To ensure stable grid operation in the future, the proportion of grid-forming systems is currently being drastically increased. For this reason, current research is increasingly focussing on possible interactions between grid-forming inverters and existing systems. These existing systems include grid-connected photovoltaic and battery inverters at the low-voltage level. A high proportion of solar generation is found at the low-voltage level in particular, which is why the integration of grid-forming systems, especially at the low-voltage level, is currently being actively promoted by the German grid development plan.

For effective parallel operation with existing systems, it makes sense for grid-connected systems to fulfil the requirements placed on converters in accordance with VDE-AR-N 4105 as precisely as possible. This also includes static voltage maintenance, which makes it possible to counteract the voltage increase caused by feed-in and to significantly increase the absorption capacity of distribution grids for decentralised energy systems. This motivates the investigations published here in the “Fuchstal-leuchtet” and “Verteilnetz 2030+” projects as to whether grid-forming systems are also capable of implementing the static voltage maintenance methods in accordance with VDE-AR-N 4105. This is a major challenge, particularly due to the fact that grid-forming systems directly influence the voltage.

In a previous work, the ability of a grid-forming inverter to realise a Q(U) characteristic curve in a laboratory environment in parallel operation with a grid-following inverter has already been demonstrated. In this work, the dynamic interactions of the two inverters are considered. This includes different start-up time constants of the Q(U) control and varying positions of the inverters in the grid. In addition, the reaction to rapid voltage changes is analysed.