A robust multilevel hybrid compensation system for 25kV electrified railway applications

On many electrified railway systems with single-phase 25kV industrial frequency supply, the power quality can be particularly poor when conventional SCR-based locomotives are operating, and this constrains the amount of power that can be delivered to the locomotives. This paper presents a shunt compensation system consisting of alternatively a cascaded or a reduced topology multilevel active power filter, and a low rating damping filter. The active filter is controlled by a novel hysteresis current regulation strategy, and both mitigates low order voltage harmonic distortion along the feeder and provides RMS voltage support. The passive filter damps harmonic resonances that are typical in such 25kV traction systems. The results show that the filter system can significantly increase traction system power transfer capacity with only a relatively small capital investment, allowing older thyristor based locomotives and increased traffic levels to be supported without necessarily requiring a complete system upgrade. Simulation and experimental results are included showing the performance of the filter for both steady state and transient conditions.