Demonstrating the Need for Adaptive Droop Control for Grid-Forming Inverters in Islanded and Grid-Connected Modes

Grid-forming (GFM) inverters are critical grid assets for ensuring voltage and frequency stability in modern microgrid systems, and they also provide grid stabilizing capability for bulk power systems. These GFM inverters primarily rely on droop control for decentralized operation in both islanded and grid-connected modes; however, using fixed droop settings-particularly tight droop coefficients-across both operational modes can lead to system instability, especially in grid-connected scenarios. In this study, we investigate the behavior of a pure hardware microgrid comprising multiple GFM inverters from different vendors, multiple grid-following inverters, and a diesel generator. Experimental observations demonstrate that tight droop settings destabilize grid-connected operation due to interactions among GFM inverters and between GFM inverters and the grid. We propose an adaptive droop strategy, including a rule of thumb for modifying droop coefficients based on grid operational status (islanded/grid-connected) and loading conditions. Our findings highlight the importance of adaptive droop control to ensure stable and reliable microgrid operation.