Analysis of 1 Hz Oscillations Induced by Grid-Forming Plant in Kaua'i Power System

The real-world sub-synchronous oscillation events arising from grid-forming (GFM) inverters have been rarely reported so far. This paper analyzes one real-world 1 Hz oscillation event in Kaua'i Island power system with two GFM-based plants. First, our measurement-based analysis shows that one droop-based GFM plant is the source of the 1 Hz oscillations. Thereafter, our model-based studies reveal that the 1 Hz oscillations in Kaua'i are probably caused by the power plant controller of the droop-based GFM plant, instead of the inverter-level droop control. This is supported by our comprehensive simulation studies and smallsignal analysis. Thus, we believe that the GFM controller at the inverter level bears no responsibility in this specific event. Finally, we perform the Electromagnetic Transients (EMT) simulation study to validate that properly tuning PPC can address the Kaua'i 1 Hz oscillation event.