Stability Analysis and Loop Reshaping for DC-link Voltage Synchronized Grid-Forming PVs

As the penetration of photovoltaic (PV) sources in the power system continues to rise, it is important for PV converters to operate in grid-forming (GFM) mode. Since the dc voltage is not controlled by the PV side, GFM PVs should have capabilities for dc voltage regulation and ac side support. However, most research on GFM PVs has concentrated on the stability related to dc-link voltage synchronization control (DVSC), neglecting the impact of ac voltage amplitude control (AVC) on the stability of GFM PVs. In this paper, a new low-frequency oscillation issue in GFM PVs is firstly identified. To investigate the destabilization mechanism of this phenomenon, a small-signal model is derived, revealing that the low-pass filter of the reactive power introduced in the AVC loop and the AVC loop coupled into the DVSC loop are two key factors leading to instability. This paper proposes an effective approach to suppress the low-frequency oscillations, which feeds forward reactive power dynamics into the DVSC loop through a proportional gain. This strategy not only addresses low-frequency oscillations but also improves the transient performance of the GFM PVs. Moreover, this strategy enables the GFM converter to operate stably across a wide range of short-circuit ratios (SCR). Finally, simulation and experimental results validate the accuracy of the theoretical analysis and the effectiveness of the proposed method.