Evaluation of negative-sequence-current compensators for high-speed electric railways

This paper evaluates three alternative topologies for a static compensator (StatCom), suitable to eliminate the negative-sequence currents in a railway system. The compensator is placed in a transformer substation, which feeds two separate sections of the catenary. The major requirements of this converter is to: (i) symmetrize the railway load as seen from the high-voltage side, (ii) control the catenary voltage to a defined level, and (iii) compensate for the harmonics generated by the rolling stock. The topologies investigated here are: (a) a 2-level converter with a common dc-bus capacitor, (b) a delta-connected modular multilevel converter (MMC) based on full-bridge submodules, and (c) a double-star MMC based on half-bridge submodules. The evaluation is based on the minimum device-rating requirements of each converter that fulfill the application requirements in the steady-state. Simulation results are provided to support the conclusions of this evaluation.