar day. As the sun comes out, inertia decreases.

During solar-heavy parts of the day, inertia drops to the lowest levels, meaning that the grid can slide out of control much faster.

It’s worth noting that we don’t yet understand the cause of the blackout, and fossil-fuel power plants often cause blackouts too. But it’s certainly true that low inertia would have made it harder to contain the trip to a regular local outage.

How do we fix this?

A grid that is dominated by renewables needs to be managed differently. We need a system that lets us take advantage of abundant renewables, whilst increasing the resiliency of our supply.

In the past, inertia came “for free” as a byproduct of fossil fuel generation. Now, as renewable penetration increases, the price of wholesale power will fall, but the value of inertia must increase. Market design should represent the value of resiliency, and market mechanisms should ensure that sufficient inertia is procured at all times.

There are plenty of ways to add inertia to the grid, but they do need to be correctly incentivised. Solutions include:

• Rapidly acting demand response from distributed assets
  • Electric Vehicle chargers, home batteries and HVAC that can pause in less than a second.
• Grid-forming inverters that allow solar, batteries and wind to provide instantaneous “synthetic inertia”
  • Many current installations use “grid-following” rather than “grid-forming” inverters. Grid-following inverters adopt the current grid frequency, whilst grid-forming inverters can help drag the frequency of the grid back to 50 Hz. Our market design should value the latter.
• Decommissioned fossil fuel power plants can be re-purposed as “synchronous condensers” - flywheels that add inertia to the system

Onwards

A grid that enjoys an abundance of renewable energy should not sacrifice resiliency. When the sun comes out and power prices plunge, batteries and EV chargers across the country should switch on intelligently. In systems with lower inertia, fast-acting demand response is more valuable, as flexible assets can respond rapidly to any grid fluctuations during high renewable periods. This is the future we’re building at Axle Energy, and we’re hiring the people to make it happen. Join us!

Footnotes

* Rough estimate based on Spain’s domestic generation mix. Does not include interconnectors or domestic synchronous condensers.

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