Real-time simulation of Grid-Forming inverters as DC/AC interfaces for DC-nanoGrid in grid-connected and islanded AC Microgrids

The transition toward renewable energy sources has led to widespread adoption of inverter-based generation systems employing Grid-Following control, which reduces the natural inertia of power systems and increases the risk of frequency instability. To mitigate this issue, Grid-Forming inverters have emerged as a key technology, enabling decentralized frequency and voltage control. Among Grid-Forming approaches, Virtual Synchronous Generator strategies aim to replicate the dynamic response of synchronous machines, offering support for frequency and voltage stability. This paper proposes a Grid-Forming control model for inverters interfacing DC-nanoGrids with the power system, integrating Load Frequency Control principles to enhance grid stability. The proposed strategy enables the DC-nanoGrid to contribute to frequency regulation by adjusting its power output in response to system disturbances, thereby providing both synthetic inertia and primary frequency support. Additionally, depending on the operating mode, the control can either contribute to restoring the nominal frequency (secondary frequency regulation) or track an active and reactive power reference. To validate the proposed control model, Real-Time Hardware-in-the-Loop simulations are performed. In these simulations, the control model is integrated into a microcontroller and interfaced with a high-fidelity physical system, modeled in a Real-Time simulator. This approach allows for a realistic evaluation of the system's dynamic response and control performance. The control is tested in two scenarios: grid-connected mode, where the nanoGrid provides services such as synthetic inertia and primary frequency regulation while tracking an active and reactive power reference in steady-state condition; off-grid (islanded) mode, where the nanoGrid's ability to maintain both voltage and frequency at nominal values is assessed. The simulation results highlight the effectiveness of the proposed control model in stabilizing frequency during transient conditions and in ensuring reliable operation both under grid-connected and islanded modes.