Impedance modeling and stability analysis of AT-fed flexible traction power system for heavy haul railways

The autotransformer (AT)-fed flexible traction power system (FTPS), integrated with energy storage (ES), supplies power to freight trains (FTs). However, the widespread deployment of power electronic converters introduces low-frequency voltage oscillations, posing significant challenges to system stability. In this paper, a sequence impedance model of the FTPS is developed, incorporating both ES converters and FT converters under various control strategies. The model achieves an accuracy error of less than 4% for FT and less than 8% for ES. Moreover, it effectively captures the dynamic interaction between ES and FT under different traction network configurations. Based on the generalized Nyquist stability criterion, the impacts of traction network operation modes, controller parameters, and the number of ES and FT units connected to the system on overall stability are thoroughly analyzed. Experimental results demonstrate that the Scott-connected transformer configuration in the fully parallel traction power supply mode offers the highest system stability.