Abstract-For AC railway power supply systems with a different frequency than the public grid, high-voltage AC (HVAC) transmission lines are common, connected to the catenary by transformers. This paper suggests an alternative design based on an HVDC (High Voltage DC) feeder, which is connected to the catenary by converters. Such an HVDC line would also be appropriate for DC-fed railways and AC-fed railways working at public-grid frequency.The converter stations between the public grid and the HVDC feeder can be sparsely distributed, not denser than on 100 km distances, whereas the converters connecting the HVDC feeder to the catenary are distributed denser. Their ratings can be lower than present-day substation transformers or converters, since the power flows can be fully controlled.Despite a relatively low power rating, the proposed converters can be highly efficient due to the use of medium frequency technology. The proposed feeding system results in lower material usage, lower losses and higher controllability compared to present solutions.Simulations of the proposed solution show clear advantages regarding transmission losses and voltages compared to conventional systems, especially for cases with weak feeding, and when there are substantial amounts of regeneration from the trains.

AbsRact-Design considerations are presented for a medium frequency transformer in a line side power conversion system for electric railway traction. The system employs soft commutation in order to reduce switching losses and allow for high operating frequency of the transformer.Two different designs are evaluated; The first one is dry insulated and the second one is liquid-immersed. Calculations indicate that a 1 MVA transformer operating at 4 kHz could he realized with au active weight of helow 150 kg for both dry and liquid-immersed transformer concepts.The transformers are modeled magnetically, electrically and thermally, and a geometric optimization procedure based on a cost function is applied lo achieve an optimum design.

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