Gyuláné Vincze, Gergely György Balázs
Budapest University of Technology and Economics Department of Electric Power Engineering
The urban electric vehicles are operating with DC voltage network. The nominal voltage of the overhead line is different. For example in Budapest: 600V (tram), 825V (metro), 1100V (suburban train), 1500V (cog wheel train), the permissible voltage range from the nominal value is +20%...-30%. In urban traffic conditions it is characteristic when the stops and the reasons that stop the vehicle are frequent therefore the load of the overhead line is dynamically varying. Between two stops launching, accelerating, coasting, braking, stopping, waiting phases are repeating. For energy saving purposes the power-off coasting - that has no energy demand - and at modern vehicles the regenerative braking is the often used. With regenerative braking the 20-30% of the energy consumption of the urban electric vehicles can be saved.
The DC voltage of the overhead line is produced by a diode rectifier circuit connected to the three-phase public supply network through a transformer (Fig.3.1.a). Therefore the vehicles cannot recuperate the braking energy into the public supply network. Considering the fact that in urban traffic a few vehicles powered from the same contact line, the energy transmission can be achieved between the vehicles. Therefore the recuperated energy (current) of a braking vehicle can be consumed by other vehicles. In the aspect of the vehicles the regenerative braking mode can be achieved, if provided that the contact line voltage not exceeds the permissible value. If the maximum voltage level is reached, other braking mode should be selected e.g.: resistive braking.
The contact line is generally overhead line (Fig.3.1.b.), at trolleys two overhead lines (Fig.3.1.c.). The energy is supplied by a third rail at the metro and the millennium underground train. The contact line is distributed to segments that can be separately released, the energy supply of the segments can be unidirectional or bidirectional. The energy supply of the busy, radial-connected junctions causes problems for the urban vehicles.