Identification of Voltage and Current Dynamic-Induced Impedance Contributions for Insights into VSG Low-Frequency Instability

The low-frequency stability of grid-connected virtual synchronous generators (VSGs) can be degraded if power controller parameters are not properly tuned under high active power transfer and strong grid conditions. The impedance model of VSGs is an essential indicator that can reveal small-signal instability violation mechanisms. However, the whole impedance model mixes the dynamics of all control loops, making it difficult to gain insights into how high active power transfer degrades low-frequency stability via active power control. To overcome this issue, this paper decomposes the entire impedance model into three parts that are dominated by the voltage and current control, the capacitor voltage dynamic, and the output current dynamic, respectively. By identifying the capacitor voltage and output current dynamics, the proposed decomposition method provides a clearer understanding of how high active power transfer degrades low-frequency stability. Specifically, the decomposed impedance model shows that the qq-axis low-frequency phase angles of the capacitor voltage and output current dynamic-induced impedance components start from 270° and 360°, respectively, leading to possible low-frequency instability issues under strong grid conditions.