Low-Frequency Stability Analysis of Single-Phase System With $dq$-Frame Impedance Approach—Part II: Stability and Frequency Analysis

Nowadays, the power electronics-based systems sometimes present the low-frequency instability owing to the interaction between the power converters and the grid, such as the low-frequency oscillation (LFO) happening in the single-phase vehicle-grid (electric multiple units (EMUs)-traction power grid) systems of high-speed railway. To address this problem, the impedance-based stability analysis method is adopted. Given the grid impedance and the $dq$-frame converter impedance in the EMUs, several popular impedance-based stability analysis methods are compared, and their features and applicability are summarized. The generalized Nyquist stability criterion is then applied to more cases to validate the proposed impedance model for the single-phase converters, and excels in good estimation of the stability boundary and oscillation frequency. The analysis results are validated by the simulation and dSPACE tests. Through the comprehensive analysis in this paper, the mechanism of LFO in high-speed railway system is further elucidated, and it is suggested that the negative impedance of the converters will contribute to the LFO.