Railway electrification provides faster and reliable train journeys compared to those of diesel trains and a strong reduction of pollution in busy stations and the country-side. However, many national programmes for the electrification of new and existing railway lines have required a substantial investment for the railway infrastructure. This is because railway electrification uses AC single-phase power that requires connection to high-voltage transmission lines, which are not always available in the intended places where the railway feeder stations should be located and usually require complicated and expensive modifications of the existing layouts.
In addition, the typical power levels of heavy railways and even high-speed railways is compatible with the capabilities of medium-voltage distribution systems. However, the connection of railway feeder stations to the power distribution network would be possible only by adopting schemes that does not introduce any imbalance in the power distribution system. In contrast to the single-phase AC electrification system, DC systems satisfy this requirement, However, the level of the DC voltage is limited to around 3 kV for the limitation on the maximum short-circuit breaking current of circuit breakers, which in turns limit the maximum power of the railway. Additionally, a higher voltage of the power supply would pose problems for the traction system of the trains, which operates at voltage levels of few kV.
The aim of Medium Voltage DC Electric Railway Systems (MVDC-ERS) project is to propose a new type of MVDC traction power supply based on controlled bidirectional converters to improve the connectivity of the railway to the grid and to integrate renewable power sources to the railway electrification system. This would not only improve the efficiency of the railway supply, but it will give additional capacity to the power distribution grid, as railway electrification lines could be used to provide extra capacity between the nodes where the substations are connected. This would be especially important for future scenarios where a higher proportion of renewable energy sources will be introduced in the power system and the control of the power flows will be vital to maintain the correct functionality of the power system. With reference to the on-board traction system, the project investigates DC Power Electronic Traction Transformers (PETT) to adapt the catenary voltage for the traction system of the trains.
The key objectives of the project are to introduce new high-efficiency topologies of power conversion systems to convert medium-voltage AC power into medium-voltage DC power with the capability of limiting the short-circuit current, to introduce high-power density topologies of power converters for on-board DC transformers, to investigate the impact of the forthcoming wide band-gap semiconductor devices in terms of efficiency and voltage level for the converters of the feeder stations and in terms of weight and volume for the traction converters, to understand how the new railway electrification system should be controlled and protected when renewable power sources are integrated, to understand how on-board energy storage can be exploited and how it can optimise the operations of the network, and to work with industrial stakeholders to investigate the marketability of the new electrification system and trains.
