Due to the loss of power coming from synchronous generators, it is important from a grid stability perspective to provide new power electronic-based generation with grid-forming (GFM) control as an alternative source of inertia in the power grid. The discussion goes as far as whether GFM inverters should be introduced to the low-voltage (LV) grid. This would have the further advantage that in the event of a power outage, the LV grid could permanently continue to operate stably as an island, thereby increasing resilience and security of supply. However, this is in contradiction to the disconnection of grid-following (GFL) inverters required by the standards in the event of an unwanted island grid formation for safety reasons.

For islanding detection, an active procedure is often implemented, which uses a disturbance signal large enough to specifically attempt to bring the frequency or voltage within an islanded grid outside the safe operating range in order to cause the plant to be switched off. If GFM systems are integrated into the low-voltage grid, the question arises as to what extend they would challenge the islanding detection of GFL inverters due to their voltage and frequency-influencing properties.

To answer this question, a test grid is set up in the laboratory consisting of a GFM inverter in power consumption mode and a GFL inverter in power generation mode connected to the utility grid that is represented by a four-quadrant amplifier. The grid is switched off at different power balances between the GFM and GFL inverter and the islanding event is investigated for various GFM control parameters such as inertia and droop functionality.

The tests demonstrate that in many cases the GFL inverter does not detect the islanded grid when the GFM inverter is connected. In case the GFL inverter does stop the power generation immediately after the islanding, a re-energizing several seconds later can be observed for distinct test cases. Furthermore, it is shown that the islanded system balances at a new power setpoint to which the grid-stabilizing P(f)-control of the GFL inverter is contributing.