That's the question that still has not been definitively answered as of today (May 11), almost two weeks after the April 28 power outage that plunged much of Spain and Portugal into darkness for almost 24 hours. Why could renewable energy sources such as wind and solar power have contributed to the blackout? The answer isn't simple, but as more and more countries derive more of their energy from renewables, it's a question that deserves examination.
What we do know about the blackout is this. The Iberian Peninsula is a little like Texas in that its power grid is nearly autonomous, with only small interties to the rest of the European continent. A little after noon, some "oscillations" appeared in the grid and were "detected and mitigated." Operating a large power grid is a delicate balancing act in which the fluctuating demand must be met by appropriate generating capacity at all times. And across the entire grid, all the generating plants must produce power in synchronism at a rate of 50 Hz (in Europe—60 Hz in the U. S.).
A prime indicator of the health of the grid is how close the grid's frequency is to its nominal frequency. The grid is like a symphony orchestra in which all the instruments are tuned to the same pitch. The entire system is designed for optimum efficiency at 50 Hz, and as little as only 1 Hz deviation above or below that can lead to serious problems and ultimately damage or destroy millions of dollars' worth of transformers and other gear. So grid operators have both automatic and manually backed-up systems to keep the grid frequency near its nominal value, and to vary the amount of power being generated as demand varies.
For reasons that are not yet clear, at 12:33 PM three generators tripped off the grid. This meant that the system lost 2.2 GW of capacity instantly. In response, the grid frequency began to fall from 50 Hz, and when it reached 48 Hz, automatic protection circuitry began to disconnect more generators from the grid, leading to a cascade that shut the entire system down in a matter of seconds.
Once a thing like this happens, it takes hours to communicate among the now-isolated generating plants and organize an effort to re-synchronize and reconnect parts of the grid in a way that will not lead to further problems. In the meantime, most communications and transportation systems in Spain and Portugal were severely crippled, thousands of people had to be evacuated from electric trains, and seven people died as a result of the blackout.
At the time of the grid failure, over half of the grid's power was being produced by solar, wind, or hydroelectric plants. Assuming most of this was wind or solar, the grid was therefore missing something that power grids used to have an abundance of: "spinning reserve." And spinning reserve is an important way that grids can stabilize themselves.
Simply put, spinning reserve is the energy stored in the mechanical momentum of the turbines and generators used to produce power at nuclear, fossil-fueled, and hydropower plants. A generator-turbine shaft, armature, and blades weighing many tons cannot be stopped on a dime, and the fact that it's spinning, typically at thousands of revolutions per minute, means that there's a lot of energy stored in it.
When a sudden increase or decrease in load occurs on such a generator, the spinning reserve means that its speed (which directly determines its frequency) does not change instantly. If the load increases (as it would if generators elsewhere suddenly tripped off the line), the spinning reserve automatically keeps the frequency from dropping instantly. This factor can be used in designing stability into the grid, and historically spinning reserve has been an asset in making grids stable.
When renewable sources began to be connected to power grids, the approach taken by designers was that the renewables would always be a small fraction of the total power generated. So when they designed the devices to interface solar or wind power to the grid (called "inverters") they simply designed them to follow whatever frequency the grid was producing at the time. Electronics has no mechanical momentum, so renewable sources can adjust their frequency instantaneously. As long as they represent a small fraction of the total power generated, like a few monkeys riding on the back of an elephant, the fact that renewables do not contribute spinning reserve was not important. The monkeys go where the elephant goes, and they're just along for the ride.
But reports say that at the time of the blackout, the fraction of power being made by renewables was on the order of 58%. So the monkeys outweighed the elephant in this case. Engineers have studied and modeled these situations, and presumably know what they're doing, but there is an undercurrent of suspicion that under some circumstances, having too large a fraction of renewables on a power grid that is isolated, like the Iberian Peninsula's is, can lead to trouble. The question is, was last month's blackout an example of the kind of trouble renewables can cause? We will have to wait on the results of the investigation to find out.
There is a way to make renewable power sources act like they have spinning reserve, but it's not cheap. That energy has to come from somewhere, and either the renewable source has to hold its maximum capacity in reserve (which is wasteful), or you have to add capacity in the form of batteries. But with suitable inverter design, a wind or solar source with batteries can be made to act like it has a certain amount of spinning reserve.
If we find that the blackout was in fact worsened by inverter-based renewables, something like the battery-spinning-reserve idea may need to be implemented as a safety precaution. There are other good reasons to put battery storage on grids with a lot of renewable energy. A windless night produces no wind or solar power, and it's handy in such cases to have energy stored up somewhere that you can use in such situations.
Batteries are improving steadily and may come in very handy to avert the next blackout. If it turns out that renewables contributed to the problem, we have a solution, but it's not going to be cheap.
Sources: I referred to an article in Wired at https://www.wired.com/story/what-caused-the-european-power-outage-spain-blackout/, an article on batteries supplying spinning reserve at https://www.renewableenergyworld.com/energy-storage/battery/spinning-reserve-displacement-using-batteries-for-a-more-efficient-and-cleaner-way-to-back-up-power/, and the Wikipedia article "2025 Iberian Peninsula blackout."
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