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Efficient recovery of braking energy through a reversible dc substation

The full regeneration of the braking energy is one of the most promising sources of energy savings to transport system operators. This paper presents the outcome of a RailEnergy cooperative research program focusing on reduction of energy consumption in transport systems. Today, urban transportation systems are generally fed at 750 Vdc or 1,500Vdc via rectifier bridges and trains are equipped with modern 3-phase ac traction packages. This allow for easier and more effective implementation of dynamic braking over a wider range of speed and voltage with the possibility of feeding back some of the energy via the 3rd rail or OCS to adjacent trains. The target of this project is to improve the line receptivity of dc power systems by transferring the excess energy to the ac side and thus regenerate it, via the transformer, to the ac medium voltage distribution network that is naturally receptive. This concept transforms the traditional unidirectional dc traction into a reversible one. It is suitable to all known types of dc traction power supply systems from 600Vdc up to 3,000Vdc. The key benefits expected from reversible dc traction substations are: • Regeneration of 99% of the braking energy at all time, while maintaining priority to natural exchange of energy between trains; this will allow eliminating the braking resistors, and thus reduce the train mass and heat release; • Regulation of its output voltage in traction and regeneration modes to reduce losses, and increase the pick-up of energy from distant trains, and • Reducing the level of harmonics and improvement of the power factor on the ac side. Under an Alstom funded internal R&D programme two 750 Vdc prototypes dc reversible substations were built and tested on a dedicated tramway test-track at the Alstom plant in La Rochelle France. To reduce development time & costs the prototypes were built using state-of-the-art industrial variable speed drive converters, active harmonic filters, high power semiconductors modules, modern controls and protections, modularity, high integration while taking into account the specific requirements of the railway industry. The test results confirmed the theoretical findings and demonstrated that the reversible substations allowed regenerating all of the excess available kinetic energy of the tramway.