The central research question addressed by the “Fuchstal leuchtet” project is whether and how stable grid operation can be achieved within a purely inverter-based island grid (grid-forming and grid-following converters) based on their applicable technical connection rules (TCR). This includes island grid operation as well as the transition between synchronous and island mode, and vice versa. Given recent advancements (e.g., VDE FNN Guideline “Technical requirements for grid-forming capabilities including provision of inertia”), operational requirements now define the synchronous control behavior of grid-forming (GFM) inverters, supplementing the established TCRs for grid-following (GFL) systems. While another paper examines the process of intentional islanding with GFM and GFL inverters, which will be examined practically in the 3rd field trial, this study focuses on the evaluation of the synchronous grid operation capability of the two GFM inverters, based on the manufacturer's parameterization according to the VDE FNN Guideline.

EMT simulations are conducted to analyse the grid behavior of the GFM inverters within the available grid section of the “Fuchstal leuchtet” research project. For this purpose, the entire grid section (cables, transformers), the overlying high-voltage grid, and the two grid-forming inverters are modeled in PSCAD, utilizing manufacturer-specific EMT simulation models for the GFM units. The parameterization, and consequently the control behavior, of the GFM inverters is based on the manufacturer's implementation of the VDE FNN Guideline. It is important to note that these inverters are older inverters which can now act as grid-forming units through software modifications. To evaluate their grid behavior, the following aspects are examined:

• Voltage source behavior
• Active power response to a phase angle step
• Effective impedance
• Response during underfrequency and overfrequency ranges (PCNB)
• Continuous voltage control – Setpoint response
• Damping of power-frequency-oscillations
• Response to steep frequency gradients (RoCoF)

The full paper provides a detailed explanation of the individual tests according to the VDE FNN guideline and their possible implementation in the field lab, demonstrating the extent to which the GFM inverters comply with these requirements. Building on the simulation results, a field test will be conducted in coordination with the distribution system operator (DSO). This practical trial aims to validate the simulation results using specific, feasible scenarios thereby demonstrating the reliable synchronous operation of GFM units in the distribution network.