Stability Criterion for Near-area Grid-forming Converters Under the Weak Grid Condition

This paper introduces a streamlined stability criterion to systematically examine the interaction mechanisms among grid-forming wind turbine generators (GFM-WTGs), particularly within the same wind farm. To elucidate the interaction mechanisms among multiple GFM-WTGs operating under identical conditions and controller parameters, the concepts of common-mode and differential-mode currents, voltages, and impedances, which significantly simplifies the stability criterion, are presented in this paper. Since the internal dyanmics of the machine-side converter of GFM-based permanent magnet synchronous generator (PMSG) are disragraded, GFM-WTG is simplified as GFM-based voltage source inverter (GFM-VSI) without losing generality. The stability criterion reveals the characteristics of common-mode and differential-mode currents, allowing for the identification of the dominant factors contributing to system instability. Besides, the influence of connection strengths between GFM-VSI and controller parameters on system stability is examined. Both hardware-in-the-loop and physical experiments based on two paralleled GFM-VSIs validate the effectiveness of the proposed stability criterion.