Should We Care about Grid Inertia?
The recent massive power outage in Spain and Portugal has led to discussions around renewables and grid inertia, so it's probably worth a deeper understanding
The recent power outage in Spain and Portugal dominated headlines and there were many excellent Substack posts on this, such as this, this and this. But for now, I just wanted to do a deeper dive into a term that has come up a lot — grid inertia — and used a paper written in 2020 to explore this concept.
The outage raised critical questions about the resilience of modern electricity grids with high levels of renewable energy penetration. Specifically, the event prompted the crucial debate: could the high percentage of renewables like solar PV and wind have contributed to grid instability during the incident?
And if so, how are other countries with significant wind and solar power in their electricity mix (such as Denmark, the Netherlands, and Germany) managing these challenges seemingly without experiencing similar widespread system collapses?
Ricky Lanusse from AntarcticSapiens wrote a wonderful post on this explaining why this may be happening to Spain (emphasis mine):
Spain remains one of Europe’s most isolated energy systems. Its interconnection capacity with the rest of the continent sits at a paltry 3% of its installed power. The EU target is 15% by 2030. Spain isn’t even close. When the system starts to crash, there’s not enough bandwidth to borrow strength from neighbors. That means one thing: you either stabilize internally, or you fall.
And Spain and its outdated grid fell.
Blackouts don’t discriminate by energy source.
Countries like Denmark and Netherlands benefit from strong international interconnections with the continental European grid and also a more advanced grid management and technology, providing them with greater flexibility to manage fluctuations despite high renewable energy generation.
Moving beyond immediate speculation over the outage’s trigger, this post will delve into the grid inertia concept. The paper we will use is titled ‘Inertia and the Power Grid: A Guide Without the Spin’. Published by the US National Renewable Energy Laboratory and University of Colorado Boulder in 2020, this paper provides an excellent primer on understanding grid inertia and especially on how it relates to renewables.
Let’s begin!
What is grid inertia?
If you are short for time, this accompanying video to the paper provides a great overview on inertia’s role in maintaining a reliable power system:
In this paper, the authors use the analogies of of driving a vehicle or riding a bicycle to understand inertia. An important concept here is grid frequency, which is the rate of direction switching of electric current. The frequency used in the US is 60 Hz (cycles per second) while in Europe is 50 Hz.
In a power grid, inertia is derived from hundreds or thousands of generators that are synchronised — meaning they are all rotating in lock step, so if one generator is out of sync, it will be disconnected from the grid unless its frequency gets back in range, as illustrated below:
Why renewables do not have inertia?
A key difference between conventional power sources (like coal or nuclear) and renewables is that the former turn the spinning energy of a turbine into electricity via a synchronous generator, a process that inherently produces alternate current (AC) electricity.
Renewables, such as wind and solar PV, produce direct current (DC) electricity, which must then be converted into AC. The converter is also known as an inverter, hence these energy sources are also commonly referred to as inverter-based resources (IBRs).
Going back to the vehicle analogy, the large spinning masses in traditional turbines provide inertia, like a car or bicycle that continues to roll for a while even after you stop accelerating or pedalling. This inherent inertia in conventional power plants offers valuable seconds of resistance against rapid frequency changes, providing crucial time for other grid control systems to respond and stabilise the grid after a disturbance.
Conversely, IBRs like most solar PV and wind turbines lack these heavy spinning components directly coupled to the grid frequency. Hence, they do not naturally provide the same type or magnitude of synchronous inertia.
Is this necessarily a problem?
However, the paper highlights that the absence of inherent synchronous inertia from IBRs does not automatically render them problematic for grid stability.
First, the authors of the paper stated that using electronic-based resources, it is possible to develop ‘fast frequency response’ (FFR) services. These electronic-based responses can be significantly faster than the mechanical responses of traditional generators.
FFR can rapidly inject or absorb active power in response to frequency deviations, helping to stabilise the grid in a low-inertia environment. Battery energy storage systems (BESS) are a prime example of technology capable of providing very fast and precise frequency response, as they are able to significantly increase output or switch between charging and discharging in less than a second with proper control.
In this area, it does seem that Spain or Portugal may have fallen short. Reports showed that Spain has an installed grid-scale battery energy storage system (BESS) of 60MW as of April 2025, significantly lower than the UK’s 5.6 GW and Italy’s 1 GW despite similar projected storage needs for these three countries.
Designing a modern grid for IBRs
The paper also offers an interesting perspective that the traditional reliance on synchronous inertia is largely a consequence of the historical dominance of synchronous generators.
‘Zero-inertia microgrids’ are small (often isolated) systems that have demonstrated the technical feasibility of operating solely with inverter-based resources. By utilising advanced inverter controls that can independently form and maintain grid voltage and frequency, they can essentially create their own stable grid without the need for synchronous machines.
While some such microgrids have been operational for a period, this approach is still largely in development and primarily implemented at a small scale (often in systems less than 10 MW). This concept represents a more frontier approach to grid design.
Summary
While the causes of the Spain-Portugal power outage are still under investigation, the high share of renewable generation in these countries has attracted attention.
This led us to investigate the idea of grid inertia. A technical paper helped us understand why IBRs may have lower inherent inertia than traditional generators. However, this is not necessarily a problem if there are sufficiently fast responses that help manage deviations in grid frequency.
These options include: [1] a well-connected grid with neighbouring systems to share resources and balance loads; and [2] fast-acting resources like battery storage solutions that could provide rapid frequency response.
In the case of Spain and Portugal, we know that the Iberian peninsula is an ‘energy island’ and the fact that it was only connected to France during the outage did not help. On point number two, Spain lags behind peers on grid-scale BESS investments, which may have led to increased vulnerability.
Spain and Portugal are not the only countries that are adding more renewables to their power grids. While vulnerabilities may arise due to the lack of grid inertia in IBRs, the paper pointed to useful resources and strategies, such as Fast Frequency Response (FFR), that can effectively manage deviations in grid frequency. Given these capabilities, it may not be entirely fair to solely pin the blame for the outage on renewables.
Countries like Denmark and the Netherlands, despite having even higher shares of renewables than Spain, appear to be better positioned to prevent such outages. This highlights the crucial role played by more extensive grid interconnections and investment in solutions like battery storage and advanced grid management technologies.