Grid-forming inverters ensure stable power supply

Grid-forming inverters ensure stable power supply

Technology News |
By Christoph Hammerschmidt

In our everyday lives, we trust that electricity will reliably come out of 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 the electrical lines of Europe has a sinusoidal alternating voltage with an approximately constant frequency of 50 Hz (North America 60 Hz). This stability is made possible by the physical properties of synchronous generators in large power plants. These bring inertia and thus the so-called instantaneous reserve into the system via their rotating mass. They can compensate for any generation deficits in the short term via the stored kinetic energy and thus bridge the time until further protective measures such as the provision of control reserves are activated. Thus, even in critical situations, such as the unplanned loss of large generation capacities or the disintegration of the grid into grid sections, a so-called system split, there are no immediate nationwide power failures.

Now, however, large nuclear and coal-fired power plants are increasingly being taken off the grid and replaced by renewable forms of power generation at present and in the coming years. “This means that synchronous generators, which are a very essential basis for grid control, will be lost,” 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 to maintain grid stability.

Inverters are power electronic devices whose primary task is to convert direct current into alternating current. Depending on the power class, there are such devices with outputs up to the megawatt range. 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 behaviour 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 that the devices react correctly as a reflex 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.”


Rogalla explains that there is now a consensus among transmission grid operators that grid-forming inverters will be necessary for a large proportion of the plants that are newly connected to the grid. Thus, Fraunhofer ISE is available to advise various electricity grid operators and has been working with various cooperation partners from science and industry in the “VerbundnetzStabil” project since 2017. In this project, competences from the field of power electronics and control engineering are combined with competences in grid dynamics and interconnected grid control in order to be able to take a holistic view of the operation of and the exact requirements on grid-forming inverters.

In the first project step, the requirements for future electricity 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 reconstruct the model of a 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 disturbance scenarios.

The researchers are very satisfied with the results. “Our investigations once again clearly show that a conversion from synchronous generators to grid-forming inverters works and is also becoming increasingly urgent,” emphasises Singer. “At the same time, we were able to clearly define what the grid of the future really needs and, with the help of a test guideline that we developed, provide suggestions for important technical details where there is no clear standard yet,” 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.”

The report of the project is currently being finalised. At the same time, the researchers want to test their devices and findings on the real power grid in a smaller grid. In another research project, which is currently being planned, the technology developed will be implemented in a large photovoltaic storage power plant and grid interactions will be investigated under real-world conditions.

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