Single-phase cascaded H-bridge multi-level active power filter based on direct current control in AC electric railway application

A single-phase cascaded H-bridge multi-level active power filter (APF) to deal with the power quality problems in the AC electric railway systems is presented. The APF can connect to the traction busbar directly without any transformer or resonance branch. The direct current control combining the current decoupling transformation is introduced in the control strategy. This strategy simplifies the control system, enabling the least amount of current measurement with the existing sensors and removing complex current reference extraction algorithms. The control system design process, especially for the parameter tuning, is mainly discussed. Finally, simulation and experimental results verify the effectiveness of the proposed APF system applied in the AC electric railway systems.