Stability Prediction and Damping Enhancement for MVdc Railway Electrification System

As three-phase ac-dc power converters have been extensively employed in dc systems, the grid-interface interaction stability is currently an important concern for the power industry. In particular, the interaction between a flexible traction substation (FTS) and dynamic electric trains may cause dc system instability. This paper presents a methodology to perform stability investigation of a medium-voltage direct current (MVdc) railway electrification system (RES). A small-signal model in the synchronously rotating dq frame of an MVdc RES, consisting of an FTS, electric trains, and a catenary network, has been derived. Results show that dynamic changes of trains (e.g., number, location, power, controller parameters, and capacitor aging), control parameters of FTSs, ac grid impedance, and the renewable energy source connection substantially affect the system stability. This analysis facilitates a design-oriented study. A virtual damping control strategy in the FTS is selected to enhance the system stability. Finally, the theoretical analysis is validated by performing a semiphysical experiment.