System resilience: A market for inertia

The heavy rotating masses of large thermal power plants, and the inertia they introduce, ensure stability in the electricity system. When fossil fuels are gradually taken off the grid, it is primarily inverter-based systems such as battery storage, wind and solar plants that will provide the necessary instantaneous reserve. A market is then needed to control what inertia previously achieved of its own accord.

By Eva Augsten

Passivity, stagnation, inactivity, dullness, apathy are just some of the synonyms that can be found for the term “inertia.” What sounds so negative in a thesaurus has a number of positive consequences in the physical world – even if it doesn’t appear so at first. For example, when we drop a cup, and all the planet’s gravity pulls it toward the center of the Earth, it still accelerates only gradually. Yes, it still moves fast enough to shatter when it reaches the floor, but without its inertia, it would strike the ground at a speed as though it had fallen from an aircraft – and you’d be sorry if you didn't manage to pull your foot away quickly enough. In other words, inertia makes our world more predictable. It gives us time to react to changes.

This is also true in the electricity system – thankfully, because this is the only way to keep the power supply frequency stable even though consumption and supply change every second. Huge quantities of steel, from which turbines, shafts and generators are made, rotate in large thermal power plants. Even if consumption suddenly plummets, inertia ensures that the turbines do not accelerate immediately. Inertia doesn’t react; it’s there from the beginning. The stabilizing effect of the inert masses is therefore also called the “instantaneous reserve.”

However, inertia can only slow down change; it can’t stop it altogether. If generation and consumption are not in harmony again within a fraction of a second, the power supply frequency deviates from the deadband of 49.99 to 50.01 Hertz. Within a few seconds, the primary operating reserve then needs to kick in. Battery storage systems, flexible gas turbines or controllable loads such as electrode boilers then receive or supply power, as required, and thereby stabilize the power supply frequency. After about a minute, the secondary reserve is activated in order to bring the frequency back to within the target range. Through the interplay of these mechanisms, the power supply frequency is practically always in the range between 49.8 and 50.2 Hertz. In the integrated European grid, current and voltage are in harmony – from Aalborg to Algiers and from Lisbon to Luhansk.

With fossil energy sources, reserves are simply there

As we all know, our energy supply is undergoing a fundamental transformation. Wind and solar power plants are set to take over from fossil-fuel power stations. Countless scenarios have simulated where and when they need to be built, and what storage capacities and flexible consumers are needed to ensure that sufficient energy is always available.

Wind turbines and solar cells produce direct current. Inverters are needed to convert this into usable alternating current.

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The distribution of tasks will change at the same time as the technical remodeling. On the regular power market, market mechanisms and competition found their feet decades ago. All manner of kilowatt hours package sizes can be purchased – from a base load subscription fixed for several years to a spontaneous, bite-size quarter of an hour. Since 2001, there has also been a defined market where transmission grid operators can obtain balancing energy, which balances out small fluctuations. To date, however, there is no way to buy inertia and the instantaneous reserve it provides. Inertia is simply there.

But as the rotating masses gradually disappear from power generation, someone will have to take over their function. Battery storage systems, wind power plants and photovoltaic systems, connected to the grid via current converters, are the candidates of choice for this. They do not effect a mechanical moment of inertia on the power grid, but they could easily emulate this – and this is referred to as virtual or synthetic inertia. Manufacturers insist that modern devices would not need new hardware for this, merely a software update.

Tests to avoid “zombie grids”

What sounds so simple is a paradigm shift. Today, all current converters automatically adapt to the voltage and frequency of the grid. If certain thresholds are exceeded, the devices turn off. This is an elementary safety function in the low-voltage grid. If a grid that was thought to be de-energized were to continue to carry current in an uncontrolled manner, it could be fatal for maintenance teams and might also damage equipment. It is therefore essential that systems are able identify this kind of situation.

A grid-forming inverter, on the other hand, dictates the target values for voltage and frequency itself. Depending on the state of the network, either more or less power automatically flows into the grid. Like inertia, this effect is also inherent. Numerous microgrids, which supply power to hotel complexes, villages and mining operations in remote areas, prove that it is possible to operate a stable power grid with grid-forming inverters.

But scaling this up is not straightforward. “In principle, grid-forming inverters are nothing new. However, we can’t simply translate the experience gained in a limited microgrid to our large existing network,” says Professor Jutta Hanson, head of the Department of Electrical Power Supply with Integration of Renewable Energy at the Technical University of Darmstadt and board member of VDE ETG. Instead, we need to feel our way gradually. How high does the proportion of grid-stabilizing inverters need to be? How can they be integrated at the different voltage levels?

