Quadratic Sensitivity Models for Flexible Power Quality Improvement in AC Electrified Railways

With the development of flexible traction power systems (TPSs), more active and flexible power quality improvement is becoming feasible for ac electrified railways. However, the standard definitions of power quality indexes of electrified railways are strongly nonlinear, which poses challenges to the optimal real-time power quality control of flexible TPSs. Therefore, this letter establishes universal and concise mathematical models that directly link the active and reactive power outputs of the traction transformer to the negative sequence current (NSC) and effective power factor (EPF) of traction substations (TSSs). With that, the quadratic sensitivity models (QSMs) of the power outputs of the traction transformer to the NSC and EPF of TSSs are developed. These QSMs can accurately estimate the varieties of NSC and EPF with the varieties of the power outputs of the traction transformer. To demonstrate their utility, two application schemes of the proposed QSMs for optimal real-time power quality control in flexible TPSs are explored and verified via hardware-in-the-loop experiments.