Understanding a Type of Forced Oscillation in Grid-Forming and Grid-Following Inverter Connected Systems

This paper investigates a novel oscillation phenomenon in systems with grid-forming (GFM) and grid-following (GFL) inverters. Unlike previous studies that primarily focus on small-signal stability, this work explores forced oscillations (FOs), where GFL inverters act as excitation sources, inducing oscillations in GFM inverters. Specifically, interharmonics generated by maximum power point tracking (MPPT) control trigger low-frequency oscillation modes in GFM inverters. To analyze this phenomenon, the paper first examines the power characteristics of the interharmonics injected by MPPT and leverages this understanding to develop a P-ω admittance model for GFM and GFL inverter connected systems. This model enables precise prediction of FO amplitudes and provides insights into the influence of factors such as the number of GFM inverters on oscillation behavior. To validate findings, hardware-in-the-loop experiments are performed to verify the observed behavior in a system consisting of photovoltaic farms with GFL control and battery energy storage systems with GFM control. Finally, merits and limitations of this work are explicitly highlighted to provide a balanced perspective.