If the lights of houses and factories stay on, it is also thanks to something that cannot be seen: heavy masses of steel turning. They are cylinders that weigh nearly a hundred tons at a constant speed of around 1,500 rpm, ready to react in fractions of a second if the power grid falters. They do not produce electricity: they stabilize it. They are synchronous compensators, rotating machines that return to the grid that physical inertia that photovoltaics, by its very nature, does not possess, and that was once provided automatically by the large turbines of power plants. They work very well in Italy; in Spain, where inertia was in short supply, a blackout stranded an entire country for almost a day last summer.
It is the paradox of the transition: the more intermittent renewables we put in, the less “mass” spins in the system. For more than a century, spinning turbines have generated almost all electricity. They produce alternating current, that is, a voltage that oscillates between negative and positive values at a predetermined frequency. The grid must be in fine balance at all times. Too much production can bring it down, while too much demand can cause blackouts. Sudden changes in frequency-for example, due to the shutdown of a power plant-propagate jolts throughout the system.
The role of inertia
One element helps operators govern frequency: inertia. It is the physical principle that a body maintains its state of motion (or stillness) until an external force changes it. The greater the mass in motion, the greater the inertia. On conventional grids, spinning turbines react automatically by speeding up or slowing down and bringing the frequency back toward 50 Hz. If the jolts are too abrupt–above 1 Hz–centers or entire portions of the grid shut down. In extreme cases, such as Spain’s, entire countries can go dark.
Once upon a time, when a country like Italy had a few dozen places of electricity production, everything was easier. Today there are almost a million: we are in the age of renewables. And this safety net is no longer there. Photovoltaic cells are the only major source of generation without moving parts, and therefore cannot provide inertia. They produce electricity when solar radiation strikes silicon, a semiconducting material, setting in motion the flow of electrons.
The safety net
Rebuilding the protective grid requires installing synchronous compensators, which spin at the right speed, and in case, can be used to react as a “stability reservoir” to prevent a local disturbance from becoming a national incident. These turbines, which often weigh more than 100 tons, stay spinning at the exact frequency and help the grid regain balance even if, suddenly, a wind or solar plant disconnects.
In Europe and around the world, however, grid investments do not always keep pace with new megawatts. According to BloombergNEF, the EU 27 and Britain spend an average of 0.7 euros on upgrading grids for every euro invested in renewables. Spain is the tail end, with just 0.30 cents. And there is no shortage of technical and regulatory bottlenecks: specifications thousands of pages long, complex tenders, years between design and commissioning. Meanwhile, PV or wind grows faster than the capacity to absorb it without risk.
Terna’s business plan
And in Italy? Terna has put the issue at the center of its plans. Between 2023 and 2025, the power grid company has initiated the purchase of multiple units, for an investment in the order of hundreds of millions. In the Italian plans, the approach is complementary: to place grid-forming inverters alongside synchronous compensators on plants and storage systems, so as to extend stability services even where the grid is “weak.”
The business plan calls for the installation of machines that can regulate voltage and stability. These include Statcoms – high-performance static machines that can dampen fluctuations on the grid -, synchronous compensators and resistors, static machines smaller than Statcoms that absorb active power (to constitute a stabilizing load) or reactive power (as a distributed regulation resource). In the case of more rapid and severe events, Terna’s defense system corrects imbalances very quickly, even in tens of milliseconds, beyond the reach of manual control. This is thanks to a network of monitoring equipment (Wams) distributed not only in Italy but also abroad, which can detect very quickly (every 20 milliseconds) critical phenomena for the entire European area.
Someday the grid may no longer have rotating machines. The task would be done by inverters, which can create “synthetic inertia” with computer-controlled electrical signals. Inverters are connected to solar, wind and battery systems: they convert the direct current produced by these systems into alternating current with frequency and voltage aligned with the grid. Today, however, inverters are mostly “grid-following”: they merely follow the parameters of the grid, which is of little use in case of disturbances. By equipping them with advanced controls, they become “grid-forming”: instead of amplifying faults, they correct them, injecting additional power to stabilize voltage and frequency when needed. But they are a new technology, and grid operators do not yet fully trust the results.
