Image: Fraunhofer IEE

Grid control 2.0: Power converters can keep the interconnected grid stable even with very high shares of renewable energies

In the future, the power supply system can also be operated without the stabilizing effect of the synchronous generators of conventional power plants. This was jointly stated by the experts of the joint project “Grid Control 2.0” at the final conference in Kassel today. The research project was able to show that plants with grid-forming power converters can provide instantaneous reserve and thus stabilize the system even in extreme situations. At the end of the project, power grid operators will discuss the detailed results and questions regarding the introduction of the new technology with technical experts and representatives of the Federal Ministry of Economics and Climate Protection and the Federal Network Agency.

Up to now, it has mainly been the synchronous generators of large power plants that have ensured that the frequency and voltage requirements in the power grid are met. However, with the energy transition, power plants are increasingly being replaced by wind energy and photovoltaic systems that are coupled to the electrical grid with power converters. The joint project “Grid Control 2.0”, coordinated by Fraunhofer IEE, was able to demonstrate that generation plants with grid-forming power converters can provide instantaneous reserve and thus stabilize the system even in extreme situations. At the final conference, power grid operators will discuss the results of the research project and issues relating to the introduction of the technology with experts and representatives of the German Federal Ministry of Economics and Climate Protection and the German Federal Network Agency. On the previous day, grid-forming power converters were demonstrated in operation in laboratories on the new campus of Fraunhofer IEE in Kassel, among others.

The results of the joint project “Grid Control 2.0” on control and stability in the converter-dominated interconnected grid contribute to the further development of application rules. Sponsored by the program, which started in 2018 and is now coming to an end, is the
joint project from the German Federal Ministry for Economic Affairs and Climate Protection with around 10 million euros.

Alexander Folz, Government Director in the System Security Division of the Federal Ministry of Economics and Climate Protection states, “The Grid Regulation 2.0 project has addressed key challenges for future system operation. The results come at just the right time for the Roadmap System Stability of the Federal Ministry of Economics and Climate Protection.”

“We are convinced that the interconnected grid – and, in the event of a fault, subgrids as well – can be kept stable even with very high converter shares. However, this requires suitable control methods. We have determined the requirements for these procedures and developed control procedures to ensure that the power converters can serve safe and stable system operation,” explains project manager Dr. Philipp Strauß, deputy director of Fraunhofer IEE in Kassel. Particular attention is paid to the development of a suitable transformation path. “New technologies must be seamlessly integrated into existing grid control processes. The transition must be designed so that the emerging system is at least as stable as the current one,” says Strauß.

In addition to Fraunhofer IEE, the project involves the Technical University of Braunschweig and the University of Kassel, the distribution grid operator EWE NETZ GmbH, E-ON SE with its subsidiaries Westnetz GmbH and Mitteldeutsche Netzgesellschaft Strom mbH, the converter manufacturers and system providers SMA Solar Technology AG and SiemensEnergy Global GmbH & Co. KG, the European Distributed Energy Resources Laboratories (DERlab e.V.) for international networking, the Deutsche Energie-Agentur GmbH (dena), the four operators of the German transmission grid, and the Forum Netztechnik und Netzbetrieb (FNN) in VDE as the institution defining the grid connection rules. The project thus includes representatives of all the main stakeholder groups affected by this issue. “This was essential to the success of our work,” Strauß notes.

Continental European interconnected network in view

The new control procedures and their stabilizing influence were simulatively evaluated under extreme scenarios, such as a system split – i.e. a grid separation across Europe. Strong voltage dips were not only calculated, but also carried out in the accredited laboratory according to existing standards. A particular challenge for the grid-forming power converters here are suitable and extremely fast current limiting processes. The research teams were able to demonstrate that even under such harsh conditions, a contribution to network formation can be made.

Grid-forming properties are now also required of systems with very large converters, which are used, for example, for power factor correction in the transmission grid or in head-end stations of high-voltage direct current transmission (HVDC). In addition, the project will contribute to how these requirements can be reflected in the technical application rules for generation plants in the future. In all of this, the project partners also take into account the international perspective – after all, the German power grid is embedded in the continental European interconnected grid.

Research questions of the project

At the final conference, Philipp Strauß summarized the results on the research questions: “In the project, grid-forming control methods were further developed with optimized current limiting methods. Spatial distribution of grid-forming equipment is necessary and instantaneous reserve can be provided in the transmission and distribution grids. New islanding detection methods have been developed. Battery systems, wind turbines, photovoltaic systems, rotating phase shifters, statcoms, and electric loads, among others, can be grid-forming with the new control methods. A pure converter grid by grid-forming control is possible and the seamless transition with different shares of synchronous machines is realizable. A specification and new test methods have been developed for electrical inertia, net formation and damping, among others.”

A central topic here is the interaction between power converters and synchronous generators. For example, the partners analyzed in which combination and at which voltage level synchronous generators as well as current-influenced or voltage-influenced converters are necessary or permissible in order to maintain system stability. In addition, the experts evaluated the voltage quality at the various operating points of the future system against the background that the share of synchronous generators will vary more and more with the further expansion of renewable energies. In this context, the experts also investigated which robustness requirements result from this for the stress imprinting controls.

Last but not least, they also addressed the question of the extent to which stable system behavior is possible even with completely converter-based generation. The project thus makes an important contribution to achieving Germany’s climate protection targets.

Further need for research

“The instantaneous reserve, which has so far come from the rotating masses of conventional, fossil-fuel power plants and stabilizes the power system’s grid frequency, can also be effectively provided in a decentralized manner by grid-forming inverters at all voltage levels of the distribution grid. “, specifies Prof. Dr. Bernd Engel of TU Braunschweig, “In a further research project, we want to address remaining technical challenges, such as possible power oscillations between the inverters, compatibility with existing protection devices and the risk of unwanted islanding.”