Introduction: A flexible traction substation provides power to the trains and the communication and signal facilities at the same time, taking into account the scarcity of strong power sources in the weak power grids. However, the severe impact of single-phase traction loads gives rise to negative sequence (NS) and voltage sag (VS) problems, which compromise the safe operation of electrical equipment. Methodology: To address these issues, this paper proposes a flexible traction power supply topology that integrates a power flow controller, a quasi-Z-source inverter, a dynamic voltage restorer, and photovoltaic (PV) cells, together with a comprehensive compensation strategy for NS and VS based on power quality standards. Results: The strategy employs the three-phase voltage unbalance factor (VUF) as a constraint and quantifies the influence of NS on VS detection accuracy, thereby achieving optimal NS compensation and in-phase VS compensation. Discussion: Hardware-in-loop (HIL) experiments confirm that the proposed structure and strategy reduce compensation power demand, maintain the VUF within 2%, and stabilize the grid voltage at 1 p.u. by providing active power support via the PV system, thereby reducing the capacity requirement of the primary compensation devices. Conclusion: These results demonstrate the capability of the integrated system to mitigate the impacts of single-phase traction loads and enhance power quality in weak grids.
Sun et al. (Fri,) studied this question.