As part of the transition to an energy system with 100% renewable generation, renewable energy power plants must increasingly take over responsibility for system stability. A key element in ensuring system stability are grid forming units. In Germany, scenarios predict a significant expansion of renewable energy generation and storage systems in distribution networks. Therefore, it makes sense to investigate which of these devices should be equipped with grid-forming characteristics and to what extent. Such integration of grid-forming systems into distribution networks raises several open questions that must be addressed. This contribution presents current research findings on some aspects of operating distribution networks with grid-forming systems.

Power system oscillations: The integration of grid-forming converters can pose challenges related to oscillations in the distribution grid. This investigation focuses on Virtual Synchronous Machine converters, which emulate the dynamics of traditional synchronous generators by using the swing equation. However, this emulation can unintentionally lead to oscillation issues similar to those found in transmission systems. The study reveals that as the level of virtual inertia in the distribution grid increases, these oscillations may become more pronounced. This highlights the importance of implementing effective damping measures to address potential instabilities.

Operation of weak networks: The impact of distributed generation connected to distribution grids is investigated in a case study evaluating SCR, eSCR and fault currents.A decreasing system strength also impacts voltage control. Therefore, a continuous voltage control was introduced with the update of the German standards (VDE-AR-N 4120/4110). This continuous voltage control plays a crucial role in maintaining stable voltage levels in the distribution grid with short-circuit ratio SCR≥3 without being explicitly grid-forming. Thus, the number of required grid-forming units might be reduced. The setup and the dynamic performance of the continuous voltage control is analyzed.

Temporary island operation: In principle, grid-forming inverters enable the stable continued operation of critical sub-grids after a fault-related disconnection from the supply grid. The continued operation of disconnected sub-grids can increase the resilience and supply reliability of the power supply. Synchronization provides the opportunity to reconnect the sub-grid to the interconnected grid without first blacking it out. This function is simulated in the laboratory using a setup consisting of a grid-forming inverter, parallel generators and loads on a grid connection. Both disconnection and interception of the grid, as well as resynchronization and reconnection of the sub-grid with active generators and consumers are simulated in a series of laboratory tests and the basic suitability of this technology and the necessary requirements for this application are explained.

The research presented in this paper is supported by the German Federal Ministry for Economic Affairs and Energy in the frame of the project “Verteilnetz2030plus” (FKZ 03EI4067). The authors are solely responsible for the content of this publication.