© Fraunhofer ISE

Stable power supply due to grid-forming inverters

Large power plants with their synchronous generators ensure stability in the power grid, but are gradually being shut down in the course of the energy transition. To address this problem, researchers at the Fraunhofer Institute for Solar Energy Systems ISE are investigating how grid-forming inverters can ensure a reliable supply of sinusoidal alternating current and stable grid frequency in the future.

Switching on the lamp, charging the mobile phone, putting the milk in the fridge and quickly vacuuming the flat – in our everyday lives we rely on electricity coming reliably from the socket. But our power grid is very complex in its structure and the equilibrium in which it normally finds itself is fragile. Ideally, the current flowing through Europe’s electrical lines has a sinusoidal alternating voltage with an approximately constant frequency of 50 Hertz. This stability is made possible by the physical properties of synchronous generators in large power plants. These bring inertia and thus the so-called momentum reserve into the system via their rotating mass. They can compensate for any generation deficits in the short term using the stored kinetic energy and thus bridge the time until further protective measures such as the provision of control reserves are activated. This means that even in critical situations, such as the unplanned failure of large generation capacities or the disintegration of the grid into grid sections, a so-called system split, nationwide power failures do not occur immediately.

Now, however, large nuclear and coal-fired power plants are increasingly being taken off the grid and replaced by renewable forms of energy generation. “In this way, the synchronous generators are lost, which are a very essential basis for grid control,” explains Dr. Sönke Rogalla, head of the Power Electronics and Grid Integration department at Fraunhofer ISE. He and his research team see grid-forming inverters as a promising alternative for maintaining grid stability.

The right programming makes the difference

Inverters are power electronic devices whose primary task is to convert direct current into alternating current. Depending on the power class, they vary in appearance from small battery storage systems to large megawatt systems. Their electrical behaviour is not physically defined, but must first be determined accordingly via certain control algorithms. Nowadays, inverters are usually programmed to feed a desired power into a rigidly assumed power grid provided by powerful large-scale power plants. Grid-forming inverters, on the other hand, are programmed to behave like a voltage source. Comparable to the behavior of conventional power plants, grid-forming inverters thus react at short notice to the demand of the grid and provide instantaneous reserves.

“It is important, for example, that the devices react correctly and reflexively in special cases such as overload situations, defective lines or system splits and keep the grid stable,” says Roland Singer, group leader of converter-based grids. “To this end, we are researching the development of devices and algorithms. We can test various application scenarios with the help of simulations as well as using the test infrastructure in our institute’s own multi-megawatt lab in Freiburg.”

Holistic view in the “VerbundnetzStabil” project

Rogalla explains that there is now a consensus among transmission system operators that grid-forming inverters will be necessary for a large proportion of the plants that are newly connected to the grid. For example, Fraunhofer ISE provides consulting services to various electricity grid operators and has been working with various cooperation partners from science and industry in the “VerbundnetzStabil” project since 2017 (see info box below). “In a unique constellation, we have succeeded in bringing together competences from the field of power electronics and control technology with competences in grid dynamics and grid control. This allowed us to take a holistic look at the use of and the exact requirements for grid-forming inverters on a larger scale,” says Rogalla.

In the first project step, the requirements for future power grids were clarified and critical situations defined. This formed the basis for concrete device development and programming together with the inverter manufacturer KACO new energy. In the Multi-Megawatt Lab, the researchers were then able to recreate a small-scale power grid and investigate how the proportion of synchronous machines and grid-forming inverters, as well as the implemented controls, affected voltage stability in various fault scenarios.

Rogalla and Singer are very satisfied with the results. “Our investigations once again clearly show that a switch from synchronous generators to grid-forming inverters works and is also becoming increasingly urgent,” emphasizes Singer. “At the same time, we were able to clearly define what the grid of the future will really need and, with the help of a test guideline that we developed, provide suggestions for important technical details where there is still no clear standard,” adds Rogalla. “In this way, we want to provide the industry with assistance in the technical evaluation of suitable devices for the upcoming market launch of grid-forming inverters.”

Currently, the final report of the project is being written. At the same time, the researchers want to test their devices and findings on the real power grid in one of the institute’s office wings. In a further research project, which is currently being planned, the technology developed is to be implemented in a large photovoltaic storage power plant and grid interactions are to be investigated under real conditions.