On the use of frequency in the primary control of fully inverter based islanded microgrid

The present work focuses its attention on a no-inertia Microgrid (MG) with the combined presence of a Grid Forming (GFo) and a Grid Feeding (GFe) inverter. During Normal Operation (NO), the GFe inverter injects a specified amount of active and reactive power, while the GFo one acts as a slack bus. This way, the active and reactive power balance of the MG is respected until physical limits of the energy source are reached. At that time, the GFe inverter has to curtail its power. The paper aims to analytically show that the frequency measured by a Phase Locked Loop (PLL) connected at the GFe inverter busbar increases when this happens. This can be used as a signal for its control system to activate the power curtailment without the necessity of communication between the inverter controllers. This way, it is possible to mimic what happens in traditional networks, even if there are no rotating units directly connected to the network itself. Comparison with a detailed simulation is performed to assess the impact of the assumptions made to develop the analytical model and, thus, to show its effectiveness.