Low-Frequency Stability Analysis of Vehicle-Grid System With Active Power Filter Based on dq-Frame Impedance

To eliminate the low-frequency oscillation (LFO) phenomenon in the vehicle-grid system of high-speed railways, a single-phase cascaded H-bridge multilevel active power filter (APF) directly accessed to the 27.5-kV traction network is proposed in this article, which can provide the dynamic reactive and interharmonic compensation. To explore the mechanism of APF on LFO suppression, the dq-frame impedance model of vehicle-grid system with APF is established first. The impact of the second-order generalized integrator model and the duty ratio model on the impedance accuracy is discussed in detail. Then, based on the generalized Nyquist stability criterion and system power consumption criterion, the stability of vehicle-grid system and its sensitivity to parameters in different cases are compared. The results indicate that the proposed APF can significantly improve the low-frequency stability of vehicle-grid system and owns much better performance than the STATCOM proposed in previous literatures. Finally, the time-domain simulations in MATLAB and the experiments on the controller hardware in the loop platform are implemented, which further validate the accuracy of theoretical analysis and the superior performance of APF in LFO elimination.