To date, converters have been used in small-scale grids. They cannot simply be applied in the same way to a national power grid.

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Enno Wieben, from EWE Netz, has also pinpointed a few open questions. In his employer’s grid area, more power is already being fed in than consumed on balance over the year. Therefore, it is not uncommon for wind and solar energy plants to be able to keep the grid alive even when the central supply fails or is deliberately switched off for maintenance. To prevent this from happening, modern power converters provide the grid with a number of small current or voltage nudges and measure the reaction. In a small microgrid, these nudges are felt immediately, while a large grid remains unfazed. “If there are a lot of grid-forming inverters in the grid in future, it is possible that they might absorb each other’s fluctuations and fail to notice that they are creating a microgrid,” says Wieben. This poses a risk to equipment and, in the worst case, also to people’s safety. We therefore need thorough studies as well as laboratory and field tests to avoid such “zombie grids.”

System Stability Roadmap points the way

If a system transformation of this magnitude is to successfully take place during ongoing operation, all those involved will have to work together. “Who does what when?” is the big question; the System Stability Roadmap from the Federal Ministry for Economic Affairs and Climate Action (BMWK) aims to provide the answer. More than 150 people from over 80 organizations were involved in its production. It is by no means a finished plan for system transformation. Many of its 18 milestones involve defining further details. For example, this year VDE’s Forum Network Technology/Network Operation (VDE FNN) will draw up the technical requirements and test regulations for the grid-forming converters, so that the market for instantaneous reserve can start in 2025. The Federal Network Agency’s policy establishment procedure has already been under way since the end of 2023. “We don’t want to specify the type of equipment that must be used here, but rather how it needs to behave,” says Christoph Wulkow from VDE FNN, project manager of the responsible project group. This means that not only grid-forming capability is needed, but also the ability to reliably produce the offered instantaneous reserve. It is likely that battery storage systems will very quickly be suitable for this. However, wind power installations and certain consumers are also very promising options for providing instantaneous reserve promptly. In the United Kingdom, the national transmission grid operator ESO already took this step in 2019 with the launch of the Stability Pathfinder program. In three stages, it defines various “products” it needs for system stability. The first tender procedures have now taken place for all the products. The result of the first tender phase, a 200-megawatt battery storage system, which will later grow to 300 megawatts, is currently under construction in Blackhillock, Scotland. According to the operator, Zenobe, it is set to be the first battery “to provide the full suite of active and reactive power services in the world” – including the instantaneous reserve.

Reliability, even without safeguarding everything

For Germany, the System Stability Roadmap plans for all new current converters to be grid-forming as of 2028. From 2030, they should then make a “significant contribution” to system stability. At the moment, it is impossible to put an exact figure on this, because the question of how stable the grid needs to be is as yet unanswered. “It is neither technically possible nor economically feasible to safeguard against all conceivable events,” as the roadmap states, addressing an uncomfortable truth. However, even if it isn’t possible to safeguard against “everything” – from a blizzard to a volcanic eruption – there are still scenarios that certainly have to be addressed. One of these is what is known as a system split. The last-but-one incident of this kind began in Emsland, when in November of 2006, a heavily utilized 400 kV line was switched off to allow a cruise ship to pass. The most recent system split had its beginnings in Croatia. In both cases, the faults cascaded and triggered ever-larger safety mechanisms. In the final step, the integrated European grid separated into several sub-grids – the system split.

The consequence of a system split: all at once, energy stops flowing over the coupling points. On one side of the separation line, there is suddenly far too much power in the grid and the frequency skyrockets, while it falls drastically on the other side. “This power imbalance now has to be rectified on both sides through sufficient instantaneous reserve, and it has to happen immediately. In conjunction with the primary control measures, the frequency can then be stabilized,” says Wulkow. In the last two system splits, it was possible to contain this system disruption. A few hours later, the grids were synchronized and connected again.

There is still work to be done to ensure that the integrated European grid and thus the power supply in Germany remains reliable in the future. In addition to the instantaneous reserve, the inverter-based plants will have to provide short circuit current as a grid service, for example. However, the new distribution of tasks and decentralized structure also brings opportunities. A new dimension in resilience will open up when decentralized and inverter-based plants are able to form a grid themselves. “In the future, it could even be possible to operate a medium-voltage grid in micro-operation if there is a failure in the transmission grid,” says Wieben from EWE Netz. Many steps and details still need to be clarified here, for example how such microgrids are later synchronized with the overall grid again.

The System Stability Roadmap alone contains 18 milestones to be reached by 2030. If this is to succeed, no-one can afford to be inert.

Eva Augsten is a freelance journalist in Hamburg, Germany, who specializes in renewable energy.