A Hybrid Cascaded Active Power Quality Conditioner for Railway Power System

Series-type hybrid railway power conditioners (S-HRPCs) have been proposed to compensate the negative sequence currents (NSCs) and reactive power in the railway power systems (RPSs) with reduced converter capacity. However, their practical application is limited by mismatched intermediate DC voltages. To address this issue, a novel thyristor-controlled LC-coupled (TCLC) hybrid cascaded active power quality conditioner (HC-APQC) is proposed in this article. Owing to its distributed DC capacitor configuration, the HC-APQC inherently eliminates the DC voltage mismatch issue. Furthermore, leveraging the adaptive impedance of the TCLC, the HC-APQC significantly reduces both the required operating DC voltage and the capacity of the cascaded converter. The operating principle of the TCLC is first analyzed, followed by an investigation of the DC voltage mismatch mechanism in S-HRPCs. Then, a two-stage power flow distribution strategy is developed to determine the HC-APQC reference currents for NSC and reactive power compensation under fluctuating traction loads and unbalanced grid conditions. Considering converter capacity constraints, the proposed strategy ensures real-time cascaded submodules (SMs) capacitor voltage equalization and current limitation within the rated range. Finally, experimental results are presented to validate the effectiveness of the proposed system and its control strategy, demonstrating that the HC-APQC achieves a 45% reduction in the required DC voltage compared with conventional APQCs.