The research project “Fuchstal-leuchtet” aims to investigate whether and how a stable grid operation can be achieved in practice within a network that lacks a centralized control structure and is powered and loaded exclusively by power converters. A key component of the research project is the investigation of the interaction between multiple power inverter systems and the identification and analysis of undesirable interactions between the control strategies of grid-following and grid-forming inverters under real-world conditions. Therefore, the first field test focused on studying the interaction of two grid-forming inverters with varying parameter configurations and control algorithms in an island grid. By selectively switching different loads, the dynamic and static f(P)- and V(Q)-behaviour of the grid-forming inverters was validated, and critical parameter configurations along with the resulting control interactions were identified.

Building on this, the second field test expands the existing islanded grid by integrating four wind turbines, each with a rated apparent power of 3 MVA. These turbines operate in accordance with the currently applicable technical connection guidelines for grid-following generation units in medium-voltage networks. The island mode operation conducted during the field test allows for the reconfiguration of the control parameters of the Farm Control Unit. This enables both the dynamic and static Q(V)- and P(f)-behaviour of the grid-following inverters to be defined and modified.

In preparation of this field test, parameter settings and test scenarios based on manufacturer specific RMS simulation models are developed to analyse the influence of outer-loop P(f)- and f(P)-control strategies of grid-forming (GFM) and grid-following (GFL) inverters on frequency behavior. Varying the control parameters enables the identification of undesired interactions between grid-forming and grid-following units, which can be triggered by disadvantageous parametrisations of both GFM and GFL inverters.

Within the scope of the paper, the following investigations are carried out in preparation for the second field trial, and the corresponding findings are presented:

• Influence of outer-loop control strategies of grid-forming and grid-following inverters on frequency behavior in the island grid

• Identification of critical parameter sets and their representation using RMS simulation models

• Evaluation of existing technical connection rules for grid-following generation units with respect to their applicability in island grid operation