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 islanded 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.

Similar to the first field test, the control parameters responsible for the dynamic and static behavior of both grid-forming and grid-following inverters are varied. In particular, this includes the V(Q)- and f(P)-control parameters of the grid-forming battery inverters. By modifying the P- and I-parameter of the active and reactive PI-controller, dP/dt and dQ/dt-gradients as well as the P(f)- and Q(U)-characteristic curves the dynamic and static behaviour of the grid-following wind power inverters can be changed [3,4]. The entire islanded grid is EMT- and RMS-modelled in PSCAD and PowerFactory using manufacturer-specific simulation models of both grid-forming and grid-following inverters. Varying the control parameters enables not only the identification of undesired interactions between grid-forming and grid-following units, but also the derivation of concrete recommendations for grid-supportive parameterization of generation units operating within the network.

The full paper provides a detailed overview of the control parameters of both grid-forming and grid-following inverters that were varied within the simulations. Based on the observed frequency and voltage behaviour in the simulations, recommendations for appropriate parameter sets are derived to ensure grid stability of the island grid with a SCR ~ 0.5 as well as in interconnected grid operation.