Impact of DC Voltage Reference on Subsynchronous Dynamics in Grid-Forming Inverters

The influence of dc-side dynamics in grid-forming inverters has emerged as a critical area of study due to its implications for stability and control. A key yet unresolved question in the literature concerns the choice between measured and nominal dc voltage when determining the modulation index for the inverter's output voltage. This letter provides new insights into the distinct dynamic behaviors associated with these approaches. Through theoretical analysis, it demonstrates that the use of nominal dc voltage introduces unique low-frequency dynamics, leading to a previously unreported kind of subsynchronous oscillation. These oscillations, rooted in the interaction between dc-side dynamics and grid-forming control, pose potential challenges for system stability. Theoretical predictions are validated through hardware-in-the-loop studies.