Large-scale fossil-fueled power plants with their synchronous generators provide stability in the power grid. With the progressive transition of the energy system, though, these will be successively switched off. This means that in the future, grid-forming inverters from wind and solar power plants will be needed to ensure a reliable supply of sinusoidal alternating current and a stable grid frequency.
The current that flows through the electrical lines in Europe ideally has a sinusoidal alternating voltage with an approximately constant frequency of 50 hertz. The physical properties of synchronous generators in large power plants make this stability possible. They introduce inertia into the system via their rotating mass and, with it, instantaneous reserve. This means that if too little electricity is available, the system is balanced out for a short time via the stored kinetic energy. This bridges the time until other protective measures, such as the provision of control reserves, can be activated.
Up to now, inverters have operated downstream of the grid, in line with the current grid connection rules. This means that they inject a current with the same frequency into the grid for a given voltage system. In the future, though, grid-forming inverters will be needed to provide instantaneous reserve; they represent a source of three-phase voltage for the grid – similar to a synchronous generator.
Why is instantaneous reserve important?
“In the future, grid-forming inverters will play a central role in making the energy transition a reality. They become essential when there are high levels of green electricity in the power generation mix and when grids are weak,” reveals Professor Christof Wittwer. He heads the Power Electronics, Grids and Smart Systems division at Fraunhofer ISE in Freiburg. This is the only way that grid stability can be maintained in the future without rotating mass. European manufacturers of inverters have all the expertise they need to test the technical requirements. They would also have the ability to set the technical standards internationally, says Wittwer at Conexio-PSE’s digital conference Future Power Grids (Zukünftige Stromnetze).
“Just like you can’t significantly accelerate or decelerate the speed of a heavy goods train, the inertia of the rotating mass ensures that the frequency in the grid can only change relatively slowly,” describes Professor Bernd Engel from the Technical University of Braunschweig. He runs the department of components for sustainable energy systems. This is important for system stability, he says, because all measures taken by the transmission grid operators, such as control power, are tuned to the slow rate at which the frequency changes, for example, to keep the frequency within the maximum permitted green traffic light range of 49.8 to 50.2 hertz. “Outside the frequency range of 47.5 to 51.5 hertz, there’s actually a large-scale collapse of the interconnected grid,” Engel explains. This is known as a blackout.
Hybrid power plant and storage concepts offer some help
Up to now, the inverters of the photovoltaic installations have been controlled by the interconnected grid. However, with their grid detection, they provide support in the event of a fault, for instance if there is a frequency deviation or voltage fluctuations. “The more installed wind and photovoltaic systems there are in the grids, the more likely it is that operating states with purely green power generation will accumulate – for example on windy or sunny days,” explains ISE’s Professor Wittwer. Shutting down large fossil-fuel power plants in the next few years will mean a lack of rotating mass that can stabilize the grid. This is precisely why inverters with the ability to form a grid will have to ensure stable grid operation as we go forward. Experts refer to this as voltage-imprint operation.
As more and more storage systems are being installed, this task is also falling to the inverters of electricity storage systems, explains Wittwer. These types of hybrid power plant and storage concepts are already being developed. In addition to instantaneous reserve, these systems can help compensate fluctuations in the grid and active grid filtering of harmonic waves or the like.
As it stands, inverters are primarily designed to convert direct current into alternating current. However, their electrical behavior is not physically determined, but defined by certain algorithms. “In addition to minor hardware changes, the grid-forming inverter primarily requires a completely new control system,” says Professor Engel, who also worked for the manufacturer SMA for a long time. This calls for new software programming. “Of course, grid connection rules, parameterizations and certifications must be adapted for this new type of inverter beforehand,” he emphasizes.
Insights from the VerbundnetzStabil project
According to Sönke Rogalla, the consensus among transmission grid operators is that grid-forming inverters will be needed for most new plants that are connected to the grid. Rogalla, who has a doctorate in engineering, heads the Power Electronics and Grid Integration department at Fraunhofer ISE. He also coordinates the VerbundnetzStabil project, which is currently developing and testing various devices and algorithms.
Since 2017, Rogalla and his ISE team have been working closely with several partners such as the power grid operator TransnetBW and Kaco New Energy from industry. “We’ve succeeded here in bringing together expertise from the field of power electronics and control engineering with expertise in grid dynamics and interconnected grid control in a unique configuration,” describes Rogalla. This made it possible to examine the use and the exact requirements of grid-forming inverters on a larger scale. Grid-forming inverters are programmed to behave like a voltage source. Like conventional power plants, they cover the grid’s demand for instantaneous reserve.
Rogalla is visibly satisfied with the results: Once again, the tests showed clearly that a conversion from synchronous generators to grid-forming inverters works. At the same time, the project was able to clearly determine what the grid of the future really needs. Proposals for important technical details can now be made based on a test guideline that has been developed. That’s because there still isn’t a clear standard, he adds. “We want to use this to support the industry in the technical evaluation of suitable devices for the upcoming market launch of grid-forming inverters,” says Rogalla. To ensure grid operation with grid-forming inverters on a large scale, testing must start today.
Conclusion: Purely power-electronic grid operation is possible
In the joint research project Grid Control 2.0 (Netzregelung 2.0), funded by the Federal Ministry for Economic Affairs and Climate Action, evidence was also provided that even larger system disturbances such as grid splits in the transmission grid can be controlled with a completely electronic grid. For this to be possible, at least 30 percent of the inverter power must be grid-forming. The last time there was a system split of this kind was on 8 January 2021: Due to massive frequency disturbances, there was a split into sub-grid areas in the continental European interconnected area at that time.
“The technical operation of purely power-electronic grids also works without rotating masses,” confirms Wittwer. But economic representation is also important, he says. Every location has different prerequisites that have to be taken into account. Wittwer: “For example, in a country with a lot of hydropower, such as Austria or Switzerland, it might still make sense to use a synchronous generator, but if there’s a lot of wind power in the grid, pure inverter operation is more suitable.”
From Niels Hendrik Petersen
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