Current-Aware Adaptive Virtual Inertia Control for a VSM-Controlled Grid-Forming MMC in a Low-Inertia Grid

The increasing penetration of renewable energy sources and converter-interfaced generation reduces the natural mechanical inertia of modern power systems, leading to higher rate of change of frequency (RoCoF), deeper frequency deviations, and reduced dynamic stability margins following active-power disturbances. Grid-forming (GFM) converters provide an effective means of restoring inertial support by establishing voltage and frequency references and emulating synchronous-machine dynamics. This paper presents a simulation-based adaptive virtual inertia control strategy for a virtualsynchronous-machine (VSM)-controlled modular multilevel converter (MMC) connected to a low-inertia grid. The proposed controller adjusts the virtual inertia according to frequency deviation and RoCoF while incorporating measured converter current as a supervisory variable to coordinate frequency support with converter operating capability. A MATLAB/Simulink model is used to compare grid-following operation, GFM operation with fixed virtual inertia, and GFM operation with adaptive virtual inertia. Under the studied 10 MW active-power disturbance, the adaptive-inertia strategy reduces the maximum RoCoF from 3.0 Hz/s to 1.5 Hz/s relative to fixed-inertia GFM operation and improves the frequency nadir from 49.994 Hz to 49.997 Hz while maintaining the same reported settling time of 0.100 s. The assessment is further extended using the electricalresponse results of the developed model under higher active-power disturbances, including converter-current magnitude, DClink voltage, frequency response, RoCoF, and virtual-inertia variation. For the most severe investigated case, the results show that adaptive and current-aware operation modifies the inertia demand according to the disturbance severity while maintaining the converter current within the imposed operating range. The current-aware supervisory action also provides improved coordination between the adaptive inertia command and the converter electrical response, particularly in terms of DC-link and inertia behavior. Overall, the results demonstrate that the proposed VSM-based adaptive inertia framework improves lowinertia frequency response while providing a practical means of coordinating dynamic inertia support with converter electrical constraints in GFM-MMC systems.