Unintended islanding refers to the disconnection of a grid section in a sequence of faults and uncontrolled continued operation of this section detached from the interconnected grid by chance. This condition should be avoided at the low-voltage level in particular, especially to ensure personal protection during repair work and to prevent damage to devices and equipment or blockages of the AWE. Therefore, current inverters are equipped with methods that recognize this condition and disconnect from the mains. It is believed that grid-forming technologies increase the risk of these islands developing and operating uncontrollably. There is therefore a need to investigate the effects of their future integration into the grid and, if necessary, to develop measures to safely detect and prevent the unwanted formation of islanded grids. The integration of grid-forming converters into the interconnected grid raises the question of how this will affect the ability of existing systems to detect islanding. This was examined in more detail in this study.

With the gradual exit from fossil and nuclear based power generation, a replacement for the reducing grid inertia is urgently needed. Due to this reason, grid-forming inverter technology is discussed as possible future replacement but will need to be implemented in massive scales to be an adequate source of transient grid stability. Therefore, different projects, such as ‘Grid Control 2.0’ or ‘VerbundnetzStabil’ have looked into the potential for grid integration of grid-forming inverters down to the low-voltage level to utilize as much future renewable grid integration for inertia provision as possible. However, the incorporation of grid-forming converters that lack dependable islanding detection mechanisms may significantly diminish the efficacy of current procedures employed by existing grid-following converters. Consequently, islanded grids that emerge due to faults or planned maintenance may not be reliably deactivated during a fault. This paper presents a solution for detecting islanding in grid-forming converters. The authors analyze the limitations of a commonly used active islanding detection method (slip-mode frequency shift) on a grid-following inverter, whose basic assumptions are undermined if a grid-forming system is operated in parallel and propose an adapted detection procedure for grid-forming converters. The proposed ‘-Q-f-droop’ method is shown to be capable of reliable islanding detection in both standalone and interconnected operation with existing plants by non-detection zones created with a suitable setup in simulation. The results are verified through laboratory tests using grid-following and grid-forming PHIL inverters. The developed method ensures reliable shutdown of formed island grids even with the integration of grid-forming systems into the low-voltage level.